CCN Proteins Are Distinct from, and Should Not Be Considered Members of, the Insulin-Like Growth Factor-Binding Protein Superfamily

In a recent letter, Baxter et al. (1) proposed renaming a family of extensively studied proteins, known in the literature as the CCN family (reviewed in Refs. 2–5), as insulin-like growth factor binding protein-related proteins (IGFBP-rPs). This proposed name change lacks functional or biological basis, has been suggested unilaterally without consensus or consultation with those working on these proteins, and serves to confuse rather than clarify the literature. The prototypic members of the CCN family (CTGF, CYR61, and NOV) were discovered in our laboratories in the early 1990s (6–8). Additional members of the family have been identified, including Elm-1/WISP-1, Cop-1/WISP-2, and WISP-3 (Refs. 9–11 ; Fig. 1). These highly conserved cysteine-rich proteins share four conserved modular domains with sequence similarities to IGFBP, von Willebrand factor, thrombospondin, and a cysteine knot characteristic of some growth factors including platelet-derived growth factor, nerve growth factor, and transforming growth factor-β (2). Cop-1/WISP-2 is unique because it lacks the carboxyl-terminal cysteine knot domain. Each of the modular structural domains is encoded by a separate exon, suggesting that genes of the CCN family arose through exon shuffling of preexisting genes. Sequence similarity with IGFBPs exists only in the N-terminal domain encoded by one exon. From the sequence perspective, CCN proteins are no more related to IGFBPs than to von Willebrand factor, thrombospondin, or growth factor cysteine knots (Refs. 2–5 ; Fig. 1). Schematic representation of CCN proteins compared with Mac25 and IGFBP-1. Following the secretory signal (open oval), CCN proteins exhibit four discrete conserved domains: IGFBP, VWC (von Willebrand factor type C repeat), TSP1 (thrombospondin type 1 repeat), and CT (carboxyl-terminal cysteine knot). A central variable region separate the proteins into two halves. The overall percent amino acid sequence identities and homologies including conservative substitutions (%ID/HO) of the human sequences of each protein compared with CTGF are listed. Within the N-terminal domain, CTGF shows 48–60% amino acid sequence identity compared with CCN family members, but only 31–33% compared with Mac25 and IGFBP-1. Outside of the N-terminal domain, CCN proteins, Mac25, and IGFBP-1 share no sequence similarity. Functionally, a number of biologically significant activities have been clearly demonstrated for CCN proteins, none of which has any apparent relationship to IGF binding (12–27). Purified CCN proteins have been demonstrated to mediate and promote cell adhesion, migration, proliferation, and survival (3–5, 12). As matrix-associated, heparin-binding proteins (13), CYR61 and CTGF are novel ligands of the integrinsα Vβ3 andα IIbβ3 (14–16), and NOV interacts with fibulin 1C (17), suggesting their involvement in cell adhesion signaling. Both CYR61 and CTGF induce angiogenesis in vivo (15, 18) and chondrogenesis in vitro (19, 20). CTGF is expressed in fibroblasts during wound healing (21, 22) and can induce fibrosis in vivo (22). Furthermore, CTGF has been demonstrated to mediate both the mitogenic and matrigenic activities of transforming growth factor-β (23–25). Other studies have revealed that CYR61 promotes tumor growth (18), whereas Cop-1/WISP-2 or Elm-1/WISP-1 can inhibit tumor growth (9, 10). It has also been established that the expression of NOV is abnormal in tumor cells (26, 27) and that expression of an amino-truncated form of NOV is transforming but full-length NOV inhibits fibroblast growth (8), suggesting an involvement of this proto-oncogene in malignancy. If a unified nomenclature were to be proposed for these multifunctional proteins, such nomenclature should reflect their demonstrated biological activities. The term “IGFBP-related proteins” does not fulfill this need. Is there any functional or biological basis for addressing the CCN proteins as members of the IGFBP superfamily? The only data in existence is that CTGF (28) and NOV (29) bind IGF in vitro with a 100- to 1000-fold lower affinity than authentic IGFBPs. Inasmuch as no IGF binding to NOV was observed under standard ligand blotting assay conditions (27), the low-affinity binding for IGF remains controversial. No published data speak to any potential binding of CYR61 to IGF. Clearly, these proteins cannot compete with the high-affinity IGFBPs that are so abundant in serum. More importantly, to date, there is no demonstrated physiological significance of IGF binding by any member of the CCN family. Thus, proposing to abandon the established names of the CCN proteins and to rename and reclassify them on a speculative basis does not make sense and serves only to divert attention from the carefully documented and published work that has identified specific biological activities of these molecules. The proposal of Baxter et al. (1) to reclassify substantively different molecules under the same rubric is misleading, exemplified in this case by the placement of CCN proteins in the same category as Mac25, a protein homologous to the activin-binding protein follistatin (30). Even a cursory inspection of Fig. 1 reveals that the CCN proteins form a distinct family, separate and apart from Mac25 and IGFBPs. The proposed renaming misleadingly suggests an intimate relationship among CCN proteins Mac25 and IGFBPs that does not exist and implies that the biological activities of CCN proteins function through an IGF-binding activity, which has not been demonstrated in any context. Changes in nomenclature often make good sense in a field where clarity and focus can be served based on accumulated new information. However, this should be done with the consensus of those who work in the field, rather than unilaterally. In this instance, because the very low-affinity binding of IGF by CCN proteins has no demonstrated biological significance, this proposed name change serves no scientific or intellectual purpose. The proposed renaming of the CCN family as IGFBP-rPs simply ignores the multitude of well-documented and established biological activities of these proteins (3–27). The use of superfluous names such as IGFBP-rP serves only to add confusion rather than insight into the functions and activities of this complex and important emerging family of proteins.