BLOCKERS OF PLATELET-DERIVED GROWTH FACTOR-ACTIVATED NONSELECTIVE CATION CHANNEL INHIBIT CELL-PROLIFERATION

In serum-deprived G(o)-arrested cells, the addition of serum or growth factors initiates a cascade of events that culminates in DNA synthesis and mitosis. Recently, we showed that in mouse L-M(TK-) fibroblasts a 28-pS nonselective cation channel (NS channel) becomes quiescent at G(o) arrest and rapidly active within seconds of platelet-derived growth factor (PDGF) or serum addition, placing this response very early in the postreceptor signaling cascade. However, lack of specific channel blockers hindered determination of whether channel activation was necessary for mitogenesis. Derivatives of N-phenylanthranilic acid (DCA) have been reported to block a pancreatic nonselective channel. Therefore, using single-channel analysis, we examined the effect of these agents on the L-M(TK-) NS channel. Flufenamic acid and mefenamic acid rapidly produced reversible channel block with an inhibitory constant (K(i)) almost-equal-to 10-mu-M. Furthermore, the component of the macroscopic K+ efflux shown to be mediated by the NS channel was blocked with a similar K(i) value. DCA effects on cell proliferation were tested by measuring cloning efficiency and growth rate. Both were inhibited over the range of concentration that affected channel activity, and a 50% inhibitory dose of 50-100-mu-M was determined. This observation further substantiates the hypothesis that NS channel activation forms a necessary component in the transduction of the mitogenic signal from the PDGF receptor.