Abstract 208: Down Regulation of the Novel Transcriptional Regulator of Cardiac Hypertrophy, Krüppel-Like Factor 15, is Mediated by TGF-Beta Induced p38 Activation in vivo and in vitro.
Multiple mechanisms that promote cardiac hypertrophy (LVH) are known, but only few transcriptional inhibitors of hypertrophy have been identified. In TGR(mren2)27 (Ren2), a model of angiotensin II (AngII) driven LVH, microarray analysis revealed significant down-regulation of Krüppel-like factor 15 (KLF15). In situ hybridization showed that KLF15 expression was particularly suppressed in hypertrophied cardiac myocytes (CMs) of Ren2 rats, which was also seen in aortic stenosis patients. Knock-down of KLF15 by shRNA resulted in a 10-fold increase in BNP expression (p<0.005). Overexpression of KLF15 suppressed activity of MEF2, a major transcriptional regulator of hypertrophy. Since KLF15 seems an important novel inhibitor of myocyte hypertrophy, we sought to answer what suppresses KLF15 in CMs, thereby allowing LVH. We revealed that TGF-β robustly repressed expression of KLF15 in rat CMs (p=0.03). Next we tested the role of TGF-β in vivo by employing a cross of MerCreMer mice with mice where the TGF-β type I receptor had been floxed, which allowed to circumvent developmental effects of loss of the TGF-β type I by adding tamoxifen only in adult mice. After tamoxifen, the mice received AngII. AngII induced LVH and decreased KLF15 expression in wild type mice, but failed to induce LVH nor suppress KLF15 in the TGF-β type I receptor floxed mice, suggesting that TGF-β signaling is necessary to suppress KLF15 in-vivo. AngII increased Phospho-p38 in wild type mice (p<0.05) but failed to induce phospho-p38 in TGF-b receptor floxed mice. To evaluate the role of p38 in suppression of KLF15, CMs were treated with MKK6 (a p38 upstream kinase) which induced a 3.5 fold increase in BNP. MKK6 decreased KLF15 levels 4 fold. In rats with exercise induced physiological hypertrophy, no down regulation of KLF15 was observed, indicating that KLF15 is only suppressed in pathological hypertrophy. Taken together, we show that KLF15 is a novel transcriptional inhibitor of pathologic hypertrophy, possibly via its ability to inhibit MEF2. Suppression of KLF15 allows pathologic LVH, and is conferred by TGF-β via recruitment of p38 MAPK. This demonstrates a novel pathway by which TGF-β can directly induce pathologic hypertrophy by removing the transcriptional repression by KLF15.