EXPRESSION AND ACTIVITIES OF CYCLINS AND CYCLIN-DEPENDENT KINASES IN DEVELOPING RAT VENTRICULAR MYOCYTES

The molecular mechanisms responsible for the alterations in proliferat ive capacity of cardiac myocytes during development remain unknown; ho wever, cell cycle dependent molecules may be involved, We have determi ned the expression of cyclins A, D1-3 and E, and cyclin-dependent kina ses (CDKs) 2, 4, 5 and 6 and cdc2 in freshly isolated rat cardiac myoc ytes from fetal (18 days gestation), neonatal (2 days post-natal) and adult animals by immunoblotting. Our results show a dramatic decrease in expression of these proteins during normal cardiac development, suc h that levels are highest in fetal myocytes but are significantly down -regulated in adult cells (P<0.05, in each case), We also have determi ned the in vitro kinase activities of cdc2, CDK2, CDK4, CDK5 and CDK6 immunocomplexes in fetal, neonatal and adult myocytes. There was a con sistent and significant loss of cdc2, CDK2, CDK4 and CDK6 kinase activ ities in adult cardiac cell lysates (5.3-, 10.6-, 1.5- and 1.9-fold de creases, respectively) when compared to neonatal samples (P<0.05); CDK 5 activity showed a similar trend but failed to reach significance. In conclusion, our results show that the expression and activities of va rious positive regulators of the cell cycle are down-regulated signifi cantly during development of the cardiac myocyte, concomitant with the loss of proliferative capacity in adult myocytes, Down-regulation of these proteins may be pivotal in the withdrawal of the cardiac myocyte from the cell cycle. (C) 1997 Academic Press Limited.