GH influences multiple metabolic pathways. Excess GH (acromegaly) causes a distinct form of cardiomyopathy, which may progress to fulminant heart failure. AMPK is an energy conservation enzyme that modulates multiple areas of the cell stress response, inhibiting anabolism and promoting catabolism. AMPK is activated by phosphorylation at Thr172 and measurement of Thr172 phosphorylation is thought to correlate with enzyme activity. We investigated the influence of GH on cardiac AMPK in transgenic mouse models of chronic GH excess.
Methods: Bovine-GH-overexpressing (bGH) transgenic mice were produced by the microinjection of bGH cDNA into the pronucleus of C57BL/6J embryos. Animals were sacrificed at 2 and 8 months of age. Cardiac tissue was homogenised and AMPK immunoprecipitated using AMPK-α1 and -α2 antibodies. Functional AMPK assay was performed using the synthetic target peptide SAMS and 32P-ATP. Western blotting was performed for total AMPK (tAMPK) and Thr172 phospho-AMPK (pAMPK) using monoclonal antibodies. Bands were corrected for the house-keeping protein GAPDH.
Results: There were no significant differences in cardiac AMPK activity, measured by functional assay, between transgenic mice and their appropriate littermates at either timepoint. However, 2 month-old bGH mice demonstrated an increase in pAMPK/tAMPK ratio compared to controls and 8 month-old bGH mice demonstrated a reduction in pAMPK/tAMPK ratio compared to controls.
Conclusion: Chronic GH excess is associated with alterations in cardiac AMPK Thr172 phophorylation which do not appear to influence its activity. A discrepancy between AMPK Thr172 phosphorylation levels and the functional assay is unusual and may represent other factors influencing AMPK function, such as phosphorylation at inhibitory sites. The low levels of pAMPK/tAMPK in older bGH transgenics may be a contributing factor to the heart failure of acromegalic cardiomyopathy.