TLR4-mediated activation of the ERK pathway following UVA irradiation contributes to increased cytokine and MMP expression in senescent human dermal fibroblasts

Exposure to ultraviolet (UV) radiation is a major contributing factor to premature aging (photoaging) and skin cancer. In vitro models of cellular senescence have proven to be very useful for the study of slow and progressive accumulation of damage resulting in the growth arrest of aging skin cells. In this study, we compared UVA-induced cellular responses in non-senescent (NS) vs. Senescent (S) human dermal fibroblasts (HDFs). HDFs were irradiated with a single dose of UVA (7.5 J/cm2) and QuantSeq 3' mRNA sequencing was performed to assess differential gene expression. Both NS and S HDFs expressed similar numbers of differentially expressed genes, although distinct sets of genes were differentially expressed between the two groups. Higher expression of matrix metalloproteinases (MMPs) and Toll-like receptor (TLR) pathway genes, such as TLR4, MyD88, and CXCL-8, was detected in S HDFs as compared with NS HDFs, and UVA exposure led to a downregulation of collagen genes, such as COL8A2 and COL5A3. Consistent with gene expression profiling, enhanced IL-6 and IL-8 secretion was observed in S HDFs compared with NS HDFs, in response to UVA. Furthermore, we show that TLR4-mediated ERK pathway is responsible for the UVA-mediated mitochondrial dysfunction as well as increased secretion of MMP-1 and IL-8 in S HDFs. Taken together, our results demonstrate the UVA-induced common and distinct molecular patterns of cellular responses between NS and S HDFs and suggest TLR4/ERK pathways as candidate targets to reduce senescent phenotypes.