The recent discovery of posttranscriptional and posttranslational mechanisms of NKG2D ligand regulation has added more complexity to the picture (). IFN-γ was reported to decrease the expression of human MICA and ULBP-2 on melanoma cells (15
) and of the mouse NKG2D ligand H60 on sarcomas (16
). An IFN-γ–inducible microRNA was shown to mediate the down-regulation of MICA (17
). MICA and MICB can also be shed from tumor cells by metalloproteinases (18
) in a process that requires the cell surface endoplasmic reticulum 5 protein (ERp5) (20
). Soluble MICA/B and ULBP-2 can be detected in the serum of cancer patients, but not in healthy individuals, suggesting that tumors may exploit down-regulation of NKG2D ligands to escape from NK cell recognition.
Intriguingly, transcripts of mouse MULT1 have been detected in healthy tissues, including the thymus, although cell surface expression on thymocytes was not detected. In this issue, Nice et al. show that mouse MULT1 is ubiquitinated in normal cells, leading to its lysosomal degradation and preventing its expression at the cell surface (6
). In response to heat shock or UV irradiation, ubiquitination of MULT1 was reduced. Genotoxic shock, on the other hand, had no effect on MULT1 ubiquitination. Therefore, heat shock might operate by two distinct mechanisms to up-regulate different NKG2D ligands. It activates the transcription of ligands such as MICA and MICB that contain heat shock promoter elements, and it inhibits the posttranslational degradation of other ligands, such as MULT1.
UV irradiation might likewise control different NKG2D ligands by different mechanisms. UV irradiation induces the activity of AP-1, which has been shown to control RAE-1ϵ expression, most likely by direct transcriptional regulation. And, as shown by Nice et al., UV irradiation also inhibits the ubiquitination and degradation of MULT1 (6
). Whether UV irradiation and heat shock operate via similar mechanisms to induce MULT1 cell surface expression is currently unknown. Some tumor cells, such as YAC-1 cells, constitutively express MULT1, although it is unclear why MULT1 is not degraded in these cells. Tumor cells might constitutively activate heat shock responses that potentially counteract the degradation of MULT1, or they might be missing thus far unidentified components of the ubiquitination and/or degradation pathways. Future studies will be required to determine whether proteasome inhibitors further up-regulate MULT1 expression on tumor cells that already constitutively express the protein.
ULPB transcripts were initially found in various tissues in healthy individuals, including heart, lung, liver, and testis (21
). MICA and MICB transcripts were also found in multiple tissues, with the exception of the central nervous system (22
). Notably, MICA protein expression was exclusively detected on enterocytes by immunohistochemistry staining (23
). It is possible that cell surface expression of the transmembrane proteins MICA, MICB, and RAET-1G, which possess lysine residues within their cytoplasmic tails, is also controlled by ubiquitination. An additional posttranslational mechanism must exist to control the cell surface expression of ULBP family members that are GPI-anchored proteins.
The concept that NKG2D ligand expression can be controlled by ubiquitinylation is not completely new. The E3 ubiquitin ligase K5 from Kaposi sarcoma virus was shown to down-regulate cell surface expression of MICA by ubiquitination of lysine residues in the protein's cytoplasmic tail (24
). However, this ubiquitination did not correlate with an increased rate of degradation, but rather in a redistribution of MICA from the plasma membrane to an intracellular compartment. Therefore, even subsequent to ubiquitination, distinct mechanisms exist to control expression of different NKG2D ligands.
Multiple checkpoints operating at different levels of NKG2D ligand protein synthesis not only facilitate the expression of NKG2D ligands during different types of stress but might also fine tune the kinetics of cell surface expression. It is feasible that posttranslational regulation allows more rapid expression of NKG2D ligands on the surface of virus-infected or tumor cells than could be achieved via transcriptional regulation, thus ensuring rapid elimination of unhealthy cells.