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. 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. Recent reports suggest that AMP-activated protein kinase (AMPK), 2The abbreviations used are:AMPKAMP-activated protein kinaseα2i TG micetransgenic mice expressing an inactive form of α2 in skeletal muscleα1i TG micetransgenic mice expressing an inactive form of α1 in skeletal muscleα1/2i TG micetransgenic mice expressing an inactive form of both α1 and α2 in skeletal muscleEDLextensor digitorum longusAICAR5-aminoimidazole-4-carboxamide-1-β-4-ribofuranosideGLUT1glucose transporter 1GLUT4glucose transporter 4IRSinsulin receptor substrateKRBKrebs-Ringer bicarbonateDNPdinitrophenolCLAMSComprehensive Lab Animal Monitoring SystemZMP5-aminoimidazole-4-carboxamide ribonucleotide 2The abbreviations used are:AMPKAMP-activated protein kinaseα2i TG micetransgenic mice expressing an inactive form of α2 in skeletal muscleα1i TG micetransgenic mice expressing an inactive form of α1 in skeletal muscleα1/2i TG micetransgenic mice expressing an inactive form of both α1 and α2 in skeletal muscleEDLextensor digitorum longusAICAR5-aminoimidazole-4-carboxamide-1-β-4-ribofuranosideGLUT1glucose transporter 1GLUT4glucose transporter 4IRSinsulin receptor substrateKRBKrebs-Ringer bicarbonateDNPdinitrophenolCLAMSComprehensive Lab Animal Monitoring SystemZMP5-aminoimidazole-4-carboxamide ribonucleotide a member of a metabolite-sensing protein kinase family, controls blood glucose homeostasis by regulating glucose transport in skeletal muscle and glucose production in the liver (1Rutter G.A. 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AMP-activated protein kinase transgenic mice expressing an inactive form of α2 in skeletal muscle transgenic mice expressing an inactive form of α1 in skeletal muscle transgenic mice expressing an inactive form of both α1 and α2 in skeletal muscle extensor digitorum longus 5-aminoimidazole-4-carboxamide-1-β-4-ribofuranoside glucose transporter 1 glucose transporter 4 insulin receptor substrate Krebs-Ringer bicarbonate dinitrophenol Comprehensive Lab Animal Monitoring System 5-aminoimidazole-4-carboxamide ribonucleotide AMP-activated protein kinase transgenic mice expressing an inactive form of α2 in skeletal muscle transgenic mice expressing an inactive form of α1 in skeletal muscle transgenic mice expressing an inactive form of both α1 and α2 in skeletal muscle extensor digitorum longus 5-aminoimidazole-4-carboxamide-1-β-4-ribofuranoside glucose transporter 1 glucose transporter 4 insulin receptor substrate Krebs-Ringer bicarbonate dinitrophenol Comprehensive Lab Animal Monitoring System 5-aminoimidazole-4-carboxamide ribonucleotide AMPK is a serine/threonine kinase consisting of a catalytic α subunit and regulatory β and γ subunits (15Hardie D.G. 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Hirshman M.F. N. S.A. Witters L.A. Goodyear L.J. Diabetes. PubMed Scopus Google Scholar) and muscle in L.G. Foufelle F. Barnes K. Baldwin S.A. Woods A. Carling D. Biochem. J. 2002; 363: 167-174Crossref PubMed Scopus (156) Google Scholar). To AMPK is for hyperosmolarity-induced glucose transport, isolated EDL muscles from α2i TG and wild type mice were in with or In to AICAR- and glucose transport, sorbitol-induced glucose transport was not by the lack of α2 activity in skeletal muscle α1 activity was only inhibited in α2i TG mice not Glucose in TG J. O. M. Birnbaum M.J. PubMed Scopus Google Scholar) that glucose transport was in muscles from AMPK transgenic mice J. O. M. Birnbaum M.J. PubMed Scopus Google Scholar), whereas Jorgensen S.B. B. Andreelli F. C. P. Vaulont S. Richter E.A. Wojtaszewski J.F. 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In muscle glucose transport measured in vitro was reduced in the α2i TG mice with wild type mice to reduced muscle the in vitro In of was a muscle and glucose transport and the that production was was no in glucose transport wild type and α2i TG mice suggest that AMPK α2 activation is not essential for glucose transport measured in we also that glucose transport measured in vivo was normal in multiple muscles of the α2i TG mice is now using both and transgenic that α2 activation is not essential for glucose transport in skeletal not the mechanism for reduced muscle with in vitro in the α2i TG mice. is that reduced to of AMPK α2 that transgenic mice expressing an inactive AMPK α2 in of measured by N. N. L. I. Hirshman M.F. Goodyear L.J. R. 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J. 2004; PubMed Scopus Google Scholar) that muscle glucose transport is in α1 mice. observations may suggest no role for α1 in glucose transport. both AICAR- and glucose transport in α2i TG mice, α1 activity increases to that with in vitro that α1 activation for α2 in regulating glucose transport. we that skeletal muscle of α1 protein and that a of the α1 activity in of whole muscle from and muscle and In and blood and and isolated from wild type mice α1 expression with skeletal muscle and L. both the α1 and α2 inactive to a with α2 in the skeletal In normal muscle glucose transport in the whole body α1 mice also be by the of α1 protein to In we that of AMPK α2 activity AICAR- and glucose transport in skeletal that AMPK α2 is for of insulin-independent glucose transport in skeletal In AMPK α2 activity is not essential for muscle contraction- and hyperosmolarity-induced glucose transport, both stimuli of AMPK suggest that muscle contraction- and hyperosmolarity-induced glucose transport regulated by a redundant mechanism in which AMPK α2 is one of multiple signaling pathways. for critical and for also C. Kahn and for the of the muscle kinase