A cardiac myocyte-restricted Lin28/let-7 regulatory axis promotes hypoxia-mediated apoptosis by inducing the AKT signaling suppressor PIK3IP1

•Hypoxia induces Lin28 expression in a cardiac myocyte-specific manner to downregulate members of the miR-let-7 family.•Downregulation of let-7 promotes cardiac myocyte cell death during hypoxia-reoxygenation.•Mir-let-7g targets a negative regulator of PI3K, PIK3IP1 to the RISC.•Hypoxia represses PI3K-Akt signaling in cardiac myocytes by reducing let-7, and replacinglet-7 restores PI3K signalling.•A similar Lin28-let-7 axis appears to control PIK3IP1 in human myocardium.

RationaleThe let-7 family of microRNAs (miRs) regulates critical cell functions, including survival signaling, differentiation, metabolic control and glucose utilization. These functions may be important during myocardial ischemia. MiR-let-7 expression is under tight temporal and spatial control through multiple redundant mechanisms that may be stage-, isoform- and tissue-specific.ObjectiveTo determine the mechanisms and functional consequences of miR-let-7 regulation by hypoxia in the heart.Methods and resultsMiR-let-7a, -7c and -7g were downregulated in the adult mouse heart early after coronary occlusion, and in neonatal rat ventricular myocytes subjected to hypoxia. Let-7 repression did not require glucose depletion, and occurred at a post-transcriptional level. Hypoxia also induced the RNA binding protein Lin28, a negative regulator of let-7. Hypoxia ineither induced Lin28 nor repressed miR-let-7 in cardiac fibroblasts. Both changes were abrogated by treatment with the histone deacetylase inhibitor trichostatin A. Restoration of let-7g to hypoxic myocytes and to ischemia-reperfused mouse hearts in vivo via lentiviral transduction potentiated the hypoxia-induced phosphorylation and activation of Akt, and prevented hypoxia-dependent caspase activation and death. Mechanistically, phosphatidyl 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 showing that miR-let-7g specifically targets Pik3ip1 to the cardiac myocyte Argonaute complex 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.ConclusionA conserved hypoxia-responsive Lin28-miR-let-7-Pik3ip1 regulatory axis is specific to cardiac myocytes and promotes apoptosis during myocardial ischemic injury.