Protein Interaction And Antileukemic Effects Of Arsenic And Its Active Methylated Metabolites

Arsenic is a renowned metalloid that has been found all over the world. Arsenic has been recognized as a human carcinogen causing cancer of bladder, lung, liver and skin and thereby, has gained significant attention globally. Inorganic arsenic (iAsⅢ) is enzymatically known to be methylated into different arsenic metabolites by arsenic methyltransferase; AS3MT in the presence of glutathione and S-adenosylmethionine. The different arsenic biomethylated metabolites include; mono- and dimethylated metabolites such as monomethylarsonous acid (MMAⅢ), monomethylarsonic acid (MMAV), dimethylarsinous acid (DMAⅢ) and dimethylarsinic acid (DMAV). Thereby, to elucidate the mechanisms responsible for arsenic induced toxicity and its clinical aspects it is essential to promote the better understanding of arsenic biotransformation pathway, as the toxicity and therapeutic efficacy of arsenic can be greatly acknowledged by relating it to the pathway involved in arsenic metabolism. Despite of various toxic events related with arsenic it has also been recognized as a novel anticancer, where As2O3 have recently been used for treatment of APL patients as an effective anticancer agent by targeting of PML-RARa which results in cell differentiation via degradation of oncoprotein. As, a deeper glance is required to link the gaps between the toxic and therapeutic effects of arsenicals, thereby, a detailed discussion regarding the mechanistic insights of arsenic-induced carcinogenicity and anticancer efficacy has been made over here particularly focusing on the paradoxical effects of arsenic with reference to its dose, metabolic pathway, various cellular targets and mechanisms involved in the cytotoxic effects of arsenicals.One of the major pathways through which arsenic can induce cellular toxicity includes activation of phosphorylation-dependent signaling pathways. On the other hand, there is an opposing signaling pathway called as protein tyrosine phosphatase (PTP) whose activity is known to regulate the phosphorylation pathway of these signaling pathways originated at the level of receptor tyrosine kinases. Reversible tyrosine phosphorylation, which is governed by the balanced action of protein tyrosine kinases and phosphatases, regulates important signaling pathways that are known to be involved in the control of cell proliferation, adhesion and migration. Thereby, an attempt in initial study was made to focus on the interaction of toxic trivalent arsenite (iAsⅢ) and its intermediate metabolites such as monomethylarsonous acid (MMAⅢ) and dimethylarsinous acid (DMAⅢ) with cellular PTPs in vitro. Moreover, using recombinant PTPs (e.g., PTP1B and CD45), the probable binding site in PTP for the binding of arsenic was also evaluated. As a result, it was interestingly found that both methylated metabolites (i.e., MMAⅢ and DMAⅢ) but not by iAsⅢ were able to strongly inhibit the activities of PTP1B (cytoplasm-form) or CD45 (receptor-linked form). Moreover, this study also suggested that the organic intermediate, DMAⅢ might directly bound to the active site cysteine residue of PTP1B (e.g., Cys215), resulting in inhibition of enzyme activity. These results suggest that arsenic exposure may disturb the cellular signaling pathways through PTP inactivation. Though the interaction of arsenic with the biomolecules is usually associated with the toxic effects of arsenic species leading to induction of various diseases including cancers, recently inhibitors of the PTPs are also being recognized to have therapeutic value with unique modes of action.As As2O3 is being used froⅢ thousands of years in Chinese, Mongolian and Tibetan traditional medicine and has also proven to be an effective drug for the treatment of acute promyelocytic leukemia by targeting the fusion oncoprotein, this fruitful approach has invigorated ideas of many scientists to utilize this metalloid for the treatment of other malignancies. Hence, in further study human myeloid leukemia HL-60 cell line was used and an attempt was made to explore the probable antileukemic effects of trivalent arsenicals including iAsⅢ, MMAⅢ and DMAⅢ along with the probable mechanism underlying their antileukemic effect as trivalent arsenicals have been recognized to be more toxic than their pentavalent metabolites. The trivalent methylated metabolites showed to be more potent than their precursor; iAsⅢ by significantly decreasing cell survival at low concentrations. Similarly as compared to iAsⅢ, the methylated metabolites; MMAⅢ and DMAⅢ remarkably induced cellular apoptosis in HL-60 cells. On further exploring the probable pathways underlying the apoptotic effect of MMAⅢ and DMAⅢ, ROS generation was revealed to be one of the major reasons for induction of cell apoptosis. On additional investigation, as compared to iAsⅢ, both MMAⅢ and DMAⅢ showed to induce DNA damage evident by DNA laddering. The DNA damage was verified by the detection of y-H2AX foci in cellular nuclei. The effect of iAsⅢ, MMAⅢ and DMAⅢ was also observed on the activation of the caspase cascades and apoptosis-related proteins, where a remarkable activation of caspase-3 and-9 in the cells exposed to MMAⅢ and DMAⅢ was observed. Similarly, PARP cleavage was also evident in cells after exposure to both the trivalent methylated metabolites; however, iAsⅢ did not apparently show any relevant effects at the same dose. On the basis of these findings it can be suggested that arsenic intermediate metabolites MMAⅢ and DMAⅢ may be of help in future with reference to their antileukemic effect observed in HL-60 cells induced via genotoxicity and cellular apoptosis. Hence, there is a need to further explore the tremendous capabilities of this metalloid and the probable role of its active trivalent intermediate metabolites in the treatment of other numerous neoplastic diseases beyond APL.