Abstract 2559: Targeting mTORC1 may provide selectivity for inhibiting proliferation of hair follicle stem/progenitor cells during tumor promotion

Activation of Akt occurs during tumor promoter treatment of mouse skin and plays an important role in skin tumor promotion through multiple downstream pathways, including mTORC1. In this study, we examined the impact of rapamycin on mTORC1 signaling in relation to its effect on TPA-induced epidermal proliferation and skin tumor promotion. TPA activates mTORC1 signaling in a time-dependent manner in cultured primary mouse keratinocytes and an immortalized mouse keratinocyte cell line. Early activation (15-30 min) of mTORC1 signaling induced by TPA was mediated in part, by PKC activation, whereas the later activation (2-4 hr) of mTORC1 was mediated by activation of EGFR and Akt. BK5.AktWT transgenic mice where Akt1 is overexpressed in basal epidermis were significantly more sensitive to TPA-induced epidermal proliferation and two-stage skin carcinogenesis. Targeting mTORC1 effectively inhibited TPA-induced epidermal hyperplasia as well as tumor promotion in epidermis of both wild-type and BK5.AktWT mice in a dose-dependent manner. We also examined the effect of rapamycin on the proliferation and migration of bulge-region KSCs from the hair follicles induced by TPA. Ten-day old mice were injected with BrdU every 12h over a 48h time period and then 70 days later mice were treated with acetone, TPA or rapamycin and TPA. Seventy days after the last injection of BrdU, only the label retaining cells (LRCs) were seen in the bulge region of the hair follicles. BK5.Akt1WT transgenic mice, which overexpress Akt1 in the K5 compartment of mouse epidermis (which includes bulge region KSCs) led to significant expansion of LRCs (∼3-fold) per hair follicle compared with wild-type FVB mice. There was also a significant expansion of the K15 expressing cell population, especially in the hair follicle that coincided with expression of phospho-Akt. In addition, a significant increase in phospho-S6K and phospho-PRAS40 was seen in the hair follicles, suggesting an important role of mTORC1 signaling in the alterations seen in terms of LRCs and K15 expressing cells. After two weeks of TPA treatment, LRCs were reduced in the bulge region and moved upward into the interfollicular epidermis. Treatment with rapamycin inhibited the migration and proliferation of the LRCs from the hair follicles to upper epidermal compartments induced by TPA. Furthermore, LRCs remained in the bulge region of hair follicles. Since the bulge-region KSCs are considered to be the key target population for tumor development in the two-stage skin carcinogenesis model, these data indicate that signaling through mTORC1 contributes significantly to the process of skin tumor promotion through effects on proliferation of multiple keratinocyte populations including bulge-region KSCs. Thus, blocking this pathway may be an effective strategy for prevention of epithelial carcinogenesis. 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 2559. doi:1538-7445.AM2012-2559