Activation of the hypoxia response pathway protects against age-induced cardiac hypertrophy

Aims: We have previously demonstrated protection against obesity, metabolic dysfunction, atherosclerosis and cardiac ischemia in a hypoxia-inducible factor (HIF) prolyl 4-hydroxylase-2 (Hif-p4h-2) deficient mouse line, attributing these protective effects to activation of the hypoxia response pathway in a normoxic environment. We intended here to find out whether the Hif-p4h-2 deficiency affects the cardiac health of these mice upon aging. Methods and results: When the Hif-p4h-2 deficient mice and their wild-type littermates were monitored during normal aging, the Hif-p4h-2 deficient mice had better preserved diastolic function than the wild type at one year of age and less cardiomyocyte hypertrophy at two years. On the mRNA level, downregulation of hypertrophy- associated genes was detected and shown to be associated with upregulation of Notch signaling, and espe- cially of the Notch target gene and transcriptional repressor Hairy and enhancer-of-split-related basic helix-loop- helix (Hey2). Blocking of Notch signaling in cardiomyocytes isolated from Hif-p4h-2 deficient mice with a gamma-secretase inhibitor led to upregulation of the hypertrophy-associated genes. Also, targeting Hey2 in isolated wild-type rat neonatal cardiomyocytes with siRNA led to upregulation of hypertrophic genes and increased leucine incorporation indicative of increased protein synthesis and hypertrophy. Finally, oral treat- ment of wild-type mice with a small molecule inhibitor of HIF-P4Hs phenocopied the effects of Hif-p4h-2 defi- ciency with less cardiomyocyte hypertrophy, upregulation of Hey2 and downregulation of the hypertrophy- associated genes. Conclusions: These results indicate that activation of the hypoxia response pathway upregulates Notch signaling and its target Hey2 resulting in transcriptional repression of hypertrophy-associated genes and less car- diomyocyte hypertrophy. This is eventually associated with better preserved cardiac function upon aging. Activation of the hypoxia response pathway thus has therapeutic potential for combating age-induced cardiac hypertrophy. 1. Introduction Cardiovascular diseases are the leading cause of death on a global scale [1], and cardiac impairments of varying etiology are estimated to affect over 37 million people [2]. Heart failure is commonly caused by chronically increased cardiac workload, leading to a loss of cardiomyocytes or pressure overload due to hypertension. In response to increased loading, the heart undergoes pathophysiological processes, characterized by structural and functional remodeling of the myocar- dium, such as cardiomyocyte hypertrophy and fibrosis [3,4]. The prevalence of heart failure increases dramatically upon aging [5], and increased arterial stiffness [6], elevated blood pressure [7].