Phenylarsine oxide increases intracellular calcium mobility and inhibits Ca2+-dependent ATPase activity in thymocytes

A rise in intracellular Ca2+ levels has been implicated as a regulatory sig nal for the initiation of lymphocyte proliferation. In the present study th e mechanism underlying the elevation of [Ca2+] induced by phenylarsine oxid e [PAO] was investigated in thymocytes. This agent inhibits HIV-1 replicati on and also NF-kappa B-mediated activation. It has been reported that the P AO-induced Ca2+ elevation results from an enhanced plasma membrane calcium permeability in T cells. Here, we present biochemical evidence that the PAO -induced Ca2+ increase was independent of external Ca2+. Consistent with th ese facts, when [Ca2+](i) was depleted by prolonged incubation of the cells in Ca2+-free medium, PAO addition did not lead to [Ca2+](i) increase. Thes e data indicate the involvement of intracellular organelles of thymocytes a s the source of Ca2+. Moreover, evidence is presented that PAO inhibited Ca 2+-dependent ATPase activity from thymocytes and sarcoplasmic reticulum fro m skeletal muscle. This inhibition was dose-dependent, with a IC50 of about 30 mu M for both preparations of enzyme. The ability of PAO to inhibit Ca2 +-dependent ATPase represents a novel mechanism of action for this drug. Pr esent data suggest that the PAO-dependent [Ca2+](i) increase could be mainl y the result of inhibition of Ca2+-dependent ATPase. In addition, we descri be also a Ca2+-dependence for PAO effect on tyrosine phosphorylation. (C) 1 999 Academic Press.