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Interactions Cellulaires Neuroendocriniennes, UMR 6544-Centre National de la Recherche Scientifique, Université de la Méditerranée, Institut Jean-Roche, Faculté de Médecine Nord, Marseille, France
1 Service de Neurochirurgie, Centre Hospitalo-Universitaire Timone, Marseille, France
2 Département de Thérapie Génique et Thérapie Cellulaire-Etablissement Français du Sang, Marseille, France
(Requests for offprints should be addressed to A Barlier; Email: barlier.a{at}jean-roche.univ-mrs.fr)
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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-subunit promoter (-subunit 1 –520, +33 bp, and -subunit 2 –907, +33 bp) were tested. In gonadotroph adenomas, the percentage of identified fluorescent cells and the fluorescence intensity analyzed by fluorescence-activated cell sorting indicated that the strength of the -subunit 1 and -subunit 2 promoters were comparable to that of the PGK promoter. Primary cultures of rat pituitary cells showed that -subunit 1 is more selective to thyreotroph and gonadotroph phenotypes than -subunit 2. GH promoter activity appeared weak in somatotroph adenomas. The human GH enhancer did not increase the GH promoter activity at all but the human prolactin promoter (–250 bp) allowed 4-fold more fluorescent cells to be obtained than the GH promoter. Several cell lines appeared too permissive to test cell-specificity of pituitary promoters. However, on human non-pituitary cell cultures, the tested pituitary promoters seemed clearly selective to target endocrine pituitary phenotypes. This study gives a starting point for a gene-therapy program using lentiviral vectors to transfer therapeutic genes in human pituitary adenomas. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Gene transfer by viral vectors can be considered as a promising way to achieve a specific, efficient treatment. The human pituitary tumors have a slow growth rate in vivo and are practically quiescent in vitro (Atkin et al. 1997). Adenoviral vectors could be therefore considered as an appropriate choice for genetically modifying pituitary cells, given their high infecting capacity in non-dividing cells (Castro et al. 1997, Lee et al. 1999, Southgate et al. 2000, 2001). However, serious concerns have been raised about the safety of using the first generation of adenoviral vectors because of their capacity to elicit severe inflammatory responses. In fact, pituitary cellular infiltration has been reported after regional in vivo stereotaxic pituitary injection of adenovirus in sheep (Davis et al. 2002), and, contrary to what has been observed after the infection of brain parenchyma (Kajiwara et al. 2000), circulating anti-adenovirus-neutralizing antibodies have been detected consistently in rat receiving such injections. Furthermore, a significant decrease of transgene expression was seen after 3 months (Southgate et al. 2001). Newer ‘high-capacity adenoviral vectors’ could open new perspectives founded on both long-term transgene expression and absence of inflammatory reactions (Thomas et al. 2000).
Lentiviral vectors have recently been used to transduce a variety of low-proliferating or quiescent cells, such as differentiated neurons (Naldini et al. 1996a, b). They have enough cloning capacity to allow more complex expression than vectors derived from the murine leukemia virus (Kafri et al. 2000), and transduction in vivo of a variety of tissues, such as brain, liver, skeletal muscle or pancreas, has shown a sustained expression lasting for 6 months (Blomer et al. 1997). These qualities render lentiviral vectors potentially useful for the treatment of human pituitary adenomas.
Beside direct therapeutic effects, a major aim of gene therapy is to prevent damage of non-target tissues. Taking advantage of the fact that hormonal specificity is preserved in adenomas, these tumors represent an interesting model in the search of targeted expression of genes using cell-specific promoters. Several promoters of human pituitary hormones driving marker or toxic genes in adenovirus have been tested on animals (Lee et al. 1999, 2002, Southgate et al. 2000, 2001, Davis et al. 2001). These experiments have shown a restricted expression of reporter gene under the control of pituitary promoters, though they have not been applied to human pituitary adenomatous cells; only one study used human prolactinoma cells in vitro to transfer a gene by an adenoviral vector (Freese et al. 1996).
