Bcl-2/Bcl-xL Regulates Cell Cycle Through a Novel Mechanism in Addition to Cell Survival

Abstract

CANCER BIOLOGY

BCL-2/BCL-XL REGULATES CELL CYCLE THROUGH A NOVEL MECHANISM IN ADDITION TO CELL SURVIVAL

YELENA MELKUM JANUMYAN

Dissertation under the direction of Professor Elizabeth Yang

Bcl-2 and Bcl-xL are anti-apoptotic and inhibit proliferation. During cell cycle arrest, Bcl-2 or Bcl-xL expression causes elevation of the CDK inhibitor p27, decrease in cell size and RNA content. This enhanced state of G0 results in delay of G0-G1 transition and S phase entry upon cell cycle stimulation. We investigated the mechanism of the Bcl-2/Bcl-xL cell cycle function and the relationship between apoptosis regulation and cell cycle control. To assess the role of mitochondrial functions in Bcl-2/Bcl-xL-mediated cell cycle delay, we monitored ATP levels and found that cells expressing Bcl-2 or Bcl-xL exhibit a delay in peak ATP during cell cycle entry; however, exogenous elevation of ATP did not reverse the delay in cell cycle entry. In mtDNA-free 143B ?0 cells defective in electron transport, Bcl-xL is still able to delay cell cycle entry but not enhance G0 arrest. To determine whether enhanced G0 and subsequent delay in cell cycle are consequences of increased survival, we inhibited cell death by means other than Bcl-2/Bcl-xL expression. Increasing survival by caspase inhibition partially recapitulated the cell cycle arrest phenotype of Bcl-2/Bcl-xL, while cells expressing the survival kinase Akt exhibited none of the cell cycle phenotypes of Bcl-2/Bcl-xL. These data indicate that the cell cycle delay by Bcl-2/Bcl-xL is not mediated through the regulation of ATP/ADP exchange, and does not require normal mitochondrial membrane potential. Bcl-2/Bcl-xL may regulate quiescence partially through the inhibition of caspase-dependent apoptosis, but cell survival is not the sole mechanism of the cell cycle regulation by Bcl-2/Bcl-xL. In addition to inhibition of mitochondrial apoptosis, mechanisms requiring an intact electron transport chain are necessary for the full cell cycle function of Bcl-2/Bcl-xL.