Cell cycle control of PDGF-induced Ca²⁺ signaling through modulation of sphingolipid metabolism

The effects of growth factors have been shown to depend on the position of a cell in the cell cycle. However, the physiological basis for this phenomenon remains unclear. Here we show that the majority of both CEINGE clone3 (cl3) and human embryonic kidney 293 cells, when arrested in a quiescent phase (G₀), responded to platelet-derived growth factor BB (PDGF- BB) with non-oscillatory Ca²⁺ signals. Furthermore, the same type of Ca²⁺ response was also observed in CEINGE cl3 cells (and to a lesser extent in HEK 293 cells) blocked at the G₁/S boundary. In contrast, CEINGE cl3 cells synchronized in early G₁ or released from G₁/S arrest responded in an oscillatory fashion. This cell cycle-dependent modulation of Ca²⁺ signaling was not observed on epidermal growth factor and G-protein-coupled receptor stimulation and was not due to differences in the expression of PDGF receptors (PDGFRs) during the cell cycle. We demonstrate that inhibition of sphingosine-kinase, which converts sphingosine to sphingosine-1-phosphate, caused G₀ as well as G₁/S synchronized cells to restore the oscillatory Ca²⁺ response to PDGF-BB. In addition, we show that the synthesis of sphingosine and sphingosine-1-phosphate is regulated by the cell cycle and may underlie the differences in Ca²⁺ signaling after PDGFR stimulation.