Human pituitary adenomas are difficult to obtain. Fortunately, the cells can be put into in vitro conditions mimicking some aspects of the natural disease, provided various stringent requirements be fulfilled to preserve for several weeks the molecular and pharmacological properties of tumoral cells (Jaquet et al. 1985). Using this in vitro model, we present for the first time a lentiviral vector derived from human immunodeficiency virus type-1 (HIV-1) as an effective vehicle to genetically manipulate human pituitary adenoma cells. The strength and the specificity of human promoters of both -subunit of glycoprotein hormones (-subunit) and GH were evaluated on gonadotroph and somatotroph tumors as well as in other cell populations. Considering that the utilization of strong promoters can reduce both the viral doses employed and side effects, an increment of GH-promoter strength was attempted. Considering that the majority of somatotroph adenomas are in fact somatolactotroph tumors, a human GH-promoter enhancer (Jin et al. 1999) was used and compared with the PRL promoter.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The lentiviral vector pRRLT-PGK-eGFPsin18 (Naldini et al. 1996a, b) encodes eGFP placed under control of phosphoglycerate kinase (PGK), an ubiquitous gene promoter. In the present study, pRRLT-PGK-eGFPsin18 containing a central DNA flap sequence (Zennou et al. 2000) is called lenti-PGK-eGFP. From this plasmid, six different lentiviral vectors containing human pituitary promoters at the place of PGK promoter sequence were generated (Fig. 1
). The PGK promoter was removed using BamHI/BstXI digestion. Human promoters of the glyco-protein hormone -subunit (-subunit 1 –520, +33 bp, and -subunit 2 –907, +33 bp) and of GH (–481, +54 bp) were obtained after PCR amplification from human genomic DNA, using the following primers. For -subunit 1: 5'-CAACTTTGCGTTCTTTGG-3' and 5'-CTTCGTCTTATGAGTTCTCAGTAAC-3'; for -subunit 2: 5'-CCTTCTTAGAATTGCCTCATACCT-3' and 5'-CTTCGTCTTATGAGTTCTCAGTAAC-3'; for GH: 5'-AATCATGCCCAGAACCCCCGCAATC-3' and 5'-TAGTGAGCTGTCGACAGGACC-3'. These fragments were subcloned into the PCRscript Amp SK(+) plasmid at an SrfI site (Stratagene, Amsterdam, The Netherlands). The fragments of -subunit 1 and -subunit 2 were excised from the PCRscript Amp SK(+) plasmid using NotI and EcoRI enzymes and ligated to RRLT-X-eGFPsin18 to generate the new vectors, RRLT--subunit 1-eGFPsin18 and RRLT--subunit 2-eGFPsin18, named lenti--subunit 1-eGFP and lenti--subunit 2-eGFP, respectively. A NotI-blunted XhoI fragment was generated to recover the GH promoter from PCRscript Amp SK(+) and inserted instead of PGK into the lenti-PGK-eGFP vector plasmid at a BamHI-blunted XhoI site. This new RRLT-hGH-eGFPsin18 vector was named lenti-GH-eGFP. The plasmid containing a GH enhancer was a gift from Dr P Cattini (University of Manitoba, Canada). The 1.6 G fragment was excised using EcoRV and ClaI enzymes and inserted upstream of the GH promoter. The vector was named lenti-1.6 GH-eGFP. A region of 327 bp at the 3' end of the 1.6 G fragment was obtained by PCR amplification from the 1.6 G plasmid using the primers 5'-ACACTAGCCCCAAAGTTAATGAAG-3' and 5'-CTTAACCCTCCTGCCCCTGACTT-3'. The vector containing this portion upstream of the GH promoter was named lenti-enhGH-eGFP. The human PRL promoter (–250 bp) was a gift from Dr J Martial (Université de Liège, Liège, Belgium; Quentien et al. 2002). The fragment was excised using HindIII and BglII enzymes and used instead of the PGK promoter to generate the RRLT-PRL-eGFPsin18 vector, named lenti-PRL-eGFP. Each lentiviral plasmid was verified by DNA sequencing (Ceq 8000; Beckman Coulter, Roissy, France).
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Lentiviral particles were generated by using HEK 293T cells submitted to a triple polyethylenimine-mediated cotransfection with each lentiviral vector plasmid, conjointly with the vesicular stomatitis virus-G-encoding plasmid pMDG and the packaging plasmid pCMV
R8.91 (Naldini et al. 1996a, 1996b). The supernatants were collected twice at 1-day intervals, treated with DNase I (37 °C for 15 min), concentrated by ultracentrifugation, aliquoted and stored at –80 °C. All the supernatants were free of replication-competent virus as determinated by the replication-competent retrovirus (RCR) assay. HeLaP4 cells which stably contain the lacZ gene under the control of the tat-dependent HIV promoter were transduced and cultivated for 2 weeks, trypsinization being done every 3–4 days to favour the free expansion of cells. After infection with replicating HIV, HeLaP4 cells constantly show a clear-cut expression of ß-galactosidase (ß-gal) and they form syncitia. In our material, the consistent absence of ß-gal expression and the absence of syncitia indicated the absence of HIV-like replication-competent viruses.
Titration of lentiviral vector supernatants
Titration of lenti-PGK-eGFP, lenti--subunit 1-eGFP and lenti--subunit 2-eGFP vector supernatants was performed on HeLaT cells. In this cell line, both -subunit promoters as well as the PGK promoter were able to induce eGFP transcription. No eGFP expression under GH and PRL promoters was found in HeLaT cells. Consequently, titration of lenti-GH-eGFP, lenti-enhGH-eGFP, lenti-1.6 GH-eGFP and lenti-PRL-eGFP vector supernatants was performed on GH4C1 cells (a rat mammosomatotroph cell line) following the same procedure employed on HeLaT cells. Briefly, 1 x 105 cells were plated in 12-well dishes; 24 h later, cells were transduced overnight in 500 µl appropriate medium (Gerolami et al. 2000, Le Pechon-Vallée et al. 2000) containing 8 µg/ml polybrene and 0.1–1 µl concentrated viral supernatant: next day, the transduction medium was removed and cells were grown in 10% appropriate medium for 3 days. The percentage of transduced cells was determined by fluorescence-activated cell sorting (FACS), and the viral titer was calculated as previously described (Limon et al. 1997). Titers around 1 x 108 transducing units per ml were obtained. The titers of lenti-PGK-eGFP supernatants were similar when evaluated on HeLaT or GH4C1 cells and so the titrations obtained from these two cell lines could be safely compared.
