Because the fixed NK cells cannot secrete the cytotoxic granules, it is highly likely that this increased NK cytotoxicity against cancer cells pretreated with the CK2 inhibitors might be mediated by interactions between death ligands on NK surface and death receptors on cancer cell surface. CK2 Ro 48-8071 fumarate sensitizes many cancer cells, such as rhabdomyosarcoma, colon carcinoma, breast malignancy, cervical cancer and gastric cancer to Ro 48-8071 fumarate FasL- and TRAIL-induced apoptosis [21,36,41C43]. However, it has not been exhibited directly whether CK2 inhibition can also enhance the cytotoxicity of killer cells, which actually use the death ligands to kill malignancy cells with either agonistic antibodies or soluble forms of death ligand proteins [22C28]. The resistance of HCCs against death receptor-mediated apoptosis might be an essential step in escaping from host immune surveillance. Interestingly, however, our data showed that HCCs can be sensitized to death ligand-induced apoptosis by inhibiting the CK2 with emodin. CK2 inhibition by emodin appeared to increase rTRAIL-induced cell death significantly in HepG2 and Hep3B (Fig. 1), as well as increase the agonistic monoclonal anti-Fas antibody (CH11)-induced cell death in HepG2 (Fig. 3). MCM2 The death ligand-induced apoptosis was generally proportional to the dose of the CK2 inhibitor (Fig. 2). Furthermore, death ligand-induced apoptotic cell death in the presence of CK2 inhibitor was also correlated with the death receptorexpression level on HCCs. These results suggest that CK2 also plays a critical role in the death receptor-mediated apoptosis of HCCs, as in other malignancy cells. It is well known that NK cells effectively kill many leukaemia cells and that the process is usually mediated primarily by perforin and granzymes [4,9C11]. Perforin induces the necrosis of the target cells and granzymes induce the apoptosis of the target cells [15C17]. More recent studies showed that NK cells can also effectively kill many solid tissue-derived tumour cells and that the non-secretory/apoptotic pathway plays a more important role when NK cells eliminate solid tumour cells [7,8]. The non-secretory/apoptotic pathway is usually mediated by FasL/Fas, TNF/TNF receptor and TRAIL/TRAIL receptor interactions [5,6,8,14C17], and the death receptor-mediated apoptotic pathway is also involved in the cytotoxicity of NK cells against many leukaemia cells [12,18]. Previously, we showed that NK cells can effectively eliminate HCCs, like other solid tumours, and that the death receptor-mediated apoptotic pathway is usually involved in the NK cell-mediated HCC killing . In this study, we demonstrated that this inhibition of CK2 by emodin or TBB augments NK cytotoxicity against HCCs and HeLa cells in the 2 2 h [3H]-thymidine release assay (Fig. 5a and b), which steps primarily the extent of apoptotic target cell death [7,8,24]. Furthermore, we exhibited that this mildly fixed NK cells are as effective as untreated NK cells in the cytotoxicity against target cells pretreated with emodin or TBB (Fig. 6a). Because the fixed NK cells cannot secrete the cytotoxic granules, it is highly likely that this increased NK cytotoxicity against cancer cells pretreated with the CK2 inhibitors might be mediated by interactions between death ligands on NK surface and death receptors on cancer cell surface. This is supported further by the fact that this CK2 inhibition of cancer cells did not increase the NK cytotoxicity against these target cells in the 4 h 51Cr-release assay (Fig. 5c and d), which steps primarily necrotic cell death mediated by cytotoxic granules [7,8,24]. Like NK cells, cytotoxic T cells kill target cells by using the secretory cytotoxic granule-mediated pathway and the death receptor-mediated apoptotic pathway . Because the death receptor-mediated apoptotic target cell killing mechanism of cytotoxic T cells is exactly the same as that of NK cells, it is tempting to speculate that this inhibition of CK2 could also enhance cytotoxic T cell-mediated target cell killing. This suggests that CK2 inhibitors could increase the host’s innate and adaptive immunity against cancer by Ro 48-8071 fumarate enhancing the cytotoxicity of NK cells and cytotoxic T cells which are armed with death ligands. Acknowledgments This study was supported in part by a faculty research grant of Yonsei University College of Medicine for 2006 (No. 2006-0052) and by a medical research centre grant (R13-2002-054-02002-0) of the KOSEF..