CCN Proteins Are Distinct from, and Should Not Be Considered Members of, the Insulin-Like Growth Factor-Binding Protein Superfamily
Gary R. Grotendorst,
Lester F. Lau and
Bernard Perbal
Department of Cell Biology and Anatomy (G.R.G.), University of Miami
School of Medicine, Miami, Florida 33136; Department of Molecular
Genetics (L.F.L.), University of Illinois College of Medicine, Chicago,
Illinois 60607; and Laboratoire dOncologie Virale et
Moléculaire (B.P.), UFR de Biochimie, Université Paris 7,
75005 Paris and Unité INSERM 515, Hôpital Saint-Antoine,
Paris 75012, France
To the editor:
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, 3, 4, 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, 7, 8).
Additional members of the family have been identified, including
Elm-1/WISP-1, Cop-1/WISP-2, and WISP-3 (Refs. 9, 10, 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-ß (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, 3, 4, 5 ; Fig. 1
).

View larger version (26K):
[in this window]
[in a new window]
|
Figure 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 4860% amino acid sequence identity
compared with CCN family members, but only 3133% 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, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27). Purified CCN
proteins have been demonstrated to mediate and promote cell adhesion,
migration, proliferation, and survival (3, 4, 5, 12). As
matrix-associated, heparin-binding proteins (13), CYR61
and CTGF are novel ligands of the integrins
Vß3 and
IIbß3
(14, 15, 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-ß (23, 24, 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, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 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.
Received February 10, 2000.
References
-
Baxter RC, Binoux MA, Clemmons DR, et al. 1998 Recommendations for nomenclature of the insulin-like growth factor
binding protein superfamily. J Clin Endocrinol Metab. 83:3213.[Free Full Text]
-
Bork P. 1993 The modular architecture of a new
family of growth regulators related to connective tissue growth factor. FEBS Lett. 327:125130.[CrossRef][Medline]
-
Grotendorst GR. 1997 Connective tissue growth
factor: a mediator of TGF-ß action on fibroblasts. Cytokine Growth
Factor Rev. 8:171179.[CrossRef][Medline]
-
Lau LF, Lam SC-T. 1999 The CCN family of
angiogenic regulators: the integrin connection. Exp Cell Res. 248:4457.[CrossRef][Medline]
-
Brigstock DR. 1999 The connective tissue growth
factor/cysteine-rich 61/nephroblastoma overexpressed (CCN) family. Endocr Rev. 20:189206.[Abstract/Free Full Text]
-
OBrien TP, Yang GP, Sanders L, Lau LF. 1990 Expression of cyr61, a growth factor-inducible immediate
early gene. Mol Cell Biol. 10:35693577.[Medline]
-
Bradham DM, Igarshi A, Potter RL, Grotendorst GR. 1991 Connective tissue growth factor: a cysteine-rich mitogen secreted
by human vascular endothelial cells is related to the SRC-induced
immediate early gene product CEF-10. J Cell Biol. 114:12851294.[Abstract]
-
Joliot V, Marinerie C, Dambrine G, et al. 1992 Proviral rearrangements and overexpression of a new cellular
gene (nov) in myeloblastosis-associated virus type-1 induced
nephroblastomas. Mol Cell Biol. 12:1021.[Abstract]
-
Hashimoto Y, Shindo-Okada N, Tani M, et al. 1998 Expression of the Elm1 gene, a novel gene of the CCN (connective
tissue growth factor, Cyr61/cef10, and nephroblastoma overexpressed
gene) family, suppresses in vivo tumor growth and metastasis
of K-1735 murine melanoma cells. J Exp Med. 187:289296.[Abstract/Free Full Text]
-
Zhang R, Averboukh L, Zhu W, et al. 1998 Identification of rCOP-1, a new member of the CCN protein family, as a
negative regulator for cell transformation. Mol Cell Biol. 18:61316141.[Abstract/Free Full Text]
-
Pennica D, Swanson TA, Welsh JW, et al. 1998 WISP
genes are members of the connective tissue growth factor family that
are up-regulated in Wnt-1 transformed cells and aberrantly expressed in
human colon tumors. Proc Natl Acad Sci USA. 95:1471714722.[Abstract/Free Full Text]
-