Cell cultures
Human pituitary adenomas Fragments of human pituitary adenomas were obtained from patients submitted to trans-sphenoidal surgery. Gonadotroph and somatotroph tumors were selected according to the clinical hormonal status and immunocytochemical data (Table 1; Barlier et al. 1998). The present study was approved by the Ethics Commitee of the University of Aix-Marseille (France) and was undertaken after informed consent of patients and participants.
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Cell lines Various cell lines were tested to check the strength and specificity of pituitary promoters. MCF7 (a human breast cancer cell line), U118 (a human glioblastoma cell line), T3 (a mouse gonadotroph cell line; Windle et al. 1990) and LßT2 (a mouse gonadotroph cell line; Turgeon et al. 1996) were cultured in DMEM supplemented with 10% FCS and penicillin/streptomycin at 37 °C in humidified air atmosphere containing 5% CO2. GH4C1 and GC (a rat somatotroph cell line) cells were cultured in Ham’s F-10 supplemented with 2.5% FCS and 7.5% horse serum with penicillin/streptomycin at 37 °C in humidified atmosphere containing 7% CO2 (Le Pechon-Vallée et al. 2000).
Human non-pituitary primary cultures Human thyroid tissue were obtained from surgical fragments of patients bearing Grave’s disease. The fragments were collected in Coon’s modification of Ham’s F-12 medium (Eurobio, Les Ulis, France) and were dissociated mechanically and enzymatically (Rasmussen et al. 1996). Cells were washed and plated in Coon’s modification of Ham’s F-12 medium supplemented with 5% FCS enriched with bovine thyroid-stimulating hormone (TSH; 1 IU/l), human insulin (10 mg/l), somatostatin (10 µg/l), human transferrin (6 mg/l), hydrocortisone (10–8 M) and glycylhistidyl-acetate (10 µg/l), at 37 °C, in a humidified atmosphere containing 5% CO2 (Rasmussen et al. 1996). Human lymphocytes were cultured in RPMI medium supplemented with 10% FCS and penicillin/streptomycin at 37 °C in humidified atmosphere containing 7% CO2. Cells from primary culture of human glioma were cultured in DMEM/Ham’s F-12 supplemented with 10% FCS and penicillin/streptomycin, at 37 °C in a humidified atmosphere containing 7% CO2. Finally, primary cultures of stroma cells from human adenomas were tested. These cells were obtained from primary cultures of human pituitary adenomas cultured for 3 months, which allowed stroma cells to proliferate as adenomatous cells progressively died. After this period, only stroma cells, mainly fibroblasts, were present, according to both immunocytochemical identification and the absence of detectable hormone in the medium. These cells were collected in new plates and cultured in DMEM supplemented with 10% FCS and penicillin/streptomycin at 37 °C in a humidified atmosphere containing 7% CO2.
Rat anterior pituitary primary culture As previously described (Moreau et al. 1997), 50 female Wistar rats (201–225 g; Charles River, Iffa Credo, L’Arbresle, France) were decapitated and the pituitaries rapidly dissected out, cut into small pieces and enzymatically dispersed in trypsin-DMEM at 37 °C. DNase (1 mg/ml) was added for 1 min. Trypsin activity was stopped and a Ca2+/Mg2+-free medium containing EDTA was used to facilitate cell dissociation. Cells were suspended and centrifuged for 10 min at 300 g. After counting, they were seeded over glass coverslips in 12-well dishes at 2 x 105 cells/well.
Transduction of cells
After 5 days of culture, pituitary adenoma cells were transduced using various multiplicities of infection (MOIs) of each lentiviral vector in the presence of 6 µg/ml polybrene and appropriate medium to get a final volume of 500 µl. Cells were incubated overnight in this medium. The following day, considered as day 1 post-transduction, the medium was removed and replaced with 1 ml DMEM supplemented with 10% FCS. The expression of eGFP was followed up by fluorescence microscopy and/or FACS. For the long-term studies, the culture medium was renewed every week and cells were trypsinized and plated onto new coated wells to prevent fibroblastic invasion.
Cell lines, human non-pituitary cells and human pituitary-derived fibroblasts were plated at 105 cells/well in 12-well dishes. Then, 24 h later, cells were infected with each lentiviral construct in the presence of polybrene at 8 µg/ml in 500 µl appropriate medium. At day 1 post-transduction, the medium was changed. The fluorescence was analyzed by FACS 3 days later.
Primary cultures of rat pituitary were transduced following the same procedure. Briefly, after 48 h of culture, dishes were infected overnight with a 5-MOI load of viral particles conveying either the PGK, the -subunit 1 or the -subunit 2 promoter. At day 5 the cultures were fixed in 10% buffered nascent formaldehyde, cryo-protected in 2 M buffered sucrose and stored at –80 °C until analysis.
