INSULIN-RESISTANCE IS ASSOCIATED WITH ABNORMAL DEPHOSPHORYLATION OF ASYNTHETIC PHOSPHOPEPTIDE CORRESPONDING TO THE MAJOR AUTOPHOSPHORYLATION SITES OF THE INSULIN-RECEPTOR

Insulin resistance in the ob/ob mouse model is associated with a reduc tion in insulin-induced protein-tyrosine phosphorylation in tissues su ch as liver. To ascertain whether this decrease in phosphorylation may be due to increased phosphatase activity, protein-tyrosine phosphatas e (PTPase) activity was determined in particulate and soluble fraction s from livers of 5- to 23-week-old ob/ob mice and age-matched lean lit termates, PTPase activity was measured using a synthetic phosphopeptid e. TRDIY(P)ETDY(P)Y(P)RK, as the substrate, corresponding to residues 1142 to 1153 of the insulin receptor and containing the major autophos phorylation sites of the regulatory domain. The ob/ob mice were hyperi nsulinemic across all age groups, but only the youngest mice (aged 5 t o 7 weeks) were hyperglycemic, Most PTPase activity was present in the liver particulate fraction and was 19% to 114% greater in ob/ob mice as compared with controls. PTPase activity in the liver soluble fracti on was 26% less than control values in the youngest ob/ob mice (5 to 7 weeks), but increased with age and was 41% and 131% above control val ues at 21 to 23 and 25 to 27 weeks of age, respectively. Oral administ ration of the PTPase inhibitor sodium orthovanadate (0.6 mg/mL in drin king water for 2 weeks) to young ob/ob mice caused a significant reduc tion in the elevated particulate PTPase activity, with concomitant dec reases in plasma insulin and plasma glucose. Assessment of PTPase acti vity with a monophosphate form of the same synthetic peptide, TRDIY(P) ETDYYRK, showed lower PTPase activities as compared with the triphosph ate form and no significant differences between ob/ob and control prep arations. In conclusion, these findings demonstrate that the mechanism of dephosphorylation of insulin receptor autophosphorylation sites is altered in the liver of the ob/ob mouse. This abnormal PTPase activit y may contribute to the decrease in insulin action characteristic of t his insulin-resistant model. Copyright (C) 1995 by W.B. Saunders Compa ny