Microtubules as a Critical Target for Arsenic Toxicity in Lung Cells in Vitro and in Vivo

To understand mechanisms for arsenic toxicity in the lung, we examined effects of sodium m-arsenite (As(3+)) on microtubule (MT) assembly in vitro (0–40 µM), in cultured rat lung fibroblasts (RFL6, 0–20 µM for 24 h) and in the rat animal model (intratracheal instillation of 2.02 mg As/kg body weight, once a week for 5 weeks). As(3+) induced a dose-dependent disassembly of cellular MTs and enhancement of the free tubulin pool, initiating an autoregulation of tubulin synthesis manifest as inhibition of steady-state mRNA levels of βI-tubulin in dosed lung cells and tissues. Spindle MT injuries by As(3+) were concomitant with chromosomal disorientations. As(3+) reduced the binding to tubulin of [(3)H]N-ethylmaleimide (NEM), an -SH group reagent, resulting in inhibition of MT polymerization in vitro with bovine brain tubulins which was abolished by addition of dithiothreitol (DTT) suggesting As(3+) action upon tubulin through -SH groups. In response to As(3+), cells elevated cellular thiols such as metallothionein. Taxol, a tubulin polymerization agent, antagonized both As(3+) and NEM induced MT depolymerization. MT–associated proteins (MAPs) essential for the MT stability were markedly suppressed in As(3+)-treated cells. Thus, tubulin sulfhydryls and MAPs are major molecular targets for As(3+) damage to the lung triggering MT disassembly cascades.