There were no significant associations between prostate cancer risk or its aggressiveness and serum levels of oxidized protein as measured by protein carbonyls in this large nested case-control study. While the cancer risk associated with the highest oxidized protein levels was slightly elevated in the finasteride arm, the association did not reach statistical significance. Finasteride did not appear to impact serum protein carbonyl levels when comparing baseline measurements to those obtained after 2 yrs on finasteride. There were also no significant associations between factors considered to be associated with increased oxidative stress and oxidized protein levels.
In our prior large study of breast cancer, protein oxidation, defined as high plasma levels of protein carbonyl groups, was significantly associated with a 60% increased risk of breast cancer.7
However, in contrast to the present study, bloods were collected on average 3 months after diagnosis and thus could have been impacted by disease. Previous smaller studies have provided conflicting evidence on the association between protein oxidation and cancer risk, with positive results for Hodgkin’s lymphoma9
and bladder cancer,10
but not for lung11
cancer. None of these studies were prospective.
While aging is accompanied by increasing levels of oxidative damage, including protein oxidation (reviewed in 13
), no associations with age were observed in our study, perhaps due to the narrow age range of our participants. Significantly higher levels of oxidized proteins in smokers than in nonsmokers have been observed11;14
but we found no association between serum protein carbonyl levels and smoking status. Conflicting data have been observed for an association with fruit and vegetable intake.7;15–18
The lack of an association between these factors believed to be associated with increased oxidative stress and serum oxidized protein concentrations suggests that this measure may not be sensitive to environmental factors that increase oxidative stress.
In summary, in this large study using bloods collected before diagnosis, we found no association between serum protein carbonyl levels and prostate cancer risk. Among controls, oxidized protein levels were not significantly associated with factors thought to be associated with oxidative stress. It is possible that serum levels of oxidized proteins do not accurately reflect oxidative damage in the prostate, where there may be a higher inflammatory environment; studies examining prostate tissue for oxidative damage may help clarify the role of oxidative stress in prostate cancer etiology.