Abstract. Although a number of studies have shown that vitamin K possesses antitumor activities on various neoplastic cell lines, there are few reports demonstrating in vivo antitumor effects of vitamin K, and the antitumor effect on colorectal cancer (CRC) remains to be examined. Therefore, antitumor effects of vitamin K on CRC were examined both in vitro and in vivo. Vitamins K2, K3 and K5 suppressed the proliferation of colon 26 cells in a dose-dependent manner, while vitamin K1 did not. On flow cytometry, induction of apoptosis by vitamins K2, K3 and K5 was suggested by population in sub-G1 phase of the cell cycle. Hoechst 33342 staining and a twocolor flow cytometric assay using fluorescein isothiocyanateconjugated annexin V and propidium iodide confirmed that vitamins K2, K3 and K5 induced apoptotic death of colon 26 cells. Enzymatic activity of caspase-3 in colon 26 cells was significantly up-regulated by vitamins K2, K3 and K5. The pan-caspase inhibitor, benzyloxycarbonyl-Val-Ala-Aspfluoromethyl ketone, substantially prevented vitamin Kmediated apoptosis. In vivo study using syngeneic mice with subcutaneously established colon 26 tumors demonstrated that intravenous administration of vitamins K2, K3 and K5 significantly suppressed the tumor growth. The number of apoptotic tumor cells was significantly larger in the vitamin K-treated groups than in the control group. These results suggest that vitamins K2, K3 and K5 exerted effective antitumor effects on CRC in vitro and in vivo by inducing caspasedependent apoptotic death of tumor cells, suggesting that these K vitamins may be promising agents for the treatment of patients with CRC.
IntroductionWorldwide, colorectal cancer (CRC) ranks third in cancer incidence after lung and breast, and is a major cause of cancer mortality (1). In 2000, there were 943,000 new cases diagnosed and 510,000 deaths worldwide, corresponding figures for Europe were 363,000 cases and 199,000 deaths (2). Although surgery alone is the standard approach in localized malignancy, approximately 50% of early-stage patients present disease relapse (3). Furthermore, approximately 30% of CRC is diagnosed when it is already at an advanced stage. There are also a considerable number of patients who cannot undergo surgery even at an early stage of the disease due to severe complications, such as chronic heart failure, chronic renal failure and chronic obstructive pulmonary diseases. Fluoropyrimidine-based chemotherapy is now considered the standard treatment for patients with inoperable CRC, but approximately 90% of patients do not respond to chemotherapy based on 5-fluorouracil (5-FU)/folinic acid (4). Thus, the development of new treatment modalities is necessary to improve the overall survival rate of patients with CRC.Vitamin K is an essential vitamin that was discovered as a fat-soluble anti-hemorrhagic agent. Vitamin K is also known as an obligatory cofactor in the post-translational γ-carboxylation of glutamic acid residues (5-8). Vitamin K-dependent proteins include prothrombin (fact...
