Protein kinase C-ζ activation markedly enhances β-cell proliferation: An essential role in growth factor-mediated β-cell mitogenesis

OBJECTIVE - Diabetes results from a deficiency of functional β-cells. Previous studies have identified hepatocyte growth factor (HGF) and parathyroid hormone-related protein (PTHrP) as two potent β-cell mitogens. The objective of this study is to determine 1) whether HGF and PTHrP have additive/synergistic effects on β-cell growth and proliferation; 2) the signaling pathways through which these growth factors mediate β-cell mitogenesis; and 3) whether activation of this/these signaling pathway(s) enhances human β-cell replication. RESEARCH DESIGN AND METHODS - We generated and phenotypically analyzed doubly transgenic mice overexpressing PTHrP and HGF in the β-cell. INS-1 and primary mouse and human islet cells were used to identify mitogenic signaling pathways activated by HGF and/or PTHrP. RESULTS - Combined overexpression of HGF and PTHrP in the β-cell of doubly transgenic mice did not result in additive/synergistic effects on β-cell growth and proliferation, suggesting potential cross-talk between signaling pathways activated by both growth factors. Examination of these signaling pathways in INS-1 cells revealed atypical protein kinase C (PKC) as a novel intracellular target activated by both HGF and PTHrP in β-cells. Knockdown of PKCζ, but not PKCι/λ, expression using specific small-interfering RNAs blocked growth factor-induced INS-1 cell proliferation. Furthermore, adenovirus-mediated delivery of kinase-dead PKCζ completely inhibited β-cell proliferation in primary islet cells overexpressing PTHrP and/or HGF. Finally, adenovirus-mediated delivery of constitutively active PKCζ in mouse and human primary islet cells significantly enhanced β-cell proliferation. CONCLUSIONS - PKCζ is essential for PTHrP- and HGF-induced β-cell proliferation. PKCζ activation could be useful in therapeutic strategies for expanding β-cell mass in vitro and in vivo.