Curcumin, a plant polyphenol derived from the spice turmeric, possesses anti-cancer and anti-inflammatory properties, and can be safely ingested with a low toxicity profile. Curcumin decreases cellular proliferation by inhibiting a variety of molecular targets, including Akt, a downstream effector of phosphatidylinositol 3-kinase (PI3K). However, the effects of curcumin on the related growth and survival signal, mammalian target of rapamycin (mTOR), have not been investigated in colon cancer cells. The purpose of this study was to: (i) determine the sensitivity of certain colon cancer cells to the antiproliferative effects of curcumin, and (ii) examine the effects of curcumin on the PI3K-mTOR pathway. METHODS. (i) Human colon cancer cells HT29 and KM20 were treated with varying doses of curcumin (10-200 μM) for 24, 48, or 72 h and cell proliferation was measured. (ii) Cytotoxicity was assessed using a colorimetric assay for lactate dehydrogenase (LDH), which is released from cells when their plasma membrane is damaged. (iii) Western blotting was used to detect changes in protein expression of the epidermal growth factor receptor (EGFR), PI3K catalytic and regulatory subunits (p110α and p85α, respectively), phosphorylated Akt, phosphorylated mTOR, raptor and rictor (subunits required for assembly of mTOR complexes 1 and 2, respectively), p70S6K, and 4EBP1.
Results: . (i) Cell viability after 48 h of treatment with a moderate dose of curcumin (25 μM) was 32% and 75% in KM20 and HT29 cells, respectively. (ii) Cytotoxicity after 16 h treatment with curcumin (50 μM), was 120% for KM20 and 5% for HT29 cells. (iii) Expression of mTOR, rictor and raptor subunits was decreased after 2 h treatment with low doses of curcumin (5-20 μM). Protein expression of EGFR and the PI3K subunits was decreased after overnight treatment with high-dose curcumin (100 μM). Expression of pAkt, p70S6K and 4EBP1 was also decreased in most cells after curcumin treatment.
Conclusions: . We show, for the first time, that curcumin decreases protein expression of raptor and rictor, proteins required for the assembly of mTOR complexes 1 and 2. The inhibition of mTOR and PI3K subunits by curcumin, in addition to its established effects on Akt activation, is likely to contribute to the anticancer activity in colorectal cancer.