AbstractThe hypothesis tested in the study suggests that mechanisms of the earlier described delayed or accelerated tumor progression may be regulated by the antiapoptotic and proapoptotic cellular programs activated in stress reactions of transformed cells to the host normal cellular environment. Therefore, spontaneously transformed hamster cell line STHE, its bcl‐2‐transduced line STHE‐Bcl‐2, and 64 of their descendant tumor cell variants naturally selected in two in vivo regimes (local tumor growth versus dissemination) were examined. The role of Bcl‐2 and the possible activation of endogenous death‐signaling Bax, Ras, and HSP90/HSP70 stress proteins in STHE (Bcl‐2+/−) tumor cell variants were studied in dynamics of in vivo tumor progression. The data demonstrate: (1) Immediate in vivo activation of Bax and of HSP90/HSP70 stress proteins in disseminated STHE cells on the background of accelerated tumor progression; (2) No immediate activation of Bax and the gradual downregulation of Bcl‐2 in STHE‐Bcl‐2 cells on the background of delayed tumor progression; (3) Alternative and mutually suppressive character of Bcl‐2 and Bax expression in both regimes of tumor progression; (4) In the later stages of tumor progression, the regular transit of the initial Bcl‐2 antiapoptotic, Bax‐suppressing program, and the delayed tumor progression towards Bcl‐2 loss, activation of Bax, and acceleration of tumor progression. Thus, the delay of tumor progression is apparently determined by the ability of Bcl‐2‐expressing tumor cells to extinguish the cell‐damaging environmental stress signals and Bax activation, while its acceleration correlates with Bcl‐2 loss, activation of proapoptotic Bax, and tumor cells damage. J. Cell. Biochem. 101:1148–1164, 2007. © 2007 Wiley‐Liss, Inc.