CSPG4, a potential therapeutic target, facilitates malignant progression of melanoma

Abstract

Chondroitin sulfate proteoglycan 4 (CSPG4), a transmembrane proteoglycan originally identified as a highly immunogenic tumor antigen on the surface of melanoma cells, is associated with melanoma tumor formation and poor prognosis in certain melanomas and several other tumor types. The complex mechanisms by which CSPG4 affects melanoma progression have started to be defined, in particular the association with other cell surface proteins and receptor tyrosine kinases (RTKs) and its central role in modulating the function of these proteins. CSPG4 is essential to the growth of melanoma tumors through its modulation of integrin function and enhanced growth factor receptor-regulated pathways including sustained activation of ERK 1,2. This activation of integrin, RTK, and ERK1,2 function by CSPG4 modulates numerous aspects of tumor progression. CSPG4 expression has further been correlated to resistance of melanoma to conventional chemotherapeutics. This review outlines recent advances in our understanding of CSPG4-associated cell signaling, describing the central role it plays in melanoma tumor cell growth, motility, and survival, and explores how modifying CSPG4 function and protein-protein interactions may provide us with novel combinatorial therapies for the treatment of advanced melanoma.

Figure 1

Chondroitin sulfate proteoglycan 4 (CSPG4)/NG2 Structure. CSPG4/NG2 is composed of three major structural components: the extracellular domain, the transmembrane region, and the cytoplasmic C-terminal domain (CTD). The extracellular domain contains an N-terminal globular subdomain (D1) consisting of laminin G-type regions (LG) and disulfide bonds. The D2 subdomain consists of 15 CSPG repeats and, in NG2, a single chondroitin sulfate glycosaminoglycan (CS-GAG) chain. This region of the core protein is known to bind certain soluble growth factor ligands, and the CS-GAG is responsible for CSPG4 binding to integrin and matrix metalloproteinases (MMPs). CS modification is also associated with distinct membrane distribution patterns of CSPG4/NG2 on the cell surface. CSPG4/NG2 can be expressed with or without CS modification. Proximal to the plasma membrane, the D3 globular subdomain contains sites for N-linked carbohydrate modification, binding sites for lectins (e.g., galectin 3), and proteolytic cleavage by MMPs or other proteases. The transmembrane region of CSPG4 contains a cysteine residue (C) at position 2230 that may play a role in CSPG4 membrane localization, although this is yet to be evaluated. The CTD contains tyrosine residues (T) that serve as phosphoacceptor sites for PKCa and ERK 1,2 (CSPG4 residues 2252 and 2310, respectively). The proline-rich region (PR) may comprise a non-canonical SH3 protein interaction domain, and the C-terminus contains a 4 residue PDZ domain-binding motif (PDZ) that is responsible for interactions with various PDZ domain-containing binding partners.

Figure 2

Chondroitin sulfate proteoglycan 4 (CSPG4) Signaling Pathways. CSPG4 functions to activate two major overlapping but distinct signaling cascades: integrin/focal adhesion kinase (FAK) signaling (A) and MAPK pathway signaling (B). Through these two branches, CSPG4 ultimately promotes tumor progression through a variety of cellular functions. (A) CSPG4 influences integrin function and signaling pathways. CSPG4 promotes Src-FAK complexing through its interaction with the scaffold protein Syntenin. This leads to FAK activation by Src, prompting a number of signaling cascades including FAK–integrin–extracellular matrix (ECM) complex assembly, activation of Rac through p130cas, and activation of the PI3K/AKT/NFκB signaling cascade; activation of these pathways mediate the effects of CSPG4 on cytoskeletal reorganization, survival, chemoresistance, and migration. Activation of MMPs by CSPG4 via direct binding to the CS chain leads to local invasion of cancer cells. CSPG4/NG2 can also enhance survival as a result of its constitutive activation of integrin-related signals. (B) CSPG4 promotes MAPK signaling through receptor tyrosine (RTK)-dependent and independent mechanisms. In human melanomas, CSPG4 impacts activation of the ERK 1,2 pathway, likely by impacting on the growth factor-induced activation of RTKs. Note the expression of BRAF^V600E in this pathway, which results in the constitutive activation of this kinase (see text). In human melanoma cells expressing this BRAF-activating mutation, constitutive activation of the ERK 1,2 pathway requires the presence of CSPG4. There are several possible downstream oncogenic targets of the ERK 1,2 pathway, including MITF and activated c-Met, which are implicated in epithelial to mesenchymal transition. Inhibition of IKK by ERK 1,2 downstream of CSPG4 could also lead to cell survival and chemoresistance.