Evaluation of β-Cell Replication in Mice Transgenic for Hepatocyte Growth Factor and Placental Lactogen Comprehensive Characterization of the G1/S Regulatory Proteins Reveals Unique Involvement of p21cip

We hypothesized that combined transgenic overexpression of hepatocyte growth factor (HGF) and placental lactogen in islets would lead to even greater increases in β-cell mass and replication than either growth factor alone. This did not occur, suggesting that β-cell replication is saturable or subject to molecular restraint. We therefore performed the first comprehensive G 1 /S cell cycle survey in islets, cataloguing the broad range of kinases, cyclins, and kinase inhibitors that control the G 1 /S transition in islets from normal, HGF, placental lactogen, and doubly transgenic mice. Many of the G 1 /S checkpoint regulators (E2Fs; pRb; p107; p130; cyclins D 1 , 2 , 3 , A, and E; cdk-2; cdk-4; p15; p16; p18; p19; p21; p27; MDM2; p53; c-Myc; and Egr-1) are present in the murine islet. Most of these proteins were unaltered by overexpression of HGF or placental lactogen, either alone or in combination. In contrast, p21 cip was uniquely, dramatically, and reproducibly upregulated in placental lactogen and HGF islets. p21 cip was also present in, and upregulated in, proliferating human islets, localizing specifically in β-cells and translocating to the nucleus on mitogenic stimulation. Homozygous p21 cip loss releases islets from growth inhibition, markedly enhancing proliferation in response to HGF and placental lactogen.