Human arsenic exposure has been referred to as the worst environmental poisoning in modern human history. Arsenic toxicity in exposed humans causes cancer in diverse target tissues, including lung, bladder and skin. Despite incontrovertible epidemiology that repeatable confirms arsenic carcinogenicity, no definitive mechanism explaining the carcinogenic action of arsenic exists. The phosphatase and tensin homologue (PTEN) gene is a tumor suppressor gene that holds a unique place in human cancer research, in that it is arguably the only tumor suppressor gene that is commonly involved through loss of function in both familial and sporadic cancers. The principal tumor-suppressive mechanism of PTEN is through it inactivation of the Akt pathway, a central effector of cell survival and proliferation. Akt activation is a frequently reported consequence of chronic arsenic exposure in vitro, including cell culture models of arsenic-induced malignant transformation. This project seeks to test a novel mechanism by which arsenic exposure could inactivate PTEN, leading to loss of its tumor-suppressive function, leading to Akt activation and arsenic-induced carcinogenesis. Using a cell culture model amenable to in vitro studies of malignant transformation, our preliminary data establish that a general effect of inorganic trivalent arsenic (arsenite) exposure is the disruption of cellular metabolism, including the induction of aerobic glycolysis and elevation of intracellular NADH level. This is intriguing because PTEN activity has been demonstrated to be inhibited by elevated NADH. We hypothesize that an important mechanism of arsenic's carcinogenic action is its ability to disturb fundamental cell metabolism, leading to elevated NADH levels, and the functional loss of the tumor suppressor, PTEN.
Two specific aims will test this hypothesis.
Aim 1 will define the temporal relationship and associations between PTEN activity, Akt activation, and key phenotypes associated with the acquisition of malignancy in BEAS-2B cells chronically exposed to a non-cytotoxic, environmentally relevant concentration of arsenite.
Aim 2 is designed to incorporate data collected in Aim 1 to affect sustained, experimentally modulated, increased and decreased NADH level in derived BEAS-2B cell lines. These BEAS-2B cell lines will be subjected to the malignant transformation protocol used in Aim 1 (1 um arsenite for approximately 17 weeks). Our hypothesis predicts that upward or downward modulation of NADH level will result in enhanced or diminished malignant transformation, respectively. This R03 project proposes an initial test of a novel mechanism involving metabolic control of cell signaling in environmental carcinogenesis.
This project tests a novel mechanism to explain the cancer-causing effect of environmental exposure to arsenic, an exposure that occurs on a massive global scale and exacts a tremendous societal burden. Proving the validity of this novel mechanism, centered on arsenic disrupting the way that cells produce energy from carbohydrates, could offer hope for exciting new ways to prevent environmentally acquired cancer, from arsenic as well as potentially other environmental toxicants.