Tumor is one of the major challenges that modern medicine must face. In order to find effective diagnosis and treatment methods, researchers actively explore the process and molecular basis of its occurrence and development. It is worth noting that some biomolecules found in the research process that the role of the tumor is not simply to promote or inhibit the tumor, but to play a diametrically opposite function as the environment or tumor development changes.
CD95 (also known as Fas and APO-1) is a classic death receptor, mainly by inducing apoptosis to maintain tissue homeostasis in the immune system. Existing studies have shown that during cancer progression, CD95 is often down-regulated in tumor cells or makes cells resistant to apoptosis, thereby allowing tumor cells to escape apoptosis. However, complete loss of CD95 is rarely seen in human cancer. Many cancer cells express large amounts of CD95 in vitro and are highly sensitive to CD95-mediated apoptosis. In addition, cancer patients often have elevated levels of CD95 and CD95L. These data indicate that CD95 may have its non-apoptotic activity to promote tumor growth.
To test the function of endogenous CD95 in tumor cells, CD95-specific short hairpin (sh) RNA lentivirus was used to reduce the expression of CD95 in various human cancer cell lines. Infection with CD95-mediated lentiviral shRNA to the CD95-mediated apoptosis in vitro CD95 overexpressing ovarian cancer cell line HeyA8 greatly reduced the CD95 protein and surface expression, resulting in decreased CD95 apoptosis sensitivity. Interestingly, the abolition of CD95 expression also resulted in a substantial reduction in cancer cell growth. This has been confirmed by another shRNA targeting CD95 (R4) and another ovarian cancer cell line SKOV3.ip1, which expresses a very small amount of CD95 and is completely resistant to CD95L-induced apoptosis. Rebuilding SKOV3.ip1 CD95 knockdown cells and short-term anti-interference (si) RNA CD95 cells to endogenous levels can restore the growth of SKOV3.ip1 cells. In addition, growth inhibition of CD95 expression was also found in cell lines derived from colon cancer (HCT116), renal cancer (CAKI-1), breast cancer (MCF7), and liver cancer (HepG2). It is worth noting that 6 days after lentivirus infection, knockdown of CD95 in MCF7 cells with R6 virus resulted in growth arrest. However, 26 days after infection, the growth of cells expressing moderate CD95 levels was partially restored. However, neither MCF7 nor CAKI-1 cells benefited from the overexpression of CD95, indicating that regardless of absolute levels, cancer cells maintain CD95 expression at a level sufficient to promote optimal growth based on CD95 expression.
Further research shows that stimulation of CD95 in 22 tumor cell lines does not lead to increased proliferation. However, incubating cells with neutralized CD95L monoclonal antibody (mAb) NOK-1 reduced cell growth, indicating that the small amount of CD95L produced by tumor cells can help their growth. To further test this hypothesis, the researchers used three independent shRNAs specific to CD95L based on lentivirus to knock down CD95L. As a result, it was found that when the knockout efficiency was comparable, these viruses caused different degrees of growth inhibition, from reduced growth to complete loss of growth. In addition, knocking out CD95L will also reduce the growth of all other cancer cell lines, suggesting that CD95L is essential for the growth of many tumor cells.
In short, the CD95/CD95L system can promote the growth of cancer. CD95 activates neuronal stem cells and acts as a tumor promoter for glioblastoma by activating Src kinase. In addition, it was recently discovered that mice expressing only soluble CD95L can induce large histiocytic sarcoma in the liver, which may be due to the lack of apoptosis induction and the carcinogenic activity of CD95L. The data also shows that CD95 plays a role in promoting growth mainly through the pathways involving JNK, Jun, Erg1 and Fos.