Rapamycin Is a Potent Inhibitor of Skin Tumor Promotion by 12-O-Tetradecanoylphorbol-13-Acetate

Abstract

Aberrant activation of PI3K/Akt signaling has been implicated in the development and progression of multiple human cancers. During the process of skin tumor promotion induced by treatment with the phorbol ester TPA, activation of epidermal Akt occurs as well as several downstream effectors of Akt, including the activation of mTORC1. Rapamycin, an established mTORC1 inhibitor, was used to further explore the role of mTORC1 signaling in epithelial carcinogenesis, specifically during the tumor promotion stage. Rapamycin blocked TPA-induced activation of mTORC1 as well as several downstream targets. In addition, TPA-induced epidermal hyperproliferation and hyperplasia were inhibited in a dose-dependent manner with topical rapamycin treatments. Immunohistochemical analyses of the skin from mice in this multiple treatment experiment revealed that rapamycin also significantly decreased the number of infiltrating macrophages, T-cells, neutrophils, and mast cells seen in the dermis following TPA treatment. Using a two-stage skin carcinogenesis protocol with DMBA as initiator and TPA as the promoter, rapamycin (5–200 nmol per mouse given topically 30 minutes prior to TPA) exerted a powerful anti-promoting effect, reducing both tumor incidence and tumor multiplicity. Moreover, topical application of rapamycin to existing papillomas induced regression and/or inhibited further growth. Overall, the data indicate that rapamycin is a potent inhibitor of skin tumor promotion and suggest that signaling through mTORC1 contributes significantly to the process of skin tumor promotion. The data also suggest that blocking this pathway either alone or in combination with other agents targeting additional pathways may be an effective strategy for prevention of epithelial carcinogenesis.

Overexpression of PRAS40T246A in the Proliferative Compartment Suppresses mTORC1 Signaling, Keratinocyte Migration, and Skin Tumor Development

“…BK5.PRAS40 T246A mice were significantly less sensitive to TPA-induced epidermal hyperproliferation measured by labeling index (LI) and epidermal thickness (Figures 3a, 3b; p <0.0001, Mann-Whitney U test). These data together with the data in Figure 2 indicate that overexpression of PRAS40 T246A in basal keratinocytes effectively blocked TPA-induced epidermal hyperproliferation, which reproduces the effect of rapamycin observed in our previous studies (Checkley, Rho, 2011, Rho, Kiguchi, 2013). …”

“…As shown in supplemental Figure S1b and S1c, reduction of TPA-induced mTORC1 activation in BK5.PRAS40 T246A mice produced a inhibitory effect similar to that observed with 5 nmol of rapamycin treatment. This dose of rapamycin was previously shown to produce an ~ 50% inhibition in skin tumor promotion by TPA in wild-type mice (Checkley et al , 2011)…”

“…Previous studies from our laboratory revealed that Akt and mTORC1 signaling pathways play an important role in epithelial carcinogenesis using the mouse skin model (Checkley et al , 2011, Lu et al , 2007). Rapamycin (an mTORC1 inhibitor) dramatically blocked mTORC1 downstream signaling, epidermal proliferation and skin tumor promotion induced by TPA in both wild-type (WT) and BK5.Akt1 transgenic mice (Checkley et al , 2014, Checkley, Rho, 2011, Rho et al , 2013). Treatment of mouse epidermis with TPA leads to increased phosphorylation of Akt and PRAS40 (proline-rich Akt substrate of 40 kDa) (Lu, Rho, 2007, Rho, Kiguchi, 2013).…”

Effect of Metformin, Rapamycin, and Their Combination on Growth and Progression of Prostate Tumors in HiMyc Mice

“…Previous studies from our laboratory demonstrated a strong local anti-inflammatory effect of low dose RAPA when given topically to mice during the process of skin tumor promotion (37). Furthermore, other studies have shown anti-inflammatory effects of RAPA (36), and more recently anti-inflammatory effects of MET (38, 39).…”

Rapalogs in cancer prevention

“…117 Rapamycin also prevents skin tumors induced by phorbol esters. 118 It was assumed that rapalogs delay cancer by targeting cancer cells directly. Yet, there were some indications that the effect might be indirect via targeting normal cells.…”