Hepatic insulin resistance in KKA y mice and its amelioration by pioglitazone do not involve alterations in phospholipase C activity

Abstract

It has been proposed that an abnormality in the regulation of cytosolic-free Ca 2+ may be the cause of some forms of insulin resistance. In support of this proposition, it was reported that phospholipase C-catalyzed hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP 2) by liver plasma membranes from obese patients with non-insulin-dependent diabetes mellitus (NIDDM) was abnormally augmented (Thakker et al., J. Biol. Chem. 264, 7169–7175). The objective of this investigation was to determine if a novel antidiabetic agent, pioglitazone, ameliorated hepatic insulin resistance in KKA y mice and to identify any alterations in PIP 2-phospholipase C activity of liver plasma membranes that may accompany changes in insulin sensitivity. Treatment of KKA y mice for 4 days with pioglitazone (20 mg/kg per day) decreased blood glucose and insulin and improved a variety of indices of hepatic insulin resistance, but did not alter the rate of PIP 2 hydrolysis by liver plasma membranes. Acute treatment of isolated liver plasma membranes with pioglitazone (1–100 μM) also failed to alter PIP 2-phospholipase C activity. Furthermore, the specific activity, Ca 2+-requirement, pH-dependence and sensitivity to guanosine 5′-thiotriphosphate of the PIP 2-phospholipase C in KKA y liver membranes were indistinguishable from those of C57BL/6J (normal) mice. Among C57BL/6J and KKA y mice fed either a control or pioglitazone-supplemented diet, there was no correlation between PIP 2-phospholipase C activity in isolated liver membranes and either glucose or insulin concentrations in the circulation. These data indicate that an alteration in PIP 2-phospholipase C activity of liver plasma membranes is neither a cause nor an obligatory consequence of insulin resistance in KKA y mice or its amelioration by pioglitazone. Alterations of liver membrane phospholipase C activity in NIDDM, therefor, may reflect diabetic pathology other than the insulin resistance associated with this disease.