Oxidative stress in malignant melanoma and non-melanoma skin cancer

Solar ultraviolet (UV) radiation is considered to be a major aetiological factor in melanoma and non-melanoma skin cancer. A growing body of evidence indicates that oxidative stress is involved in photocarcinogenesis. However, in vivo data for human skin are still lacking. Reactive oxygen species participate in a number of pathophysiological processes including DNA damage and lipid peroxidation (LPO) and are considered to be a key factor in tumour progression.

We hypothesized that in human skin cancer the natural redox balance is disturbed and that this imbalance may result in an accumulation of LPO products.

To test this, skin biopsies of superficial spreading melanoma were compared with age-matched benign melanocytic naevi and young healthy controls. Additionally, non-melanoma skin cancers (basal cell carcinoma, squamous cell carcinoma) and actinic keratosis were investigated (n = 18 each). Expression of the antioxidant enzymes, copper-zinc superoxide dismutase, manganese superoxide dismutase and catalase was analysed by immunohistochemical techniques. To detect LPO products, protein-bound malondialdehyde (MDA) was visualized.

In human melanoma biopsies, a significant overexpression of the antioxidant enzymes was found when compared with surrounding non-tumour tissue, benign melanocytic naevi, and young controls. Intriguingly, the LPO marker MDA was significantly increased in melanoma tissue. MDA was located not only in typical melanoma cells, but also occurred in surrounding keratinocytes. In contrast, a severely disturbed antioxidant balance with diminished antioxidant enzymes was found in non-melanoma tumours, whereas MDA was elevated only in squamous cell carcinomas.

These findings indicate that oxidative stress may play different roles in the pathogenesis of human skin cancers. In non-melanoma skin cancer, a diminished antioxidant defence caused by chronic UV exposure might contribute to multistep carcinogenesis, whereas melanoma cells exhibit increased oxidative stress which could damage surrounding tissue and thus support the progression of metastasis.