The In Vitro Anti-cancer Effects Of Curcumin And Its Influence On The IGF-1 System In Human Cancer Cell Lines

Curcumin (diferuloylmethane), isolated from the rhizome of the plant Curcuma Longa Lin,is a natural compound present in turmeric. It is extensively used as a dietary spice andpigment in Asian. Curcumin exhibits potent anti-cancer activities and chemopreventiveproperties in various cell lines and animal models. Studies have shown that curcumin preventscancer in many tissues of mice and rats, and has been associated with regression ofestablished solid malignancies in humans. Further studies have revealed that curcuminmediates its anti-cancer effects by modulation of several important molecular targets,including transcription factors, enzymes, cell cycle proteins and receptors. However, themechanisms of action for curcumin are not well understood. Contradictory results have alsobeen obtained.The insulin-like growth factor-1 (IGF-1) system, including IGFs (IGF-1 and IGF-2),IGF-1R (IGF-1 receptor) and IGFBPs (IGF binding proteins), has been implicated to play acritical role in cancer etiology and the development of human cancer. The aim of the presentstudy is to investigate whether the growth inhibitory effects of curcumin are related tochanges of the IGF-1 system and to elucidate the cellular and molecular mechanisms bywhich curcumin inhibits the IGF-1 pathway in human cancer cell lines.In this study, the inhibitory effects of curcumin on various human cancer cell lines such asHT-29, MCF-7, PC-3, Hela cells and normal vascular endothelial cell ECV-304 wereexamined. The results show that treatment with curcumin resulted in a dose andtime-dependent inhibition of all these 5 cells lines. The results of electron micrograph, flowcytometry with only PI-staining and with Annexin-Ⅴconjugated with PI staining show thatcurcumin induces apoptosis of HT-29 in a dose-dependent maner. Curcumin exposure causedmembrane potential decrease, with cytochrome C releases from mitochondria to cytosol, anddecresed Bcl-2 protein level with the increasing of Bax protein, and activation of Caspase-3.It is concluded that the hypopolarization of mitochondria is an early event ofcurcumin-induced apoptosis and this initiated the mitochondrial pathway of apoptosis inducedby curcumin.Further Experiments were used to evaluate whether the anti-cancer effects of curcuminwere related to the IGF-1 system. IGF-1 (50μg/L) in serum free medium produced maximum proliferation and minimized apoptosis. However, curcumin exhibited a potent ability to bluntIGF-1-stimulated HT-29 and MCF-7 cell growth and reverse the IGF-1-induced apoptosisresistance. To determine whether curcumin intervenes IGF-1 or IGFBP-3 secretion, HT-29and MCF-7 cells were incubated in serum free medium in the presence of variousconcentrations of curcumin for indicated time periods. As a result, curcumin decreased thesecretion of IGF-1 with a concomitant increase of IGFBP-3 in a dose-dependent manner. Thedata further demonstrated that curcumin reduced endogenous IGF-1 production at bothmRNA and protein level, which were involved in decreased IGF-1 secretion into the culturemedium supematant as determined by radioimmunoassay.IGF-1R has been identified as a general and promising target for anticancer therapy.Curcumin decreased IGF-1R transcripts and IGF-1R proteins in a dose-dependent manner.Receptor tyrosine kinase assays revealed that IGF-1-stimulated IGF-1R tyrosine kinaseactivation was also abrogated by curcumin. Further, treatment with IGF-1 results in a rapidand sustained phosphorylation of IGF-1R in HT-29 cells, whereas this phosphorylation effectwas inhibited by curcumin in a dose-dependent manner. In addition, curcumin treatment inHT-29 cells substantially inhibited IGF-1-mediated Akt and Erk1/2 phosphorylation.The peroxisome proliferator-activated receptor-γ(PPAR-γ), a ligand-dependenttranscription factor belonging to the family of nuclear receptors, has been implicated in theregulation of cell growth and differentiation. Curcumin causes upregulation of PPAR-γprotein. The antagonist to PPAR-γ, GW9662, pretreatment abrogated curcumin-induced cellgrowth inhibition and reduction of IGF-1R.In conclusion, this study demonstrates that curcumin could not only inhibit cell growth andinduce apoptosis in HT-29 and MCF-7 cells, but also blunt IGF-1-stimulated HT-29 andMCF-7 cell growth and apoptosis resistance, and these effects may be mediated, at leastpartially, by suppression of IGF-1 system and IGF-1R-mediated signaling pathway.Furthermore, curcumin-induced suppression of the gene expression of IGF-1R may bepartially mediated by ativation of PPAR-γpathway. These results provided not only a novelinsight into the roles and mechanisms of curcumin in inhibition of cancer cell growth, butpotential therapeutic strategies for treatment of human cancer.