EXPRESSIONS AND ACTIVITIES OF CELL-CYCLE REGULATORY MOLECULES DURING THE TRANSITION FROM MYOCYTE HYPERPLASIA TO HYPERTROPHY

The role of cell cycle dependent molecules in controlling the switch f rom cardiac myocyte hyperplasia to hypertrophy remains unclear, althou gh in the rat this process occurs between day 3 and 4 after birth. In this study we have determined (1) cell cycle profiles by fluorescence activated cell sorting (FACS); and (2) expressions, co-expressions and activities of a number of cyclins, cyclin-dependent kinases (CDKs) an d CDK inhibitors by reverse transcriptase-polymerase chain reaction (R T-PCR), immunoblotting and in vitro kinase assays in freshly isolated rat cardiac myocytes obtained from 2, 3, 4 and 5-day-old animals. The percentage of myocytes found in the S phase of the cell cycle decrease d significantly during the transition from hyperplasia to hypertrophy (5.5, 3.5, 2.3 and 1.9% of cells in 2-, 3-, 4- and 5-day-old myocytes, respectively, P<0.05), concomitant with-a significant increase in the percentage of G(0)/G(1) phase cells. At the molecular level, the expr essions and activities of G(1)/S and G(2)/M phase acting cyclins and C DKs were downregulated significantly during the transition from hyperp lasia to hypertrophy, whereas the expressions and activities of G(1) p hase acting cyclins and CDKs were upregulated significantly during thi s transition. In addition, p21(CIP1)- and p27(KIP1)-associated CDK kin ase activities remained relatively constant when histone H1 was used a s a substrate, whereas phosphorylation of the retinoblastoma protein w as upregulated significantly during the transition from hyperplasia to hypertrophy. Thus, there is a progressive and significant G(0)/G(1) p hase blockade during the transition from myocyte hyperplasia to hypert rophy. Whilst CDK2 and cdc2 may be pivotal in the withdrawal of cardia c myocytes from the cell cycle, CDK4 and CDK6 maybe critical for maint aining hypertrophic growth of the myocyte during development, (C) 1998 Academic Press.