Pretranslational suppression of a glucose transporter protein causes insulin resistance in adipocytes from patients with non-insulin-dependent diabetes mellitus and obesity.

Abstract

A major portion of insulin-mediated glucose uptake occurs via the translocation of GLUT 4 glucose transporter proteins from an intracellular depot to the plasma membrane. We have examined gene expression for the GLUT 4 transporter isoform in subcutaneous adipocytes, a classic insulin target cell, to better understand molecular mechanisms causing insulin resistance in non-insulin-dependent diabetes mellitus (NIDDM) and obesity. In subgroups of lean (body mass index [BMII = 24±1) and obese (BMI = 32±2) controls and in obese NIDDM (BMI = 35±2) patients, the number of GLUT 4 glucose transporters was measured in total postnuclear and subcellular membrane fractions using specific antibodies on Western blots. Relative to lean controls, the cellular content of GLUT 4 was decreased 40% in obesity and 85% in NIDDM in total cellular membranes. In obesity, cellular depletion of GLUT 4 primarily involved low density microsomes (LDM), leaving fewer transporters available for insulin-mediated recruitment to the plasma membrane (PM). In NIDDM, loss of GLUT 4 was profound in all membrane subfractions, PM, LDM, as well as high density microsomes. These observations corresponded with decrements in maximally stimulated glucose transport rates in intact cells. To assess mechanisms responsible for depletion of GLUT 4, we quantitated levels of mRNA specifically hybridizing with human GLUT 4 cDNA on Northern blots. In obesity, GLUT 4 mRNA was decreased 36% compared with lean controls, and the level was well correlated (r = +0.77) with the cellular content of GLUT 4 protein over a wide spectrum of body weight. GLUT 4 mRNA in adipocytes from NIDDM patients was profoundly reduced by 86% compared with lean controls and by 78% relative to their weight-matched nondiabetic counterparts (whether expressed per RNA, per cell, or for the amount of CHO-B mRNA). Interestingly, GLUT 4 mRNA levels in patients with impaired glucose tolerance (BMI = 34±4) were decreased to the same level as in overt NIDDM.We conclude that, in obesity, insulin resistance in adipocytes is due to depletion of GLUT 4 glucose transporters, and that the cellular content of GLUT 4 is determined by the level of encoding mRNA over a wide range of body weight. In NIDDM, more profound insulin resistance is caused by a further reduction in GLUT 4 mRNA and protein than is attributable to obesity per se. Suppression of GLUT 4 mRNA is observed in patients with impaired glucose tolerance, and therefore, may occur early in the evolution of diabetes. Thus, pretranslational suppression of GLUT 4 transporter gene expression may be an important mechanism that produces and maintains cellular insulin resistance in NIDDM. (J. Clin. In-

Peripheral insulin resistance develops in transgenic rats overexpressing phosphoenolpyruvate carboxykinase in the kidney

“…In contrast, muscle GLUT4 protein content was not different between the two groups of rats, despite white gastrocnemius muscle being insulin resistant in the PEPCK transgenic rats. This tissue-specific reduction of GLUT4 is similar to that reported in patients with Type 2 diabetes with a decrease in adipose tissue but normal skeletal muscle GLUT4 content [48,49,50]. Tissue-specific regulation of GLUT4 has also been described in animals fed a high fat diet, which resulted in decreased GLUT4 mRNA expression in white adipose tissue but not skeletal muscle [51,52].…”

Chronically increased glucose uptake by adipose tissue leads to lactate production and improved insulin sensitivity rather than obesity in the mouse

“…In obesity and type 2 diabetes, insulin resistance leads to decreased adipose tissue glucose uptake, which parallels the downregulation of glucose transporter 4 production in adipose tissue [9][10][11][12]. In adipose-specific Glut4 (also known as Slc2a4) knockout mice, insulin-stimulated glucose transport, glucose disposal and insulin sensitivity in adipose tissue are impaired [13][14][15].…”

Expression of human GLUT4 in mice results in increased insulin action

“…This may be related to the extent of the diabetic state. Similarly, in obese humans and those with NIDDM, the level of GLUT 4 protein is markedly reduced in adipocytes [3,4], but not in skeletal muscle [30][31][32]. Thus, data from rodent models and humans point to a mechanism other than an absolute reduction in GLUT 4 as the principal reason for impaired insulin-stimulated glucose transport in skeletal muscle, although it could be sufficient to account for the impairment in adipose tissue.…”