Alterations in growth and apoptosis of insulin receptor substrate-1-deficient β-cells

Insulin and IGF-I activate antiapoptotic pathways via insulin receptor substrate (IRS) proteins in most mammalian cells, including beta-cells. IRS-1 knockout (IRS-1KO) mice show growth retardation, hyperinsulinemia, and hyperplastic but dysfunctional islets without developing overt diabetes, whereas IRS-2KOs develop insulin resistance and islet hypoplasia leading to diabetes. Because both models display insulin resistance, it is difficult to differentiate islet response to insulin resistance from islet defects due to loss of proteins in the islets themselves. We used a transplantation approach, as a means of separating host insulin resistance from islet function, to examine alterations in proteins in insulin/IGF-I signaling pathways that may contribute to beta-cell proliferation and/or apoptosis in IRS-1KO islets. Islets isolated from wild-type ( WT) or IRS-1KO mice were transplanted into WT or insulin-resistant IRS-1KO males under the kidney capsule. The beta-cell mitotic rate in transplanted islets in IRS-1KO recipients was increased 1.5-fold compared with WT recipients and was similar to that in endogenous pancreases of IRS-1KOs, whereas beta-cell apoptosis was reduced by similar to 80% in IRS-1KO grafts in IRS-1KO recipients compared with WT recipients. Immunohistochemistry showed a substantial increase in IRS-2 expression in IRS-1KO islets transplanted into IRS-1KO mice as well as in endogenous islets from IRS-1KOs. Furthermore, enhanced cytosolic forkhead transcription factor (FoxO1) staining in IRS-1KO grafts suggests intact Akt/PKB activity. Together, these data indicate that, even in the absence of insulin resistance, beta-cells deficient in IRS-1 exhibit a compensatory increase in IRS-2, which is associated with islet growth and is characterized by both proliferative and antiapoptotic effects that likely occur via an insulin/IGF-I/IRS-2 pathway.