Arsenic is a known human bladder
carcinogen; however, the mechanisms underlying arsenical-induced bladder
carcinogenesis are not understood. Previous research has demonstrated that exposure of a nontumorigenic human urothelial cell line, UROtsa, to 50 nM
monomethylarsonous acid (
MMA(III)) for 52 weeks resulted in malignant transformation. To focus research on the early mechanistic events leading to
MMA(III)-induced
malignancy, the goal of this research was to resolve the critical period in which continuous
MMA(III) exposure (50 nM) induces the irreversible malignant transformation of UROtsa cells. An increased growth rate of UROtsa cells results after 12 weeks of
MMA(III) exposure. Anchorage-independent growth occurred after 12 weeks with a continued increase in colony formation when 12-week exposed cells were cultured for an additional 12 or 24 weeks without
MMA(III) exposure. UROtsa cells as early as 12 weeks
MMA(III) exposure were tumorigenic in
severe combined immunodeficiency mice with tumorigenicity increasing when 12-week exposed cells were cultured for an additional 12 or 24 weeks in the absence of
MMA(III) exposure. To assess potential underlying mechanisms associated with the early changes that occur during
MMA(III)-induced
malignancy, DNA methylation was assessed in known target gene promoter regions. Although DNA methylation remains relatively unchanged after 12 weeks of exposure, aberrant DNA methylation begins to emerge after an additional 12 weeks in culture and continues to increase through 24 weeks in culture without
MMA(III) exposure, coincident with the progression of a tumorigenic phenotype. Overall, these data demonstrate that 50 nM
MMA(III) is capable of causing irreversible malignant transformation in UROtsa cells after 12 weeks of exposure. Having resolved an earlier timeline in which
MMA(III)-induced malignant transformation occurs in UROtsa cells will allow for mechanistic studies focused on the critical
biological changes taking place within these cells prior to 12 weeks of exposure, providing further evidence about potential mechanisms of
MMA(III)-induced
carcinogenesis.