Effect of aging and habitual sun exposure on the genetic response of cultured human keratinocytes to solar-simulated irradiation.

Aging and chronic sun exposure are known to be associated with decreased cutaneous immune function, changes in the balance between epidermal proliferation and differentiation, and a greatly enhanced risk of photocarcinogenesis. However, their specific effects on the response of human keratinocytes to ultraviolet (UV) irradiation are unknown. We therefore asked whether aging and photoaging modulate the response at the mRNA level to UV-inducible genes implicated in immunomodulation and/or growth control. Cultured human keratinocytes derived from newborn, young adult, and old adult donors were exposed to a single physiologic dose of solar-simulated UV or sham irradiation and harvested at 1, 4, 8, 24, and 48 h post-irradiation for northern blot analysis. Specific mRNA was detected using cDNA probes encoding the proto-oncogenes c-fos and c-myc and the growth-arrest and DNA damage (GADD153) gene, all recently shown by our laboratory to be modulated by UV in newborn keratinocytes; interleukin (IL)-1a, IL-1b, and the IL-1 receptor antagonist (IL-1ra), two keratinocyte cytokines and their competitive inhibitor, implicated in the immunomodulatory effect of UV; and SPR2, a recently cloned gene known to be induced during normal keratinocyte differentiation and by lethal UV-C irradiation. Our data suggest that aging alone strikingly increases the baseline expression of SPR2 and IL-1ra but has relatively little effect on the response to UV for the other genes examined. In contrast, the combination of aging and habitual sun exposure, so-called photoaging, markedly increases c-fos inducibility and decreases baseline expression of SPR2 and IL-1ra relative to that in cells from sun-protected skin of the same donors. The implied alterations in signal transduction and differentiation state observed in cells derived from habitually sun-exposed sites of old adults may explain in part the predisposition to photocarcinogenesis in photoaged skin.