Vitamin D inhibits the activation of stress-activated protein kinases by physiological and environmental stresses in keratinocytes.

In addition to its known effects on keratinocyte proliferation and differentiation, the hormonal form of vitamin D, 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), has been shown to protect keratinocytes from UV- and chemotherapy-induced damage. Epidermal keratinocytes contain both the machinery needed to produce 1,25(OH)(2)D(3) and vitamin D receptors. The activation of the stress-activated protein kinases (SAPKs), such as c-Jun N-terminal kinase (JNK) and p38, is an early cellular response to stress signals and an important determinant of cell fate. This study examines whether modulation of these SAPKs is associated with the effects of 1,25(OH)(2)D(3) on keratinocytes under stress. HaCaT keratinocytes were exposed to heat shock, hyperosmotic concentrations of sorbitol, the epidermal growth factor receptor tyrosine kinase inhibitor AG1487, the pro-inflammatory cytokine tumor necrosis factor alpha, and H(2)O(2). These stresses activated both SAPKs. Pretreatment with 1,25(OH)(2)D(3) inhibited the activation of JNK by all stresses and the activation of p38 by heat shock, AG1478 and tumor necrosis factor alpha. Under the same conditions, treatment with 1,25(OH)(2)D(3) protected HaCaT keratinocytes from cytotoxicity induced by exposure to H(2)O(2) and hyperosmotic shock. The effect of 1,25(OH)(2)D(3) was dose-dependent, already apparent at nanomolar concentrations, and time-dependent, maximal after a 24-h pre-incubation. We suggest that inhibition of SAPK activation may account for some of the well-documented protective effects of 1,25(OH)(2)D(3) on epidermal cells during exposure to UV or chemotherapy and may also be related to the anti-inflammatory actions of the hormone in skin.