Abstract

Bryostatin 1, a macrocyclic lactone that has been widely characterized as an ultrapotent protein kinase C (PKC) activator, displays marked pharmacological differences with the typical phorbol ester tumor promoters. Bryostatin 1 impairs phorbol 12-myristate 13-acetate (PMA)-induced tumor promotion in mice and is in clinical trials as an anticancer agent for a number of hematopoietic malignancies and solid tumors. In this study, we characterized the effect of bryostatin 1 on LNCaP prostate cancer cells, a cellular model in which PKC isozymes play important roles in the control of growth and survival. Although phorbol esters promote a strong apoptotic response in LNCaP cells via PKCδ-mediated release of TNFα, bryostatin 1 failed to trigger a death effect even at high concentrations, and it prevented PMA-induced apoptosis in these cells. Mechanistic analysis revealed that bryostatin 1 is unable to induce TNFα release, and it impairs the secretion of this cytokine from LNCaP cells in response to PMA. Unlike PMA, bryostatin 1 failed to promote the translocation of PKCδ to the plasma membrane. Moreover, bryostatin 1 prevented PMA-induced PKCδ peripheral translocation. Studies using a membrane-targeted PKCδ construct revealed that the peripheral localization of the kinase is a requisite for triggering apoptosis in LNCaP cells, arguing that mislocalization of PKCδ may explain the actions of bryostatin 1. The identification of an antiapoptotic effect of bryostatin 1 may have significant relevance in the context of its therapeutic efficacy.