Cytochemical identification of cells
Cytospinning of human pituitary adenoma cells In order to get human adenoma cells without fragments of culture matrix and fibroblasts, a gentle enzymatic dissociation was performed for 1 min with 100 µl trypsin-EDTA (Invitrogen). Detached cells were resuspended in 100 µl of appropriate medium. Each aliquot was dropped down into a disposable cytofunnel (Shandon, Pittsburgh, PA, USA) and centrifuged against a gelatin-coated cytoslide (Shandon, Eragny, France) at 1000 r.p.m. for 5 min in a Cytospin 2 apparatus (Shandon, Cheshire, UK). After centrifugation the slides were immersed for 1 h in 10% buffered nascent formaldehyde, pH 7.4. Fixed cells were either conserved at –80 °C in 2 M buffered sucrose or mounted immediately in Mowiol for microscopic observation and cell counting.
Immunocytochemistry Cultured tumoral cells and fibroblasts were fixed for 30 min in buffered nascent formaldehyde; residual free aldehyde groups were quenched by buffered 50 mM ammonium chloride; unspecific epitopes were neutralized by BSA. After cell permeabilization with 0.2% Triton-X100 (Sigma-Aldrich) the cells were incubated overnight (at 4 xC) with 1:100 dilutions of either a monoclonal anti-human -subunit of pituitary glycoprotein hormones antibody (Dako, Trappes, France), a polyclonal anti-human chromogranin A antibody (Immunotech Beckman Coulter, Roissy, France) or a monoclonal anti-human fibroblast antibody (Cymbus Biotechnology, Hants, UK). Specificity was controlled by substituting the main antibody with buffered BSA. A second incubation followed for 2 h at 4 °C with either FITC-conjugated F(ab')2 donkey-anti-mouse IgG fragments (Jackson ImmunoResearch Laboratories, West Grove, PA, USA) for detecting the -subunit, with Texas Red-conjugated similar donkey anti-rabbit fragments for detecting chromogranin A, or with Texas Red-conjugated donkey anti-mouse fragments for the identification of fibroblasts.
Immunofluorescence labelling was also performed to identify the hormone phenotype of infected rat adenohypophyseal cells (Moreau et al. 1997). Thawed cells were refixed in 10% buffered nascent formaldehyde, quenched, permeabilized and neutralized. Primary antibody incubation was done overnight at 4 °C with polyclonal antibodies against rat PRL, luteinizing hormone, GH and TSH elaborated by Dr A.F. Parlow (Harbor-UCLA Medical Center, Torrance, CA, USA) and with a polyclonal anti-rat adrenocorticotrophin (ACTH) 1–24 antibody elaborated by Dr M. Grino (Institut Jean Roche, Faculty of Medicine, Marseille, France). TRITC (tetramethylrhodamine ß-isothiocyanate)-conjugated F(ab')2 anti-rabbit IgG fragments were used as secondary layers to detect PRL, ß-TSH and ACTH; GH and ß-luteinizing hormone were detected with Protein A coupled to Texas Red (Jackson ImmunoResearch Laboratories).
Confocal microscopy Intracellular localization of eGFP was visualized 3 and 5 days after infection. Fixed and permeabilized cells were counterstained on slides with 2 µg/ml propidium iodide (Molecular Probes P-3566) in Mowiol mounting medium; this procedure gives a relatively low background fluorescence from RNA and counterstains cell nuclei in optimal contrast conditions with respect to eGFP. Specimens were observed on a Leica TCS laser-scanning confocal device coupled to a DMR inverted microscope equipped with planachromatic 20/0.4 and 63/1.4 objectives (Leica, Heidelberg, Germany). The argon/krypton laser source generated spectral lines at 488 and 568 nm; signals were recovered through bandpass filters centered at 520 nm (green emission) and 600 nm (red emission). The Leica Scanware algorithm was used to store optical sections. Infected adenoma cells labelled for human chromogranin A were also observed under confocal microscopy.
Estimation of eGFP-expressing cells
FACS To follow up eGFP expression, FACS analysis was done. Cells were trypsinized and resuspended in 3 µl of iodure propidium (50 µg/ml; BD Biosciences, Pharmingen, San Diego, CA, USA) in 500 of appropriate medium and run on a FACSsort (Becton Dickinson, San Jose, CA, USA). Data were analyzed with the CellQuest program (Becton Dickinson). 10 000 events were acquired for each analysis.
Quantitative microscopy For nine non-functioning gonadotroph adenomas and for primary cultures of rat pituitary, quantitative microscopy of fluorescent cells was performed using a Nikon TE-300 inverted microscope supplied with a Plan Fluor ELWD 20x/0.45 PH1 DM objective and a Hamamatsu C4742–95 digital camera monitored with the Twain Two driver. For each slide, 10–102 microscopic fields (430 µm longx345 µm wide) were digitized according to a systematic random sampling procedure (Gundersen & Jensen 1987). For human adenomas, each field was recorded twice: once under phase contrast to count the total number of cells per field, and once under epifluorescence (barrier filter centered at 480 nm) for identifying and counting eGFP-emitting cells. The capture time was constantly fixed at 111 ms, the time at which any fluorescent cell was neither visualized nor detected by the camera in non-infected cells; the weakest eGFP emission was 37 times more brilliant than the background noise. The total number of cells counted per experimental condition ranged from 521 to 4136. For cultured rat pituitary cells, each field was captured three times: once under phase contrast, once under epifluorescence with barrier filter centered at 480 nm (eGFP cells), and once with a barrier filter centered at 560 nm to detect hormone-labelled cells. The total number of cells counted per hormone phenotype ranged from 2460 to 7341. All cell counts were performed directly by one single investigator, without any image-processing algorithm. Doubtful decisions were cleared by overlaying fluorescence and phase-contrast images with Photoshop Software (Adobe System, Mountain View, CA, USA).
