Photocarcinogenesis and inhibition of intercellular adhesion molecule 1 expression in cells of DNA-repair-defective individuals

Abstract

Cells from patients with xeroderma pigmentosum complementation group D (XP-D) and most patients with trichothiodystrophy (TTD) are deficient in excision repair of ultraviolet (UV) radiation-induced DNA damage. Although in both syndromes this defect is based on mutations in the same gene, XPD, only XP-D, not TTD, individuals have an increased risk of skin cancer. Since the reduction in DNA repair capacity is similar in XP-D and TTD patients, it cannot account for the difference in skin cancer risk. The features of XP-D and TTD might therefore be attributable to differences in the immune response following UV-irradiation, a factor which is presumed to be important for photocarcinogenesis. We have measured the capacity of UVB radiation to inhibit expression of the immunological key molecule intercellular adhesion molecule 1 (ICAM-1) in cells from three healthy individuals in comparison to cells from three XP-D and three TTD patients. Cells from XP-D patients, but not from TTD patients, exhibited an increased susceptibility to UVB radiation-induced inhibition of ICAM-1 expression. Transfection of XP-D cells with the wild-type XPD cDNA, but not with XPC cDNA, corrected this abnormal phenotype. Thus, the skin cancer risk in DNA repair-defective individuals correlated with the susceptibility of their cells to UVB radiation-induced inhibition of ICAM-1 expression, rather than with their defect in DNA repair. The XPD protein has dual roles: in DNA repair and transcription. The transcriptional role might be important for the control of expression of immunologically relevant genes and thereby contribute to the skin cancer risk of a DNA-repair-deficient individual.

Figure 1

Effect of UVB radiation on the inhibition of IFN-γ-induced ICAM-1 mRNA and surface expression in DNA-repair-deficient versus normal cells. (a) RT-PCR on ICAM-1 (Left) and GAPDH (Right) mRNA expression in normal (N46), XP-D (XP16BR), and TTD (TTD1BEL) cells. Cells were left unstimulated (lane 1) or stimulated with rhIFN-γ (lanes 2–6). In lanes 1 and 2, cells were sham irradiated, in lanes 3–6, cells were exposed to decreasing doses of UVB radiation (lane 3, 100 J/m²; lane 4, 50 J/m²; lane 5, 25 J/m²; and lane 6, 12.5 J/m²). IFN-γ (500 units/ml) was added immediately after UVB radiation exposure and cells were harvested after a 4-h incubation period. Data are shown as fluorescence of ethidium bromide-stained gels and represent one of three essentially identical experiments. (b) Summary of RT-PCR results from three XPD (⋄), three normal (○), and three TTD (▵) cell strains. IFN-γ-induced ICAM-1 mRNA expression is given as percent of ICAM-1 mRNA expression in sham-irradiated, IFN-γ-stimulated cells and is plotted against UVB radiation (J/m²). Each curve represents mean values of three independent experiments. Standard deviation for each mean was less than ±15%. (c) Summary of FACS analysis results from three XPD (⋄), three normal (○), and three TTD (▵) cell strains. IFN-γ was added immediately after UVB radiation exposure and cells were harvested after a 24-h incubation period. IFN-γ-induced ICAM-1 surface expression is given as percent of ICAM-1 surface expression in sham-irradiated, IFN-γ-stimulated cells and plotted against UVB radiation (J/cm²). Each curve represents mean values of three independent experiments. Standard deviation for each mean was less than ±15%.

Figure 2

Effect of UVB radiation on inhibition of IFN-γ-induced ICAM-1 mRNA expression in genetically engineered XP-D cells. Summary of RT-PCR results from XP-D cells transfected with functional XPD cDNA (□), nontransfected XPD cells (○), and XP-D cells transfected with XPC cDNA (▵). Suppression of IFN-γ-induced mRNA expression is given as percent of ICAM-1 mRNA expression in sham-irradiated, IFN-γ-stimulated cells and is plotted against UVB radiation (J/m²). Each curve represents mean values (SD < 15%) of three independent experiments.