The ultraviolet B (UVB) component of sunlight causes non‐melanoma skin cancers due to the damage it inflicts on genomic DNA. The response of epidermal keratinocytes to sunlight depends on the dose of UVB received and the severity of the damage to the DNA. Mild DNA damage typically induces DNA‐repair pathways and cell survival, while severe DNA damage provokes apoptosis. Primary human keratinocytes grown in serum‐free media respond in a similar manner to UVB irradiation. However, we observed that keratinocytes are exquisitely more susceptible to UVB‐induced apoptosis if the growth medium is depleted of exogenous growth factors. Therefore, an exogenous growth factor could provide protection from UVB‐induced apoptosis. We found that the only growth factor that provided protection from UVB‐induced apoptosis was insulin and that the protective effect elicited by insulin was not due to binding the insulin receptor but, rather, to activation of the insulin‐like growth factor‐1 (IGF‐1) receptor. Additionally, activation of the IGF‐1 receptor in combination with UVB irradiation induced keratinocytes to become post‐mitotic. This survival function of the IGF‐1 receptor in response to UVB irradiation was influenced by activation of phosphatidylinositol‐3 kinase and MAP kinase. Prior to UVB irradiation, insulin or IGF‐1 had little to no effect on cell growth or viability. Therefore, activation of the IGF‐1 receptor in conjunction with UVB irradiation promotes keratinocyte survival at the expense of cell proliferation. Int. J. Cancer 80:431–438, 1999. © 1999 Wiley‐Liss, Inc.

p53 is a tumor suppressor gene that has been implicated in a number of important cellular processes, including DNA repair and apoptosis. Genomic damage in human keratinocytes caused by ultraviolet B (UVB) irradiation has been shown to induce both apoptosis and p53 expression. We have previously observed that p53 expression in cultured normal human keratinocytes is predominantly perinuclear; however, exposure of cells to UVB radiation induces a major shift of p53 expression to the nucleus. Using terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling, internucleosomal DNA ladders and flow cytometry, we correlated observed changes in p53 expression with the induction of apoptosis at low, intermediate and high doses of UVB radiation. High doses of UVB radiation induced cells to undergo apoptosis, whereas UVB radiation at low doses did not induce apoptosis but appeared to stimulate repair of the DNA damage induced by UVB radiation. Intermediate doses of UVB radiation induced a heterogeneous population of cells to undergo either DNA repair or apoptosis. The level of UVB radiation dose also influenced the induced cellular localization of p53. These observed differences in p53 cellular localization correlated with the induction of DNA repair or apoptosis. In cells undergoing apoptosis, p53 protein was found within the blebs of the degenerating nuclei. Our data give support to increasing evidence that p53 may play a role in both the repair of UV-radiation-induced DNA damage and the induction of apoptosis, and may function as a central control checkpoint in response to UVB-radiation-induced DNA damage.

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Cutaneous LCH may be associated with underlying MDS in adults and should be considered in the differential diagnosis of cutaneous eruptions in patients with MDS.

Normal human keratinocytes are stimulated to proliferate in serum-free medium containing subphysiological concentrations of calcium (0.09 mM, low calcium). In this study, we examined the effect of increased levels of extracellular calcium (2.0 mM, normal calcium) on UVB-induced apoptosis. Apoptosis was assessed by changes in cellular morphology, annexind V-FITC flow cytometry, and the formation of internucleosomal DNA ladders. High doses of UVB induced keratinocytes grown in low calcium medium to undergo apoptosis. In contrast, keratinocytes grown for 72 h in normal calcium medium were completely resistant to UVB-induced apoptosis. No apoptosis was observed even at UVB doses as high as 1200 J/m2. However, despite the lack of UVB-induced cell death, keratinocytes grown in normal calcium medium lost the ability to proliferate following high levels of UVB irradiation. High doses of UVB also increased the expression of the differentiation-specific proteins involucrin and cytokeratin 10 in a dose-dependent manner. In addition, growth in normal calcium medium lowered the UVB-induced stimulation of the p53 protein and altered the normal subcellular localization pattern of p53. UVB irradiation of human keratinocytes grown in normal calcium medium may be inducing further cell differentiation in the absence of overt cell death.