Journal of Experimental & Clinical Cancer Research 2009, 28:135 http://www.jeccr.com/content/28/1/135 bles represents an additional, potential source of bias. Neither the analyses conducted by subsets including only one of the outcome variables, nor the analyses performed by case-case and control-control comparison between subject with and without missing data items showed sig- nificant results. We conducted a systematic search of the literature and combined the available results in a meta-analysis. We found significant evidence supporting the protective role of the metabolic pathway favoring 2-hydroxylation over 16α-hydroxylation in Pca development. This increases our confidence in the single studies' results, which were consistent, and might indicate that the lack of significance was mainly due to the limited sample size of the single studies. Despite their historical use in prostate cancer treatment, our knowledge regarding the effects of estrogens on pros- tate, their role in cancer development and the mecha- nisms mediating their action as therapeutic agents is quite limited. The published literature mainly focuses on the effects of circulating estrone and estradiol in relation to prostate cancer risk, providing inconsistent evidence [17,18,25,26]. A wide variety of methodological issues ranging from the restricted sample size to possible bias introduced by uncontrolled sources of hormonal variabil- ity might provide a partial explanation to the cited incon- sistency. It is also plausible that the surmised exposures have not been captured over periods comparable by degree of prostate sensitivity to hormonal influences across the different studies. The lack of consideration for factors potentially relevant to the overall estrogenic activ- ity, namely, hydroxylated metabolites of E1 and E2, might provide a further explanation that would integrate the aforementioned hypotheses. The dominating hydroxylation pathway significantly affects the biological activity of estrogen metabolites. Indeed, 16α-OHE1 binds with high affinity the estrogen receptor and exerts a strong estrogenic action that leads to increased cell proliferation and DNA synthesis [27,28]. Conversely, 2-OHE1 exerts a weak agonist effect on the oestrogen receptor and shows anti-angiogenic properties [29,30]. Little epidemiologic evidence exists with regard to the hypothesis investigated in the present study. Our previous study results support the association between elevated 2- OHE1 urinary levels and a reduced Pca risk (OR 0.83 95% CI 0.43-12.44), whereas elevated16α-OHE1 urinary levels are associated with increased risk (OR 1.69 95% CI 0.93- 3.06, p for linear trend 0.002) [13]. In their case-control study, Yang and colleagues found no significant difference in the median levels of 2-OHE1 and 16α-OHE between the compared groups. However, the sample size was very limited and the number of cases extremely low [24]. In their cross-sectional study, Teas et al evaluated the varia- bility of the urinary levels of 2-OHE1 and 16αOHE1 in a sample of African-American men attending prostate can- cer screening clinics and investigated any possible relation of these two metabolites with PSA. They reported an over- all significant reduction in 2-OHE1 per each 1.0 ng/ml increase in PSA [31]. Further evidence of the role of sex steroid hormones in prostate cancer emerges from studies focusing on the role played by estrogen metabolites in breast carcinogenesis. Several case-control and cohort studies show that women who metabolize a larger proportion of estrogens via the 16α-hydroxy pathway may be at a significantly higher risk of breast cancer compared to women who metabolize proportionally more estrogens via the 2-hydroxy pathway [16,32-34]. We observed a 40% breast cancer risk reduc- tion in women whose 2-hydroxyestrone (2-OHE1) to 16α-hydroxyestrone ratio was in the highest tertile of the distribution compared to those in the lowest tertile [35]. In summary, in the context of a still limited scientific evi- dence base, our study and meta-analysis provide data sup- porting a differential role of the estrogen hydroxylation pathway in prostate cancer development. The small sam- ple size of our original study prevents us from drawing strong conclusions, but the results of our meta-analysis including the second study provide us with greater evi- dence in support of the investigated association and the need for further studies. Competing interests The authors declare that they have no competing interests. Authors' contributions MB contribution to data analysis, results interpretation, manuscript drafting, review coordination LY laboratory assays HJS methodological advice, critical revision of the manu- script, systematic review conception FS and SG data analysis SS, KW, GB, MG critical revision of the manuscript PM case-control study conception and design, methodo- logical advice, critical revision of the manuscript All authors have read and approved the final version of the manuscript Page 7 of 8 (page number not for citation purposes)