Interaction between acylphosphatase and SERCA in SH-SY5Y cells

Ca2+ transport by sarco/endoplasmic reticulum, tightly coupled with the enz ymatic activity of Ca2+-dependent ATPase, controls the cell cycle through t he regulation of genes operating in the critical G(1) to S checkpoint. Expe rimental studies demonstrated that acylphosphatase actively hydrolyses the phosphorylated intermediate of sarco/endoplasmic reticulum calcium ATPase ( SERCA) and therefore enhances the activity of Ca-2+ pump. In this study we found that SH-SY5Y neuroblastoma cell division was blocked by entry into a quiescent G(0)-like state by thapsigargin, a high specific SERCA inhibitor, highlighting the regulatory role of SERCA in cell cycle progression. Addit ion of physiological amounts of acylphosphatase to SY5Y membranes resulted in a significant increase in the rate of ATP hydrolysis of SERCA. In synchr onized cells a concomitant variation of the level of acylphosphatase isoenz ymes opposite to that of intracellular free calcium during the G(1) and S p hases occurs. Particularly, during G(1) phase progression the isoenzymes co ntent declined steadily and hit the lowest level after 6 h from G(0) to G(1 ) transition with a concomitant significant increase of calcium levels. No changes in free calcium and acylphosphatase levels upon thapsigargin inhibi tion were observed. Moreover, a specific binding between acylphosphatase an d SERCA was demonstrated. No significant change in SERCA-2 expression was f ound. These findings suggest that the hydrolytic activity of acylphosphatas e increase the turnover of the phosphoenzyme intermediate with the conseque nces of an enhanced efficiency of calcium transport across endoplasmic reti culum and a subsequent decrease in cytoplasmic calcium levels. A hypothesis about the modulation of SERCA activity by acylphosphatase during cell cycl e in SY5Y cells in discussed.