Tumor promoting phorbol esters, such as PMA, are potent activators of the PKC pathway. The bryostatins, marine natural products, activate PKCs similarly to phorbol esters but paradoxically inhibit most phorbol ester responses, including skin tumor promotion. Because of this inhibitory activity on the PKC pathway, bryostatin 1 has been the subject of extensive clinical trials. Serious impediments to clinical development, however, have been the scarcity of natural supplies coupled with great structural complexity, making synthesis problematic. Recent synthetic advances are now yielding simplified structural derivatives. Here, we compare the behavior of the bryostatin derivative Merle 23 with that of bryostatin 1 and phorbol 12-myristate 13-acetate (PMA) in mouse epidermal cells. While having similar binding activity on PKC as does bryostatin 1, Merle 23 behaves like PMA rather than like bryostatin 1 in U937 human leukemia cells but conversely behaves more like bryostatin 1 in the human prostate cancer cell line LNCaP. As a first step in assessing whether Merle 23 possesses or lacks skin tumor promoting activity, its activity in mouse primary keratinocyte cultures was assessed. PMA induces a prominent, prolonged morphological response in keratinocytes. Bryostatin 1 induces a similar initial response, which is however very transient. The morphological response to Merle 23 is more prolonged than for bryostatin 1 but much shorter than for PMA. For initial activation of PKCδ or induction of ERK1/2 phosphorylation, all three ligands show similar activity. By 24 hrs, however, bryostatin 1 is much more potent for down regulation of PKCα, whereas Merle 23 and PMA are similar. For PKCδ, bryostatin 1 at higher concentrations protects PKCδ from down regulation whereas Merle 23 does not. Transcriptional responses of various inflammation genes were monitored by qPCR. PMA causes the greatest increase in transcript levels for all genes investigated. In many cases bryostatin 1 induces a smaller response and the response to Merle 23 generally resembles that of bryostatin 1. While these studies are ongoing, they suggest that Merle 23 largely behaves like bryostatin 1 in the mouse epidermal cells. Our findings emphasize the important role of cellular context in determining the pattern of behavior of Merle 23. An important implication is that different cellular environments reveal different structure activity relations for bryostatin 1-like behavior. Finally, our studies raise the possibility that Merle 23, which is significantly simplified relative to bryostatin 1 in its upper two rings, may be non-promoting, at least in the mouse skin tumor promotion system.

Citation Format: Jessica Kelsey, Noemi Kedei, Christophe Cataisson, Mark Petersen, Stuart Yuspa, Gary Keck, Peter Blumberg. Comparison of the activity of the bryostatin derivative Merle 23 with that of bryostatin 1 and phorbol ester in mouse epidermal cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2454. doi:10.1158/1538-7445.AM2013-2454