Study On The Anticancer Activities Of Honokiol In Human Colorectal Cancer RKO Cells

The improved understanding of oncogenesis and the involvement of oncogenes and tumor suppressor genes, has led to a shift from a single cytotoxicity to a specific target-directed anti-cancer drug development. Cancer genes have been found to be important not only in the control of cell proliferation but also in the mediation of processes such as apoptosis, angiogenesis, drug resistance, metastasis, and differentiation. And the target-directed anticancer agents are signaling transducer, angiogenesis inhibitors, mult-drug resistance reversing agents, metastasis inhibitors, and differentiation promoters.On screening of anticancer compounds from the naturally-occuring products, we found honokiol (C18H18O2, MW= 266.33), an active component isolated from the herb 'Houpo' (Magnolia officinalis) displayed potential anticancer activities by targeting more than one point of signaling pathways of cancer development. With a pair of hydroxy groups (-OH) in its structure, honokiol, for the first time, was proved to be an effective anti-oxidant agent[1] It induced the apoptotic cell death in some types of human cancer cells[2-4], but it is still unclear whether honokiol induced apoptosis is mediated by the important cancer suppressor gene p53.In the first part of this paper, the molecular mechanism of honokiol-mediated apoptotic process was examined in human colorectal cancer RKO cells. Here, we found that honokiol-induced apoptotic cell death was accompanied by up-regulation of Bid, down-regulation of Bcl-XL, and activation of caspases. Proteolytic activation of caspase-3 and cleavage of caspase-7 were observed in honokiol-treated RKO cells. However, honokiol had no effect on the levels of p53, Bcl-2 and Bax proteins. Furthermore, it had similar cytotoxicity to three different p53 expressing cells. In conclusion, modulation of BC!-XL andBid proteins, activation of caspase cascades participated in honokiol-triggered apoptotic process in human colorectal cancer RKO cells. This is a p53 -independent apoptosis.Besides its cytotoxicity activity, Bai X reported that honokiol blocked VEGF's growth signal in endothelial cells[4]. In this study, we found that honokiol had the potential to prevent angiogenesis by targeting HIF-1. HIF-1 is a key modulator in the process of tumor angiogenesis[5]. It can be induced by hypoxia or chemical hypoxia mimicked by CoCl2[6]. We showed in the second part of this paper that, honokiol (0.1-lug/ml) down-regulated CoCl2 (150uM) induced HIF-1 a accumulation, and thus inhibited its target VEGF expression. In addition, honokiol demonstrated to inhibit HIF-1 protein degradation by down-regulating iNOS and HSP70, the former mediating the VHL-dependent HIF-1 degradation, and the latter preventing HIF-1 a degradation by non VHL-dependent pathway. We concluded in this part that, honokiol inhibits tumor angiogenesis by targeting HIF-1 in human colorectal cancer RKO cells.Besides tumor angiogenesis, hypoxia also mediates tumor resistance to radiotherapy and chemotherapy[7]. Drug resistance is generally accepted as an important cause of treatment failure for patients with neoplastic diseases. Molecular mechanisms underlying drug resistance include the action of drug efflux pumps belonging to the super-family of ATP binding cassette (ABC) proteins, such as P-glycoprotein (P-gp), and MDR-associated protein (MRP), which mediate the cellular extrusion of a large variety of therapeutic drugs, a phenotype that is referred to as multidrug resistance (MDR)[8]. P-gp is encoded by mdrl gene. Overexpression of P-gp or enhancement of its function, confers resistance to a variety of structurally and functionally unrelated antitumor drugs[9l According to recent report, mdrl is one of the direct target genes of HIF-1[10] We have concluded in part II that, honokiol can inhibit HIF-1 a protein accumulation and thus down-regulating its target genes in human colorectal cancer RKO cells. Does it mean honokiol has the potential in suppressing tumor MDR? To answer this question, we designed several experiments with a d...