HYPERTHERMIA INDUCES RESISTANCE TO ULTRAVIOLET LIGHT-B IN PRIMARY ANDIMMORTALIZED EPIDERMAL-KERATINOCYTES

Environmental exposure to UVB (290-320 nm) wavelengths of the solar sp ectrum causes major damage, including carcinogenesis, in the skin. The refore, cellular responses that protect against UVB damage are of part icular interest in cutaneous epithelial cells. In cultured keratinocyt es, mild hyperthermia generates a classical stress response with acqui red thermotolerance and elevated stress protein synthesis (E. V. Mayti n, J. Biol. Chem., 267: 23189-23196, 1992). To test the ability of thi s stress response to protect against UVB damage, monolayers of primary murine keratinocytes or BALB/MK keratinocytes were heated at 42-degre es-C for 1 h and then exposed to UVB at 6 h (typical dose, 40 mJ/cm2). Survival was assessed by fluorescein diacetate/ethidium bromide vital dye uptake and video microscopy. With heat-conditioning prior to UVB, a significant increase in both the percentage viability (2- to 3-fold ) and in the absolute number of living (fluorescein diacetate-positive ) cells was measurable at 24-48 h. Steady-state incorporation into [H- 3]DNA and S-35-protein, while suppressed immediately after UVB, showed greater recovery in heat-conditioned cultures compared to sham-condit ioned cultures at 48 h. Increased metabolic activity was accompanied b y increased proliferative potential since colonies of BALB/MK cells ob served at 72 h were larger, more numerous, and more active in the upta ke of 5-bromo-2'-deoxyuridine in heat-conditioned cultures. A time cou rse for the development of UVB resistance showed maximal protection wh en heat and UVB were spaced approximately 6 h apart. Hyperthermic cond itioning could induce UVB protection in nonproliferating cells, indica ting that cell cycle arrest was not primarily responsible for the UVB- protective effect. In summary, hyperthermia induces a mechanism in epi thelial cells which can ameliorate damage from UVB.