1,25-Dihydroxyvitamin D₃ regulation of cardiac myocyte proliferation and hypertrophy

We previously demonstrated that 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] inhibits myocyte maturation (T. D. O'Connell, D. A. Giacherio, A. K. Jarvis, and R. U. Simpson. Endocrinology 136: 482-488, 1995). To define further the role of 1,25(OH)₂D₃ in regulating myocardial development, we examined the effects of 1,25(OH)₂D₃ on proliferation and growth of primary cultures of ventricular myocytes isolated from neonatal rat hearts. When neonatal myocytes were grown in a serum-supplemented medium, cell number approximately doubled, and treating these myocytes with 1,25(OH)₂Da₃ inhibited their proliferation by 56.56% after 4 days. Flow cytometry revealed that 1,25(OH)₂D₃ reduced the percentage of cells in the S phase of the cell cycle by 31.39% after 4 days. We show for the first time that proliferating cell nuclear antigen protein levels were specifically reduced by 1,25(OH)₂D₃. Protooncogene c-myc protein levels were also reduced by this hormone. Interestingly, a phorbol ester had a similar effect on myocyte proliferation. Furthermore, 1,25(OH)₂D₃ increased myocyte protein levels and increased cell size, suggesting that it induces cardiac myocyte hypertrophy. Our findings indicate that 1,25(OH)₂D₃ and phorbol esters directly regulate myocyte proliferation and induce myocyte hypertrophy. Finally, the data demonstrate that the mechanism by which 1,25(OH)₂D₃ regulates myocyte proliferation involves blocking entry into the S phase of the cell cycle.