The antitumor agent bryostatin 1 and the tumorpromoting phorbol esters function as structural mimetics of the second lipid messenger diacylglycerol (DAG) by binding to the C1 domain of DAG receptors. However, bryostatin 1 and the phorbol esters often differ in their cellular actions. In mouse skin, the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) is a potent tumor promoter, whereas bryostatin 1 lacks this activity and antagonizes the tumor-promoting effects of TPA. Although protein kinase C mediates many of the effects of DAG on skin, the exact mechanisms responsible for the biology of bryostatin 1 and TPA in the epidermis have not been elucidated. We recently reported that the novel DAG receptor RasGRP1 is expressed in mouse keratinocytes and mediates TPA-induced Ras activation. This finding prompted us to examine the regulation of RasGRP1 by bryostatin 1. We found that whereas TPA induced translocation of RasGRP1 to both the plasma and internal membranes of the keratinocytes, bryostatin 1 recruited RasGRP1 only to internal membranes and the nuclear envelope. In addition, TPA led to a concentration-dependent down-regulation of RasGRP1, whereas bryostatin 1 failed to induce full RasGRP1 downregulation. Interestingly, bryostatin 1 was less effective than TPA at activating Ras. The results presented here suggest the possibility that a differential modulation of RasGRP1 by bryostatin 1 compared with TPA could participate in the disparate responses of the epidermal cells to both DAG analogues. This result may have implications in the understanding of the antitumor effects of bryostatin 1 in the skin. [Mol Cancer Ther 2006;5(3):602 -10]