The bryostatins inhibit growth of B16/F10 melanoma cells in vitro through a protein kinase C-independent mechanism: dissociation of activities using 26-epi-bryostatin 1.
Bryostatin 1 is a potential cancer chemotherapeutic agent in Phase II clinical trials, with positive responses observed for malignant melanoma, among other tumors. The bryostatins are known to be potent ligands for protein kinase C (PKC), functioning as partial antagonists. In the present study, we explore the mechanism by which the bryostatins inhibit growth to B16/F10 mouse melanoma cells in vitro. Three experimental approaches suggest that the growth inhibition is independent of PKC. First, we characterized in detail the translocation and down-regulation of the PKC isozymes alpha, delta, and epsilon in response to phorbol ester and bryostatin 1 in these cells. Although the dose-response curves obtained for the translocation-activation of PKC isozymes showed good correlation with the growth-enhancing activity of phorbol 12-myristate 13-acetate, for no PKC isozyme was there a good correlation with the growth-inhibitory activity of bryostatin 1. Second, inhibition PKC, inhibited the growth of the B16/F10 melanoma cell lines with potency similar to that of bryostatin 1. We confirmed here that 26-epi-bryostatin 1 showed 60-fold reduced affinity for PKC and 30-60-fold reduced potency to translocate and downregulate PKC isozymes compared with bryostatin 1. We presumed that the principal toxicity of bryostatin 1 reflects its interaction with PKC, and we would thus predict that epi-bryostatin 1 would be less toxic. Indeed, we found at least 10-fold reduced toxicity of 26-epi-bryostatin 1 in C57BL/6 mice compared with bryostatin 1. We conclude that the growth inhibition of the bryostatins, at least in this system, does not result from interaction with PKC. As exemplified by 26-epi-bryostatin 1, this insight permits the design of analogues with comparable growth inhibition to bryostatin 1 but with reduced toxicity.
Szallasi
Z
Z
Laboratory of Cellular Carcinogenesis and Tumor Promotion, National Cancer Institute, Bethesda, Maryland 20892, USA.
Du
L
L
Levine
R
R
Lewin
N E
NE
Nguyen
P N
PN
Williams
M D
MD
Pettit
G R
GR
Blumberg
P M
PM
D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology* / Q000627:therapeutic use; D054713:Bryostatins; D002455:Cell Division / Q000187:drug effects; D007783:Lactones / Q000494:pharmacology* / Q000627:therapeutic use; D018942:Macrolides; D008546:Melanoma, Experimental / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D051379:Mice; D011493:Protein Kinase C / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects*; D014407:Tumor Cells, Cultured
D000970:Antineoplastic Agents; D054713:Bryostatins; D007783:Lactones; D018942:Macrolides; C046785:bryostatin 1; D011493:Protein Kinase C