While other studies have employed presurgical models to test the effects of complementary therapies on prostate cancer (44
), to our knowledge, this has been the largest effort to date . Not only does it demonstrate the feasibility of implementing complex prevention trials within the community setting, it also suggests that flaxseed is well-accepted, safe to use, and may affect tumor proliferation rates. These effects appear independent of dietary fat intake, though we lacked adequate power to detect interactions. Furthermore, mean decreases in fat intake to only 25–28.4% of total calories, instead of the prescribed level of <20%, may have jeopardized our ability to observe effects that may have accompanied better adherence. The low-fat diet has shown success in other studies, though these studies also included exercise and endpoints differed (10
Our observation of lower proliferation rates with flaxseed supplementation is consistent with our previous in vitro
work in LNCaP, DU-145 and PC-3 prostate cancer cell lines which also found inhibited cell growth with exposure to flaxseed-derived lignans (8
). Lower cellular proliferation, and reduced tumor burden and urogenital weight also was found in our preclinical study using the transgenic adenocarcinoma mouse model which compared 5% flaxseed supplementation vs. an elemental control diet (AIN-76A) (7
). Furthermore, the lower proliferation rates observed with flaxseed supplementation in the current study, parallel findings from our previous clinical studies, one which also used a presurgical model and found lower proliferation indexes among 25 patients assigned to a flaxseed-supplemented, low-fat diet as compared to historic controls matched on Gleason sum, PSA at diagnosis, disease laterality, race and age (5
), and another which found reduced pre-post proliferation rates in the benign epithelium of patients with abnormal biopsies scheduled for rebiopsy (6
). Reduced proliferation rates with flaxseed also have been observed by Thompson et al. who employed a presurgical model in breast cancer (n=32) and found a 34.2% reduction in the Ki-67 labelling index (p=.001) (36
); the strength of this study is that the tumor proliferation rate in biopsy specimens serves as a strong baseline measure from which to assess change in tumors in the breast, whereas in prostate cancer the ability to assess change from biopsy to surgery is limited given its multi-focal and biologically-diverse nature. Animal studies by Thompson and colleagues also support reduced proliferation rates with flaxseed supplementation (46
). Therefore, all published studies have observed lower or reduced proliferation rates with flaxseed supplementation, thus providing a consistent finding.
However, unlike previous reports, including one of our recent studies which found that flaxseed-derived lignans induced apoptosis in LNCaP cells via a mitochondrial-mediated case-dependent pathway (4
) we did not observe differences in apoptosis between treatment arms. Indeed, we observed little variation in TUNEL scores in this study, since apoptosis was either absent or negligible in the majority of our samples. Reasons for this are unknown.
In contrast to our previous studies conducted among men with prostate cancer and those with abnormal biopsies that showed high grade prostatic intraepithelial neoplasia or foci of atypical cells (5
), we did not observe differences in PSA change between the study arms. Curiously, all study arms experienced significant decreases in both PSA and testosterone during the presurgical period. While Nakashima et al. (48
) report consistent decreases in testosterone among patients from pre-anesthesia to 7 days post-prostatectomy, there are no antecedent reports of decreases in testosterone or PSA during the presurgical period. A handful of reports exist; however, describing declines in testosterone with acute stress imposed in the laboratory setting or observed in community-dwelling subjects under a variety of situational factors (49
). Therefore, the decreases in testosterone observed in this study may relate to the acute stress attendant with impending surgery-a decline in testosterone that then drives PSA downward. Further study is needed to support or refute this conjecture. The decreases in PSA noted within the control arm also point to the importance of a randomized controlled design during this period of time, and provide evidence that subjects are unable to serve as their own controls.
Our initial premise, that flaxseed exerts its effects through androgen and IGF pathways was unsupported, at least with respect to the biomarkers tested. Indeed, it is possible that other biomarkers assessed along these pathways might be responsible for the effects that we witnessed. For example, reductions in intracellular 5oc-reductase, effects on IGFBP-1 or -2, or other mechanisms may be at play, such as natural killer cell activity, vascular endothelial growth factor, etc (2
). Eicosanoid-related pathways may hold particular promise since our data suggest that the ALA in flaxseed may be converted to EPA in both the erythrocytes and the target tissue. Therefore, membrane-mediated events that directly relate to the mechanical integrity of cell membranes or to signal transduction also warrant further exploration, as do mechanistic studies that build on recent work suggesting that ω3FAs may impact HER2 (erbB-2) oncogene expression and thus hold promise for both breast and prostate cancer (50
). In addition to mechanistic studies, investigations also are needed to determine dose-response and effects among patients who manifest recurrent disease after surgery or those electing expectant management.
