Characterization of mice doubly transgenic for parathyroid hormone-related protein and murine placental lactogen: A novel role for placental lactogen in pancreatic β-cell survival

Transgenic overexpression of either parathyroid hormone-related peptide (PTHrP) or mouse placental lactogen type 1 (mPL1) in pancreatic β-cells, using the rat insulin II promoter (RIP), results in islet hyperplasia either through prolonged β-cell survival or through increased β-cell proliferation and hypertrophy, respectively. For determining whether the two proteins might exert complementary, additive, or synergistic effects on islet mass and function when simultaneously overexpressed in β-cells in vivo, RIP-FTHrP and RIP-mPL1 mice were crossed to generate mice doubly transgenic for PTHrP and mPL1. These double-transgenic mice displayed marked islet hyperplasia (threefold), hypoglycemia, increased β-cell proliferation (threefold), and resistance to the diabetogenic and cytotoxic effects of streptozotocin compared with their normal siblings. Although the phenotype of the double-transgenic mice was neither additive nor synergistic relative to their single-transgenic counterparts, it was indeed complementary, yielding the maximal salutary phenotypic features of both individual transgenes. Finally, mPL1, for the first time, was shown to exert a protective effect on the survival of β-cells, placing it among the few proteins that can improve function and proliferation and prolong the survival of β-cells. Placental lactogen 1 is an attractive target for future therapeutic strategies in diabetes.