Shravan K. Chintala, Ph.D. and Jasti S. Rao, Ph.D.

Department of Neurosurgery, Box 064, The University of Texas M.D. Anderson Cancer Center, 1515 Holcombe Blvd., Houston, TX 77030, USA.

Received 9/28/96; Accepted 10/17/96; On-line 11/01/96

5. ECM RECEPTORS AND CELL ADHESION MOLECULES

As they invade the tissue, tumor cells must adhere to a variety of ECM components and cell surface molecules. These interactions have a profound effect on the tumor cells invasive phenotypic behavior. Over the last several years, extensive progress has been made in identifying ECM components and particular sites on the components that mediate cellular adhesion / invasion. Tumor cells interact with the extracellular matrix by adhesion molecules or matrix receptors. Several classes of matrix receptors have been described, including integrins, gangliosides, CD44, and some members of the immunoglobulin superfamily such as the neural cell adhesion molecule (NCAM) (92).

5.1. Gangliosides

Gangliosides are a class of sialic acid-containing glycosphingolipids that are selectively localized in the cell membrane (93, 94). Although their functional role is not well documented, the observation that cells that had high migratory capacity concomitantly express more gangliosides gave rise to the speculation that gangliosides may be involved in migratory and invasive behavior of glial tumor cells (22). In this context, Merzak et al. (95) showed for the first time that cell-membrane gangliosides are involved in glioma cell invasion in vitro. Moreover, the same group showed that exogenous addition of gangliosides enhanced the adhesion of human glioma cell lines to fibronectin, laminin, vitronectin and collagen I (96), thus consolidating the role of gangliosides in adhesion. The identification of a ganglioside binding site on fibronectin supported the role of gangliosides in fibronectin-mediated cell adhesion (97). Moreover, expression of GD3 ganglioside was found elevated in malignant astrocytomas (98) and gangliosides were shown to regulate the function of integrins (99-101).

5.2. Cluster differentiation 44

Originally known as the lymphocyte homing receptor (also known as Hermes antigen, Pgp-1), cluster differentiation 44 (CD44) is a polymorphic family of glycoproteins that carries N- and O-linked sugars and glycosaminoglycan side chains found to be the major receptor for hyaluronic acid (HA) with a wide cellular distribution (102, 103). CD44 has two isoforms with a molecular mass of 80-90 kDa and 150 kDa respectively (104). CD44 is expressed on both normal and neoplastic astrocytes (105), and it has been shown to be upregulated in neoplastic cells (106). Recent studies have shown that CD44 mediates attachment of cells to various ECM proteins including HA, chondroitin sulfate, laminin, fibronectin, vitronectin, types I and IV collagen and modified basement membrane matrigel (107). Koochekpour et al. (19) recently reported that antibodies against CD44 inhibited the invasion of human glioma cells and they have showed that the factor responsible for the invasion is hyaluronic acid.

5.3. Neural cell adhesion molecule (NCAM)

NCAMs are perhaps the most widely studied adhesion molecules that are expressed on developing neurons in normal brain (108-111). NCAMs are implicated in mediating adhesion to neural elements and have been shown to bind to several types of collagens (112). A glycoprotein, NCAM, is composed of three polypeptides, NCAM-A, NCAM-B, and NCAM-C, with a molecular mass of 180 kDa, 140 kDa, and 120 kDa, respectively. The A and B proteins are phosphorylated, integral membrane proteins, while C is a nonphosphorylated peripheral polypeptide linked to the plasma membrane by a phosphatidylinositol anchor (113, 114). During migratory events in embryogenesis, NCAM is down-regulated, and it is re-expressed once the target organ has been reached and differention is initiated (21). Recent studies (115) showed that NCAM expression is correlated with different modes of invasion of rat glioma cell lines, both in vivo (116), and in vitro (117). NCAM expression has also been shown to correlate with downregulation of 92-kDa gelatinase. The finding that MMP-1 was downregulated by NCAM-B transfection but not in NCAM-C clones, suggested that the NCAM molecule's transmembrane domain, but not its extracellular domain, is involved in signal transduction (117).

5.4. Integrins

Integrins are heterodimeric integral plasma membrane cell-surface receptors that modulate cell-cell and cell-ECM interactions (118). They consist of noncovalently linked alpha and ß subunits. To date, 8ß and 16alpha subunits have been identified; their combinations produce 20 distinct integrins (119-121). Integrins are proposed to be involved in signal transduction from the ECM to the nuclei of cells. The cytoplasmic domain interacts with the cytoskeletal proteins in the vicinity of the cell membrane and is also responsible for the cell adhesion, shape and migration. The binding of cells to different ECM proteins, fibronectin, laminin, thrombospondin, von Willebrand's factor, and tenascin depends on the specific combination of alpha and ß subunits (120). The subunits (alpha, 120-180 kDa; ß, 90-110 kDa) are noncovalently associated and expressed on a wide variety of cell types. Integrins recognize an (RGD) sequence in fibronectin and vitronectin, in addition to Lys-Gln-Ala-Gly-Asp-Val (KQAGDV) in fibrinogen, Glu-Ile-Leu-Asp-Val (EILDV) in alternatively spliced segment of fibronectin, and Gly-Pro-Arg-Pro (GPRP) in fibrinogen (122, 118).

The cytoplasmic domain of the integrin molecule interacts with cytoskeletal proteins such as actin and talin, and activates specific kinases and phosphatases. The extracellular domain of the integrin interacts with ECM components and with integral plasma membrane receptors. Divalent cations, such as calcium and magnesium, play an important role in the function of integrins; a distinct calcium binding site has been identified on an integrin subunit (118). Altered expression of integrins has been shown to correlate with the malignant phenotype of tumors (123, 124). Integrin subunits link the cell's extracellular environment with internal cytoskeletal elements via linking proteins, such as talin and vinculin (125).