001 RE-EXPRESSION OF A CONSTITUTIVELY-ACTIVE CYCLIN B1:CDC2 COMPLEX IN ADULT RAT CARDIOMYOCYTES IS SUFFICIENT TO REINITIATE CELL DIVISION

K. A. Bicknell*, G. Brooks.School of Pharmacy, The University of Reading, Reading Berskhire RG6 6AJ

The ability of the myocardium to repair itself following a myocardial infarction is severely impaired by the limited capacity of adult ventricular cardiomyocytes to divide. Consequently, the damaged region of the myocardium is replaced by scar tissue and cardiac function is often irreversibly impaired. We previously have shown that adult cardiomyocytes normally do not divide as a consequence of cell cycle arrest in G₀/G₁ and G₂/M. However, during pressure overload-induced hypertrophy, a partial reactivation of the cell cycle machinery occurs that permits adult cardiomyocytes to progress through the G₁/S transition prior to their accumulation in G₂/M. Interestingly, expression and activity of the G₂/M phase cyclin:CDK complex, cyclin B1:CDC2, is not detected in normal adult cardiomyocytes nor in cardiomyocytes undergoing hypertrophy. Thus, we hypothesised that expression and activity of cyclin B1:CDC2 might limit adult cardiomyocyte proliferation. We have used recombinant adenoviruses to re-express cyclin B1 and/or a constitutively active form of CDC2, CDC2AF, in isolated adult rat cardiomyocytes. Re-introduction of cyclin B1 or CDC2AF alone did not result in an increase in cyclin B1:CDC2 kinase activity nor proliferation in adult myocytes compared to control cultures. However, cyclin B1:CDC2 kinase activity was increased 2.9±0.2-fold in cyclin B1 and CDC2AF co-infected cultures compared to controls. Furthermore, re-expression of both cyclin B1 and CDC2AF in adult cardiomyocytes resulted in a significant increase in the number of tropomyosin-positive myocytes (144%±16%), 24 hours after infection, compared to control cultures (100%±8%, n = 5, p<0.05). Interestingly, an increased number of smaller mononuclear tropomyosin-positive myocytes was observed in cyclin B1 and CDC2AF co-infected cultures (17.5%), compared to controls (7.1%). In this study, we have shown that over-expression of the cyclin B1:CDC2 complex reinitiates …