MULTIPLE DEFECTS IN THE ADIPOCYTE GLUCOSE-TRANSPORT SYSTEM CAUSE CELLULAR INSULIN-RESISTANCE IN GESTATIONAL DIABETES - HETEROGENEITY IN THENUMBER AND A NOVEL ABNORMALITY IN SUBCELLULAR-LOCALIZATION OF GLUT4 GLUCOSE TRANSPORTERS

Mechanisms causing cellular insulin resistance in gestational diabetes mellitus are not known. We, therefore, studied isolated omental adipo cytes obtained during elective cesarean sections in nondiabetic (contr ol) and GDM gravidas. Cellular insulin resistance was attributable to impaired stimulation of glucose transport; compared with control subje cts, basal and maximally insulin-stimulated transport rates (per surfa ce area) were reduced 38 and 60% in GDM patients, respectively. To det ermine underlying mechanisms, we assessed the number, subcellular dist ribution, and translocation of GLUT4, the predominant insulin-responsi ve glucose transporter isoform. The cellular content of GLUT4 was decr eased by 44% in GDM patients as assessed by immunoblot analysis of tot al postnuclear membranes. However, GDM patients segregated into two su bgroups; half exhibited profound (76%) cellular depletion of GLUT4 and half had GLUT4 levels in the normal range. Cellular GLUT4 was negativ ely correlated with adipocyte size in the control subjects and GDM pat ients with normal GLUT4 (r = 0.60), but fell away below this continuum in GDM patients with low GLUT4, indicating that heterogeneity was not caused by differences in obesity. All GDM patients exhibited abnormal ities in GLUT4 subcellular distribution. In basal cells, increased amo unts of GLUT4 were detected in membranes fractionating with plasma mem branes and high-density microsomes (such that the plasma membrane GLUT 4 level in GDM patients was equal to that observed in insulin-stimulat ed cells from control subjects). Furthermore, insulin stimulation indu ced translocation of GLUT4 from low-density microsomes to plasma membr anes in control subjects but did not alter subcellular distribution in GDM patients. In other experiments, cellular content of GLUT1 was nor mal in GDM patients, and GLUT1 did not undergo insulin-mediated recrui tment to plasma membranes in either control subjects or GDM patients. A faint signal was detected for GLUT3 only in low-density microsomes a nd only with one of two different antibodies. In GDM, we conclude that insulin resistance in adipocytes involves impaired stimulation of glu cose transport and arises from a heterogeneity of defects intrinsic to the glucose transport effector system. GLUT4 content in adipocytes is profoundly depleted in approximately 50% of GDM patients, whereas all patients are found to exhibit a novel abnormality in GLUT4 subcellula r distribution. This latter defect is characterized by accumulation of GLUT 4 in membranes cofractionating with plasma membranes and high-de nsity microsomes in basal cells and absence of translocation in respon se to insulin. The targeting relegate GLUT4 to a membrane compartment from which insulin cannot recruit transporters to the cell surface and have important implications regarding skeletal muscle insulin resista nce in GDM and NIDDM.