Overexpression of Hexokinase II in Transgenic Mice

Abstract

Hexokinase II (HKII) is the predominant isozyme expressed in peripheral insulin-responsive tissues. To explore the role of HKII in muscle glucose metabolism, two lines of transgenic mice were generated where overex-pression was restricted to striated muscle; HKII protein levels and activity were increased by 3-8-fold. Oral glucose tolerance, intravenous insulin tolerance, and insulin and lactate levels were unaffected in transgenic mice. There was a trend toward increased levels of muscle glycogen; however, glucose-6-phosphate levels were increased by 43% in transgenic skeletal muscle following in vivo glucose and insulin administration. Using 2-[ 3 H]deoxyglucose as a tracer, in vitro basal and insulin stimulated glucose uptake were determined in ex-tensor digitorum longus, soleus, and epitrochlearis muscles. Maximal insulin-stimulated glucose uptake was increased by 17% (extensor digitorum longus), 34% (so-leus), and 90% (epitrochlearis) in transgenic muscles; basal and submaximal glucose uptake was also modestly increased in soleus and epitrochlearis. These data suggest that increased muscle HKII (corresponding to the upper end of the physiologic range) may not be sufficient to augment net in vivo glucose homeostasis. However , glucose phosphorylation can represent a rate-limiting step for skeletal muscle glucose utilization since muscle glucose-6-phosphate levels are increased during in vivo hyperinsulinemia and hyperglycemia; furthermore , basal and insulin-mediated muscle glucose uptake can be increased by a selective increase in HKII expression. Glucose phosphorylation to glucose-6-phosphate (G6P) 1 by hexokinase represents the obligate first step in glucose metabolism. Of the three "low K m " mammalian hexokinases (types I, II, and III), hexokinase II (HKII) is the predominant isozyme expressed in insulin-responsive tissues (1-3).