Downregulation of polo-like kinase correlates with loss of proliferative ability of cardiac myocytes.

Cardiac myocytes rapidly increase the cell number during the fetal and early neonatal period, but they lose their proliferative ability soon after birth. To understand the mechanism of how cardiac myocytes exit from the cell cycle, we examined the role of a newly identified serine/threonine kinase, polo-like kinase (Plk), in the process of proliferation of cardiac myocytes. Northern blot analysis revealed that Plk gene was abundantly expressed in cardiac myocytes and non-myocytes of fetal and neonatal rats but not in cardiocytes of adult rats. Western blot analysis showed that Plk protein was also detected only in fetal and neonatal hearts. During the early stage of cardiac differentiation. Plk expression was well correlated with the proliferative ability of cardiocytes. Plk mRNA was most abundant in undifferentiated embryonic stem (ES) cells and the mRNA levels decreased along with cardiac differentiation in the developing ES cell system. Once serum was deprived from the culture media, expression levels of Plk were markedly decreased and DNA was not synthesized in both cardiac myocytes and non-myocytes of neonatal rats. Re-addition of serum stimulated Plk gene expression and DNA synthesis in non-myocytes but not in cardiomyocytes. All these results taken together with the critical role of Plk in DNA synthesis in many cell types suggest that downregulation of Plk is important for the permanent withdrawal of cardiomyocytes from the cell cycle.