Statistical analysis
For quantitative microscopy, the statistical analysis was done taking into consideration the first 500 cells systematically random-sampled and computed for each experimental condition. Statistical significance was determined by one-factor ANOVA followed by Dunnett’s test. For FACS quantitation, significance was determined by non-parametric Wilcoxon test or Mann–Whitney test. Data were expressed as the mean±S.E.M.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Human pituitary adenomas contain non-dividing tumoral cells proper and stroma cells, mainly fibroblasts, actively proliferating in vitro. Considering that the primary culture of adenoma cells could be invaded by fibroblasts, we first characterized the cells actually present after several days of culture using cytospinning and immunocytochemistry. Cells collected from the culture after gentle trypsinization and cytospun were arranged mainly into clusters composed of a few cells to several tens of cells, displaying two different morphologies (Fig. 2A). The great majority consisted of small, circular or elliptical cells, about 12–15 µm in diameter, positively labelled with antibodies directed against chromogranin A and/or -subunit of pituitary glycoprotein hormones; these cells were considered as adenomatous cells proper (Fig. 2B). Conjointly, some clusters included a few much larger and polymorphic cells (25–40 µm) positively labelled with the anti-human fibroblast antibody (Fig. 2A) or with the anti--subunit antibody; the first group was considered to be composed of fibroblasts and the second of large adenoma cells. Cell counts showed that fibroblasts did not exceed 0.3% of the total number of cells at day 20; large adenoma cells did not exceed 0.8%.
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To evaluate transduction effciency only the small eGFP-positive cells were quantified (Fig. 2B
). After transduction, variable eGFP fluorescence intensity from one adenoma to another was observed. Gonadotroph cells expressed eGFP as a green cytoplasmic and nuclear fluorescence of variable intensity (Fig. 3A and B). Both cytoplasmic and nuclear fluorescences consistently increased with the concentration of lentiviral particules. At day 5, the mean percentage of eGFP-fluorescent cells was 6.1% at 1 MOI. The percentage of eGFP-positive cells rose abruptly to 43.4% at 2.5 MOI, attaining 67.8% after a viral load of 100 MOI (Fig. 2C–H and Fig. 4). The augmentation of the nuclear fluorescence followed a more regular course: 0.6, 21, 30, 59 and 74% of cells with an eGFP-emitting nucleus were found at 1, 2.5, 10, 50 and 100 MOI, respectively (Fig. 4).
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). The outburst of expression happened between days 2 and 5 (Fig. 2K and L, and Fig. 5) and was still present at day 20 (Fig. 2M and N). Fibroblast-free adenoma cells derived from two clinically non-functioning tumors (NF8, NF10) showed a strong eGFP fluorescence (similar to Fig. 2N) 90 days after a 10 MOI infection.
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eGFP expression driven by -subunit 1, -subunit 2 and GH promoters in human gonadotroph and somatotroph adenomas
The -subunit 1 promoter was found to be a strong promoter in gonadotroph adenomas. In fact, the mean percentages of fluorescent cells were not significantly different from values obtained after infection with lenti-PGK-eGFP. Transduction effciency significantly decreased when the -subunit 2 promoter was used (Fig. 6). Driven by the GH promoter, eGFP was detected in only 5.7% of gonadotroph cells. In somatotroph adenomas, the mean percentage of fluorescent cells was not significantly different under PGK, -subunit 1 or -subunit 2 promoters and reached only 31% under the GH promoter.
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-subunit 1 and -subunit 2 promoters a great variability in fluorescence intensity was observed from one tumor to another (Table 2). In all cases, fluorescence intensity under the GH promoter was significantly lower than that observed under the PGK promoter. In human somatotroph cells, it represented only 4% of the mean fluorescence intensity under the PGK promoter. Under the -subunit 2 promoter, both types of tumor showed a lower, but not significant, fluorescence than that observed under the -subunit 1 promoter.
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-subunit 1-eGFP and lenti--subunit 2-eGFP at 5 and 10 MOI (Table 3). In the same tumors, increasing doses of lenti-GH-eGFP did not improve the percentage of fluorescent cells. The time-course variation of eGFP expression driven by the -subunit 1 and -subunit 2 promoters in two gonadotroph adenomas (NF11, NF12) showed comparable fluorescent cell percentages (about 50%) and brilliances at days 8 and 20 (about 250 arbitary units).
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-subunit 1, -subunit 2 and GH promoters in pituitary and non-pituitary cell lines
To test the cell specificity of -subunit 1, -subunit 2 and GH promoters, eGFP expression was assessed in gonadotroph (T3, LßT2), somatotroph (GH4C1, GC) and non-pituitary (MCF7, U118) cell lines, and compared with the eGFP expression driven by the ubiquitous PGK promoter. Considering the mean percentages of fluorescent cells (Fig. 7A), -subunit 1 and -subunit 2 promoters showed a similar activity to the PGK promoter in gonadotroph cell lines and, surprisingly, in somatotroph cell lines. These percentages were significantly lower in non-pituitary cell lines (MCF7, U118) but it is noteworthy that they remained high: more than 50% of cells expressed eGFP under the -subunit 1 and -subunit 2 promoters. The mean percentages of fluorescent cells under the GH promoter were similar to that observed under the PGK promoter in somatotroph cell lines and was signficantly lower in gonadotroph cell lines and in non-pituitary cell lines.
