Platelet-derived growth factor (PDGF)-induced Ca²⁺ signaling in the CG4 oligodendroglial cell line and in transformed oligodendrocytes expressing the β-PDGF receptor

Ca²⁺ signaling induced by platelet-derived growth factor (PDGF) was investigated in the oligodendroglial cell lines CG4 and CEINGE clone 3, using fura-2 microfluorimetry and video imaging. CEINGE cl3 cells, immortalized with polyoma middle T antigen, were found to uniformly express the polyoma middle T antigen protein as well as 2',3'-cyclic nucleotide 3'- phosphodiesterase, a specific marker for oligodendroglia. PDGF-BB induced both oscillatory and non-oscillatory Ca²⁺ responses in CEINGE cl3 cells as well as in CG4 cells, grown either as O-2A progenitors or differentiated oligodendrocytes. However, in CG4 cells the percentage of oscillatory Ca²⁺ responses was higher than that observed in CEINGE cl3 cells. In contrast, oscillatory Ca²⁺ responses were not observed in PC-12 cells transfected with β-PDGF receptor (PDGFR) or in NIH 3T3 fibroblasts. CG4 cells expressed only the α-PDGFR, whereas CEINGE cl3 cells expressed both α and β isoforms. When CEINGE cl3 cells were exposed to PDGF-AA, which binds only to the α-PDGFR, the percentage of oscillatory Ca²⁺ responses was higher than that observed after PDGF-BB stimulation. We previously reported that block of the enzyme sphingosine kinase, and a consequent increase in intracellular sphingosine levels in CEINGE cl3 cells caused an increase in the percentage of oscillatory Ca²⁺ responses induced by PHGF-BB. However, in CG4 cells block of sphingosine kinase did not increase the oscillatory Ca²⁺ response elicited by PDGF-BB, although the addition of exogenous sphingosine induced an oscillatory Ca²⁺ response in 77% of cells studied. We hypothesize that the α-PDGFR is less effective than the β-PDGFR in stimulating the activity of sphingosine kinase. The results also suggest that α- and β-PDGFRs may differently regulate sphingolipid metabolism.