Abstract 3815: Comparison of transcriptional response to phorbol ester, bryostatin 1, and bryostatin analogues in LNCaP and U937 cancer cell lines provides insight into their differential mechanism of action

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Bryostatin 1 has attracted great interest as a cancer chemotherapeutic agent with a unique mechanism of action. Whereas bryostatin 1 binds to and activates protein kinase C (PKC) like the phorbol esters, it paradoxically antagonizes many but not all phorbol ester responses. We wish to understand its mechanism(s) of action and how the newly emerging synthetic bryostatin analogs functionally resemble or differ from bryostatin 1. Previously, we have compared patterns of biological response to bryostatin 1, the phorbol ester phorbol 12-myristate 13-acetate (PMA), and the synthetic bryostatin 1 derivative Merle 23 in two human cancer cell lines, LNCaP and U937. Bryostatin 1 fails to induce a typical phorbol ester biological response in either cell line, whereas the bryostatin analog Merle 23 resembles PMA in the U937 cells and bryostatin 1 in the LNCaP cells. Here, we have compared patterns of transcriptional response to bryostatin 1, PMA, and Merle 23 in the same two human cancer cell lines. We examined by qPCR the transcriptional response as a function of dose and time for a series of genes highly regulated downstream of protein kinase C. In both cell lines, bryostatin 1 differed from phorbol ester primarily in having a shorter duration of transcriptional modulation. In LNCaP cells bryostatin 1 induced the majority of the genes similarly to PMA at 2 hours but to much lower levels at 6, 12, and 24 hrs. In U937 cells the bryostatin 1 induced response was similar to that of PMA at 2 hrs but progressively less than that of PMA at 8 hrs and 24 hrs. This was not due to bryostatin 1 instability, since bryostatin 1 suppressed the PMA response at these later times. A notable difference between the two cell lines was in the extent of transcriptional response to PMA, which was much greater in the LNCaP cells than in the U937 cells. In both cell lines Merle 23 induced a pattern of transcription largely like that of PMA rather than bryostatin 1 although with a modest reduction in expression at later times (12 and 24 hrs) in the LNCaP cells. We thus conclude that the difference in biological response of the two cell lines to Merle 23 most likely does not reflect an underlying difference in mechanism but rather reflects the greater absolute magnitude of the differences in response in the LNCaP cells compared to the U937 cells. For a series of bryostatins and synthetic analogues which ranged from bryostatin 1-like to phorbol ester-like in activity on the U937 cells for proliferation and differentiation, the duration of transcriptional response correlated with their pattern of biological activity, suggesting that this may provide a useful endpoint for structure activity analysis. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3815. doi:1538-7445.AM2012-3815