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). The activity of gonadotroph promoters was the same as that of the PGK promoter in gonadotroph cell lines and was only a little lower than what was observed with the PGK promoter in other cell lines, except in U118 in which both lenti--subunit 1-eGFP and lenti--subunit 2-eGFP exhibited only 7% of the activity detected for lenti-PGK-eGFP. The activity of the GH promoter was found constantly to be lower than that of the PGK promoter even in somatotroph cell lines, in which the intensities under the GH promoter represented only 14 and 24% of the intensities observed for PGK in GH4C1 and GC cells.
Cell-type-specific eGFP expression driven by -subunit 1, -subunit 2 and GH promoters in human non-pituitary cells and human pituitary-derived fibroblasts
To test the cellular specificity of pituitary promoters, four human primary cultures were used: fibroblasts, lymphocytes, glioma cells and thyrocytes. The experiments were run under the same conditions as for cell lines. In all cultures, the mean percentages of fluorescent cells as well as the mean fluorescence intensities dramatically decreased under pituitary promoters compared with the PGK promoter (Fig. 8). The mean percentages of fluorescent cells under pituitary promoters did not rise beyond 6%, except in thyrocytes where it attained 21% with both the -subunit 1 and -subunit 2 promoters; however, the corresponding fluorescent intensities in this type of cells were low, representing only 10% of the intensity measured after the PGK promoter. In fibro-blasts, under increasing loads from 5 to 50 MOI of vectors containing eGFP driven by the pituitary promoters, the number of fluorescent cells as well as the fluorescence intensities remained unchanged (Fig. 9). Similar results were obtained in the rest of human non-pituitary cultures.
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-subunit 1, -subunit 2 and GH promoters in rat pituitary primary culture
To test the cellular specificity of pituitary promoters with respect to pituitary endocrine cells, the eGFP expression was assessed on rat pituitary primary culture transduced by lenti--subunit 1-eGFP, lenti--subunit 2-eGFP and lenti-GH-eGFP and compared with the transduction obtained by lenti-PGK-eGFP. Under the PGK promoter, all endocrine cells of the anterior pituitary were able to be transduced. Under the -subunit 1 promoter, the maximal percentage of fluorescent cells was observed in thyreotroph and gonadotroph cells. Thyreotroph cells attained 86% and gonadotroph cells 43% of PGK-fluorescent cells. In other cell types, the percentages of -subunit 1-fluorescent cells represented only 8% of PGK-fluorescent PRL cells, 5% of PGK-fluorescent GH cells and 5% of PGK-fluorescent ACTH cells. The -subunit 2 promoter appared less efficient than -subunit 1 in both thyreotroph and gonadotroph phenotypes. Indeed, the percentages of fluorescent cells under -subunit 2 were higher than those under -subunit 1 in PRL, GH and ACTH cells: they represented 29, 33 and 52% of PGK-fluorescent cells, respectively (Fig. 10). No fluorescent cells were seen under the GH promoter.
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To increase the percentage of fluorescent cells after transduction of human somatotroph cells by lentiviral vector containing pituitary promoter, the 1.6 G fragment from the locus-control region in the human GH gene, known to be a distal enhancer, and a shorter fragment, enhGH containing Pit-1 sequences (Jin et al. 1999), were put upstream of the GH promoter. Any significant increase of the percentage of fluorescent cells was observed in GC and GH4C1 cell lines as well as in somatotroph adenoma cells. With the majority of human somatotroph adenomas being somatolactotroph adenoma, the PRL promoter, inserted instead of the GH promoter, was evaluated in the same conditions. In these cells, the mean percentages of fluorescent cells increased by a factor of 4 (Fig. 11); a 2-fold mean fluorescence intensity was observed under the PRL promoter in comparison to the GH promoter (data not shown). These results were obtained on two adenomas (A5, A6) considered as ‘pure’ somatotroph adenomas since they contained only 10% of PRL cells, according to data from the Surgical Pathology Department of Timone Hospital, Marseille, France (D Figarella-Branger, unpublished data). The PRL promoter appeared highly specific: the mean percentages of fluorescent cells (Fig. 11) as well as the mean fluorescence intensities (data not shown) dramatically decreased under the PRL promoter compared with the PGK promoter in human non-pituitary cells, human pituitary-derived fibroblasts and non-pituitary cell lines (U118 and MCF7) as well as in one human gonadotroph adenoma cell line (NF13). The mean fluorescence intensities represented only 4% of the mean intensities observed for PGK in human gonadotroph adenoma cells.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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After using eGFP expression as a marker gene under the control of the PGK promoter to assess gene-transfer efficiency, both gonadotroph and somatotroph adenomas were found efficiently transduced even with moderate viral loads: up to 97% of cells were able to express eGFP 5 days after infection. The percentage of eGFP-positive cells increased drastically between days 2 and 5 and was still present 90 days after infection. This sudden increment cannot be attributed to proliferation of adenoma cells (Atkin et al. 1997) and/or infected fibroblasts. Nevertheless, it is well known that two mechanisms can artificially increase eGFP: first, pseudotransduction, i.e. the direct transfer of marker gene protein by its presence in vector supernatants or its incidental incorporation into vector particules (Liu et al. 1996), and secondly, the transient expression of the marker gene inside a target cell prior to the vector’s integration into the specific target cell’s genome (Haas et al. 2000). In the present study, two facts contribute to excluding biased eGFP expression: 1, the fluorescence at day 1 post-infection was absent in both transduced and non-transduced cells; this observation eliminates pseudotransduction; 2, the time course of events showed that the increment of adenomatous cells expressing eGFP was quite in advance of any increment produced by cell proliferation; this elimates the second bias.
