Abstract
One of the prominent causes of heart failure is atherosclerosis of the coronary arteries. Obstruction of blood flow within the coronary arteries results in inadequate perfusion, or ischemia, of the heart muscle. The let-7 family of microRNAs (miRs) regulates critical cell functions, including survival signaling, metabolic control and glucose utilization, which may be important during myocardial ischemia. MiR-let-7 expression is under tight temporal and spatial control through multiple redundant mechanisms. The mechanisms and functional consequences of miR-let-7 regulation in ischemic myocardium largely remain unknown. Here we show that miR-let-7c, -7a and -7g are downregulated in the adult mouse heart early after coronary occlusion, and in neonatal rat ventricular myocytes subjected to hypoxia. This occurs post-transcriptionally and is independent of glucose depletion. Hypoxia also induced Lin28, a negative regulator of let-7. Both changes were abrogated by treatment with the histone deacetylase inhibitor trichostatin A. Let-7 repression and Lin28 induction were absent in cardiac fibroblasts. Restoration of let-7g to hypoxic myocytes and to ischemia-reperfused mouse hearts in vivo potentiated the activation of Akt and prevented cell death. Mechanistically, phosphotidyl inositol 3’kinase interacting protein 1 (PIK3IP1), a negative regulator of PI3K, was identified as a novel target of miR-let-7 by a crosslinking technique the targeted PIK3IP1 to the myocyte RISC. Finally, in non-failing and failing human myocardium, we found specific inverse relationships between Lin28 and miR-let-7g, and between miR-let-7g and PIK3IP1. This study reveals a conserved hypoxia-responsive Lin28-miR-let-7-PIK3IP1 regulatory axis that is specific to cardiac myocytes and promotes apoptosis during myocardial ischemic injury.