Irs–2 Mediates the Antiapoptotic Effect of Insulin in Neonatal Hepatocytes

Abstract

To assess the role of insulin action and inaction in the liver, immortalized hepatocyte cell lines have been generated from insulin receptor substrate (IRS)-2 ؊/؊ and wild-type mice. Using this model, we have recently demonstrated that the lack of IRS-2 in neonatal hepatocytes resulted in insulin resistance. In the current study, we show that immortalized neonatal hepatocytes undergo apoptosis on serum withdrawal, with caspase-3 activation and DNA laddering occurring earlier in the absence of IRS-2. Insulin rescued wild-type hepatocytes from serum withdrawal-induced caspase-3 activation and DNA fragmentation in a dose-dependent manner, but it failed to rescue hepatocytes lacking IRS-2. In IRS-2 ؊/؊ cells, insulin failed to phosphorylate Bad. I n the liver, regulation of apoptosis is essential for development and hepatic homeostatic mechanisms. 1 Thus, liver hyperplasia during development or regeneration may be aided by inhibition of apoptosis. Conversely, liver atrophy occurs by apoptosis of liver cells in the absence of regeneration (for review, see Patel et al. 2 ). Progression of the apoptotic program can be inhibited at different levels depending on the origin of the death stimulus. In this regard, insulin promotes survival in a number of cell types. This effect is mediated by a complex network of intracellular signaling pathways. 3 The cascade begins when the activated insulin receptor (IR) phosphorylates several docking proteins, including the insulin receptor substrate (IRS) family 4 and Shc. Then, phosphorylated IRSs (IRS-1, -2, -3, and -4) connect with the phosphatidylinositol 3-kinase (PI 3-kinase) signaling pathway, 5 which plays a central role in the metabolic actions elicited by insulin, and the Ras/MAPK pathway, a major regulatory pathway for gene expression. 6 Among the members of the IRS family, IRS-2 triggers insulin actions in the liver. In fact, mice deficient in IRS-2 develop hepatic insulin resistance and fatty liver along with ␤-cell failure, 7-10 producing a severe type-2 diabetic phenotype. At the mo-

Amphiregulin Contributes to the Transformed Phenotype of Human Hepatocellular Carcinoma Cells

“…Representative images. apoptosis induced by growth factor withdrawal and TGF-h treatment is the activation of the expression of the BH3-only protein Bim (33)(34)(35). Bim is a proapoptotic member of the Bcl-2 family that binds and neutralizes its prosurvival relatives, such as Bcl-X L , priming the cell for apoptosis (33).…”

“…As mentioned before, the EGFR/Erk1/2 pathway is crucial in preventing anoikis (36) and has been recently identified as a key pathway in the protection from TGF-h-induced apoptosis in late-stage liver tumorigenesis (49). Interestingly, both protein kinases (JNK and Erk1/2) play opposing roles in the regulation of the expression and activity of the proapoptotic BH3-only protein Bim (33,(35)(36)(37)(38). Consistent with this, we observed a potent induction in Bim expression upon AR silencing in hepatocellular carcinoma cells.…”

Sirt1-Independent Rescue of Muscle Regeneration by Resveratrol in Type I Diabetes

“…But note these long-lived mutants lacking insulin/IGF-1 activity are in species where the adult animals are composed almost entirely of post-mitotic cells. In mice, where there are separate and promiscuous insulin and IGF-1 signaling pathways, a partial decrease in IGF-1 or growth hormone, a lack of insulin receptors in adipose tissue, or heterozygosity or lack of IRS-2 (in general or in brain) have a much less robust and at times a gender-specific positive effect on life span [31][32][33]. The organism-wide glucose metabolism remains unperturbed in these experimental models of reduced insulin/IGF-1 signaling [29].…”

Irs1 and Irs2 signaling is essential for hepatic glucose homeostasis and systemic growth

“…Cell-based experimentation with immortalized hepatocytes suggests that Irs2 is the major effector of both the metabolic and the growth-promoting actions of insulin (27). Moreover, Irs2 signaling is essential for the suppression of gluconeogenesis and apoptosis in immortalized neonatal hepatocytes (29,30). Importantly, Irs2 is highly regulated through feedback and counterregulatory cascades.…”