Expression of Glucokinase in Skeletal Muscle: A New Approach to Counteract Diabetic Hyperglycemia

Abstract

Chronic hyperglycemia is responsible for diabetes-specific microvascular and macrovascular complications. To reduce hyperglycemia, key tissues may be engineered to take up glucose. To determine whether an increase in skeletal muscle glucose phosphorylation leads to increased glucose uptake and to normalization of diabetic alterations, the liver enzyme glucokinase (GK) was expressed in muscle of transgenic mice. GK has a high Km for glucose and its activity is not inhibited by glucose 6-phosphate. The presence of GK activity in skeletal muscle resulted in increased concentrations of glucose 6-phosphate and glycogen. These mice showed lower glycemia and insulinemia, increased serum lactate levels, and higher blood glucose disposal after an intraperitoneal glucose tolerance test. Furthermore, transgenic mice were more sensitive to injection of low doses of insulin, which led to increased blood glucose disposal. In addition, streptozotocin (STZ)-treated transgenic mice showed lower levels of blood glucose than STZ-treated controls and maintained body weight. Moreover, injection of insulin to STZ-treated transgenic mice led to normoglycemia, while STZ-treated control mice remained highly hyperglycemic. Thus, these results are consistent with a key role of glucose phosphorylation in regulating glucose metabolism in skeletal muscle. Furthermore, this study suggests that engineering skeletal muscle to express GK may be a new approach to the therapy of diabetes mellitus.

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

“…Similarly, in Glut4-overexpressing mice, the majority of glucose was metabolised to lactate (70%) [17]. This is also consistent with results obtained in transgenic mice expressing Gck in skeletal muscle or in the liver, in which serum lactate concentrations were increased [24,29]. Moreover, in these mice, the increase of lactate production by hepatocytes occurred only at high glucose concentration [24].…”

“…Uptake of deoxy-D-[ 3 H]glucose was measured in isolated soleus muscles, in 3T3-L1 adipocytes and in isolatedadipocyte suspension incubated with or without insulin as previously described [4,28,29] and in the ESM. To determine glucose and pyruvate incorporation into lipids, epididymal fat pads and 3T3-L1 adipocytes were incubated for 2 h with either D-[U- 14 C]glucose or D-[2-14 C]pyruvate (PerkinElmer, Waltham, MA, USA) supplemented with glucose as stated in the figure legends.…”

Counteraction of Type 1 Diabetic Alterations by Engineering Skeletal Muscle to Produce Insulin

“…This agrees with the increased glucose disposal observed after a glucose tolerance test in Tg1 transgenic mice. Similarly, transgenic mice with increased glucose uptake because their liver or skeletal muscle has been engineered to overexpress key genes in the regulation of glucose transport, such as GLUT4 (47)(48)(49)(50) or GLUT1 (51), or glucose phosphorylation, such as glucokinase (52,53) or c-myc (33,54), show reduced blood glucose levels and improved glucose tolerance. Forty-five days after STZ-treatment, fed control mice were highly hyperglycemic, while constitutive expression of insulin in skeletal muscle of Tg1 transgenic mice led to increased insulinemia and to marked reduction of hyperglycemia.…”

Reversal of Type 1 Diabetes by Engineering a Glucose Sensor in Skeletal Muscle

“…These features allow glucose to be taken up only when it is at high concentrations, as previously reported in pancreatic ␤-cells (18). Expression of glucokinase in skeletal muscle increases glucose disposal and reduces diabetic hyperglycemia (17,19,20). However, expression of glucokinase alone cannot normalize glycemia in type 1 diabetes because of the lack of insulin-mediated glucose transport.…”

“…When diabetic, these insulin-expressing transgenic mice are normoglycemic during fasting but remain hyperglycemic under fed conditions (16). To increase glucose phosphorylation, the hepatic glucose-phosphorylating enzyme glucokinase has also been expressed in skeletal muscle (17). In contrast to hexokinase II, glucokinase has a high K m for glucose (ϳ8 mmol/l), it is not inhibited by glucose 6-phosphate, and it shows kinetic cooperativity with glucose (12).…”