Kireeva ML, Mo F-E, Yang GP, Lau LF. 1996 Cyr61,
product of a growth factor-inducible immediate-early gene, promotes
cell proliferation, migration, and adhesion. Mol Cell Biol. 16:13261334.[Abstract]
-
Yang GP, Lau LF. 1991 Cyr61, product of a growth
factor-inducible immediate early gene, is associated with the
extracellular matrix and the cell surface. Cell Growth Differ. 2:351357.[Abstract]
-
Kireeva ML, Lam SC-T, Lau LF. 1998 Adhesion of
human umbilical vein endothelial cells to the immediate-early gene
product Cyr61 is mediated through integrin
Vß3. J Biol Chem. 273:30903096.[Abstract/Free Full Text]
-
Babic AM, Chen C-C, Lau LF. 1999 Fisp12/mouse
connective tissue growth factor mediates endothelial cell adhesion and
migration through integrin
Vß3 promotes
endothelial cell survival, and induces angiogenesis in vivo. Mol Cell Biol. 19:29582966.[Abstract/Free Full Text]
-
Jedsadayanmata A, Chen C-C, Kireeva ML, Lau LF, Lam
SC-T. 1999 Activation-dependent adhesion of human platelets to
Cyr61 and Fisp12/mouse connective tissue growth factor is mediated
through integrin IIbß. J Biol Chem. 274:2432124327.[Abstract/Free Full Text]
-
Perbal B, Martinerie C, Sainson R, Werner M, He B,
Roizman B. 1999 The C-terminal domain of the regulatory protein
NOVH is sufficient to promote interaction with fibulin 1C: a clue for a
role of NOVH in cell-adhesion signaling. Proc Natl Acad Sci USA. 96:869874.[Abstract/Free Full Text]
-
Babic AM, Kireeva ML, Kolesnikova TV, Lau LF. 1998 CYR61, a product of a growth factor-inducible immediate early genes,
promotes angiogenesis and tumor growth. Proc Natl Acad Sci USA. 95:63556360.[Abstract/Free Full Text]
-
Wong M, Kireeva ML, Kolesnikova TV, Lau LF. 1997 Cyr61, product of a growth factor-inducible immediate early gene,
regulates chondrogenesis in mouse limb bud mesenchymal cells. Dev Biol. 192:492508.[CrossRef][Medline]
-
Grotendorst GR. Induction of tissue, bone or
cartilage formation using connective tissue growth factor. United
States Patent 5,837,258, November 17, 1998.
-
Igarashi A, Okochi H, Bradham DM, Grotendorst GR. 1993 Regulation of connective tissue growth factor gene expression in
human skin fibroblasts and during wound repair. Mol Biol Cell. 4:637645.[Abstract]
-
Frazier K, Williams S, Kothapalli D, Klapper H,
Grotendorst GR. 1996 Stimulation of fibroblast cell growth, matrix
production, and granulation tissue formation by connective tissue
growth factor. J Invest Dermatol. 107:404411.[Abstract]
-
Kothapalli D, Frazier K, Grotendorst GR. 1997 TGF-ß induces anchorage-independent growth of NRK fibroblasts via the
synergistic action of CTGF-dependent and CTGF-independent signaling
pathways. Cell Growth Differ. 8:6168.[Abstract]
-
Kopthapalli D, Hayashi N, Grotendorst GR. 1998 Inhibition of TGF-ß stimulated CTGF gene expression and anchorage
independent growth by elevation of intracellular cAMP. FASEB J. 12:11511161.[Abstract/Free Full Text]
-
Duncan MR, Frazier KS, Abramson S, et al. 1999 Connective tissue growth factor mediates transforming growth
factor-collagen synthesis: down-regulation by cAMP. FASEB J. 13:17741786.[Abstract/Free Full Text]
-
Li WX, Martinerie C, Zumkeller W, Westphal M, Perbal
B. 1996 Differential expression of novH and CTGF in human glioma
cell lines. J Clin Mol Pathol. 49:M91M97.
-
Chevalier G, Yeger H, Martinerie C, et al. 1998 novH: differential expression in developing kidney and a marker of
heterotypic differentiation in Wilms tumor. Am J Pathol. 52:15631575.
-
Kim HS, Nagalla SR, Oh Y, Wilson E, Roberts CTJ,
Rosenfeld RG. 1997 Identification of a family of low-affinity
insulin-like growth factor binding proteins (IGFBPs): characterization
of connective tissue growth factor as a member of the IGFBP
superfamily. Proc Natl Acad Sci USA. 94:1298112986.[Abstract/Free Full Text]
-
Burren CP, Wilson EM, Hwa V, Oh Y, Rosenfeld RG. 1999 Binding properties and distribution of insulin-like growth factor
binding protein-related protein 3 (IGFBP-rP3/NovH), and additional
member of the IGFBP superfamily. J Clin Endocrinol Metab. 84:10961103.[Abstract/Free Full Text]
-
Kato MV, Sato H, