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Institute of Biomedical and Clinical Science, Peninsula Medical School, Universities of Exeter and Plymouth, Research Way, Plymouth, Devon PL6 8BU, UK
1 Institut für Zellbiologie, Universitätsklinikum Essen, D-45122 Essen, Germany
(Requests for offprints should be addressed to N G Morgan; Email: noel.morgan{at}pms.ac.uk)
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Introduction |
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(MODY1) (Yamagata et al. 1996a), HNF1 (MODY3) (Yamagata et al. 1996b), pancreas-duodenum homeobox-1 (PDX-1; MODY4) (Stoffers et al. 1997), HNF1ß (MODY5) (Horikawa et al. 1997, Nishigori et al. 1998, Lindner et al. 1999, Bingham et al. 2000, Bingham & Hattersley 2004) and neuroD1/Beta2 (MODY6) (Malecki et al. 1999). The exception is MODY2, which is caused by mutations in the glucokinase gene (Froguel et al. 1993). In most cases, the MODY genes are known to be important for the functional competence of the pancreatic ß-cell and, as a consequence, mutations that lead to altered transcriptional or enzymatic activity are sufficient to cause ß-cell defects and hence diabetes. As yet, the MODY5 gene, HNF1ß, has not been ascribed a clear role in pancreatic ß-cells.
HNF1 and 1ß are nuclear transcription factors of the homeodomain family. Their genes are located on different chromosomes in man, but the two may have arisen by an original gene duplication event during evolution (Bach et al. 1991). The proteins share a high degree of sequence homology, but are most divergent within the C-terminal transactivation domain. HNF1 and -1ß bind to the same DNA consensus sequence and they can interact with this region either as homodimers or as an HNF1/1ß heterodimer (Mendel et al. 1991, Bach & Yaniv 1993, Cereghini 1996).
Despite the apparent similarities between HNF1 and -1ß, it is likely that they undertake distinct functional roles within the ß-cell, as evidenced by the different phenotypes seen in MODY3 and MODY5 patients. Patients with mutations in HNF1ß have an impaired insulin secretory response to glucose and sulphonylureas (Nishigori et al. 1998, Bingham et al. 2000, Ryffel 2001, Pearson et al. 2004) and they exhibit a progressive loss in basal insulin secretion, suggesting a decline in ß-cell mass. In contrast, MODY3 patients retain a robust insulin secretory response to sulphonylureas despite the attenuation of glucose-induced insulin secretion (Pearson et al. 2003, 2004). Thus, it seems likely that HNF1 and -1ß exert differential effects in the ß-cell and that the latter may regulate both secretory competence and cell viability.
Previous studies have established that HNF1 is expressed in mature ß-cells and that it promotes the transcription of a range of genes in these cells. These include several genes that are critical for the maintenance of the ß-cell phenotype, such as Glut-2, PDX-1, L-type pyruvate kinase and possibly insulin (Wang et al. 1998, Ben-Shushan et al. 2001, Shih et al. 2001). HNF1 may also be required for the proliferation of ß-cells as it has been demonstrated to regulate genes involved in control of the cell cycle, such as cyclin E, p27 and insulin-like growth factor-I (Wobser et al. 2002, Yang et al. 2002). In contrast, little is known about the role of HNF1ß in ß-cells. Homozygous HNF1ß knockout mice are non-viable, with death occurring soon after implantation of the embryo (7.5 embryo days in mice) (Barbacci et al. 1999, Coffinier et al. 1999), making the function of HNF1ß difficult to study using whole animal knockout approaches. In experiments where HNF1ß has been selectively deleted in mature mouse ß-cells (using Crerecombinase expressed under the control of the insulin promoter (RIP-Cre)), there was evidence of impaired glucose tolerance and reduced insulin secretion. These are correlated with alterations in the functional activity of other ß-cell transcription factors such that PDX-1 and HNF1 were increased and HNF4 decreased (Wang et al. 2004). These results suggest that the expression of HNF1ß may be required to maintain the differentiation state and functional activity of mature ß-cells (Coffinier et al. 1999, Wang et al. 2004). However, the experiments must be interpreted with caution, since it has subsequently been revealed that alterations in glucose homeostasis can occur in RIP-Cre mice that are unrelated to changes in the gene of interest (Lee et al. 2006).
