Expression of Bcl-XS alters cytokinetics and decreases clonogenic survival in K12 rat colon carcinoma cells

bcl-XS, a member of the bcl-2 family, has been shown to induce and/or sensitize some cells to undergo programmed cell death, and to negate the anti-apoptotic activity of bcl-XL and bcl-2 by mechanisms which are still uncertain. To help understand these mechanisms we have established stable derivatives of the K12 rat colon carcinoma cell line that express bcl-XS in a tetracycline-regulated manner, using an autoregulatory retroviral cassette. When bcl-XS expression is induced, we observe two phenotypic responses. A small fraction of cells appear to undergo spontaneous apoptosis while the majority of cells undergo a form of cytostasis. In the latter case, the cells stop dividing (or divide a limited number of times at a retarded rate) and swell to many times their original size. These cells can take on a ghostlike appearance and subsequently detach from the culture plates and die or they may remain intact in a hindered state of proliferation. Doubling times were calculated to be 31.4 h in the presence of tetracycline and 50.4 h without tetracycline, bcl-XS expression also causes dramatic alterations in the cell cycle distribution of K12 cells manifesting as a substantial decrease (approximate to 50%) in the fraction of S phase cells with a concomitant increase in the G1 population. Continuous expression of bcl-XS, at levels approximately equal to that of bcl-XL, decreased the viability of K12 cells as demonstrated by a log decline in clonogenic survival. This decrease occurred without considerable apoptosis or a compensatory increase in the level of bcl-XL. None of these phenotyes were present in control cells expressing beta-galactosidase in a similar retroviral cassette. These observations demonstrate that bcl-XS can have substantial cytokinetic effects under circumstances that produce relatively little apoptosis.