Data from Crucial Role of Phospholipase Cε in Skin Inflammation Induced by Tumor-Promoting Phorbol Ester

Abstract

<div>Abstract<p>In two-stage skin chemical carcinogenesis, phorbol ester 12-<i>O</i>-tetradecanoylphorbol-13-acetate (TPA) acts as a promoter essential for clonal expansion of the initiated cells carrying the activated <i>ras</i> oncogenes. Although protein kinase C (PKC) isozymes are the main targets of TPA, their role in tumor promotion remains controversial. We previously reported that mice lacking a Ras/Rap effector phospholipase Cε (<i>PLCε<sup>−/−</sup></i> mice) exhibited marked resistance to tumor formation in the two-stage skin carcinogenesis. <i>PLCε<sup>−/−</sup></i> mice also failed to exhibit basal layer cell proliferation and epidermal hyperplasia induced by TPA, suggesting a role of PLCε in tumor promotion. Here, we show that <i>PLCε<sup>−/−</sup></i> mice exhibit resistance to TPA-induced skin inflammation as assessed by reduction in edema, granulocyte infiltration, and expression of a proinflammatory cytokine, interleukin-1α (IL-1α). On the other hand, the proliferative potentials of keratinocytes or dermal fibroblasts in culture remain unaffected by the <i>PLCε</i> background, suggesting that the PLCε's role in tumor promotion may be ascribed to augmentation of inflammatory responses. In dermal fibroblast primary culture, TPA can induce activation of the PLCε lipase activity, which leads to the induction of IL-1α expression. Experiments using small interfering RNA–mediated knockdown indicate that this activation is mediated by Rap1, which is activated by a TPA-responsive guanine nucleotide exchange factor RasGRP3. Moreover, TPA-induced activation of Rap1 and PLCε is inhibited by a PKC inhibitor GF109203X, indicating a crucial role of PKC in signaling from TPA to PLCε. These results imply that two TPA targets, RasGRP3 and PKC, are involved in TPA-induced inflammation through PLCε activation, leading to tumor promotion. [Cancer Res 2008;68(1):64–72]</p></div>