An unexpected finding of this study was despite the fact that ALA intakes were significantly higher among flaxseed-supplemented men, we did not find any evidence that this translated into higher levels of ALA in the erythrocytes or prostatic tissue. Instead, we found evidence that EPA levels were higher, thus suggesting that conversion of ALA to higher-chained ω3FAs may occur and may not be as rate-limiting as previously thought (23
). Speculation exists as to whether ALA from various sources is metabolized differently or may be influenced by energy balance or temporal changes in the hormonal milieu, thus calling for further investigation. Therefore, more research is needed regarding ALA and prostate cancer, especially studies which control for salient risk factors and which can distinguish between markers of dietary intake or of energy balance, and those that are on the causal pathway (17
). While erythrocyte levels of fatty acids provide a reliable measure of intermediate intake (51
), in conducting further study, the use of other methods, such as radioisotope tracing to discern immediate effects on metabolism, as well as fatty acid analyses of fat biopsy tissue for longer term investigations would be of interest.
Additionally, this trial produced findings which again support the benefits of a low-fat diet in reducing serum lipids, and helping with weight management via a reduction in energy intake. While the reduction in dietary fat to 25–28.4% of total energy did not translate specifically into favorable outcomes in prostate cancer associated endpoints, since cardiovascular disease is a leading co-morbid factor among men with prostatic carcinoma (2
), this study provides favorable findings for both interventions. However, unlike a low-fat diet which has proven benefit for cardiovascular disease (53
), further studies are needed before we can definitively support flaxseed supplementation as a proven complementary therapy for prostate cancer. To date however, the evidence suggests that flaxseed is: 1) a good, low cost source of select vitamins and minerals, and fiber; 2) is well-accepted and safe to use; and 3) warrants further testing as a preventive or complementary therapy for prostate cancer.
Caution however is warranted in generalizing these findings. Limitations that are specific to the study design (i.e., lack of a placebo-control and lack of power to detect the potential impact of the low fat diet or interactions by study arm or race), the study sample (i.e., over-representation of more highly educated men) or inherent difficulties in conducting prostate cancer research (i.e., the multifocality of prostate cancer or small volume disease) may have influenced our findings. While overcoming these challenges may be difficult, e.g., the creation of a food product that could successfully mask a 30 g. dose of flaxseed, others such as, conducting further research to determine dose-response and additional mechanisms of action, as well as further studies aimed at determining the potential synergy between low fat and flaxseed regimens, or potentially stronger effects among African-Americans is of particular interest.
In summary, this pre-prostatectomy evaluation of the chemopreventive potential of two nutritional interventions utilizes surrogate endpoint biomarkers as primary endpoints. In our study, the modest sample size and short duration, together with the infrequency of cancer recurrence, precluded investigation of clinical cancer endpoints. Furthermore, the development and validation of molecular markers as modifiable surrogates for preferred clinical endpoints remains a work in progress. Although the strength of conclusions drawn from our data is limited by these factors, our study makes several important contributions to clinical intervention trial implementation in cancer prevention. Indeed, the down-regulation of Ki-67, a candidate surrogate for cell proliferation, in the flaxseed-treated arms is highly suggestive of an anti-carcinogenic effect on prostate cancer cells in vivo. Thus, this study serves to generate hypotheses for future larger trials in which flaxseed supplementation can be juxtaposed against prostate cancer recurrence, thereby testing the cancer preventive efficacy of the intervention, as well as contributing to the literature documenting the validity of Ki-67 as a surrogate endpoint biomarker. The current emphasis on biomarker development is thus well-served by this study. Therefore, this study contributes not only to development of nutritional preventive interventions for prostate cancer, but also exemplifies the successful implementation of a study model in which biomarker development is carried-out in a cancer prevention trial that targets accrual within the community setting.