CD9, a tetraspanin target for cancer therapy?

Abstract

In the present minireview, we intend to provide a brief history of the field of CD9 involvement in oncogenesis and in the metastatic process of cancer, considering its potential value as a tumor-associated antigenic target. Over the years, CD9 has been identified as a favorable prognostic marker or predictor of metastatic potential depending on the cancer type. To understand its implications in cancer beside its use as an antigenic biomarker, it is essential to know its physiological functions, including its molecular partners in a given cell system. Moreover, the discovery that CD9 is one of the most specific and broadly expressed markers of extracellular membrane vesicles, nanometer-sized entities that are released into extracellular space and various physiological body fluids and play a role in intercellular communication under physiological and pathological conditions, notably the establishment of cancer metastases, has added a new dimension to our knowledge of CD9 function in cancer. Here, we will discuss these issues as well as the possible cancer therapeutic implications of CD9, their limitations, and pitfalls.

Keywords: Antibody; CD9; cancer; exosome; extracellular vesicle; immunotherapy; tetraspanin.

Figure 1.

Membrane topology of CD9. The human CD9 protein contains four transmembrane segments (1–4), short cytoplasmic N- and C-protein termini, and two extracellular (EC) domains forming a short and a larger loop. The CD9 EC2 domain is properly folded with two disulfide bridges in which two cysteine residues are found in a conserved CCG motif among all tetraspanins. A potential N-glycosylation site is located in the EC1 domain, close to the second transmembrane domain. Several cysteine residues at the transition of the cytoplasmic domains and transmembrane segments are subject to palmitoylation. The outer and inner leaflets of the plasma membrane are shown. Therein, membrane cholesterol and GM₃ ganglioside could alter CD9 function. Note that the structure of CD9 is not represented with the appropriate scale.

Figure 2.

Cellular expression of CD9 and its functional roles. (a) The tetraspanin CD9 is associated with plasma membrane notably protruding structures such as microvillar-like projections, filopodia, and lamellipodia. An intracellular fraction of CD9 is also present in the endosomal system, notably the late endosome/multivesicular body (MVB), which correlates to its release in association with exosomes into the extracellular milieu. CD9 is also released from microvilli in association with budding microvesicles. A nuclear pool of CD9 has been reported, but no related function (?) has been described. CD9 can associate with various protein partners and regulate their activities in various cellular processes as indicated. For example, the binding of CD9 to adhesion and/or integrin molecules can suppress or promote cell–cell and cell-matrix interactions as well as cell migration. Similarly, the interaction of CD9 with claudin-1 could affect the formation of the tight junction (TJ), and favor epithelial-mesenchymal transition, or EGFR, and attenuate the EGF-EGFR signaling pathway. CD9 located on the cell surface could promote the endocytosis of CD9-positive EVs or regulate the entry of the virus or cell-cell fusion. EVs can play a role in intercellular communication. CD9 may regulate the sheddase activity of certain cell surface enzymes. (b) Silencing CD9 in the MDA-MB-231 (MDA) breast cancer cell line alters the plasma membrane. Scanning electron microscopy revealed that the cell border and microvillar-like structures at the dorsal membrane of MDA cells (left panels, asterisk and arrow, respectively) are altered in CD9-deficient MDA cells (shCD9, right panels, double and single pointing angle quotation mark, respectively). Cell culture conditions and other methods are described in Rappa et al. EE: early endosome; EGF: epidermal growth factor; nc: nucleus. Scale bars are indicated.