ACTIVATION OF A CALCIUM-PERMEABLE CATION CHANNEL CD20 EXPRESSED IN BALB C 3T3 CELLS BY INSULIN-LIKE GROWTH-FACTOR-I/

CD20 functions as a calcium-permeable cation channel. When expressed i n Balb/c 3T3 cells, CD20 accelerates the G(1) progression induced by i nsulin-like growth factor-I (IGF-I). To further characterize how CD20 modulates the action of IGF-I, we investigated whether the activity of CD20 channel was affected by IGF-I. In quiescent cells expressing CD2 0, IGF-I increased cytoplasmic free calcium concentration, [Ca2+](c), which was reversed by the removal of extracellular calcium. In contras t, IGF-I did not increase [Ca2+](c) in cells that did not express CD20 . In perforated patch clamp recordings, addition of IGF-I to the bath solution augmented the Ca2+ permeability, which was reversed by anti C D20 antibody. In cell-attached patch, calcium-permeable channel activi ty with unitary conductance of 7 picosiemens was detected, which was a bolished by anti-CD20 antibody. The single channel activities were mar kedly enhanced when IGF-I was included in the pipette solution, wherea s IGF-I added to the bath solution was ineffective. When cells were fi rst exposed to pertussis toxin, activation of the channel by IGF-I was blocked. Transfection of cDNA for Gip2, a constitutive active form of alpha(i2), activated the CD20 channel. These results indicate that th e CD20 channel is regulated by the IGF-I receptor by a mechanism invol ving pertussis toxin-sensitive G protein.