Differential regulation of insulin receptor substrate-1 degradation during mannitol and okadaic acid induced apoptosis in human neuroblastoma cells

Alpha-synuclein overexpression negatively regulates insulin receptor substrate 1 by activating mTORC1/S6K1 signaling

Alpha-synuclein (α-Syn) is a major component of Lewy bodies, a pathological feature of Parkinson's and other neurodegenerative diseases collectively known as synucleinopathies. Among the possible mechanisms of α-Syn-mediated neurotoxicity is interference with cytoprotective pathways such as insulin signaling. Insulin receptor substrate (IRS)-1 is a docking protein linking IRs to downstream signaling pathways such as phosphatidylinositol 3-kinase/Akt and mammalian target of rapamycin (mTOR)/ribosomal protein S6 kinase (S6K)1; the latter exerts negative feedback control on insulin signaling, which is impaired in Alzheimer's disease. Our previous study found that α-Syn overexpression can inhibit protein phosphatase (PP)2A activity, which is involved in the protective mechanism of insulin signaling. In this study, we found an increase in IRS-1 phosphorylation at Ser636 and decrease in tyrosine phosphorylation, which accelerated IRS-1 turnover and reduced insulin-Akt signaling in α-Syn-overexpressing SK-N-SH cells and transgenic mice. The mTOR complex (C)1/S6K1 blocker rapamycin inhibited the phosphorylation of IRS-1 at Ser636 in cells overexpressing α-Syn, suggesting that mTORC1/S6K1 activation by α-Syn causes feedback inhibition of insulin signaling via suppression of IRS-1 function. α-Syn overexpression also inhibited PP2A activity, while the PP2A agonist C2 ceramide suppressed both S6K1 activation and IRS-1 Ser636 phosphorylation upon α-Syn overexpression. Thus, α-Syn overexpression negatively regulated IRS-1 via mTORC1/S6K1 signaling while activation of PP2A reverses this process. These results provide evidence for a link between α-Syn and IRS-1 that may represent a novel mechanism for α-Syn-associated pathogenesis.

A novel domain mediates insulin-induced proteasomal degradation of insulin receptor substrate 1 (IRS-1)

Insulin receptor substrate-1 (IRS-1) plays a pivotal role in insulin signaling, therefore its degradation is exquisitely regulated. Here, we show that insulin-stimulated degradation of IRS-1 requires the presence of a highly conserved Ser/Thr-rich domain that we named domain involved in degradation of IRS-1 (DIDI). DIDI (amino acids 386-430 of IRS-1) was identified by comparing the intracellular degradation rate of several truncated forms of IRS-1 transfected into CHO cells. The isolated DIDI domain underwent insulin-stimulated Ser/Thr phosphorylation, suggesting that it serves as a target for IRS-1 kinases. The effects of deletion of DIDI were studied in Fao rat hepatoma and in CHO cells expressing Myc-IRS-1(WT) or Myc-IRS-1(Δ386-430). Deletion of DIDI maintained the ability of IRS-1(Δ386-434) to undergo ubiquitination while rendering it insensitive to insulin-induced proteasomal degradation, which affected IRS-1(WT) (80% at 8 h). Consequently, IRS-1(Δ386-434) mediated insulin signaling (activation of Akt and glycogen synthesis) better than IRS-1(WT). IRS-1(Δ386-434) exhibited a significant greater preference for nuclear localization, compared with IRS-1(WT). Higher nuclear localization was also observed when cells expressing IRS-1(WT) were incubated with the proteasome inhibitor MG-132. The sequence of DIDI is conserved more than 93% across species, from fish to mammals, as opposed to approximately 40% homology of the entire IRS-1. These findings implicate DIDI as a novel, highly conserved domain of IRS-1, which mediates its cellular localization, rate of degradation, and biological activity, with a direct impact on insulin signal transduction.

Diabetes regulates mitochondrial biogenesis and fission in mouse neurons

AIMS/HYPOTHESIS

Normal mitochondrial activity is a critical component of neuronal metabolism and function. Disruption of mitochondrial activity by altered mitochondrial fission and fusion is the root cause of both neurodegenerative disorders and Charcot-Marie-Tooth type 2A inherited neuropathy. This study addressed the role of mitochondrial fission in the pathogenesis of diabetic neuropathy.

METHODS

Mitochondrial biogenesis and fission were assayed in both in vivo and in vitro models of diabetic neuropathy. Gene, protein, mitochondrial DNA and ultrastructural analyses were used to assess mitochondrial biogenesis and fission.

