THE AMP-ACTIVATED PROTEIN-KINASE GENE IS HIGHLY EXPRESSED IN RAT SKELETAL-MUSCLE - ALTERNATIVE SPLICING AND TISSUE DISTRIBUTION OF THE MESSENGER-RNA

The AMP-activated protein kinase (AMPK) phosphorylates, and thereby in activates, a number of enzymes involved in the regulation of Lipid met abolism. We have studied the expression of the AMPK gene in a variety of rat tissues. The gene is transcribed into a message of approximatel y 9.5 kb as detected by Northern blotting. Highest expression of the A MPK message was found in skeletal muscle, which contained 20 amol/mu g total RNA as determined by competitive reverse-transcription/polymera se chain reaction (RT-PCR). In liver, kidney, brain, mammary glands, h eart and lung, AMPK mRNA levels ranged over 1-4 amol/mu g total RNA. A dipose tissue contained less than 1 amol/mu g total RNA. A second AMPK mRNA form was detected by RT-PCR that was 142 bases shorter than the functional transcript. This transcript was apparently generated by alt ernative splicing of a single exon within the 5'-coding region. The sh orter of the two messages, which is not translated into AMPK protein, contributed between 35-60% of AMPK mRNA in most tissues, but only 15-2 0% in skeletal muscle and heart. As a result, functional AMPK mRNA was sevenfold higher in skeletal muscle than in liver, although AMPK acti vity was much lower. By Western blotting, relatively large amounts of AMPK protein were detected in skeletal muscle compared to liver. AR?PK isolated from skeletal muscle was not activated by treatment with AMP K kinase under conditions where Liver AMPK was fully activated. A sing le 63-kDa polypeptide was immunoprecipitated from rat skeletal muscle using anti-peptide IgG against AMPK. In contrast, two additional polyp eptides with apparent molecular masses of 38 kDa and 36 kDa co-precipi tated with the 63-kDa AMPK protein from rat liver. These results indic ate that the muscle enzyme has a different subunit organization compar ed to the liver enzyme, which may account for its low catalytic activi ty. Together, our results indicate a physiological role for AMPK in mu scle, in addition to its previously described role in lipid metabolism .