Decipher β2-microglobulin: Gain- or loss-of-function (a mini-review)

Background

β2-Microglobulin (β2M), a structurally light chain of major histocompatibility complex (MHC) class I molecules, is expressed on the surface of all nucleated cells. β2M non-covalently interacts with the heavy chain of MHC class I to function in the presentation of antigenic peptide fragment to cytotoxic (CD8+) T lymphocytes [1]. In the absence of β2M, most MHC class I molecules become less expressed on the surface of cells [2]. Transfection of β2M-deficiency cells with ER-retained β2M restored the function of MHC class I, supporting a role of β2M as chaperone for MHC folding [3]. Nevertheless, β2M is present as a soluble form at low levels in different bodily fluids under physiological conditions. It is increasingly recovered in patients with chronic kidney disease and certain malignancies, including liquid and solid tumors [4,5]. The elevated level of β2M in body fluid is significantly associated with tumor status and poor prognosis. It is believed that cancer cells frequently downregulate the expression of MHC class I to avoid immune surveillance, allowing gain-of-function via the secretion of free β2M. Thus, the free β2M is widely implicated in association with MHC class I-loss tumors.

β2-microglobulin Gain-of-Function in Cancer Cells

There is convincing evidence that β2M acts as a growth-promoting factor and signal molecule in a variety of cancer cells [6–8]. Overexpression of β2M facilitates the invasion and migration of cancer cells of different origin, supporting the increased levels of β2M positively correlated with advanced-stage tumors. β2M gain-of-function is achieved by regulating the androgen receptor and a cyclic AMP-dependent protein kinase A signaling pathway in prostate cancer. A panel of genes related to cell growth, survival, signaling, and angiogenesis is therefore expressed to trigger tumor progression and cause bone metastasis [6]. Similar results were found in human cancer renal cells, showing that β2M promotes tumor growth via the activation of vascular endothelial growth factor axis [7]. Apart from the finding of β2M as an important prognostic and survival factor in a range of malignancies, it also becomes an attractive therapeutic target. Monoclonal antibodies (mAbs) against β2M have a surprising apoptotic effect on prostate cancer, renal cell carcinoma (RCC), myeloma, and other hematological malignancies [9–12]. Yang et al. [12] demonstrated that mAbs induce apoptosis in myeloma cells by recruiting MHC class I molecules to lipid rafts and by excluding growth factor receptors. Similarly, Huang et al. [13] revealed that β2M-specific mAb decreases lipid levels in prostate cancer cells to make a link between β2M-mediated intracellular signaling axis and lipid rafts in the cell membrane. This may explain why β2M mAbs-induced apoptosis in cancers is mediated via quite similar signaling pathways – the inhibition of PI3K/Akt, MAPK, and Bcl-2, and the activation of caspases-dependent cascade – supporting the role of β2M as a mitogenic factor. Of note, β2M-specific mAbs have a higher affinity for surface-bound rather than the soluble β2M and display differential sensitivity to malignant cells but not to normal cells, suggesting that the integrity of lipid rafts is an important factor for both apoptotic and survival signaling [9]. Strikingly, Cao et al. [14] reported the effectiveness of IgM anti-β2M mAbs exhibiting more potent activity than IgG mAbs at inducing tumor cells apoptosis in vitro and in vivo. The enhanced activity of IgM mAbs in tumor cell apoptosis mainly relies on the pentameric structure of the IgM. Apparently, these studies support that β2M-specific mAb is a promising therapeutic agent for the treatment of several types of malignancies, although further work is needed to evaluate the potential cytotoxicity of the antibody targeting approach. Based on a broad range of evidence, it is believed that β2M acts as a hormone-like molecule to trigger pleiotropic signaling via a ligand-to-receptor binding mechanism. Until recently, a noteworthy aspect to β2M in cancer cell signaling was to identify hemochromatosis (HFE) protein with which β2M induces epithelial-mesenchymal transition (EMT) and confers cancer lethality and bone metastasis in a variety of human cancer cells [8]. On the contrary, inhibition of β2M reverses EMT to mesenchymal-to-epithelial transition (MET). This suggests that β2M binds with HFE protein, namely β2M receptor, in association with transferring receptor to modulate iron homeostasis and activate iron-responsive HIF-1α (hypoxia inducible factor-1α) signaling in cancer cells. Because the HIF signaling cascade is activated by the effects of hypoxia, which keeps cells from differentiating, β2M-derived upregulation of HIF-1α mediates EMT and eventually promotes metastasis for cancer progression. Previously, Chen et al. [5] revealed that overexpression of β2M is associated with poor survival in patients with oral cavity squamous cell carcinoma (OCSCC) and contributes to oral cancer cell migration and invasion. The expression level of β2M in the cytoplasm and cytoplasm membrane of OCSCC epithelial cells at various stages was compared with that in normal oral mucosa, clearly showing that β2M localizes largely in the cytoplasm of tumor samples (~90–92% in a total of 256 cases) rather than in the plasma membrane. This suggests that the increased accumulation of β2M in the cytoplasm of OCSCC is significantly correlated with a relatively advanced tumor stage. This highlights the dramatic changes in β2M localization from cytoplasm membrane to cytoplasm between normal and tumor stages of OCSCC. This feature was also reported by Nomura et al. [7], who found that β2M expression, in a few cases, localized in the cytoplasm of human RCC. Thus, intracellular accumulation of β2M must play a pivotal role in cancer cell signaling and may present a potential target for therapy.

Conclusions

Cancerous cells in β2M loss-of-function are thought to avoid immune surveillance. Although β2M-mediated key molecular events such as tumor growth, cancer cells invasion, and metastasis can be attenuated by β2M-specific mAbs, questions about translocation of β2M from plasma membrane to cytoplasm in advanced-stage tumors remain to be answered.