The mTOR inhibitor rapamycin opposes carcinogenic changes to epidermal Akt1/PKBα isoform signaling

Abstract

Epidermal squamous cell carcinoma (SCC) is the most aggressive non-melanoma skin cancer and is dramatically increased in patients undergoing immunosuppression following solid organ transplantation, contributing substantially to morbidity and mortality. Recent clinical studies show that use of the mammalian target of rapamycin (mTOR) inhibitor rapamycin as a post-transplantation immunosuppressive significantly reduces SCC occurrence compared with other immunosuppressives, though the mechanism is not fully understood. We show that rapamycin selectively upregulates epidermal Akt1, while failing to upregulate epidermal Akt2. Rapamycin increases epidermal Akt1 phosphorylation via inhibition of the mTOR complex 1-dependent regulation of insulin receptor substrate-1. Epidermal Akt1 is commonly downregulated in SCC while Akt2 is upregulated. We now demonstrate similar Akt1 downregulation and Akt2 upregulation by ultraviolet (UV) radiation, the most important skin carcinogen. Hence, rapamycin's upregulation of Akt1 signaling could potentially oppose the effects of UV radiation and/or tumor-associated changes on Akt1 signaling. We show in skin culture that rapamycin does enhance restoration of Akt1 phosphorylation in skin recovering from UV radiation, suggesting a mechanism for rapamycin's antitumor activity in epidermis in spite of its efficient immunosuppressive properties.