The dogma that IGF-I stimulates pancreatic islet growth has been challenged by combinational targeting of IGF or IGF-IR (IGF receptor) genes as well as ␤-cell-specific IGF-IR gene deficiency, which caused no defect in islet cell growth. To assess the physiological role of locally produced IGF-I, we have developed pancreaticspecific IGF-I gene deficiency (PID) by crossing Pdx1-Cre and IGF-I/loxP mice. PID mice are normal except for decreased blood glucose level and a 2.3-fold enlarged islet cell mass. When challenged with low doses of streptozotocin, control mice developed hyperglycemia after 6 days that was maintained at high levels for at least 2 months. In contrast, PID mice only exhibited marginal hyperglycemia after 12 days, maintained throughout the experiment. Fifteen days after streptozotocin, PID mice demonstrated significantly higher levels of insulin production. Furthermore, streptozotocin-induced ␤-cell apoptosis (transferase-mediated dUTP nick-end labeling [TUNEL] assay) was significantly prevented in PID mice. Finally, PID mice exhibited a delayed onset of type 2 diabetes induced by a high-fat diet, accompanied by super enlarged pancreatic islets, increased insulin mRNA levels, and preserved sensitivity to insulin. Our results suggest that locally produced IGF-I within the pancreas inhibits islet cell growth; its deficiency provides a protective environment to the ␤-cells and potential in combating diabetes.

Reg proteins constitute a conserved family in human and rodents; their production in the pancreas (including the islets of Langerhans) is induced upon beta-cell damage. While some members of the family (Reg1 and islet neogenesis-associated protein, i.e. INGAP) have been implicated in beta-cell replication and/or neogenesis, including from in vivo studies using transgenic and knockout mice; the roles of the other five members have yet to be characterized. Among them, Reg2 was recently proposed to serve as an autoantigen on beta-cells that elicits T-cell attack in type 1 diabetes mellitus. Elucidation of their actions and identification of their molecular targets should provide insight into the biology of these proteins and lead to the design and development of novel strategies aimed at promoting the survival and function of the pancreatic islets. As the current terminology used for mammalian Reg genes/proteins is very confusing, we also proposed a uniformed classification in human and rodents through sequence alignments.

Both GH and IGF-I stimulate islet cell growth, inhibit cell apoptosis, and regulate insulin biosynthesis and secretion. GH receptor gene deficiency (GHR(-/-)) caused diminished pancreatic islet cell mass and serum insulin level and elevated insulin sensitivity. Because IGF-I gene expression was nearly abolished in these mice, we sought to determine whether that had caused the islet defects. To restore IGF-I level, we have generated transgenic mice that express rat IGF-I cDNA under the direction of rat insulin promoter 1 (RIP-IGF). Using RNase protection assay and immunohistochemistry, the IGF-I transgene expression was revealed specifically in pancreatic islets of the RIP-IGF mice, which exhibited normal growth and development and possess no abnormalities in glucose homeostasis, insulin production, and islet cell mass. GHR(-/-) mice exhibited 50% reduction in the ratio of islet cell mass to body weight and increased insulin sensitivity but impaired glucose tolerance. Compared with GHR(-/-) alone, IGF-I overexpression on a GHR(-/-) background caused no change in the diminished blood glucose and serum insulin levels, pancreatic insulin contents, and insulin tolerance but improved glucose tolerance and insulin secretion. Remarkably, islet-specific overexpression of IGF-I gene in GHR(-/-) mice restored islet cell mass, at least partially through cell hypertrophy. Interestingly, double-transgenic male mice demonstrated a transient rescue in growth rates vs. GHR(-/-) alone, at 2-3 months of age. Our results suggest that IGF-I deficiency is part of the underlying mechanism of diminished islet growth in GHR(-/-) mice and are consistent with the notion that IGF-I mediates GH-induced islet cell growth.

Liu J-L. A general and islet cell-enriched overexpression of IGF-I results in normal islet cell growth, hypoglycemia, and significant resistance to experimental diabetes.

. Activation of the Reg family genes by pancreatic-specific IGF-I gene deficiency and after streptozotocin-induced diabetes in mouse pancreas. Am J Physiol Endocrinol Metab 291: E50 -E58, 2006. First published January 31, 2006 doi:10.1152/ajpendo.00596.2005.-We have recently reported that Pdx1-Cre-mediated whole pancreas inactivation of IGF-I gene [in pancreatic-specific IGF-I gene-deficient (PID) mice] results in increased ␤-cell mass and significant protection against both type 1 and type 2 diabetes. Because the phenotype is unlikely a direct consequence of IGF-I deficiency, the present study was designed to explore possible activation of proislet factors in PID mice by using a whole genome DNA microarray. As a result, multiple members of the Reg family genes (Reg2, -3␣, and -3␤, previously not known to promote islet cell growth) were significantly upregulated in the pancreas. This finding was subsequently confirmed by Northern blot and/or real-time PCR, which exhibited 2-to 8-fold increases in the levels of these mRNAs. Interestingly, these Reg family genes were also activated after streptozotocin-induced ␤-cell damage and diabetes (wild-type T1D mice) when islet cells were undergoing regeneration. Immunohistochemistry revealed increased Reg proteins in exocrine as well as endocrine pancreas and suggested their potential role in ␤-cell neogenesis in PID or T1D mice. Previously, other Reg proteins (Reg1 and islet neogenesis-associated protein) have been shown to promote islet cell replication and neogenesis. These uncharacterized Reg proteins may play a similar but more potent role, not only in normal islet cell growth in PID mice, but also in islet cell regeneration after T1D. pancreatic islets; DNA microarray; gene-targeted mice; insulin-like growth factor I INSULIN-LIKE GROWTH FACTOR I (IGF-I) is produced from hepatocytes and many other cells and tissues, including the pancreas (9, 14, 23). Acting through its receptor IGF-IR, which is expressed ubiquitously, IGF-I promotes cell proliferation and growth and inhibits cell apoptosis. In normal growth of the bone, muscle, reproductive systems, and the whole mammalian organisms, IGF-I plays an irreplaceable, essential role (25,33). In cultured cells, IGF-I causes pancreatic islet ␤-cell mitogenesis in a glucose-dependent manner via activation of phosphatidylinositol 3-kinase (PI3K), ERK1/2, and 70-kDa protein S6 kinase (p70 S6K ) (15, 24). Although IGF-I and IGF-IR are normally expressed in both endocrine and exocrine pancreas (8, 9), their in vivo role in islet cell growth has been questioned by the results of most gene-targeted studies. For example, combined inactivation of insulin receptor and IGF-IR genes in early embryos results in a 50% decrease in the size of the exocrine pancreas without affecting the development of endocrine cells (19). Combined ablation of IGF-I and IGF-II genes results in an identical phenotype (19). Moreover, islet ␤-cellspecific inactivation of IGF-IR gene causes no change in ␤-cell mass despite hyperinsulinemia, glucose intoleranc...