RESULTS

There was greater mitochondrial biogenesis in dorsal root ganglion neurons from diabetic compared with non-diabetic mice. An essential step in mitochondrial biogenesis is mitochondrial fission, regulated by the mitochondrial fission protein dynamin-related protein 1 (DRP1). Evaluation of diabetic neurons in vivo indicated small, fragmented mitochondria, suggesting increased fission. In vitro studies revealed that short-term hyperglycaemic exposure increased levels of DRP1 protein. The influence of hyperglycaemia-mediated mitochondrial fission on cell viability was evaluated by knockdown of Drp1 (also known as Dnm1l). Knockdown of Drp1 resulted in decreased susceptibility to hyperglycaemic damage.

CONCLUSIONS/INTERPRETATION

We propose that: (1) mitochondria undergo biogenesis in response to hyperglycaemia, but the increased biogenesis is insufficient to accommodate the metabolic load; (2) hyperglycaemia causes an excess of mitochondrial fission, creating small, damaged mitochondria; and (3) reduction of aberrant mitochondrial fission increases neuronal survival and indicates an important role for the fission-fusion equilibrium in the pathogenesis of diabetic neuropathy.

Insulin receptor substrate (IRS)-2, not IRS-1, protects human neuroblastoma cells against apoptosis

Insulin receptor substrates (IRS)-1 and -2 are major substrates of insulin and type I insulin-like growth factor (IGF-I) receptor (IGF-IR) signaling. In this study, SH-EP human neuroblastoma cells are used as a model system to examine the differential roles of IRS-1 and IRS-2 on glucose-mediated apoptosis. In the presence of high glucose, IRS-1 underwent caspase-mediated degradation, followed by focal adhesion kinase (FAK) and Akt degradation and apoptosis. IRS-2 expression blocked all these changes whereas IRS-1 overexpression had no effect. In parallel, IRS-2, but not IRS-1, overexpression enhanced IGF-I-mediated Akt activation without affecting extracellular regulated kinase signaling. While IRS-1 was readily degraded by caspases, hyperglycemia-mediated IRS-2 degradation was unaffected by caspase inhibitors but blocked by proteasome and calpain inhibitors. Our data suggest that the differential degradation of IRS-1 and IRS-2 contributes to their distinct modes of action and the increased neuroprotective effects of IRS-2 in this report are due, in part, to its resistance to caspase-mediated degradation.

IGF signaling as a therapeutic target in pediatric solid tumors of the central and peripheral nervous system

Similar to many other growth factor systems, the IGF system consists of more than a single ligand interacting with a single receptor. There are three ligands (IGF-I, IGF-II and insulin) that interact with at least four receptors. In addition, the IGF system also involves six well-characterized binding proteins that regulate IGF action. Type I IGF receptor-mediated signaling plays a fundamental role in cell growth and malignant transformation and is an important mediator of anti-apoptotic signals. This review describes the roles of IGF signaling in childhood tumors of the CNS and PNS, including neuroblastoma, medulloblastoma, atypical teratoid/rhabdoid tumors and craniopharyngioma. Moreover, it describes strategies to disrupt the IGF signaling as a potential cancer therapy.

c-Jun NH2 terminal kinase activation and decreased expression of mitogen-activated protein kinase phosphatase-1 play important roles in invasion and angiogenesis of urothelial carcinomas

We here examined whether c-Jun NH(2) terminal kinase (JNK) might be involved in the progression of urothelial carcinomas. In vitro and in vivo invasion assays using Matrigel and chick embryo chorioallantoic membrane approaches showed constitutive activation of JNK to significantly increase two processes, invasion and angiogenesis, in the human urothelial carcinoma cell line kU-7, this being suppressed by a JNK inhibitor, SP600125, or cell-permeable peptides. In addition, we found that mitogen-activated protein kinase phosphatase (MKP)-1 functions as an endogenous inhibitor of JNK-mediated signals in urothelial carcinoma cells: chorioallantoic membrane assays showed UMUC14 cells with low MKP-1 expression to be more invasive and have pronounced angiogenesis compared to UMUC6 cells with high MKP-1. Furthermore, knockdown of the MKP-1 gene by siRNA transfection enhanced JNK activation in UMUC6 cells to the UMUC14 level. Immunohistochemically, JNK was found to be highly phosphorylated in high-grade and invasive carcinomas (>/=pT2) as well as carcinoma in situ but not in low-grade and noninvasive phenotypes (pTa, pT1). In contrast, MKP-1 was much more expressed in low-grade/noninvasive cancers than with the high-grade/invasive phenotype, reversely correlating with phosphorylated JNK. Taken together, JNK activation and decreased expression of MKP-1 may play important roles in progression of urothelial carcinoma.