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 , 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 . 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 dosedependent, 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.

The hormonally active vitamin D metabolite, 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), and keratinocyte growth factor (KGF) belong to the network of autocrine and paracrine mediators in the skin. Both were shown to modulate keratinocyte proliferation, to reverse epidermal atrophy, to increase wound healing, and to reduce chemotherapy-induced alopecia. The overlap between their activities may suggest that vitamin D exerts some of its actions by modulation of KGF activities in the skin. This notion was examined by using HaCaT keratinocytes cultured in serum-free medium in the absence of exogenous growth factors and in the presence of the EGF receptor tyrosine kinase inhibitor AG 1478 that blocks their autonomous proliferation. These cells could be stimulated to proliferate by different fibroblast growth factors (FGFs). The relative mitogenic efficacy of basic FGF, acidic FGF, or KGF was in correlation with their affinities for the KGF receptor (KGFR). Forty-eight hour co-treatment with 1,25(OH)(2)D(3) enhanced KGFR-mediated cell proliferation in a dose dependent manner. Both ERK1/2 and c-Jun N-terminal kinase (JNK) were activated by the FGFs. Treatment with 1,25(OH)(2)D(3) increased the activation of ERK but reduced the activation of JNK. Treatment with 1,25(OH)(2)D(3) increased the levels of KGFR in the presence but not in the absence of KGF, probably due to inhibition of ligand-induced receptor degradation. Inhibition of protein kinase C with bisindolylmaleimide did not interfere with the effect of 1,25(OH)(2)D(3) on KGFR-mediated ERK activation. Our results support the notion that the paracrine KGF-KGFR system in the skin can act in concert with the autocrine vitamin D system in keratinocytes to promote keratinocyte proliferation and survival under situations of stress and injury.