Epidemiologic studies indicate that tobacco smoking is a strong independent risk factor of facial wrinkle formation and pigmentation. We compared aging between Asian and Caucasian women and observed that Asian women develop pigment spots much earlier than age-matched Caucasian women, whereas the opposite is true for the formation of coarse wrinkles. Based on the epidemiologic study, we found an association of smoking with an increase of spots on the back of hands in Caucasian women and wrinkles on upper lip in Japanese women. Although it is now widely recognized that tobacco smoke has negative effects on skin, the molecular mechanisms underlying its skin-aging effects remain uncertain.
Water-soluble tobacco smoke extract, which predominantly produces oxidative stress when applied topically to cultured skin fibroblasts, impairs collagen biosynthesis. Matrix metalloproteinases, which degrade collagen, are induced dose-dependently by tobacco smoke extract as well as by other constituents that trigger the aryl hydrocarbon receptor (AhR), a ligand-dependent transcription factor that mediates the toxicity of several environmental contaminants, including photoproducts in the body generated by ultraviolet (UV) B radiation. Tobacco smoke contains more than 3800 constituents, including numerous water insoluble polycyclic aromatic hydrocarbons that trigger the AhR signaling pathway. Hexane-soluble tobacco extracts significantly induced MMP-1 mRNA in both human cultured fibroblasts and keratinocytes in a dose-dependent manner. To clarify the involvement of the AhR pathway, we used stable knockdown cell lines for AhR in HaCaT cells. AhR knockdown abolished the increase in transcription of the AhR-dependent gene CYP1A1/CYP1B1 and MMP-1 upon treatment with either tobacco smoke extract. Furthermore, the tobacco smoke extracts induced 7-ethoxyresorufin-O-deethylase activity, which was almost completely abolished by AhR knockdown. These findings suggest that the tobacco smoke extracts induced MMP-1 expression in human fibroblasts and keratinocytes via activation of the AhR pathway. Furthermore, We also investigated whether tobacco smoke enhances skin pigmentation by culturing normal human epidermal melanocytes with tobacco smoke extract. When cultured with tobacco extract, melanocytes grew quite large and produced more melanin. Culturing the cells with tobacco smoke extract combined with UVB irradiation significantly increased microphthalmia-associated transcription factor (MITF) expression in a dose-dependent manner. Immunocytochemical studies revealed that these activated melanocytes actively expressed aryl hydrocarbon receptors (AhR) around the nuclear membrane. The tobacco smoke-induced MITF activation was inhibited by RNA silencing of AhR. This study provides the first evidence that tobacco smoke enhances sun-induced pigmentation by AhR-mediated mechanisms inside melanocytes.
Thus, the AhR pathway may be pathogenetically involved in extrinsic skin aging.