Exogenous and autocrine growth factors significantly reduce the anti-proliferative and pro-apoptotic effects of curcumin on normal and cancerous epithelial cells

812 INTRODUCTION: Curcumin, a natural dietary substance, has been established as a chemoprotective agent that inhibits colon carcinogenesis. In contrast, growth factors such as IGF-II and progastrin (PG) have been shown to exert proliferative and anti-apoptotic effects on both normal and cancerous intestinal epithelial cells. The effects of these growth factors on the inhibitory potency of curcumin have not been examined. Since IGF-II and PG are expressed by the majority of colon carcinomas, we examined the inhibitory efficacy of curcumin on colon cancer cells in the presence of these two relevant growth factors. METHODS: Colon carcinoma (Caco-2) cells express high levels of autocrine IGF-II during their logarithmic growth phase (days 2-5 in culture). Expression of IGF-II rapidly declines as the cells begin to differentiate (days 6-11 in culture). Based on this pattern of expression, Caco-2 cells were treated with curcumin (25-100uM) after 3, 5, 7, or 9 days in culture to determine if the inhibitory efficacy of curcumin correlates with the relative expression of autocrine IGF-II. Rates of proliferation and apoptosis were quantified by cell cycle analysis. Non-transformed intestinal epithelial (IEC-18) cells do not produce autocrine growth factors, but do proliferate in response to exogenous IGF-II or PG. IEC-18 cells were treated with an optimal dose of curcumin (25uM) in the presence or absence of either PG (0.1nM) or IGF-II (10nM). Relative rates of apoptosis were measured by western blot analysis of activated caspase 9, while proliferation was determined by a count of the total number of viable cells. RESULTS: Surprisingly, curcumin was ineffective at inducing apoptosis in Caco-2 cells treated on days 3 and 5 but exerted increasing apoptotic effects in Caco-2 cells on days 7 and 9. The increase in apoptosis suggested a correlation with the naturally declining expression of IGF-II in Caco-2 cells after day 6. In IEC-18 cells, curcumin treatment decreased proliferation and increased apoptosis within 24 hours. However, in the presence of either exogenous PG or IGF-II, the protective effects of curcumin were significantly inhibited. IGF-II was approximately 3 times more effective at interfering with curcumin than was PG. CONCLUSIONS: These unexpected results demonstrate that the protective effects of curcumin can be greatly diminished by the presence of growth factors. Moreover, IGF-II is more effective than PG at overcoming the anti-proliferative and pro-apoptotic effects of curcumin. Thus, the efficacy of curcumin in a patient’s diet may be dictated by the growth factor profile of the carcinoma.