EXPRESSION OF CALCIUM-PERMEABLE CATION CHANNEL CD20 ACCELERATES PROGRESSION THROUGH THE G(1) PHASE IN BALB C 3T3 CELLS/

CD20 is a transmembrane protein that functions as a Ca2+-permeable cat ion channel (Bubien, J. K., Zhou, L. J., Bell, P. D., Frizzel, R. A., and Tedder, T. F. (1993) J. Cell Biol. 121, 1121-1132) and is involved in growth regulation of B lymphocytes. In order to further investigat e the role of calcium entry in cell cycle progression, we introduced t he cDNA encoding a Ca2+-permeable cation channel, CD20, into Balb/c 3T 3 cells. Balb/c 3T3 cells transfected with a vector containing cDNA en coding CD20 expressed the CD20 protein, which was detected by assaying the binding of a monoclonal antibody against CD20. Calcium-permeable cation channel activity was detected in CDaO-expressing cells by whole cell patch clamp recording and microfluorometric determination of the cytoplasmic Ca2+ concentration using fura-2. The expression of CD20 i nduced significant alterations in the responses of the cells to insuli n-like growth factor-I (IGF-I). IGF-I induced DNA synthesis by control cells only when they had been pretreated with both platelet-derived g rowth factor (PDGF) and epidermal growth factor (EGF). In contrast, DN A synthesis by 30% of the quiescent CD20 expressing cells was initiate d in response to IGF-I in the absence of priming with PDGF and EGF. Wh en control quiescent cells were primed with PDGF and EGF, the addition of IGF-I led to the initiation of DNA synthesis after 14 h or more, w hereas it induced DNA synthesis by CD20 expressing cells primed with P DGF and EGF 4 h earlier. The IGF-induced DNA synthesis was dependent o n extracellular Ca2+, and expression of CD20 reduced the concentration of extracellular Ca2+ required for it. Furthermore, DNA synthesis by approximately 25% of the CD20-expressing cells was initiated after pri ming with PDGF and EGF, even in the absence of the progression factor IGF-I. These results indicate that CD20 expressed in Balb/c 3T3 cells functions as a constitutively active Ca2+-permeable cation channel and that expression of CD20 accelerates G(1) progression in a Ca2+-depend ent manner.