In order to investigate the function of HNF1ß in mature ß-cells without the potential complications arising from RIP-Cre-recombinase-mediated knockout, we have used a clonal ß-cell line (INS-1) to conditionally express either wild-type (WT) HNF1ß or one of two naturally occurring mutants identified in patients with MODY5 (A263insGG and P328L329del). The mutation P328L329del (abbreviated to P328del) leads to the synthesis of a protein having a severely truncated transactivation domain, but that retains the DNA binding and dimerisation domains (Fig. 1
) (Bingham et al. 2000). From studies in HeLa cells, P328del has been reported to possess increased transcriptional activity compared with WT HNF1ß (Wild et al. 2000), although examination of the profile of genes that are up-regulated in response to the expression of this mutant in INS-1 cells suggests that it may be less active than the WT (Thomas et al. 2004). The A263insGG (A263ins) mutant has no transactivation domain and a truncated DNA-binding domain that is non-functional (Senkel et al. 2005). However, A263ins can still form dimers with WT HNF1ß and this has been suggested to result in dominant-negative activity against the native form in a variety of cell types, including the pancreatic ß-cell line MIN6 (Nishigori et al. 1998, Tomura et al. 1999, Bai et al. 2002).
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Materials and Methods |
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Pancreatic ß-cell lines (INS-1) that conditionally express human WT HNF1ß or one of two mutant forms, designated P328del or A263ins, were used in these experiments. These cell lines were recently described in detail by Thomas et al.(2004) but, briefly, cDNAs encoding WTor the mutant forms of HNF1ß were cloned downstream of the Tet operator in the plasmid pcDNA5/FRT/TO. This plasmid was integrated by site-directed Flp recombination into the insulinoma cell clone INS1-Flp-In-T-Rex. Stable cell lines containing the HNF1ß gene were obtained by hygromycin selection. The Tet operator inhibits the expression of HNF1ß gene and this can be alleviated by the addition of tetracycline.
Cells were cultured in RPMI 1640 medium (Invitrogen) containing 11 mM glucose, with 10% foetal bovine serum, 2 mM L-Gln and 50 µM ß-mercaptoethanol supplemented with 100 U/ml penicillin and 100 µg/ml streptomycin, with 10 µg/ml blasticidine and 150 µg/ml hygromycin to maintain selection. Cells were cultured at 37 ° C in 5% CO2:95% air and grown and maintained in 75 cm3 flasks. They were used in experiments or passaged when approximately 80% confluent. To study the expression of the gene of interest, cells were seeded into 25 cm3 flasks, 24-or 6-well plates for 24 h before the addition of tetracycline at up to 1 µg/ml.
Isolation of rat islets
Islets from Wistar rats were isolated by collagenase digestion of the pancreas. The dissected pancreas was distended with incubation buffer (Gey & Gey 1936) gassed with O2/CO2 (95:5), with 1 mM CaCl2 and 4 mM glucose added just before use) and then finely chopped. Collagenase type XI was added and the pancreatic tissue shaken in a water bath (37 ° C) for about 5 min until the exocrine component of the pancreas was digested, releasing free islets. The islets were hand picked using a finely drawn Pasteur pipette for use in individual experiments.
mRNA isolation
TRIZOL reagent (Invitrogen) was used to extract RNA from INS-1 cells, islets and tissue extracts. The cell lysates were passed though a pipette tip several times and the contents were transferred to sterile microfuge tubes. Chloroform (0.2 ml per 1 ml TRIZOL used) was added, the tubes vortexed, incubated at room temperature for 10 min and then spun at 12 000 g for 15 min. The upper aqueous phase containing the RNA was removed and transferred to a new tube. To this, 0.5 ml isopropanol was added for each 1 ml TRIZOL used and incubated at – 20 ° C for 1 h, before being centrifuged at 12 000 g for 20 min. The resulting pellet was washed twice in 75% ethanol, air-dried and resuspended in RNase-free (diethylpyrocarbonate) water.
