Richard C. Ho

Skeletal Muscle-Selective Knockout of LKB1 Increases Insulin Sensitivity, Improves Glucose Homeostasis, and Decreases TRB3

ABSTRACT LKB1 is a tumor suppressor that may also be fundamental to cell metabolism, since LKB1 phosphorylates and activates the energy sensing enzyme AMPK. We generated muscle-specific LKB1 knockout (MLKB1KO) mice, and surprisingly, found that a lack of LKB1 in skeletal muscle enhanced insulin sensitivity, as evidenced by decreased fasting glucose and insulin concentrations, improved glucose tolerance, increased muscle glucose uptake in vivo, and increased glucose utilization during a hyperinsulinemic-euglycemic clamp. MLKB1KO mice had increased insulin-stimulated Akt phosphorylation and a >80% decrease in muscle expression of TRB3, a recently identified Akt inhibitor. Akt/TRB3 binding was present in skeletal muscle, and overexpression of TRB3 in C2C12 myoblasts significantly reduced Akt phosphorylation. These results demonstrate that skeletal muscle LKB1 is a negative regulator of insulin sensitivity and glucose homeostasis. LKB1-mediated TRB3 expression provides a novel link between LKB1 and Akt, critical kinases involved in both tumor genesis and cell metabolism.

AMP-activated Protein Kinase α2 Activity Is Not Essential for Contraction- and Hyperosmolarity-induced Glucose Transport in Skeletal Muscle

To examine the role of AMP-activated protein kinase (AMPK) in muscle glucose transport, we generated muscle-specific transgenic mice (TG) carrying cDNAs of inactive α2 (α2i TG) and α1 (α1i TG) catalytic subunits. Extensor digitorum longus (EDL) muscles from wild type and TG mice were isolated and subjected to a series of in vitro incubation experiments. In α2i TG mice basal α2 activity was barely detectable, whereas basal α1 activity was only partially reduced. Known AMPK stimuli including 5-aminoimidazole-4-carboxamide-1-β-4-ribofuranoside (AICAR), rotenone (a Complex I inhibitor), dinitrophenol (a mitochondrial uncoupler), muscle contraction, and sorbitol (producing hyperosmolar shock) did not increase AMPK α2 activity in α2i TG mice, whereas α1 activation was attenuated by only 30–50%. Glucose transport was measured in vitro using isolated EDL muscles from α2i TG mice. AICAR- and rotenone-stimulated glucose transport was fully inhibited in α2i TG mice; however, the lack of AMPK α2 activity had no effect on contraction- or sorbitol-induced glucose transport. Similar to these observations in vitro, contraction-stimulated glucose transport, assessed in vivo by 2-deoxy-d-[3H]glucose incorporation into EDL, tibialis anterior, and gastrocnemius muscles, was normal in α2i TG mice. Thus, AMPK α2 activation is essential for some, but not all, insulin-independent glucose transport. Muscle contraction- and hyperosmolarity-induced glucose transport may be regulated by a redundant mechanism in which AMPK α2 is one of multiple signaling pathways.

Ablation of AMP-Activated Protein Kinase α2 Activity Exacerbates Insulin Resistance Induced by High-Fat Feeding of Mice

OBJECTIVE—We determined whether muscle AMP-activated protein kinase (AMPK) has a role in the development of insulin resistance. RESEARCH DESIGN AND METHODS—Muscle-specific transgenic mice expressing an inactive form of the AMPK α2 catalytic subunit (α2i TG) and their wild-type littermates were fed either a high-fat (60% kcal fat) or a control (10% kcal fat) diet for 30 weeks. RESULTS—Compared with wild-type mice, glucose tolerance in α2i TG mice was slightly impaired on the control diet and significantly impaired on the high-fat diet. To determine whether the whole-body glucose intolerance was associated with impaired insulin sensitivity in skeletal muscle, glucose transport in response to submaximal insulin (450 μU/ml) was measured in isolated soleus muscles. On the control diet, insulin-stimulated glucose transport was reduced by ∼50% in α2i TG mice compared with wild-type mice. High-fat feeding partially decreased insulin-stimulated glucose transport in wild-type mice, while high-fat feeding resulted in a full blunting of insulin-stimulated glucose transport in the α2i TG mice. High-fat feeding in α2i TG mice was accompanied by decreased expression of insulin signaling proteins in gastrocnemius muscle. CONCLUSIONS—The lack of skeletal muscle AMPK α2 activity exacerbates the development of glucose intolerance and insulin resistance caused by high-fat feeding and supports the thesis that AMPK α2 is an important target for the prevention/amelioration of skeletal muscle insulin resistance through lifestyle (exercise) and pharmacologic (e.g., metformin) treatments.