475 Int. J. Environ. Res. Public Health 2012, 9 1. Introduction Arsenic (As) is an abundant metalloid element in the Earth’s crust. Its compounds are widely used in industries, agricultures, and medicines [1]. In addition to occupational contact, consumption of contaminated drinking water is a major source of As exposure for humans. Most As in drinking water exists as inorganic forms released from either certain types of rock or man’s past activities [1,2]. An estimated 100–140 million people around the world are suffering from chronic As poisoning [3,4]. Inhaled or ingested As can be rapidly absorbed, enter the bloodstream, and distribute throughout the body [5,6]. The lung is one of the major target organs of As, either from contaminated air or from polluted water, and leading to the development of bronchiectasis, bronchitis, chronic obstructive pulmonary disease (COPD), and malignancies [2,7]. Environmental exposure to As is generally in the form of either arsenite (As3+) or arsenate (As5+) [6,8,9], with As3+ being more potent in its biological effects than As5+ [9,10]. As3+ is the predominant form in drinking water from deep (anaerobic) wells while As5+ predominates under aerobic conditions. Any As5+ in the water is rapidly reduced to As3+ as it enters cells [8,9]. Since As3+ displays high affinity for -SH groups perturbing essential proteins [11,12], As3+ compounds are of major human health concern. However, the precise mechanisms leading to lung damages and cancinogenesis by As3+ remain unclear and should be further investigated. Microtubules (MTs) are a highly organized network of filamentous structures in the cytoplasm of eukaryotic cells [13]. The principal protein of MTs is a heterodimer consisting of tightly linked globular peptides, i.e., α- and β-tubulins, which polymerize into long, hollow, cylindrical filaments within the cell. In addition, a third subunit of MTs is -tubulin, functioning in the nucleation of mitotic spindle MTs. The most prominent feature of MTs is a rapid exchange between tubulin subunits and polymers. Lung cells contain abundant MTs which play a critical role in maintaining cellular architecture and internal organization, and in regulating cell shape, motility, transport, division, etc. [14]. Many drugs and toxicants can interact with cytoskeletal MTs [13,15]. For example, colchicine and vinblastine disassemble MTs and block mitotic spindle formation. In contrast, taxol promotes tubulin assembly and stabilizes MTs. Notably, metal ions display important modulating effects on MT dynamic. As shown by us and others, Mg2+, Be2+ and Ni2+ promote MT assembly while Ca2+, Cu2+, Cd2+, Hg2+, Cr6+ and As3+ cause MT disassembly [16-21]. Thus, MT injuries may be a critical manifestation of the pathogenesis of a number of toxic agents. Our previous studies have shown As3+ disassembly of MTs in cultured Swiss 3T3 cells [21]. To further understand mechanisms for cellular toxicity of As in the lung, we examined effects of As3+, a major toxic form of As in the nature, on MT assembly in vitro, in cultured rat lung fibroblasts (RFL6) and in lung tissues of the rat animal model. Results demonstrated that As3+ targets tubulin sulfhydryls and MT-associated proteins (MAPs) disrupting the MT organization in tubulin polymerization in vitro and in rat lung cells in vitro and in vivo. 2. Materials and Methods 2.1. Materials Sodium m-arsenite (As3+, NaAsO2, analytical grade with purity ≥ 99.0%), dithiothreitol (DTT), N-ethylmaleimide (NEM), propidium iodine, and taxol were from Sigma-Aldrich Co (St. Louis, MO,