1,25(OH)(2) VITAMIN-D-3 AND RETINOIC ACID ANTAGONIZE ENDOTHELIN-STIMULATED HYPERTROPHY OF NEONATAL RAT CARDIAC MYOCYTES

1,25 (OH)(2) Vitamin D-3 (VD3) and retinoic acid (RA) function as liga nds for nuclear receptors which regulate transcription, Though the car diovascular system is not thought to represent a classical target for these ligands, it is clear that both cardiac myocytes and vascular smo oth muscle cells respond to these agents with changes in growth charac teristics and gene expression, In this study we demonstrate that each of these ligands suppresses many of the phenotypic correlates of endot helin-induced hypertrophy in a cultured neonatal rat cardiac ventricul ocyte model, Each of these agents reduced endothelin-stimulated ANP se cretion in a dose-dependent fashion and the two in combination proved to be more effective than either agent used alone (VD3: 49%; RA: 52%; VD3 + RA: 80% inhibition), RA, at concentrations known to activate the retinoid X receptor, and, to a lesser extent, VD3 effected a reductio n in atrial natriuretic peptide, brain natriuretic peptide, and alpha- skeletal actin mRNA levels, Similar inhibition (VD3: 30%; RA: 33%; VD3 + RA: 59% inhibition) was demonstrated when cells transfected with re porter constructs harboring the relevant promoter sequences were treat ed with VD3 and/or RA for 48 h. These effects were not accompanied by alterations in endothelin-induced c-fos, c-jun, or c-myc gene expressi on, suggesting either that the inhibitory locus responsible for the re duction in the mRNA levels lies distal to the activation of the immedi ate early gene response or that the two are not mechanistically couple d, Both VD3 and RA also reduced [H-3]leucine incorporation (VD3: 30%; RA: 33%; VD3 + RA: 45% inhibition) in endothelin-stimulated ventriculo cytes and, once again, the combination of the two was more effective t han either agent used in isolation. Finally, 1,25 (OH)(2) vitamin D-3 abrogated the increase in cell size seen after endothelin treatment, T hese Endings suggest that the liganded vitamin D and retinoid receptor s are capable of modulating the hypertrophic process in vitro and that agents acting through these or similar signaling pathways may be of v alue in probing the molecular mechanisms underlying hypertrophy.