Elevated Polyamine Metabolism Activates Fibroblasts via GCN2 Activation and Generates Senescent Fibroblasts to Promote Tumor Formation and Progression.

GCN2 is one of the main sensors of amino acid starvation stress, and its activation in the stressful tumor microenvironment plays a crucial role in tumor survival. We hypothesized that elevated polyamine biosynthesis and subsequent depletion of precursor arginine in the tissue microenvironment activates GCN2 and alters stromal cell metabolism to support tumor cell survival and drive myeloid immunosuppressive function. To study the effect of elevated polyamine metabolism on fibroblast activation, we used the K6/ODC transgenic model of carcinogen-initiated, polyamine-promoted skin carcinogenesis. GCN2 loss significantly delayed tumor development and decreased tumor number and tumor burden in K6/ODC; GCN2^-/- mice compared that in K6/ODC mice. Underlying dermal fibroblasts from nontumor bearing K6/ODC mice express elevated levels of genes associated with GCN2 activation and fibroblast activation. Expression of these genes was not elevated in K6/ODC; GCN2^-/- dermis. In addition, K6/ODC mice have significantly more myeloid derived suppressor cells (MDSC) compared to normal littermates, and MDSCs were decreased in K6/ODC mice deficient in GCN2. Dermal fibroblasts cultured from K6/ODC transgenic mouse skin secrete increased levels of protumorigenic factors including senescence associated secretory phenotype (SASP) factors that stimulate invasiveness of stem-like epidermal tumorspheres as well as the polarization of M2-like macrophages. Using K6/ODC; p16-3MR mice in which senescent fibroblasts can be eliminated with ganciclovir treatment, carcinogen-initiated tumor development was greatly inhibited when senescent fibroblasts were eliminated in K6/ODC; p16-3MR mice. Our studies suggest a new paradigm in which cellular stress responses resulting from increased polyamine biosynthesis accelerate fibroblast activation and a senescence phenotype to create a protumorigenic microenvironment.