If the safety of neighbouring tissues (optic nerve or hypothalamus) is considered, targeting expression of the transgene is the major challenge in gene therapy of pituitary adenomas. Tissue specificity could be attempted by in situ intratumoral injection of the viral vector bearing tissue-specific promoters during transphenoidal microsurgery as suggested by some experiments on rats (Lee et al. 2000). For the first time, in order to target gonadotroph adenomas (for which no specific therapy exists) and somatotroph adenomas resistant to somatostatin agonists, the -subunit of glycoprotein hormone promoter and the GH promoter were tested. The fragments of the promoters were chosen according to the functional sequences involved in both the expression of the gene and the cell specificity.
After transduction by adenovirus, a similar fragment to -subunit 2 ( –846 to 45 bp) has been shown to restrict to the T3 gonadotroph cell line the expression of the ß-gal marker gene or herpes simplex virus thymidine kinase toxic gene (Lee et al. 1999, 2000). However, a shorter fragment (–500 bp) seemed to be sufficient for full expression of the human -subunit gene in pituitary cell lines: the basal expression was not affected by deletion from 1800 to 442 bp upstream of the transcription start site of the gene (Horn et al. 1992), but expression was reduced with loss of promoter sequences between –442 and –391 bp (Schoderbek et al. 1992). This part of the promoter contains a pituitary glycoprotein hormone basal element (PGBE) which is able to function as a thyrotroph/gonadotroph-specific enhancer (Aylwin & Burin 1995). Consequently, two fragments of human -subunit promoter were tested, -subunit 2 and the more restricted fragment -subunit 1 (–520, +33 bp). We showed that both fragments induced eGFP expression in mouse t3 and LßT2 gonadotroph cell lines as well as in human gonadotroph adenomatous in vitro even if they did not secrete -subunit in vivo (cf. NF6). In fact, in nonfunctioning tumors, no correlation was seen between the fluorescence intensity and the in vivo hormonal status. In contrast in somatotroph tumors, one low-secreting tumor (A2) was associated with the weakest activities of the promoters. But our series was too short to be conclusive.
Using –336, +58 bp from the human GH 5' flanking region inserted in adenovirus, Lee et al.(1999) showed restricted expression of a marker gene (ß-gal) and of a toxic gene (thymidine kinase) in a somatotroph GH3 cell line. Some 95–100% of GH3 cells were ß-gal-positive, 4–5 days after infection by adenoviruses containing this fragment of GH promoter. Based on this study and on other experiments in animals (Cattini et al. 1986), we selected a similar portion of the GH promoter (–481, +54 bp) to obtain a specific expression. After transduction of two somatotroph cell lines (GH4C1 and GC) by the lentiviral vector driven by this part of the GH promoter, up to 98% of cells expressed eGFP. However, contrary to what is observed on rat somatotroph cell lines, only 31% of cells expressed eGFP in human somatotroph adenoma cells in culture. In vitro, these tumors still remain GH-secreting and keep their responsiveness to pharmacological treatments (Jaquet et al. 1985). Whatever the model used, the strength of GH promoter was weak in our study, which is in agreement with the results of authors measuring ß-gal activity (Lee et al. 1999). In that study, the ß-gal activity in GH3 cells infected by adenovirus containing GH promoter represented only 16.2% of the ß-gal activity under cytomegalovirus ubiquitous promoter.
To assess cell-specificity of these pituitary promoters, we used different cell lines. Clearly, the activity of the promoter was not restricted to those cell lines producing the hormone corresponding to the tested promoters. Even in non-pituitary cell lines, the pituitary promoter remained active; the fluorescence intensity under pituitary promoters was only low in the U118 glioma cell line. The overlapping of GH and -subunit promoter activities in pituitary cell lines was not surprising if we consider that these pituitary cells derive from a common progenitor, opening the possibility that they share common transcription factors. A similar overlapping was also observed in the study by Lee et al.(1999). What is more surprising is the activity of pituitary hormone promoters in non-pituitary cell lines. It might reflect the incompleteness of differentiation of these cells. From a practical point of view and according to our results, cell lines seem too permissive to be used as cell-specificity testers of pituitary promoters and probably of other promoters as well. Consequently, the cell specificity of the three pituitary promoters might be tested on primary cultured human cells. As we have shown, the activity of the three promoters decreased dramatically in these cells, compared with the activity of PGK promoter, confirming the selectivity of the pituitary promoters. In view of gene therapy of pituitary adenomas using stereotaxic injection, it is noteworthy that the activities of -subunit 1, -subunit 2 and GH promoters were almost absent in fibroblasts coming from human pituitary adenomas and in gliomas, even at heavy doses of lentiviral vectors. This study clearly shows the limits of animal cellular models and pituitary cell lines. In a gene-therapy program it is crucial to have the possibility of testing lentiviral constructs on human tumoral cells in vitro.