Reverse transcriptase (RT)-PCR
The expression level of the HNF1ß gene was examined by a quantitative real-time PCR approach. cDNA was first generated from total RNA using an oligo dT primer by the Thermoscript first round cDNA synthesis kit (Invitrogen). Real-time PCR was then carried out using probes to WT HNF1ß and the endogenous control gene ß-2-microglobulin (probe and primer sequences are described previously; Harries et al. 2004). Reactions contained 36 µM each primer and 8 µM probe in a total reaction volume of 20 µl on the TaqMan 7000 platform. Expression levels of HNF1ß were measured relative to ß-2-microglobulin and normalised to the expression levels in total rat kidney RNA using the 
CT method described in Applied Biosystems User Bulletin number 2 (Relative Quantitation of Gene Expression, pp. 11–15; Warrington, UK).
Western blotting
To extract whole cell protein, INS-1 cells were washed in ice-cold PBS before the addition of 0.2 ml lysis buffer (20 mM Tris, 150 mM NaCl, 1 mM EDTA and 1% Triton X, with 10 µl/ml protease inhibitor cocktail (Sigma) added just before use) per 25 cm3 flask, for 10 min on ice. The flasks were then scraped, the contents transferred to a microfuge tube (on ice) and vortexed (4 x for 15 s), with 5 s on ice between each vortexing. The protein extract was then centrifuged at 1000 g for 10 min at 4 ° C and the supernatant stored at – 80 ° C.
Equal amounts of denatured protein samples were run on a precast bis-Tris–HCl buffered 12% polyacrylamide gel (Invitrogen) at 200 V for 1 h in MOPS SDS running buffer (50 mM 3-(N-morpholino) propane sulphonic acid, 50 mM Tris base, 3.5 mM SDS, 1 mM EDTA). A prestained marker set (Amersham) was included to allow the sizes of relevant bands to be determined. Proteins were then transferred to a polyvinylidene fluoride membrane (Millipore, Watford, Herts, UK) using a ‘wet’ transfer tank (BioRad trans blot cell) for 4 h at 250 mA. The membrane was blocked overnight at 4 ° C with Tris-buffered saline containing 0.05% Tween (TTBS) and 5% low fat dried milk. Primary antibodies raised against HNF1ß, GADD45 and PAR4 were from Santa Cruz Biotech (sc-7411, sc-4100 and sc-1807 respectively; San Diego, CA, USA) anti-PDX-1 was a gift from Prof. C Wright, Vanderbilt University, Nashville, TN, USA, and anti-PTP-BL was a gift from Prof. K Erdmann, Ruhr-University Bochum, Germany. Antibodies were diluted 1 in 2000 in TTBS containing 1% milk and incubated with the membrane for 4 h at room temperature. An appropriate IgG-alkaline phosphatase conjugated secondary antibody was diluted 1 in 30 000 in TTBS containing 1% milk, added to the membrane and incubated for 1 h at room temperature. Immunoreactive bands were visualised using CPD-Star (Sigma) and exposure to X-ray film.
Insulin secretion
INS-1 cells were seeded into 24-well plates at 1 x 105 cells per well, 24 h before addition of tetracycline. At the end of the induction period, cells were washed and preincubated for 1 h in 500 µl of incubation buffer (Gey & Gey 1936) containing 6 mM glucose and 0.1% BSA. Cells were acutely stimulated with the test reagents and the incubation medium was sampled after 1 h for the measurement of insulin by RIA.
Estimation of cell viability with Trypan Blue
For the determination of cell viability, vital dye staining was used. Experiments were carried out in six-well plates with 1 x 105 cells/well seeded 24 h before induction of HNF1ß expression. Floating and attached cells were collected from each well and stained with Trypan Blue. The number of viable and dead cells was counted using a haemocytometer.
Apoptosis assays
CaspACE FITC-VAD-FMK In situ Marker (Promega), a fluoroisothiocyanate conjugate of the cell permeable caspase substrate VAD-FMK, can be localised by fluorescence detection and acts as an in situ marker for cells undergoing apoptosis. Treated cells were labelled with 10 µM CaspACE according to the manufacturer’s instructions and viewed by fluorescence microscopy. The number of green fluorescent cells was counted in a field of about 100 cells and the percentage of apoptosis was calculated.
