Background Solar ultraviolet (UV) irradiation induces the production of matrix metalloproteinases (MMPs) by activating cellular signalling transduction pathways. MMPs are responsible for the degradation and/or inhibition of synthesis of collagenous extracellular matrix in connective tissues. We mimicked the action of environmental ultraviolet on skin and investigated the effects of UVB-irradiated human keratinocytes HaCaT and IL-1α on mitogen activated protein (MAP) kinase activation, c-Jun and c-Fos (AP-1 is composed of Jun and Fos proteins) mRNA expression and MMP-1 production in UVA-irradiated dermal fibroblasts. Methods Following UVA irradiation, the culture medium of fibroblasts was replaced by culture medium from UVB-irradiated HaCaT, or replaced by the complete culture medium with interleukin (IL)-1α. MAP kinase activity expression in fibroblasts was detected by Western blot. c-Jun and c-Fos mRNA expressions were determined by reverse transcriptional polymerase chain reaction (RT-PCR); MMP-1 production in culture medium was detected by enzyme-linked immunosorbent assay (ELISA). Results Culture medium from UVB-irradiated keratinocytes increased MAP kinase activity and c-Jun mRNA expression in UVA-irradiated fibroblasts. IL-1α increased MAP kinase activity and c-Jun mRNA expression, IL-1α also increased c-Fos mRNA expression. Both culture media from UVB-irradiated human keratinocytes and externally applied IL-1α increased MMP-1 production in UVA-irradiated fibroblasts. Conclusions UVB-irradiated keratinocytes and IL-1α indirectly promote MMP-1 production in UVA-irradiated fibroblasts by increasing MAP kinase/AP-1 activity. IL-1 may play an important role in the paracrine activation and dermal collagen excessive degradation leading to skin photoaging.
