DICE: A novel tumor surveillance mechanism-a new therapy for cancer?

Mechanisms of Apoptosis Resistance to NK Cell-Mediated Cytotoxicity in Cancer

Natural killer (NK) cells are major contributors to immunosurveillance and control of tumor development by inducing apoptosis of malignant cells. Among the main mechanisms involved in NK cell-mediated cytotoxicity, the death receptor pathway and the release of granules containing perforin/granzymes stand out due to their efficacy in eliminating tumor cells. However, accumulated evidence suggest a profound immune suppression in the context of tumor progression affecting effector cells, such as NK cells, leading to decreased cytotoxicity. This diminished capability, together with the development of resistance to apoptosis by cancer cells, favor the loss of immunogenicity and promote immunosuppression, thus partially inducing NK cell-mediated killing resistance. Altered expression patterns of pro- and anti-apoptotic proteins along with genetic background comprise the main mechanisms of resistance to NK cell-related apoptosis. Herein, we summarize the main effector cytotoxic mechanisms against tumor cells, as well as the major resistance strategies acquired by tumor cells that hamper the extrinsic and intrinsic apoptotic pathways related to NK cell-mediated killing.

Site-Specific Detection of Tyrosine Phosphorylated CD95 Following Protein Separation by Conventional and Phospho-Protein Affinity SDS-PAGE

Phosphorylation of two tyrosines in the death domain of CD95 is a critical mechanism in determining the receptor's choices between cell death and survival signals. Recently, site-specific monoclonal antibodies against phosphorylated tyrosines of CD95 have been generated and used to successfully detect each phosphorylated death domain tyrosine of CD95 directly and separately by immunoblotting. Here we provide detailed protocols and useful tips for a successful site-specific detection of phosphorylated death domain tyrosine of CD95 following a protein separation by sizes (conventional SDS-PAGE) and by degrees of phosphorylation (phospho-protein affinity, mobility shift SDS-PAGE).

The role of CD95 and CD95 ligand in cancer

CD95 (Fas/APO-1) and its ligand, CD95L, have long been viewed as a death receptor/death ligand system that mediates apoptosis induction to maintain immune homeostasis. In addition, these molecules are important in the immune elimination of virus-infected cells and cancer cells. CD95L was, therefore, considered to be useful for cancer therapy. However, major side effects have precluded its systemic use. During the last 10 years, it has been recognized that CD95 and CD95L have multiple cancer-relevant nonapoptotic and tumor-promoting activities. CD95 and CD95L were discovered to be critical survival factors for cancer cells, and were found to protect and promote cancer stem cells. We now discuss five different ways in which inhibiting or eliminating CD95L, rather than augmenting, may be beneficial for cancer therapy alone or in combination with standard chemotherapy or immune therapy.

CD95 and CD95L promote and protect cancer stem cells

CD95 (APO-1/Fas) is a death receptor used by immune cells to kill cancer cells through induction of apoptosis. However, the elimination of CD95 or its ligand, CD95L, from cancer cells results in death induced by CD95R/L elimination (DICE), a type of cell death that resembles a necrotic form of mitotic catastrophe suggesting that CD95 protects cancer cells from cell death. We now report that stimulation of CD95 on cancer cells or reducing miR-200c levels increases the number of cancer stem cells (CSCs), which are more sensitive to induction of DICE than non-CSC, while becoming less sensitive to CD95-mediated apoptosis. In contrast, induction of DICE or overexpression of miR-200c reduces the number of CSCs. We demonstrate that CSCs and non-CSCs have differential sensitivities to CD95-mediated apoptosis and DICE, and that killing of cancer cells can be maximized by concomitant induction of both cell death mechanisms.