The Annexin V-Cy3 apoptosis detection kit (Sigma) uses two labels. The first, annexin-V Cy3.18 (AnnCy3), is a red fluorescent protein that binds to phosphatidylserine but cannot cross the plasma membrane. The second label, 6-carboxyfluorescein diacetate (6-CFDA), is cell permeable and is used as a measure of cell viability as it is hydrolysed by esterases present in living cells to produce the green fluorescent compound 6-carboxyfluorescein (6-CF). Dual staining with these two labels can distinguish between live, necrotic and apoptotic cells (Elliott et al. 2002). Following exposure to test reagents, INS-1 cells were labelled with the double-staining solution (AnnCy3 and 6-CFDA) according to the manufacturer’s instructions and viewed by fluorescence microscopy.
Statistical analysis
All individual experiments were performed in at least duplicate and were repeated on a minimum of three separate occasions. The results were analysed by ANOVA and were considered significant when P < 0.05.
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In initial experiments, the expression levels of native HNF1ß in ß-cells were examined. Using semi-quantitative RT-PCR, low levels (as compared to kidney) of HNF1ß mRNA were detectable in INS-1 cells and isolated rat islets (Fig. 2a). This is consistent with microarray data from INS-1 cells (Thomas et al. 2004) and with other studies in ß-cells (Coffinier et al. 1999, Wang et al. 2004, Gunton et al. 2005). Despite the clear presence of mRNA encoding HNF1ß, the protein itself was not readily detectable by Western blotting in uninduced INS-1 cells or in primary rat and human islets, which confirms previous evidence (Maestro et al. 2003, Thomas et al. 2004) that HNF1ß is not highly expressed in mature pancreatic ß-cells.
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A tetracycline inducible system was used to conditionally express either WT HNF1ß or one of two naturally occurring MODY5 mutants, P328L329del (P328del) and A263insGG (A263ins) in INS-1 cells. Tetracycline treatment (24 h) of INS-1 cells stably transfected with WT HNF1ß resulted in a dose-dependent increase in HNF1ß protein levels (Fig. 2b
) with 1000 ng/ml tetracycline yielding the highest levels of HNF1ß expression. Similar results were obtained for cells expressing the P328del and A263ins mutant forms of HNF1ß (not presented).
Effect of HNF1ß expression on insulin secretion
As MODY5 patients are characterised by impaired insulin secretory responses, the effect of increased expression of A263ins, P328del or WT HNF1ß on insulin secretion was studied.
Acute stimulation of the cells with a range of stimuli, including the metabolic fuels mono-methyl-succinate and -ketoisocaproate, a depolarising concentration of KCl or a combination of isobutylmethyl xanthine (IBMX) and phorbol myristate acetate (PMA) caused a significant increase in insulin secretion (Fig. 3), although these cells did not respond to glucose stimulation (results not shown). Induction of WT HNF1ßprotein for 24 h caused a significant decrease in insulin secretion caused by all stimuli tested (Fig. 3a). In contrast, expression of either of the mutant forms of HNF1ß (A263ins or P328del) had no attenuating effect on insulin secretion in response to any of the stimuli used (Fig. 3b and c).
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To study the genes that might be involved in HNF1ß-induced impairment of insulin secretion, microarray data from INS-1 cells expressing HNF1ß (detailed in Thomas et al. 2004) were analysed and it was noted that PDX-1 mRNA is markedly down-regulated upon induction of WT HNF1ß (Table 1). Expression of PDX-1 protein was, therefore, monitored. As expected, PDX-1 protein was strongly expressed in control (uninduced) INS-1 cells and, in agreement with the microarray data, the levels were dramatically decreased within 24 h of WT HNF1ß expression (Fig. 4a). It was notable that PDX-1 levels were not altered following the expression of P328del HNF1ß (nor when A263ins was expressed), despite the fact that P328del acts as a gain of function mutant in some assay systems (Fig. 4b).
