High glucose (HG) has been shown to induce insulin resistance in both type 1 and type 2 diabetes. However, the molecular mechanism behind this phenomenon is unknown. Insulin receptor substrate (IRS) proteins are the key signaling molecules that mediate insulin's intracellular actions.Genetic and biological studies have shown that reductions in IRS1 and/or IRS2 protein levels are associated with insulin resistance. In this study we have shown that proteasome degradation of IRS1, but not of IRS2, is involved in HG-induced insulin resistance in Chinese hamster ovary (CHO) cells as well as in primary hepatocytes. To further investigate the molecular mechanism by which HG induces insulin resistance, we examined various molecular candidates with respect to their involvement in the reduction in IRS1 protein levels. In contrast to the insulin-induced degradation of IRS1, HG-induced degradation of IRS1 did not require IR signaling or phosphatidylinositol 3-kinase/Akt activity. We have identified glycogen synthase kinase 3β (GSK3β or GSK3B as listed in the MGI Database) as a kinase required for HG-induced serine332 phosphorylation, ubiquitination, and degradation of IRS1. Overexpression of IRS1 with mutation of serine332 to alanine partially prevents HG-induced IRS1 degradation. Furthermore, overexpression of constitutively active GSK3β was sufficient to induce IRS1 degradation. Our data reveal the molecular mechanism of HG-induced insulin resistance, and support the notion that activation of GSK3β contributes to the induction of insulin resistance via phosphorylation of IRS1, triggering the ubiquitination and degradation of IRS1.