The Exit from G0into the Cell Cycle Requires and Is Controlled by Sarco(endo)plasmic Reticulum Ca2+Pump

The intracellular calcium pump sarco(endo)plasmic reticulum Ca2+(SERCA) is responsible for the formation of the Ca2+gradient across the endoplasmic reticulum membrane, and this gradient is used to generate the Ca2+signal during agonist-stimulated cell growth. In the present study, the role of SERCA in both cell cycle and growth control was investigated using cultured rat aortic endothelial cells (RAEC). Using a novel DNA transfection approach, cell lines were established that showed varying degree of SERCA activity through the down-regulation of the endogenous SERCA gene (B. F. Liu, X. Xu, R. Fridman, S. Muallem, and T. H. Kuo,J. Biol. Chem.271, 1–9, 1996). Cell proliferation studies indicated that the lower SERCA expressing cells exhibited a slower growth pattern without altering the doubling time which was similar for both parental and transfected RAEC lines. G1to S phase transition was prolonged with a smaller proportion of cells entering DNA synthesis as indicated by thymidine incorporation assay. Comparison of transfected cell lines indicated a tight coupling of SERCA activity and the length of the G1period. Down-regulation of SERCA gene expression was accompanied by increased mRNA levels of p21 (WAF1/CIP1), a universal cell cycle inhibitor. The delay in G1to S progression also coincided with the up-regulation of p53 mRNA and underphosphorylation of the retinoblastoma protein. This study suggests that Ca2+metabolism in the agonist mobilizable pool controls the cell cycle through the regulation of genes operating in the critical G1to S checkpoint.