Epidemiological and animal studies provide evidence that diets rich in omega-3 PUFAs (fish oil) reduce the risk of human cancer. Although cyclooxygenase-2 (COX-2) inhibitors are effective at high doses against colon and prostate carcinogenesis, administration of these agents over a long period of time may induce side effects. Experimental evidences reveal that omega-3 fatty acids in combination with COX-2 inhibitors reduce colon and prostate cancer effectively and minimize major side effects. However, mechanisms underlying the combined actions of these agents, which differ in their modes of action, are not well understood. Our goal was to determine the proteomic alterations mediated by the combined actions of these agents at low doses, against prostate carcinogenesis. We have separated soluble proteins from rat prostate cancer cells (I-26) treated with DHA or celecoxib individually (5μM), or in combination (2.5μM each) for 48h, using 2D gel electorpphoresis. Proteins differentially expressed were identified using mass spectrometry and Western blot analysis to confirm the identified proteins. We further validated the protein expression using immunohistochemical and immunofluorescence localization techniques. Paraffin-embedded tissue sections of prostate adenocarcinomas were used to compare the proteins expressed in cancer cells. The cancer cell growth was inhibited by low doses of DHA in combination with celelcoxib (2.5μM each) more effectively (p<0.01). The protein expression pattern varied with different treatment. A total number of 112 proteins were differentially expressed with the combined treatment of DHA and celecoxib compared to treatment with individual agents. The protein profiling indicated significant differences in the expression levels of twelve key proteins which are known to be associated with protein folding, morphogenesis, cell growth inhibition, apoptosis and differentiation. Our study revealed for the first time, an interesting observation pertaining to the dual role of molecular chaperone proteins, such as HSP70 and HSP90 against prostate carcinogenesis. Further, it is clear from our findings that rat prostate cancer cells exposed to the DHA in combination with celecoxib at low doses enhanced the expression of nuclear p53. The information on HSPs inducing cancer preventive effects in cor roboration with functional p53 is very significant and is a first time observation. Studies are in progress to see a similar effect in cancer cells with mutated p53. The biological and functional significance of the enhanced expression of HSPs induced by the combined actions of DHA and celecoxib and their role during apoptosis and differentiation process in the presence of wild-type p53 are discussed (Supported by NCI CA106296-01, and CA107813-01 grants).
[Proc Amer Assoc Cancer Res, Volume 47, 2006]