Ultraviolet B Irradiation Induces Expansion of Intraepithelial Tumor Cells in a Tissue Model of Early Cancer Progression

Abstract

Ultraviolet B irradiation is thought to enable skin cancer progression as clones of genetically damaged keratinocytes escape apoptosis and expand at the expense of adjacent normal cells. Mechanisms through which potentially malignant cells in human skin undergo clonal expansion, however, are not well understood. The goal of this study was to characterize the role of ultraviolet B irradiation on the intraepithelial expansion of early stage human tumor cells in organotypic skin cultures. To accomplish this, we have studied the effect of ultraviolet B irradiation on organotypic cultures that were fabricated by mixing normal human keratinocytes with beta-galactosidase-marked, intraepithelial tumor cells (HaCaT-ras, clone II-4), which bear mutations in both p53 alleles and harbor an activated H-ras oncogene. We found that when organotypic mixtures were exposed to an ultraviolet B dose of 50 mJ per cm2, intraepithelial tumor cells underwent a significant degree of proliferative expansion compared to nonirradiated cultures. To understand this response, organotypic cultures of nor-mal keratinocytes were exposed to ultraviolet B and showed a dose-dependent increase in numbers of sunburn cells and TUNEL-positive cells although their proliferation was suppressed. In contrast, neither the apoptotic nor the proliferative response of II-4 cells was altered by ultraviolet B in organotypic cultures. The differential response of these cell types suggested that II-4 cells were resistant to ultraviolet-B-induced alterations, which allowed these intraepithelial tumor cells to gain a selective growth and survival advantage relative to neighboring normal cells. These findings demonstrate that ultraviolet B exposure can induce the intraepithelial expansion of apoptosis-resistant, p53-mutant, and ras-activated keratinocytes, suggesting that this agent can act to promote the early stages of epithelial carcinogenesis.

Irradiation Selects for p53-Deficient Hematopoietic Progenitors

“…Ultraviolet (UV) light exposure has been shown to increase the numbers and size of p53 mutant clones in human skin [42] and to induce the expansion of p53 mutant/Ras activated premalignant cells in organotypic skin cultures [43]. The expansion of p53 disrupted clones in mouse skin required continued UV-B exposure [44], contrasting with the stable selection for p53 disruption following a single exposure to X-irradiation in our studies.…”

Nicotinic Acid Receptor Abnormalities in Human Skin Cancer: Implications for a Role in Epidermal Differentiation

“…The altered expression of the genes encoding GPR109A/B, the abnormal pattern of cellular distribution, and impaired functionality of the receptor in squamous cell carcinoma cells suggest that progression of skin damage leading to receptor defects could provide a mechanism for squamous cell cancers to avoid differentiation signals. However, the observation that poorly differentiated keratinocytes can be driven to differentiate in the presence of sufficient numbers of normal keratinocytes [4] raises the possibility that treatments that could promote epidermal differentiation in photodamaged skin could restore the balance of proliferation and differentiation required for maintaining skin homeostasis and thus may counteract the development of AK lesions and SCC. In that context, nicotinic acid receptors could be potential targets for skin cancer prevention.…”

“…The main histological characteristic of SCC is substitution of normal matured epidermal keratinocytes with poorly differentiated atypical squamous cells [3]. Mudgil et al have demonstrated in skin epidermal equivalents that transformed, poorly differentiated keratinocytes can be driven to differentiate in the presence of sufficient numbers of normal keratinocytes [4]. These observations suggest that treatments that could promote epidermal differentiation in photodamaged skin have the potential to restore the balance of proliferation and differentiation required for maintaining skin homeostasis and thus could counteract the development of AK lesions and SCC.…”

Tumor Heterogeneity: Mechanisms and Bases for a Reliable Application of Molecular Marker Design

“…Furthermore, transplantation experiments have convincingly demonstrated that different types of altered cells do not display any evidence of growth autonomy when transferred in a normal tissue environment of young animals in vivo [67]. By analogy, altered, putative initiated cells can be found in the skin of several healthy human subjects, suggesting that their presence per se is not necessarily associated with selective clonal growth, and additional (promoting/selective) influences must be enforced when the latter does occur [86]. Also epidemiologic evidence on smoking and lung cancer suggests that clonal expansion of cells is much more relevant than early mutations [87].…”