Occupational exposure to solar ultraviolet B radiation and risk of prostate cancer in Danish men

1Introduction

Prostate cancer is the most common cancer among men in most countries worldwide [1] . Both incidence and mortality rates are strongly associated with higher age, however, other firmly established risk factors are few and include Black ethnicity and family history of prostate cancer [2] . Other factors suspected to also play a role in prostate cancer development include physical inactivity, dietary factors, e.g. consumption of meat and dairy products [2] , and night-shift work [3] . In addition, geographical trends in incidence rates of prostate cancer have been observed; Scandinavians have higher incidence of the disease compared with residents in southern Europe and a similar difference in disease rates between men residing in Northern vs. Southern geographical areas in the US has been reported as well [4] . Analogous observations have been reported for a number of other cancers, e.g. breast and colon cancer, and may reflect a protective effect of ultraviolet B radiation (UV) on cancer risk. It is theorized that protection from certain cancers could be provided by UV exposure through a vitamin D -mediated mechanism [5] . Vitamin D is initially formed from 7-dehydrocholesterol after UV exposure of the skin and UV-mediated synthesis is the major source of obtainment in most countries, although ingestion of vitamin D from supplementation or the diet, e.g. fatty fish, is also considered important sources [6] . Vitamin D is first hydroxylated in the liver forming the prohormone, 25-Hydroxyvitamin D (25-OHD), and then again to the most hormonally active form, 1,25(OH) ₂ D, in the kidneys [7] . However, it has been shown that prostate cells can synthesize 1,25(OH) ₂ D intracellularly from circulating serum 25-OHD [8] and that prostate cells respond to 25-OHD levels by promoting differention of cells together with inhabitation of proliferation, invasiveness and metastasis [9] .