Objective: Heme oxygenases (HO) are the rate-limiting enzymes in heme degradation, catalyzing the breakdown of heme to equimolar quantities of biliverdin (BV), carbon monoxide (CO), and ferrous iron. The inducible HO isoform, HO-1, confers protection against ischemia/reperfusion (I/R)-injury in the heart. We hypothesized that HO-1 and its catalytic by-products constitute an antihypertrophic signaling module in cardiac myocytes. Methods and results: The G protein-coupled receptor (GPCR) agonist

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... ndothelin-1 (ET-1) (30 nmol/ l) stimulated a robust hypertrophic response in cardiac myocytes isolated from 1-to 3-day-old Sprague -Dawley rats, with increases in cell surface area (planimetry), sarcomere assembly (confocal laser scanning microscopy), and prepro-atrial natriuretic peptide (ANP) mRNA expression. Adenoviral overexpression of HO-1, but not h-galactosidase, significantly inhibited ET-1 induced cardiac myocyte hypertrophy. The antihypertrophic effects of HO-1 were mimicked by BV (10 Amol/l) and the CO-releasing molecule [Ru(CO) 3 Cl 2 ] 2 (10 Amol/l), strongly suggesting a critical involvement of BV and CO in the antihypertrophic effects of HO-1. Both BV and CO suppressed extracellular signalregulated kinases (ERK1/ERK2) and p38 mitogen-activated protein kinase (MAPK) activation by ET-1 stimulation. Moreover, BV and CO inhibited the prohypertrophic calcineurin/NFAT pathway. This inhibition occurred upstream from calcineurin because BV and CO inhibited NFAT activation in response to ET-1 stimulation but not in response to adenoviral expression of a constitutively active calcineurin mutant. Upstream-inhibition of the calcineurin/NFAT pathway by CO occurred independent from cGMP and cGMP-dependent protein kinase type I (PKG I). Conclusions: Heme oxygenase-1 and its catalytic by-products, BV and CO, constitute a novel antihypertrophic signaling pathway in cardiac myocytes. Biliverdin and CO inhibition of MAPKs and calcineurin/NFAT signaling provides a mechanistic framework how heme degradation products may promote their antihypertrophic effects.