Correlation of insulin receptor level with both insulin action and breakdown of a potential insulin mediator precursor; studies in CHO cell-lines transfected with insulin receptor cDNA

Abstract

A phosphatidylinositol-glycan (Pl-glycan) has been described previously that may serve as the precursor for a mediator of some of insulins actions. The present study further addresses the potential relevance of this compound by correlating its breakdown with other insulin actions in Chinese hamster ovary (CHO) cells which express different levels of insulin receptor. Comparisons were drawn between parent CHO cells expressing 3 · 10 3 receptors/cell and two cell-lines transfected with human insulin receptor cDNA, that expressed 600 (CHO.TH) and 300 (CHO.T) times the parent receptor level. A Pl-glycan was isolated from all cells that incorporated [ 3H]glucosamine, [ 3H]galactose and [ 3H]inositol and was rapidly turned over upon insulin stimulation. Maximal turnover by insulin of approx. 20% was achieved in each cell line consistent with the fibroblastic nature of these cells. The effect of increased insulin receptor expression was to increase the sensitivity of the PI-glycan response to insulin. Increasing receptor number from 3 · 10 3 to 0.88 · 10 6 also increased the sensitivity of response of other insulin actions measured in this study, namely activation of pyruvate dehydrogenase (PDH), glucose utilization and transport. Thus turnover of the PI-glycan is linked closely to both metabolic actions of insulin and to cell surface insulin receptor expression further supporting its potential role in insulin action.