The anti-cancer activity of the natural product, fucoidan, in haematological malignancies

Leukaemia is the most common form of haematological malignancy and includes a group of diseases characterised by an uncontrolled growth of malignant haematopoietic cells. It accounts for about 33% and 3% of cancer cases in children and adults, respectively. Worldwide, approximately 350,000 new cases of leukaemia are diagnosed each year, with more than 250,000 deaths. Acute myeloid leukaemia (AML) is the most common form of acute leukaemia, accounting for around 85% of adult cases, and a leading cause of cancer death in young adults. Although chemotherapy is often effective in the treatment of AML, at least in the short term, the agents currently in use are associated with a wide range of side effects including increasing the risk of developing therapy-related cancers. Therefore, there is considerable interest in the anti-cancer potential of natural agents with lower toxicities. Fucoidan is a fucose-rich sulphated polysaccharide that exists in the cell wall matrix of brown seaweeds. This component has shown immunomodulatory and anti-tumour activities. However, the underlying mechanisms of these activities remain largely unknown. Herein, the anti-tumour activities of fucoidan were examined in in vitro and in vivo models of AML, and its activity as a potential adjunct therapy was investigated. Investigation of the effects of fucoidan on leukaemic cells (NB4; a t(15;17) positive acute promyelocytic leukaemia, KG-1a; a minimally differentiated AML, HL60; a t(15;17) negative acute promyelocytic leukaemia and K562; an acute erythroleukaemia cell line) revealed evidence that fucoidan has a selective inhibitory effect on acute promyelocytic leukaemia (APL) and not other types of AML cells, initiating apoptosis. Examination of the pathways mediating the observed apoptotic mechanisms revealed the observed effect was caspase-dependent as it was significantly attenuated by pre-treatment with a pan-caspase inhibitor. P21/WAF1/CIP1 was significantly up-regulated leading to cell cycle arrest. Fucoidan decreased the activation of ERK1/2, and down-regulated the activation of AKT through hypo-phosphorylation of Thr(308) residue but not Ser(473). The anti-tumour activity of fucoidan was supported by in-vivo evidence which demonstrated that oral doses of fucoidan significantly delayed tumour growth in the APL xenograft model in athymic Balb/c nude mice, potentially by increasing the cytolytic activity of NK cells. This is the first study to reveal the anti-tumour activity of fucoidan on leukaemia in vivo. The selective inhibitory effect of fucoidan on APL cells and its protective effect against APL development in mice may prove that fucoidan may be useful in treatment of certain types of leukaemias. APL is one of the most aggressive types of AML characterised by differentiation arrest and accumulation of abnormal promyelocytes. Current APL therapies are associated with various side effects such as hyper-leucocytosis and differentiation syndrome which occurs in a quarter of the patients and is a serious and potentially fatal complication. There is therefore interest in the possibility of reducing the incidence of these morbidities through the use of adjunctive therapies which permit lower doses of toxic therapies to be used whilst maintaining efficacy. Herein, the synergistic effects of fucoidan on current APL therapy, arsenic trioxide (ATO) and all-trans retinoic acid (ATRA), were investigated. In vitro, the effect of fucoidan combined with both therapeutic and lower doses of these drugs was examined in APL cells. Fucoidan in combination with ATO enhanced apoptosis in APL cells at both therapeutic and lower doses of ATO as indicated by an increased sub G0/G1 population, DNA fragmentation and annexin V positive apoptotic cells. Furthermore, the combination of fucoidan with low doses of ATRA and ATO significantly enhanced cell differentiation as indicated by G0/G1 arrest and increased CD11b expression. The triple combination of these agents resulted in the greatest myeloid differentiation in APL cells compared to single or double combinations. The efficacy of fucoidan as an adjuvant to the anti-leukaemic activity of ATO or ATRA was identified in vivo in athymic Balb/c nu/nu mice bearing APL. Tumour growth was monitored by measurement of tumour size and survival. When fucoidan and sub-therapeutic doses of ATO were administered in APL-bearing mice, the median survival and tumour volume doubling time significantly increased in mice treated with fucoidan alone and combined with ATO but not ATO alone compared to the control group. When fucoidan plus low dose ATRA were administered as the therapy regimen in APL-bearing mice, the median survival and tumour volume doubling time significantly increased in all treated groups compared to the control group. Moreover, the differentiation of APL cells obtained from animal's tumour mass significantly increased in mice treated with fucoidan alone and fucoidan+ATRA but not ATRA alone compared to the control group. A further novel finding was that the differentiated APL cells derived from the excised tumour mass exhibited a down regulation of CD44 in fucoidan+ATRA treated mice. The presence of differentiated leukaemic cells with low or no expression of CD44 may be associated with decreased migration of these cells in APL. Taken together these findings provide important evidence that fucoidan may prove an effective adjuvant therapeutic agent in the treatment of selected leukaemia sub-type APL and may permit lower doses of ATO and ATRA to be employed to achieve better efficacy of treatment accompanied by lower toxicity.