Abstract
3471
Conjugated linoleic acid (CLA), an essential ω-6 polyunsaturated fatty acid, has been shown to exhibit numerous beneficial physiological effects including reduction of adiposity, modulation of immune function, and suppression of tumor development and progression in several models of carcinogenesis. Although studies over the last decade have clearly shown CLA to possess potent anti-carcinogenic properties, the molecular and cellular mechanisms through which CLA exerts its effect is not well understood. The chemotherapy drug 5-fluorouracil (5-FU) is the main stay of colon cancer treatment; however, the impact of dietary fats such as CLA on standard chemotherapeutic regimens has been vastly under-explored. Therefore, the focus of our project is to determine the effect of CLA on 5-FU mediated anti-tumor effects in the HT-29 colon cancer cell line. Our preliminary studies demonstrate that CLA induced growth inhibition through anti-proliferative and apoptosis promoting activity in the HT-29 colon cancer cell line, a cell line that contains a number of genetic abnormalities consistent with colorectal cancer including over-expression of cyclooxygenase-2 (COX-2). Our previous results showed CLA treatment down-regulated COX-2 expression (20-30% decrease) followed by suppression of PGE2 release (11-45% decrease), a prostaglandin formed from the metabolism of arachidonic acid (AA) by COX, yet new data from our lab shows that 15-delta-PGJ2 (another AA metabolite) levels were significantly increased (2-5 fold) as compared to controls. In addition, treatment of HT-29 cells with CLA results in an increase in caspase-3 activity (8-fold increase), a late endpoint marker of apoptosis, as compared to controls. Flow cytometry results show a 14-fold increase in the numbers of HT-29 cells undergoing apoptosis after treatment with CLA. CLA also inhibits bcl-2, an anti-apoptotic protein, and induces protein expression of bax, a pro-apoptotic protein. HT-29 cells are fairly resistant to 5-FU treatment as 5-FU treatment only results in a 20% decrease in cell death. However, when HT-29 cells are treated with CLA and 5-FU there is a slight increase in 15-delta-PGJ2 levels (12%) and capase-3 activity is increased 2-fold, as compared to treatment with CLA or 5-FU alone. Based on these observations, we hypothesize that the dietary fatty acid CLA, a potent anti-tumorigenic agent, alters both baseline rates and potentiates 5-FU-mediated anti-tumor efficacy by altering rates of tumor cell apoptosis and proliferation, via modulation of eicosanoid signaling pathways. We predict that CLA may selectively sensitize colon cancer cells to undergo cell death via shifts in pro- and anti-carcinogenic eicosanoids, which in turn may “prime” these cells to become more responsive to 5-FU-induced apoptosis. Future work will focus on the genetic pathways that lead to the increased sensitivity to 5-FU in the presence of CLA.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]