We have evaluated the effects of bryostatin 1 on growth of a highly malignant p53-null mouse mammary tumor line, 4T1, and the mechanism by which bryostatin 1 inhibits in vitro growth and in vivo development of tumor and metastases from the orthotopic site. Bryostatin 1 at 20-400 nM concentrations inhibits growth of 4T1 cells by approximately 60% in two-day cultures. Inhibition of growth is associated with an increase in the number of cells undergoing apoptosis with concomitant elevation in the steady state levels of bax protein and drop in bcl-2 levels. The cytotoxic effect of bryostatin 1 on 4T1 cells occurs independently of p53, since there was no evidence of p53-mediated transcriptional activity in 4T1 cells following treatment with bryostatin 1.4T1 cells respond in vivo to bryostatin 1 therapy (75 microg/kg body weight). Intraperitoneal administration of bryostatin 1 inhibits both primary and secondary tumor growth by approximately 50%. However, although bryostatin 1 has a remarkable capacity to slow tumor growth and progression, it is unable to completely eradicate tumor growth and progression due to in vivo development of tumor resistance to bryostatin 1. Levels and cellular distribution of PKCalpha and delta do not correlate with the growth inhibitory effects of bryostatin 1 on 4T1 cells; however, reduction in cytosolic PKCalpha and delta without associated increase in membrane compartment appear to correlate with bryostatin-resistance. Our results suggest that the therapeutic effects of bryostatin 1 in our system do not involve alterations in levels and distribution of PKC but rather a direct upregulation of bax/ bcl-2 ratios that is independent of p53.

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