The sulphonylurea drug, glimepiride, stimulates release of glycosylphosphatidylinositol-anchored plasma-membrane proteins from 3T3 adipocytes

Abstract

Sulphonylurea drugs stimulate glucose transport and metabolism in muscle and fat cells in vitro. The molecular basis for the insulin-mimetic extrapancreatic effects of these oral antidiabetic therapeutic agents is unknown at present. Here we demonstrate that incubation of 3T3 adipocytes with the novel sulphonylurea, glimepiride, causes a time- and concentration-dependent release of the glycosylphosphatidylinositol (GPI)-anchored ecto-proteins, 5'-nucleotidase, lipoprotein lipase and a 62 kDa cyclic AMP (cAMP)-binding protein from the plasma membrane into the culture medium. The change in the localization is accompanied by conversion of the membrane-anchored amphiphilic proteins into their soluble hydrophilic versions, as judged by pulse-chase experiments and Triton X-114 partitioning, and by appearance of anti-cross-reacting determinant (CRD) immunoreactivity of the released proteins as shown by Western blotting. Metabolic labelling of cells with myo-[14C]inositol demonstrates that inositol is retained in the major portion of released lipoprotein lipase and cAMP-binding ectoprotein. The identification of inositol phosphate after deamination of these proteins with nitrous acid suggests cleavage of their GPI membrane anchor by a GPI-specific phospholipase C. However, after longer incubation with glimepiride the amount of soluble versions of the GPI-proteins lacking inositol and anti-CRD immunoreactivity increases, which may be caused by additional drug-stimulated hydrolytic events within their GPI structure or C-termini. Since insulin also stimulates membrane release of these GPI-modified proteins, and in combination with glimepiride in a synergistic manner, sulphonylurea drugs may exert their peripheral actions in adipose tissue by using (part of) the insulin postreceptor signalling cascade at the step of activation of a GPI-specific phospholipase C.