Inhibitory effect of IGF-I on type 2 nitric oxide synthase expression in Ins-1 cells and protection against activation-dependent apoptosis: involvement of phosphatidylinositol 3-kinase.

Abstract

2 2 heterotetrameric molecule, through its ligand-induced tyrosine kinase activity, leading to the phosphorylation of the cytoplasmic domain of the -subunits (6,7). Receptor activation promotes the rapid phosphorylation on multiple tyrosine residues of insulin receptor substrate (IRS)-1 and I R S-2, which allows the binding, through the phosphorylated sequences, of s r c homology domain 2 (SH2) domains of various proteins, initiating further IGF-I-dependent signaling events (8,9). Among the proteins interacting with IRS-1 and IRS-2 are 1) Grb-2, which activates the Ras/Raf/ERK signaling pathway; 2) Shc, which in addition to the interaction with IRS-1 can be activated directly by phosphorylation through the IGF-I receptor, depending on the cell type; and 3) the p85 subunit of the phosphatidylinositol (PI) 3-kinase (8-10). PI 3-kinase is a family of isoenzymes with dual lipid and serine/threonine kinase activities that acts as a key regulator in a variety of cellular responses, including glucose transport, mitogenesis and differentiation, and membrane dynamics, and as an anti-apoptotic mediator (4,(11)(12)(13)(14). The kinase activity is located in the p110 subunit, whereas p85 acts as the regulatory part of the complex. Interaction between phosphorylated IRS-1 and p85 allows the activation of p110 and the localization of this subunit in the proximity of its substrates near the membrane, among them p85 and IRS-1 (12,14). The signaling downstream PI 3-kinase is a subject of current interest because a large number of targets appear to be modulated through this pathway (13).R e c e n t l y, it has been shown that inhibition of PI 3-kinase leads to an overexpression of nitric oxide synthase (NOS)-2, the enzyme responsible for the high-output NO synthesis in macrophages, microglia, and other cells stimulated with lipopolysaccharide (LPS) or pro-inflammatory cytokines (15,16). Expression of NOS-2 in rodent cells has been studied e x h a u s t i v e l y. The process is regulated through synergistic cooperation between the transcription factors NF-B, Stat-1, and IRF-1, at least in response to LPS and IFN-c h a l l e n g e (17,18). Some data indicate that NF-B activation is a critical event in the expression of NOS-2, and this process depends on the degradation of the corresponding inhibitory proteins

Adiponectin counteracts cytokine- and fatty acid-induced apoptosis in the pancreatic beta-cell line INS-1

“…As shown in Fig. 2, a prominent increase in caspase 3 activity (three-fourfold) was observed after treating the beta-cells with the cytokine combination IL-1β/IFN-γ or palmitic acid, respectively, in agreement with data reported in the literature [11,23]. Exposure of INS-1 cells to leptin or gAcrp30 did not modify caspase 3 activity under control conditions (not shown in the Figure).…”

“…The proapoptotic role of the NF-κB pathway in response to proinflammatory cytokines is well established in pancreatic beta cells [7,8,9,10,11]. Recent work also indicates a direct involvement of this transcription factor in the regulation of beta-cell lipotoxicity [6].…”

“…One central signalling pathway that has been extensively investigated involves the cytotoxic effects of proinflammatory cytokines on pancreatic beta cells mediated through activation of the transcription factor NF-κB [7]. NF-κB activation represents a crucial event in cytokine-regulation of expression of several chemokines and cytokines [8], Fas expression [9], iNOS and COX-2 expression [10,11], which are all regarded as essential factors for causing dysfunction and destruction of insulin-producing cells thus leading to the development of insulin-dependent diabetes. Importantly, a ceramide-dependent NF-κB-mediated up-regulation of iNOS expression was recently shown in response to palmitic acid, indicating a potential involvement of this transcription factor also in the regulation of beta-cell lipotoxicity and pathogenesis of Type 2 diabetes [6].…”

Glucagon-like peptide-1 protects beta cells from cytokine-induced apoptosis and necrosis: role of protein kinase B

“…Furthermore, exendin-4 treatment was also able to reduce the effects of cytokines on apoptosis in cells infected with constitutively-active PKB, although the protective effects of exendin-4 were not observed in all Adv-CA cells, possibly due to the extremely high levels of activated PKB already present. Nonetheless, in agreement with the finding of an important role for PKB in GLP-1 signalling, it has been shown that inhibition of PI3-K, the upstream regulator of PKB activity [23], decreases the IGF-1-mediated protection of beta cells against cytokines [41,42]. IGF-1 is a known regulator of PI3-K/PKB, whereas the mechanisms that link the G protein-coupled GLP-1 receptor to cell survival pathways are not well understood.…”

“…Nonetheless, the changes in iNOS in response to cytokines were paralleled by decreased intracellular levels of ROS. Furthermore, similar studies have shown that the iNOS/nitric oxide-induced apoptosis of INS-1 cells can be prevented by IGF-1, through a PI3-Kdependent pathway [42]. These findings suggest that iNOS may be a target gene for the PKB signalling pathway.…”