We have previously shown that isolated islets embedded in type 1 collagen gel in the presence of a defined medium undergo transdifferentiation within 96 h to duct epithelial structures. The aim of this study was to identify the factors implicated in this process. Freshly isolated canine islets were embedded in type 1 collagen gel, Matrigel or agarose for up to 120 h and cultured in (i) Dulbecco's modified Eagle's medium (DMEM)/F12 plus cholera toxin (CT), (ii) medium CMRL1066 plus CT, (iii) CMRL1066 plus forskolin and (iv) CMRL1066 alone. At 16 h, intracellular levels of cAMP (fmol/10(3) islets) were increased in groups i-iii (642+/-17, 338+/-48, 1128+/-221) compared with group iv (106+/-19, P<0.01). Epithelial differentiation correlated with the total amount of intracellular cAMP measured over 120 h. Islet-epithelial transformation during the initial 36 h was associated with a wave of apoptosis which was followed by a wave of cell proliferation. During epithelial differentiation there was a progressive loss of all islet hormones and the concomitant expression of cytoskeletal proteins characteristic of duct epithelial cells. Islets in collagen and Matrigel demonstrated high rates of epithelial differentiation (63+/-2% and 71+/-4% respectively) compared with those in agarose gel (0+/-0%, P<0.001). Islets suspended in DMEM/F12 plus CT supplemented with soluble laminin or fibronectin did not undergo transformation. Prior incubation of freshly isolated islets with an integrin-binding arginine-glycine-aspartate motif-presenting synthetic peptide also reduced islet transformation. These studies confirm the biological potential of islets of Langerhans to differentiate to duct epithelial structures. cAMP-mediated signal transduction and an appropriate integrin-matrix interaction are necessary for this process to proceed.

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