1714-P: SerpinB13 Regulates Proliferation and Cell Survival in Beta Cells

Our studies with a monoclonal antibody (mAb) to serpinB13, a protease inhibitor of cathepsin L, and serpinB13-deficient mice, demonstrated that during development, this serpin molecule downregulates the number of Ngn3-positive pancreatic endocrine progenitor cells and their conversion to insulin-producing cells. Establishing the precise role of serpinB13 in the pancreas after birth is important, as its inhibition with a mAb may provide a novel approach to augment beta cell neogenesis, proliferation, and survival in diabetes. To address this, we injected 10 micrograms of low-endotoxin, recombinant serpinB13 (or chicken ovalbumin as a control [OVA]) once a week for 4 weeks into healthy 3-week old immunodeficient Balb/c SCID mice. In addition, during the same period, these mice received 5-EdU in two i.p. injections per week (8 injections in total). At 8 weeks of age the animals were sacrificed for microscopic examination of the percentage of EdU+/insulin+ cells. In an alternative approach, to measure the impact of serpinB13 on cell survival in vitro, immortalized MIN6 beta cells were cultured with a 4 micromolar solution of staurosporine in the presence or absence of serpinB13 at 1 microgram/mL for four hours. Early and late apoptosis was examined with antibodies to Annexin V and caspase 3 antibody, respectively. We found that female mice injected with serpinB13 had significantly reduced proliferation of beta cells with a similar trend for alpha cells. This difference was observed in females rather than males and was particularly prominent for pancreatic islets less than 180 microns in diameter. Moreover, our in vitro studies revealed a significant augmentation of apoptosis and death events following exposure of MIN6 cells to serpinB13. Together our data suggest that serpinB13 has detrimental effects on beta-cell biology, both during embryogenesis and after birth, and further suggests that impeding serpinB13 function is potentially desirable to achieve improved clinical outcomes in the treatment of diabetes.