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A gradual decline in basal insulin levels (in addition to the loss of stimulated insulin secretion) has been observed in patients with HNF1ß mutations, which could indicate a reduction in ß-cell numbers during disease progression. The effect of expression of A263ins, P328del or WT HNF1ß on ß-cell growth and viability was, therefore, also investigated. Protein expression was induced for up to 96 h and changes in cell numbers were monitored (Fig. 5a–c). Uninduced INS-1 cells displayed a typical sigmoidal growth curve with the total cell number increasing almost fourfold over a 96 h period. Unexpectedly, the induction of expression of WT HNF1ß dramatically inhibited the increase in ß-cell number. This effect was evident within 48 h and it became increasingly marked as the experiment progressed. Even more strikingly, it was noted that, over 96 h, the total cell number had declined to a value that was below the 48h level. This suggests that not only had cell growth been inhibited, but that net cell death had also occurred. The latter effect was not observed with the P328del mutant since, although the expression of this mutant clearly attenuated cell growth, the total cell number did not decline below its initial value. High-level induction of the A263ins mutant had no significant effect on the growth characteristics of the cells, confirming that overexpression of these proteins per se was not responsible for the altered responses measured. Expression of A263ins HNF1ß also failed to alter the number of dead cells recovered from the medium during the 96 h growth period, suggesting that it did not promote any reduction in cell viability.
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To investigate the effects of WT and P328del HNF1ß on INS-1 cell death more directly, CaspACE (Fig. 5e) and annexin V (below) staining were employed to detect apoptotic cells. Both markers clearly revealed that expression of WT HNF1ß increased the extent of apoptosis in ß-cells. By contrast, P328del did not cause any increase in apoptosis above the control levels. The proportion of apoptotic cells detected by annexin V was: WT–uninduced 0.97 ± 0.38%, induced 13.33 ± 0.4% (P< 0.001); P328del–uninduced 0.87 ± 0.3%, induced 1.2 ± 0.3% (NS).
Genes involved in HNF1ß induction of cell death
In order to identify candidate genes that were altered by HNF1ß induction and which might be involved in cell growth and apoptosis, microarray data were used to identify candidate genes. Three genes were selected for further study on the basis that their expression was altered significantly and that they might be expected to regulate growth or viability. The genes selected encoded growth arrest and DNA-inducible protein 45 (GADD45), prostate apoptosis response-4 (PAR4) and protein tyrosine phosphatase-basophil like (PTP-BL) (see Table 1). Western blotting was then carried out to determine whether the corresponding proteins were expressed in INS-1 cells and to establish whether their expression was altered by HNF1ß induction.
GADD45 protein was not detectable in control INS-1 cells, but its expression was clearly induced by NaF, a reagent that has previously been shown to cause apoptosis in islets and ß-cell lines (Hollander et al. 1999, Sheikh et al. 2000, Elliott et al. 2002, Hildesheim et al. 2002). However, despite the evidence of increased GADD45 mRNA expression in response to WT HNF1ß in microarray studies, no GADD45 protein was detectable after either 24 or 48 h of induction of the transcription factor (results not shown).
PAR4 is a Leu zipper protein that is involved in the activation of apoptosis in many cell types (Sells et al. 1997, Rangnekar 1998, Chakraborty et al. 2001), but has not previously been identified in ß-cells. Its expression was altered 2.7-fold by WT HNF1ß at the mRNA level and thus the protein levels of PAR4 were measured in INS-1 cells. Surprisingly, it was observed that INS-1 cells (and rat islets) express abundant amounts of PAR4 protein, even under control conditions, but this was not significantly increased in response to expression of HNF1ß (results not shown).
PTP-BL is a large soluble PTP that has been implicated in regulation of the cytoskeleton and cytokinesis (Erdmann 2003, Herrmann et al. 2003). It also interacts with several proteins involved in the control of apoptosis, including Fas, adenomatous polyposis coli and nerve growth factor (Erdmann et al. 2000, Erdmann 2003, Herrmann et al. 2003). PTP-BL mRNA was increased by both WTand P328del HNF1ß as determined by microarray analysis. Uninduced INS-1 cells were found to express measurable amounts of PTP-BL protein by Western blotting and this was increased markedly by the induction of WT HNF1ß and to a lesser extent by P328del expression. By contrast, the expression of PTP-BL was not influenced by A263ins induction (Fig. 6).