Somatotroph adenomas showed a strong activity of -subunit 1 and -subunit 2 promoters. It is noteworthy that 36% of somatotroph adenomas secreted both -subunit hormone and GH (Kontogeorgos et al. 1993). Correspondingly, the GH promoter activity appeared low in gonadotroph adenomas, probably due to the promoter specificity as well as to its relative intrinsic weakness. eGFP expression under the GH promoter was too low at 10 MOI to be observed by epifluorescence in cells of human pituitary adenomas as well as of rat pituitary. The results on rat pituitary cells and on human pituitary adenomas indicate -subunit 1 promoter to be more powerful and more restricted to gonadotroph and thyreotroph phenotypes than the -subunit 2 promoter. Specific elements have been clearly identified in the portion of the -subunit promoter before –500 bp upstream, but the cis elements are not well established at present. Whatever it may be, our results suggest the presence of sequences limiting cell-type-specificity between –500 and –846 bp. Even if a residual activity of the -subunit promoter remained in GH, PRL or ACTH cells, it should not be a contraindication to attempt gene therapy targeted by this promoter, considering that the majority of patients deserving this type of therapy have already developed large, invasive tumors and several pituitary deficiencies.
For improving human somatotroph cell targeting, several strategies were attempted. A 1.6 G fragment, from the locus-control region in the human GH gene and located about 15 kb upstream of the GH transcription initiation site, was identified as enhancer of the GH promoter (Jin et al. 1999). The enhancer activity of this fragment could be localized in a 260 bp region in the 3' end of the 1.6 G which contains Pit-1-binding sites. In transfected GH3 cell line, the 1.6 G fragment induced a 5-fold increment of the human GH promoter activity (Jin et al. 1999). However, in our experiments and using lentiviral vectors as vehicles, the 1.6 G fragment as well as the 260 bp region were unable to increase the activity, whatever the model used.
In vivo, 80% of somatotroph adenomas are GH- and PRL-secreting. According to the immunocytochemical data, almost all somatotroph adenomas contain somatolactotroph and lactotroph cells (Melmed et al. 1995) During pituitary ontogenesis, somatotroph and lactotroph cells derive from a common progenitor (Dasen and Rosenfeld 2001). Consequently, the use of PRL promoter to target human somatotroph cells was attempted. Even if HIV-based vectors have a greater packaging limit than oncoretroviral vectors, the titer decreased in a semi-logarithmic manner as insert size and proviral length increased (Kumar et al. 1999). Accordingly, the first –250 bp of the PRL promoter, which contains Pit-1-binding sites, was chosen (Lemaigre et al. 1991) instead of the –4429 bp PRL promoter fragment previously tested by Davis et al.(2001). In human somatotroph adenomas transduced with lentiviral vectors the mean percentage of fluorescent cells under the PRL promoter quadrupled in comparison to the mean percentage under the GH promoter, even in somatotroph adenomas considered as ‘pure’ by immunocytochemistry. Moreover, this short fragment of PRL promoter appeared very specific to PRL and GH phenotypes as well as the –4429 bp PRL fragment in adenoviral vector on ovine pituitary (Davis et al. 2001).
In conclusion, HIV-derived lentiviral vectors are promising tools to achieve gene transfer in human pituitary adenoma cells. According to our results, an effective transduction was observed up to 3 months. The percentage of cell fluorescence reached a plateau around 5 MOI and at day 5, whichever the promoter used. The results showed that the GH promoter seems specific but poorly active in human adenoma somatotroph cells. The PRL promoter appeared more adequate to target human somatotroph cells. -Subunit 1 and -subunit 2 promoters are powerful and specific gonadotroph promoters, adequately targeting long-term gonadotroph cells from nonfunctioning adenomas. Results on rat pituitary showed -subunit 1 promoter as the most appropriate choice. Considering that long-term transgene expression can be obtained using lentiviral vectors, that receptor-mediated pharmacology can actually be efficient enough for controling the behavior of adenomas expressing receptors, and that pituitary promoters are able to target selectively pituitary cells, these tumors represent good candidates for receptor-dependent gene therapy.
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Acknowledgements |
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T3 and LT2 cell lines, Dr A Denizot (Service de Chirurgie Endocrinienne, CHU Nord, Marseille, France) for human thyroid fragments and Dr L. Ouafik (Laboratoire de Cancérologie Expérimentale, Marseille, France) for human glioma cells. We thank also Professor Peter Cattini (Manitoba, Canada) for the human GH enhancer plasmid, Dr Jean Steinberg for help in statistical analysis and Miss Henriette Gérard for supplying cytospin facilities.
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Funding |
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This work was supported by the Association pour la Recherche sur le Cancer 2001 and the Gene Vector Production Network sponsored by the Association Française contre les myopathies.
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References |
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Received 25 March 2004
Accepted 6 July 2004
Made available online as an Accepted Preprint 16 July 2004
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