Carcinogenesis Advance Access originally published online on August 20, 2009
Carcinogenesis 2009 30(10):1670-1677; doi:10.1093/carcin/bgp188
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Oncogenic Ras, but not V600EB-RAF, protects from cholesterol depletion-induced apoptosis through the PI3K/AKT pathway in colorectal cancer cells
Department of Biochemistry and Molecular Biology, Faculty of Medicine, University of Alcalá, E-28871 Alcalá de Henares, Madrid, Spain
1 Present address: Department of Physiology, University of Alcalá, E-28871 Alcalá de Henares, Spain
* To whom correspondence should be addressed. Tel: 0034 918854583; Fax: 0034 918854585; Email: antonio.chiloeches{at}uah.es
Cholesterol is necessary for proliferation and survival of transformed cells. Here we analyse the effect of cholesterol depletion on apoptosis and the mechanisms underlying this effect in colorectal cancer cells carrying oncogenic Ras or V600EB-RAF mutations. We show that chronic cholesterol depletion achieved with lipoprotein-deficient serum (LPDS) and 25-hydroxycholesterol (25-HC) treatment results in a significant increase in apoptosis in HT-29 and Colo-205 cells containing the V600EB-RAF mutation, but not in HCT-116 and LoVo cells harbouring the G13DRas mutation, or BE cells, which possess two mutations, G13DRas and G463VB-RAF. We also demonstrate that oncogenic Ras protects from apoptosis induced by cholesterol depletion through constitutive activation of the phosphatidylinositol-3 kinase (PI3K)/AKT pathway. The specific activation of the PI3K/AKT pathway by overexpression of the V12RasC40 mutant or a constitutively active AKT decreases the LPDS plus 25-HC-induced apoptosis in HT-29 cells, whereas PI3K inhibition or abrogation of AKT expression renders HCT-116 sensitive to cholesterol depletion-induced apoptosis. Moreover, our data show that LPDS plus 25-HC increases the activity of c-Jun N-terminal kinase proteins only in HT-29 cells and that the inhibition of this kinase blocks the apoptosis induced by LPDS plus 25-HC. Finally, we demonstrate that AKT hyperactivation by oncogenic Ras protects from apoptosis, preventing the activation of c-Jun N-terminal kinase by cholesterol depletion. Thus, our data demonstrate that low levels of cholesterol induce apoptosis in colorectal cancer cells without oncogenic Ras mutations. These results reveal a novel molecular characteristic of colon tumours containing Ras or B-RAF mutations and should help in defining new targets for cancer therapy.
Abbreviations: FACS, fluorescence activated cell sorting; FBS, fetal bovine serum; 25-HC, 25-hydroxycholesterol; JNK, c-Jun N-terminal kinase; LPDS, lipoprotein-deficient serum; MAPK, mitogen-activated protein kinase; MβCD, methyl-β-cyclodextrin; PARP, Poly (ADP-ribose) polymerase; PI3K, phosphatidylinositol-3 kinase
Received February 6, 2009; revised June 16, 2009; accepted July 25, 2009.