Epidermal PPARγ Signaling as a Suppressor of Toll-Like Receptor-Mediated Inflammation and Fibrosis: Relevance to Cutaneous Squamous Cell Carcinoma

Mice lacking epidermal Pparg (Pparg-/-epi) exhibit increased cutaneous carcinogenesis, while PPARγ signaling is reduced in actinic keratoses (AKs) and cutaneous squamous cell carcinomas (cSCCs). Using transcriptomic analysis, we now show that the top upregulated genes in Pparg-/-epi mouse skin, human AKs and cSCCs encode multiple damage-associated molecular patterns (DAMPs) that are TLR4 ligands, while the TLR4 agonist lipopolysaccharide (LPS) is also predicted to be the top common activated upstream regulator in both Pparg-/-epi mouse skin and in tumor datasets. By single-cell sequencing, DAMP expression was particularly elevated in myeloid cells and myofibroblasts of Pparg-/-epi mice, and these cell types exhibit transcriptional changes consistent with TLR4 signaling. Myeloid cells also exhibited a loss of Pparg expression and activity. Transcriptional analysis of published LPS-treated macrophages also reveals a decrease in PPARγ activity. Fibroblasts from Pparg-/-epi mice included cells with a gene expression profile resembling myofibroblasts found in cancer and fibrotic diseases. This was accompanied by increased dermal fibrosis in aged mice and a transcriptomic profile that indicates a key role for both TLR4 and TGFβ signaling. These data suggest that loss of epidermal PPARγ may disrupt counterbalancing PPARγ–TLR4 signals, leading to chronic inflammation and fibrosis, hallmarks of cutaneous neoplasia.