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In pancreatic β-cells, increased expression of the MODY5 gene product, HNF1β, leads to enhanced rates of apoptosis and altered regulation of the cell cycle, suggesting that control of HNF1β expression may be important for the control of β-cell proliferation and viability. It is unclear how these effects of HNF1β are mediated, but previously we have identified a protein tyrosine phosphatase, (PTP)-BL, as an HNF1β-regulated protein in β-cells and have now studied the role of this protein in INS-1 β-cells. Stably transfected cells were generated, which express either wild-type (WT) or a phosphatase-deficient mutant (PTP-BL-CS) of PTP-BL conditionally under the control of a tetracycline-regulated promoter. Enhanced expression of WT PTP-BL inhibited INS-1 cell growth dose dependently, but this effect was not observed when PTP-BL-CS was expressed. Neither construct altered the rate of apoptosis. PTP-BL has been reported to interact with components of the Wnt signalling pathway, and we observed that addition of exogenous Wnt3a resulted in an increase in cell proliferation and a rise in β-catenin levels, consistent with the operation of this pathway in INS-1 cells. Up-regulation of WT PTP-BL antagonised these responses but PTP-BL-CS failed to inhibit Wnt3a-induced proliferation. The rise in β-catenin caused by Wnt3a was also suppressed by over-expression of HNF1β, suggesting that HNF1β may interact with the Wnt signalling pathway via an increase in PTP-BL levels. We conclude that PTP-BL plays an important role in the regulation of cell cycle progression in pancreatic β-cells, and that it interacts functionally with components of the Wnt signalling pathway.

Mutations in the gene encoding hepatocyte nuclear factor (HNF)1β result in maturity-onset diabetes of the young-(MODY)5, by impairing insulin secretory responses and, possibly, by reducing β-cell mass. The functional role of HNF1β in normal β-cells is poorly understood; therefore, in the present study, wild-type (WT) HNF1β, or one of two naturally occurring MODY5 mutations (an activating mutation, P328L329del, or a dominant-negative form, A263insGG) were conditionally expressed in the pancreatic β-cell line, insulin-1 (INS-1), and the functional consequences examined. Surprisingly, overexpression of the dominant-negative mutant did not modify any of the functional properties of the cells studied (including insulin secretion, cell growth and viability). By contrast, expression of WT HNF1β was associated with a time- and dose-dependent inhibition of INS-1 cell proliferation and a marked increase in apoptosis. Induction of WT HNF1β also inhibited the insulin secretory response to nutrient stimuli, membrane depolarisation or activation of protein kinases A and C and this correlated with a significant decrease in pancrease-duodenum homeobox-1 protein levels. The attenuation of insulin secretion was, however, dissociated from the inhibition of proliferation and loss of viability, since expression of the P328L329del mutant led to a reduced rate of cell proliferation, but failed to induce apoptosis or to alter insulin secretion. Taken together, the present results suggest that mature rodent β-cells are sensitive to increased expression of WT HNF1β and they imply that the levels of this protein are tightly regulated to maintain secretory competence and cell viability.