Abstract 586: Early Characterization of the Mechanisms of ATRA-mediated Suppression of Cardiac Hypertrophy

Abstract

Pathway analyses of proteomic studies in guinea pig and rat hearts subjected to pressure overload-induced hypertrophy and heart failure (HF) suggest aberrant retinoid signaling may contribute to HF progression. Though the precise mechanisms are unknown, we recently showed that cardiac levels of the hormone all-trans retinoic acid (ATRA) are deficient in human non-ischemic HF patients. This has prompted us to revisit an old, but poorly understood, observation that ATRA suppresses phenylephrine (Phe)-induced hypertrophy in neonatal rat ventricular myocytes (NRVMs). Using confocal microscopy to measure cross-sectional area (CSA) as an index of hypertrophy, we show, consistent with prior work, that ATRA suppresses Phe-induced increases in NRVM CSA by average of 70%. Specifically, treatment groups of maintenance media (MM) alone and MM+Phe+ATRA were significantly different from the MM+Phe treatment group (p<0.005, n>58). ATRA exerts its biological actions through a class of ligand-dependent transcription factors called retinoic acid receptors (RARs), of which there are three forms, a, b and g. To better understand the mechanism by which ATRA exerts its anti-hypertrophic effect in NRVMs, we examined the impact of RARα, RARβ, and RARγ antagonists. The RARβ antagonist LE135 (1mM) fully blocked the action of ATRA (p<0.0001, n>46), while initial studies with the RARa and RARg antagonists (BMS 195614 and MM 11253) failed to significantly mitigate CSA suppression by ATRA. Thus, initial pharmacologic studies indicate ATRA elicits an anti-hypertrophic program, at least in part, through its activation of RARb. Ongoing experiments are aimed at corroborating these observations in NRVMs virally-transduced with shRNA to knock down levels of each of the RAR isoforms. These studies will provide key insights as we begin to delineate the mechanisms by which ATRA dysregulation contributes to HF pathogenesis.