Serine/Threonine Phosphorylation of IRS-1 Triggers Its Degradation

Abstract

Insulin receptor substrate (IRS)-1 protein expression is markedly reduced in many insulin-resistant states, although the mechanism for this downregulation is unclear. In this study, we have investigated the early events in the insulin pathway that trigger the degradation of IRS-1. Incubation of the adipocytes with insulin induced a fast electrophoretic mobility shift of IRS-1 and a subsequent degradation of the protein. Wortmannin and rapamycin blocked this mobility shift of IRS-1, maintained the insulin-induced tyrosine phosphorylation of IRS-1, and blocked its degradation. In contrast, a glycogen synthase kinase 3 inhibitor, a mitogen-activated protein kinase/extracellular-regulated kinase inhibitor, and various protein kinase C inhibitors had no effect. Incubation with okadaic acid increased the serine/threonine phosphorylation of IRS-1 and its degradation, mimicking insulin, and its effect was prevented by the proteasome inhibitor lactacystin, as well as by rapamycin. Treatment of the cells with the tyrosine phosphatase inhibitor orthovanadate in the presence of insulin or okadaic acid partially inhibited the degradation of IRS-1. We propose that a rapamycin-dependent pathway participates as a negative regulator of IRS-1, increasing its serine/threonine phosphorylation, which triggers degradation. Thus, regulation of serine/ threonine versus tyrosine phosphorylation may modulate IRS-1 degradation, affecting insulin sensitivity.

Insulin-like growth factor-I signaling in human neuroblastoma cells

“…The time course of IRS phosphorylation correlates with IRS expression levels; IRS-2 in N cells is rapidly degraded by IGF-I treatment, whereas the amount of IRS-1 protein in S cells remains stable. Several groups have reported that IRS proteins undergo serine phosphorylationdependent degradation by insulin, IGF-I and TNF treatment (Rui et al, 2001a;Pederson et al, 2001;Greene et al, 2003). We have also shown that IRS-2 is degraded through PI 3-K signaling in SH-SY5Y cells (Kim et al, 1998b).…”

Insulin receptor substrate (IRS)‐1 regulates murine embryonic stem (mES) cells self‐renewal

“…Since IRS-1 mRNA is still detectable after LIF withdrawn and induction of differentiation, we reason that the control of IRS-1 expression is at posttranscriptional level. Removal of LIF induces changes on IRS-1 phosphorylation status from tyrosine phosphorylation to serine phosphorylation, which has been reported to be one of the first steps for IRS-1 degradation via proteasome pathway (Pederson et al, 2001).…”

“…However, after LIF withdrawal IRS-1 became phosphorylated at residue serine 307 (Fig. 2D), a site important for the proteasome-mediated degradation of IRS-1 (Pederson et al, 2001).…”

Role of Ser/Thr kinases in the uncoupling of insulin signaling

“…Tyr-phosphorylated IRS proteins, which are key players in propagating insulin signaling, are the targets of such feedback regulatory systems. Regulation involves proteasome-mediated degradation, 5 phosphatasemediated dephosphorylation, 6 or Ser/Thr phosphorylation. The latter is an attractive regulatory mechanism because it enables multilevel control of the IRS kinases activity, and of the specific targets among nearly 100 potential Ser/Thrphosphorylation sites in IRS proteins that might play regulatory roles during the insulin response ( Figure 1).…”