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Discussion |
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In the present work, we have employed a conditional expression system to address the consequences of altered HNF1ß expression in fully differentiated ß-cells derived from an INS-1 cell clone. The characteristics of this system have been described in detail in a recent study (Thomas et al. 2004) which concluded that it is well suited for the regulated expression and functional characterisation of ß-cell transcription factors, including HNF1ß.
Initially, we examined the expression of HNF1ß in the parental INS-1 cell line (INS-1 Flp-In T-Rex) by RT-PCR and confirmed the presence of the transcript. However, HNF1ß protein was not detectable in these cells by Western blotting. A similar situation also pertained in both rat and human islets, where HNF1ß could not be detected by Western blotting in several islet preparations from either species. These data are consistent with previous observations (Coffinier et al. 1999, Maestro et al. 2003, Wang et al. 2004, Gunton et al. 2005) and imply that differentiated ß-cells express HNF1ß mRNA, but that HNF1ß protein is maintained at a relatively low level.
We observed that enforced overexpression of WT HNF1ß in INS-1 cells caused a loss of insulin secretory response, a reduction in the rate of cell growth and a net decrease in cell viability. The latter was mediated by increased apoptosis, as judged by increased caspase activity and enhanced annexin-V staining of the overexpressing cells. This potential inhibition of cell growth by increased HNF1ß expression is consistent with situations where a reduction in HNF1ß levels leads to hyperproliferation of endothelial cells (Gresh et al. 2004, Haumaitre et al. 2005), suggesting that HNF1ß may act to regulate normal cell growth. Thus, a high level of expression of HNF1ß may be detrimental to the status and functional competence of mature ß-cells and is consistent with the finding of limited expression of this protein in mature ß-cells (Maestro et al. 2003). In this context, it is interesting to note that islets from patients with type 2 diabetes may express higher levels of HNF1ß mRNA than controls (Gunton et al. 2005). Although the increase in HNF1ß expression observed in that study (approximately twofold) was not statistically significant, it nevertheless raises the possibility that an elevation of HNF1ß might contribute to the loss of secretory function in the islets of some patients with type 2 diabetes. This suggestion certainly merits further consideration.
In parallel studies to those described above in INS-1 cells, WT HNF1ß protein was also overexpressed in HEK-293 cells to control for possible non-specific effects on cell viability. High-level induction of HNF1ß did not elicit any detrimental effects on proliferation or viability in these cells, suggesting that the responses observed in INS-1 cells were not mediated by non-specific mechanisms associated with protein overexpression. This conclusion is also supported by the fact that differential effects were obtained in INS-1 cells by overexpression of WT versus mutant forms of the protein.
In an attempt to understand the molecular basis for the loss of insulin secretion seen in response to induction of WT HNF1ß, we examined the profile of genes that are influenced by this transcription factor in INS-1 cells (Thomas et al. 2004). This revealed that PDX-1 transcripts are dramatically reduced in response to WT HNF1ß induction. Since PDX-1 is essential for the regulation of a wide variety of important genes in ß-cells (Waeber et al. 1996, Watada et al. 1996, Macfarlane et al. 2000), a reduction in expression would beexpected to exert deleterious effects on insulin secretory responses. Expression of dominant-negative PDX-1 in INS-1 cells has been shown to inhibit nutrient- and KCl-induced insulin secretion (Wang et al. 2005). Monitoring of PDX-1 expression at the protein level in response to HNF1ß confirmed that it declined markedly within 24 h, suggesting that turnover of PDX-1 protein occurs rapidly in cells overexpressing WT HNF1ß.
The observation that HNF1ß induction causes a decreased expression of PDX-1 was unexpected, since it is known that the PDX-1 promoter contains an HNF1 consensus motif (Ben-Shushan et al. 2001), which might be expected to drive increased transcription of the gene. However, this motif appears to be regulated more efficiently by HNF1 in ß-cells (Ben-Shushan et al. 2001) and it is possible that, in the present studies, overexpression of HNF1ß gave rise to a net reduction in homodimeric HNF1 (by promoting heterodimer formation between HNF1 and 1ß) and thereby caused a decrease in the extent of HNF1-driven PDX-1 transcription. Irrespective of the mechanism, the finding that PDX-1 protein levels were reduced in cells expressing HNF1ß is in agreement with the report by Wang et al.(2004) that PDX-1 and HNF1ß are regulated in a reciprocal manner in ß-cells.
