AKT signaling is essential for functional and structural integrity of the heart

Abstract

The mammalian heart expresses predominantly two isoforms of protein kinase B (AKT), namely AKT1 and AKT2. Knockout mice for AKT1 and AKT2 respectively, have demonstrated that AKT1 primarily appears to regulate growth whereas AKT2 is rather involved in regulating metabolism. Here we have analyzed common functions of both AKT isoforms in the heart by generating Tamoxifen(OHTX)‐inducible, cardiac myocyte specific AKT1 and AKT2 single and double KO mice (iCMAKT1/2KO).Inactivation of AKT1 and AKT2 resulted in a maximal loss of both isoforms 5 days after start of OHTX‐treatment without a compensatory upregulation of AKT3. The basal level of Ser9 Phosphorylation of GSK3β was significantly reduced in hearts of double KO only, demonstrating that GSK3β is controlled by AKT1 and AKT2 isoforms in CM. Insulin did not stimulate GSK3β phosphorylation. Furthermore, insulin dependent phosphorylation of classical direct and indirect AKT downstream targets like mTor (Ser2448, Ser2841), p70S6 kinase (Thre 389) or S6 ribosomal protein, among others, was shown to be significantly downregulated in iCMAKT1/2KO heart.Functional analysis by echocardiography revealed a progressive decline of pump function characterized by a drop of ejection fraction from initially 62 to 37 % and 31 % on day 10 and day 21, respectively resulting in cardiac failure on day 24. In contrast, the single iCMAKT1 and iCMAKT2 KO mice showed no cardiac depression. Interestingly, wall thickness declined from day 14 to day 21. Concomitantly, cardiac mass declined to 70 %. Gene expression analysis using Agilent 60K microarrays were performed using hearts from single and double knock out mice. On day 10 after KO induction 1095 genes were differentially expressed (P<0.05) and on day 21 this number increased to 7707 (P<0.01, moderated t‐test, Benjamini‐Hochberg correction). On day 10 upon Txf induction Gdf15 (4.9 × up), a typical stress marker for diseased hearts but not ANP was induced in double knock out hearts. On day 21 this expression increased to Gdf15 (58 × up) and also ANP was induced (17.8 × up). None of these expression alterations coud be detected in Akt1 or Akt2 single knock out heart. Ingenuity Pathway analysis identified particularly metabolic pathways as well as EIF2 signalling to be substantially affected.ConclusionExpression of either AKT1 or AKT2 in cardiac myocytes is sufficient to preserve cardiac function whereas loss of both isoforms results in progessive cardiac atrophy and failure.Support or Funding InformationThis work was funded by DFG Grant IRTG1902.