RESUMEN
We previously demonstrated that Bmi-1 extended the in vitro life span of normal human oral keratinocytes (NHOK). We now report that the prolonged life span of NHOK by Bmi-1 is, in part, due to inhibition of the TGF-beta signaling pathway. Serial subculture of NHOK resulted in replicative senescence and terminal differentiation and activation of TGF-beta signaling pathway. This was accompanied with enhanced intracellular and secreted TGF-beta1 levels, phosphorylation of Smad2/3, and increased expression of p15(INK4B) and p57(KIP2). An ectopic expression of Bmi-1 in NHOK (HOK/Bmi-1) decreased the level of intracellular and secreted TGF-beta1 induced dephosphorylation of Smad2/3, and diminished the level of p15(INK4B) and p57(KIP2). Moreover, Bmi-1 expression led to the inhibition of TGF-beta-responsive promoter activity in a dose-specific manner. Knockdown of Bmi-1 in rapidly proliferating HOK/Bmi-1 and cancer cells increased the level of phosphorylated Smad2/3, p15(INK4B), and p57(KIP2). In addition, an exposure of senescent NHOK to TGF-beta receptor I kinase inhibitor or anti-TGF-beta antibody resulted in enhanced replicative potential of cells. Taken together, these data suggest that Bmi-1 suppresses senescence of cells by inhibiting the TGF-beta signaling pathway in NHOK.