Following these considerations, we also examined the results of the microarray analysis to identify additional candidate genes whose altered transcription might underlie the ability of HNF1ß to promote the loss of proliferation and viability of INS-1 cells. Several genes were identified and selected for further analysis. These included GADD45, PAR4 and PTP-BL, all of which are previously unstudied in the ß-cell.
GADD45 encodes a protein that is frequently induced by DNA damage and other cellular stresses in a variety of cells (Hollander et al. 1999, Sheikh et al. 2000, Hildesheim et al. 2002) and may play a role as a mediator of apoptosis. We were unable to detect GADD45 at the protein level in uninduced INS-1 cells, although exposure of INS-1 cells to 5 mM NaF (which induces ß-cell apoptosis (Elliott et al. 2002)) resulted in the appearance of immunoreactive GADD45. This suggests that, as in other cell types, GADD45 may play a role in regulating the apoptotic response of ß-cells to certain stimuli. However, despite the evidence for increased transcription of GADD45 in cells expressing WT HNF1ß, this was not accompanied by a detectable increase in GADD45 protein expression. Thus, we consider it unlikely that GADD45 is primarily involved in mediating the apoptotic response to HNF1ß in INS-1 cells.
A second gene product investigated was PAR4, an immediate early response gene, which was first identified in prostate cancer cells as a gene whose transcription is rapidly up-regulated during the onset of apoptosis. When over-expressed, it can sensitise cells to apoptosis mediated by a range of stimuli (Sells et al. 1997, Rangnekar 1998, Chakraborty et al. 2001). In this study, we demonstrated that INS-1 cells (as well as rat and human islets) express relatively high levels of PAR4 protein, even under control conditions. Induction of HNF1ß expression did not cause any further significant increase in PAR4 levels and we conclude that changes in expression of this protein are unlikely to mediate the loss of ß-cell viability caused by HNF1ß.
The third candidate, PTP-BL, is a soluble PTP, which serves as a central scaffolding protein for a range of cellular effectors. It contains a series of PSD 95 SA p90, discs large, ZO-1 (PDZ) domains which allow it to act as an adaptor to regulate various cellular functions, including cytoskeletal organisation and cytokinesis (Erdmann 2003, Herrmann et al. 2003). It can also control cell viability by regulating, for example, the surface expression of the death receptor, Fas, and the activity of pro-apoptotic transcription factors such as NF
B (reviewed by Erdmann 2003). We observed that induction of the expression of WTor P328del HNF1ß in INS-1 cells caused an increase in PTP-BL protein expression, suggesting that altered expression of PTP-BL could underlie the effects of HNF1ß on INS-1 cell proliferation. This was confirmed by overexpression studies showing that a small increase in PTP-BL protein expression inhibited INS-1 cell growth (Welters et al. unpublished observations).
Overall, therefore, the results presented in this study reveal that HNF1ß protein is not highly expressed in pancreatic ß-cells. Nevertheless, this gene product appears to play an important role in regulation of the functional competence of the cells, as in the case of other MODY genes. However, unlike the situation with other MODY genes, it appears that a low (rather than high) level of expression may be important for maintaining the function of mature ß-cells, since our results reveal that increased levels of HNF1ß can have detrimental consequences on the secretory competence, proliferative potential and viability of differentiated pancreatic ß-cells. This suggests that the levels of HNF1ß are tightly regulated in fully differentiated ß-cells as a means to maintain their functional competence. Thus, the control of HNF1ß expression may be an important developmental feature of ß-cells with foetal cells expressing high levels of the protein (Maestro et al. 2003) and adult cells much lower levels. It is possible that this pattern may correlate with the development of nutrient-sensitive insulin secretion in these cells.
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Acknowledgements |
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Received in final form 4 April 2006
Accepted 7 April 2006
Made available online as an Accepted Preprint 27 April 2006
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