Expression of Bax in yeast affects not only the mitochondria but also vacuolar integrity and intracellular protein traffic

Bax-induced lethality in yeast is accompanied by morphological changes in mitochondria, giving rise to a reduced number of swollen tubules. Although these changes are completely abolished upon coexpression of the Bax inhibitor, Bcl-2, coexpression of Bax with Bax inhibiting-glutathione S-transferase (BI-GST) leads to aggregation, but not fusion of the mitochondria. In addition, Bax affects the integrity of yeast vacuoles, resulting in the disintegration and eventual loss of the organelles, and the disruption of intracellular protein traffic. While Bcl-2 coexpression only partially corrects this phenotype, coexpression of BI-GST fully restores the organelles, indicating a different mode of protection exerted by Bcl-2 and BI-GST.

Tumor induction by an Lck-MyrAkt transgene is delayed by mechanisms controlling the size of the thymus

Transgenic mice expressing MyrAkt from a proximal Lck promoter construct develop thymomas at an early age, whereas transgenic mice expressing constitutively active Lck-AktE40K develop primarily tumors of the peripheral lymphoid organs later in life. The thymus of 6- to 8-week-old MyrAkt transgenic mice is normal in size but contains fewer, larger cells than the thymus of nontransgenic control and AktE40K transgenic mice. Earlier studies had shown that cell size and cell cycle are coordinately regulated. On the basis of this finding, and our observations that the oncogenic potential of Akt correlates with its effect on cell size, we hypothesized that mechanisms aimed at maintaining the size of the thymus dissociate cell size and cell cycle regulation by blocking MyrAkt-promoted G(1) progression and that failure of these mechanisms may promote cell proliferation resulting in an enlarged neoplastic thymus. To address this hypothesis, we examined the cell cycle distribution of freshly isolated and cultured thymocytes from transgenic and nontransgenic control mice. The results showed that although neither transgene alters cell cycle distribution in situ, the MyrAkt transgene promotes G(1) progression in culture. Freshly isolated MyrAkt thymocytes express high levels of cyclins D2 and E and cdk4 but lower than normal levels of cyclin D3 and cdk2. Cultured thymocytes from MyrAkt transgenic mice, on the other hand, express high levels of cyclin D3, suggesting that the hypothesized organ size control mechanisms may down-regulate the expression of this molecule. Primary tumor cells, similar to MyrAkt thymocytes in culture, express high levels of cyclin D3. These findings support the hypothesis that tumor induction is caused by the failure of organ size control mechanisms to down-regulate cyclin D3 and to block MyrAkt-promoted G(1) progression.