Attenuation of insulin-stimulated insulin receptor substrate-1 serine 307 phosphorylation in insulin resistance of type 2 diabetes.

Insulin resistance is a primary characteristic of type 2 diabetes and likely causally related to the pathogenesis of the disease. It is a result of defects in signal transduction from the cell surface receptor of insulin to target effects. We found that insulin-stimulated phosphorylation of serine 307 (corresponding to serine 302 in the murine sequence) in the immediate downstream mediator protein of the insulin receptor, insulin receptor substrate-1 (IRS1), is required for efficient insulin signaling and that this phosphorylation is attenuated in adipocytes from patients with type 2 diabetes. Inhibition of serine 307 phosphorylation by rapamycin mimicked type 2 diabetes and reduced the sensitivity of IRS1 tyrosine phosphorylation in response to insulin, while stimulation of the phosphorylation by okadaic acid, in cells from patients with type 2 diabetes, rescued cells from insulin resistance. EC(50) for insulin-stimulated phosphorylation of serine 307 was about 0.2 nM with a t(1/2) of about 2 min. The amount of IRS1 was similar in cells from non-diabetic and diabetic subjects. These findings identify a molecular mechanism for insulin resistance in non-selected patients with type 2 diabetes.