Consideration of possible effects of vitamin D on established cancer, with reference to malignant melanoma

Epidemiological studies indicate that Vitamin D has a beneficial, inhibitory effect on cancer development and subsequent progression, including melanoma (MM), and favourable MM outcome has been reported as directly related to vitamin D-3 status, assessed by serum 25-hydroxyvitamin D-3 (25[OH]D-3) levels taken at diagnosis. It has been recommended that MM patients with deficient levels of 25(OH)D-3 be given vitamin D-3. We examine possible beneficial or detrimental effects of treating established cancer with vitamin D-3. We consider the likely biological determinants of cancer outcome, the reported effects of vitamin D-3 on these in both cancerous and non-cancerous settings, and how the effect of vitamin D-3 might change depending on the integrity of tumour vitamin D receptor (VDR) signalling. We would argue that the effect of defective tumour VDR signalling could result in loss of suppression of growth, reduction of anti-tumour immunity, with potential antagonism of the elimination phase and enhancement of the escape phase of tumour immunoediting, possibly increased angiogenesis but continued suppression of inflammation. In animal models, having defective VDR signalling, vitamin D-3 administration decreased survival and increased metastases. Comparable studies in man are lacking but in advanced disease, a likely marker of defective VDR signalling, studies have shown modest or no improvement in outcome with some evidence of worsening. Work is needed in assessing the integrity of tumour VDR signalling and the safety of vitamin D-3 supplementation when defective.