Mutually antagonistic PPARgamma and TLR4 signaling in photocarcinogenesis

The most common cause of skin cancer is exposure to the damaging ultraviolet B (UVB) component of solar radiation. U.S. Veterans are at high risk for skin cancer due to high levels of sun exposure due to training and deployment. This is particularly true for soldiers deployed to areas of high ambient sun exposure such as the desert and mountain regions of Iraq and Afghanistan. Skin cancer incidence and mortality due to skin cancer goes up dramatically in Veterans who require immunosuppression for organ transplantation. Effective therapies to prevent skin cancer in high-risk sun-damaged skin are lacking. Thus, treatments that can reduce skin cancer risk in those at high risk for skin cancer development are urgently needed. Peroxisome proliferator- activated receptor γ (PPARγ) is a ligand activated nuclear receptor that has been shown to have anti- inflammatory activity. This occurs by blocking the activity of pro-inflammatory transcription factors such as NF-kB. Toll-like receptor 4 (TLR4) is cell membrane receptor that plays a key role in innate immune reactions. Activation of the TLR4 by bacterial lipopolysaccharide (LPS) activates down-stream signaling pathways that activate NF-kB. TLR4 can also be activated by a number of endogenous proteins, called damage-associated molecular patterns (DAMPs). TLR4 activation is also known to suppress the expression and activity of PPARγ. Thus, PPARγ and TLR4 have opposing effects and are also mutually antagonistic, with each suppressing the activity of the other. Tryptophan hydroxylase 1 (TPH1) is an enzyme that is necessary for producing the neurotransmitter serotonin (5-HT) outside the nervous system and plays a key role in suppressing immunity. We have shown that deletion of the PPARγ gene in the epidermis of mice (Pparg-/-epi mice) results in profound pathologic changes to the skin. This includes an increase in UVB-induced carcinogenesis, marked immune suppression, and increased dermal inflammation. We performed whole transcriptomic and single cell sequencing of Pparg-/-epi mouse skin. Compared to normal mice, Pparg-/-epi mouse skin had increased expression of DAMP mRNAs as well as TPH1 transcripts. Myeloid and lymphocytic cells that were found in Pparg-/-epi mouse skin expressed genes that are associated with immunosuppressive myeloid cells and T cells. We next compared the transcriptomic changes seen in Pparg-/-epi mouse skin with those found in actinic keratoses (AK) and cutaneous squamous cell carcinoma (SCC). Following gene set enrichment analysis, the top inhibited canonical signaling pathway was PPAR signaling and the top activated upstream regulator was LPS. In addition, DAMPs with TLR4 agonist activity were also increased in AKs and SCCs. We therefore propose that tumor formation requires a shift in the balance between these mutually antagonistic pathways that allows for unopposed TLR4 activity. This also results in increased TPH1 expression. This hypothesis will be tested in two specific aims: Aim 1 will determine the role of PPARγ signaling on TLR4 & TPH1 signaling and how inhibition of TLR4 &/or TPH1 alters the inflammatory and immune function in Pparg-/-epi mice. These studies will examine whether TLR4 inhibition blocks the immune suppression seen in Pparg-/-epi mice, determine whether TLR4 activity is necessary for TPH1 expression or 5-HT production and assess the immune cells that accumulate in Pparg-/-epi skin for functional immunosuppressive activity. Aim 2 will determine whether co-operative signaling between PPARγ and downstream TLR4 & TPH1 signaling can be targeted to for synergistic anti-cancer activity. These studies will examine how loss of Pparg in tumor epithelium alter the tumor transcriptome. We will then assess whether either TLR4 or TPH1 are necessary for the increased tumor formation in Pparg-/-epi mice. The potential that TLR4 and/or TPH1 inhibitors act synergistically with a PPARγ agonist to prevent UV-induced skin cancer will also be assessed. Project Number: 1I01BX006796-01A1 | Fiscal Year: 2026 | NIH Institute/Center: Veterans Affairs (VA) | Principal Investigator: RAYMOND KONGER | Institution: RLR VA MEDICAL CENTER, INDIANAPOLIS, IN | Activity Code: I01 | Study Section: Special Emphasis Panel[ZRD1 ONCE-A (01)] View on NIH RePORTER: https://reporter.nih.gov/project-details/11190751