Trivalent arsenic (arsenite, As3؉ ) is a human carcinogen, which is associated with cancers of skin, lung, liver, and bladder. However, the mechanism by which arsenite causes cancer is not well understood. In this study, we found that exposure of Cl 41 cells, a well characterized mouse epidermal cell model for tumor promotion, to a low concentration of arsenite (<25 M) induces cell transformation. Interestingly, arsenite induces Erk phosphorylation and increased Erk activity at doses ranging from 0.8 to 200 M, while higher doses (more than 50 M) are required for activation of JNK. Arseniteinduced Erk activation was markedly inhibited by introduction of dominant negative Erk2 into cells, while expression of dominant negative Erk2 did not show inhibition of JNK and MEK 1/2 . Furthermore, arsenite-induced cell transformation was blocked in cells expressing the dominant negative Erk2. In contrast, overexpression of dominant negative JNK1 was shown to increase cell transformation even though it inhibits arsenite-induced JNK activation. Our results not only show that arsenite induces Erk activation, but also for the first time demonstrates that activation of Erk, but not JNK, by arsenite is required for its effects on cell transformation.Arsenite is introduced into the environment during energy production based on coal, oil shale, and geothermal sources. Once in the environment, arsenite represents a potential health hazard of unknown magnitude. Arsenite is associated with increased risks of human cancer of the skin, respiratory tract, hematopoietic system, and urinary bladder (1-4). Epidemiological investigations indicated that long-term arsenic exposure results in promotion of carcinogenesis, especially in lung and skin via inhalation and ingestion (5). Many cases of skin cancer have been documented in people exposed to arsenite through medical or other occupational exposures. It has been reported that high arsenic levels in drinking water (0.35-1.14 mg/liter) increased risks of cancer of skin, bladder, kidney, lung, and colon (1, 2, 5, 6). Hence, arsenite is a well documented human carcinogen (5, 7).Previously, several hypotheses have been proposed to describe the mechanism of arsenite-induced carcinogenesis (8 -14). It has been suggested that arsenic induces chromosome aberration and sister chromatid exchange which may be involved in arsenite-induced carcinogenesis (11,12). Recently, Zhao et al. (13) reported that arsenic may act as a carcinogen by inducing DNA hypomethylation, which in turn facilitates aberrant gene expression. Additionally, it was found that arsenite is a potent stimulator of extracellular signal-regulated protein kinase (Erk) 1 and AP-1 transactivational activity and an efficient inducer of c-fos and c-jun gene expression (10,14). Induction of c-jun and c-fos by arsenite is associated with activation of JNK (10). However, the role of JNK activation by arsenite in cell transformation or tumor promotion is unclear. We have established cell culture conditions for studying arsenite-induced cell tr...

Striated muscle contraction is regulated by Ca2؉ binding to troponin, which has a globular domain and an elongated tail attributable to the NH 2 -terminal portion of the bovine cardiac troponin T (TnT) subunit. Truncation of the bovine cardiac troponin tail was investigated using recombinant TnT fragments and subunits TnI and TnC. Progressive truncation of the troponin tail caused progressively weaker binding of troponin-tropomyosin to actin and of troponin to actin-tropomyosin. A sharp drop-off in affinity occurred with NH 2 -terminal deletion of 119 rather than 94 residues. Deletion of 94 residues had no effect on Ca 2؉ -activation of the myosin subfragment 1-thin filament MgATPase rate and did not eliminate cooperative effects of Ca 2؉ binding. Troponin tail peptide TnT1-153 strongly promoted tropomyosin binding to actin in the absence of TnI or TnC. The results show that the anchoring function of the troponin tail involves interactions with actin as well as with tropomyosin and has comparable importance in the presence or absence of Ca 2؉. Residues 95-153 are particularly important for anchoring, and residues 95-119 are crucial for function or local folding. Because striated muscle regulation involves switching among the conformational states of the thin filament, regulatory significance for the troponin tail may arise from its prominent contribution to the protein-protein interactions within these conformations.Troponin is a striated muscle regulatory protein (see reviews in Refs. 1-4) that is located at periodic, 38-nm spacing along muscle thin filaments. This spacing is due to the 1:1 complex formation of troponin with tropomyosin, an elongated coiledcoil protein that stretches along seven actin monomers. Ca 2ϩ binding to troponin triggers conformational changes in the thin filament, thereby allowing actin and myosin to interact to produce force and movement. Troponin contains two domains: a globular region, which is composed of subunits TnC, 1 TnI, and the COOH-terminal portion of TnT, and a highly extended region, or tail, containing the remainder of TnT (5, 6). The globular region has a central role in regulation, because it is the site of calcium binding. In contrast, the tail region of troponin, which is the subject of this report, has an uncertain role in conformational changes of the thin filament. One possibility is that it has little direct effect on regulation, acting instead as a calcium-insensitive anchor that holds troponin onto tropomyosin (7,8). However, the details of the interactions of troponin with actin and tropomyosin are unknown, in any of the conformations of the thin filament. Moreover, there is increasing evidence that the structure of the troponin tail can alter thin filament function in a complex manner (9 -16). To better understand the troponin tail region, the present study reports the properties of a series of troponin complexes containing progressively less of this region. Cardiac TnC plus TnI was reconstituted with either cardiac TnT or a series of recombinant NH 2 -termina...

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