INVESTIGATIONS ON THE MECHANISM OF ACTION OF THE ANTIPROLIFERANT AND ION-CHANNEL ANTAGONIST FLUFENAMIC ACID

The compound flufenamic acid has been previously described as an inhib itor of chloride- and nonselective cation channels. Moreover, this com pound showed antiproliferative effects in the mouse fibroblast cell li ne LM(TK-). In this study, we investigated the effects of this compoun d on cell proliferation and membrane currents induced by mitogens (suc h as fetal calf serum, FCS) or platelet-derived growth factor (PDGF) i n LM(TK-) cells. After a brief application of FCS or PDGF (5-15 s), th e electrical response of the cells was biphasic: First, a transient po tassium conductance was activated, which appeared 8.3 +/- 0.7 s after the onset of stimulation and lasted for 30.1 +/- 2.9 s. The correspond ing single channel currents in cell-attached patches had an amplitude of 3-4 pA (at a holding potential of + 60 mV). The second effect of se rum or PDGF was the occurrence of a cation conductance for monovalent ions (sodium, potassium and cesium) and calcium. In contrast to the po tassium current, this conductance activated later (11.8 +/- 1.6 s afte r onset of fetal calf serum stimulation) and remained activated for mi nutes. Flufenamic acid inhibited the proliferation of LM(TK-) cells re versibly and in a concentration-dependent manner. This effect can be c orrelated with the inhibitory effects of flufenamic acid on mitogen-in duced membrane currents: The compound inhibited the non-selective cati on current with an IC50 of 38 mu M, whereas 135 mu M were necessary fo r halfmaximal inhibition of the potassium current; this is very close to the concentration for halfmaximal inhibition of cell proliferation (120 mu M). Hence, on the grounds of this comparison the blockade of t he non-selective cation current appears to be of only minor importance for the blockade of cell proliferation.