Fas is a known inducer of apoptosis and is important in the regulation of several aspects of the immune system, including cytotoxic killing of cells potentially harmful to the organism such as virus-infected or tumor cells. Pitti et al. described a new Fas decoy receptor, DcR3, within the TNF receptor Superfamily.1 Despite low homology with Fas, DcR3 competes for binding of Fas ligand (FasL) with similar affinity as Fas, thus demonstrating that DcR3 can inhibit FasL-induced apoptosis.
Unlike previously described decoy receptors DcR1 and DcR2, which bind TRAIL/Apo2L, DcR3 has no transmembrane sequence and is a secreted molecule. The fourth known decoy receptor, osteoprotegerin (OPG), is also a secreted molecule. DcR3 and OPG represent a subgroup of secreted members of the TNF receptor family. DcR3 shares greater homology with OPG (31%) than with DcR1 (17%), DcR2 (19%) or Fas (17%). Unlike DcR1, DcR2, and OPG, DcR3 does not bind TRAIL/Apo2L, TWEAK/Apo3L, TRANCE/OPGL, or TNF-alpha.
Fas induces apoptosis when bound by FasL. Fas-induced apoptosis is important in the removal of cancer cells by cytotoxic T cells and NK cells which express FasL. DcR3 blocking of this cytotoxicity may enable some tumors to escape this cytotoxic attack. Quantitative PCR from genomic DNA has been used to show elevated DcR3 gene copy number in some primary tumors, including lung and colon. This has been confirmed by in situ hybridization. Collectively, these results suggest that Fas/FasL interactions can limit tumor growth and cells expressing high levels of DcR3 are more likely to become cancerous.
Although Fas regulates the immune response, little is known about the regulation of FasL function. Two possible mechanisms include regulation of cFLIP (I-FLICE, FLAME, CASH) a cytoplasmic mediator of Fas signaling, or shedding of Fas. The novel decoy receptor DcR3 represents a third regulatory mechanism of FasL-induced apoptosis.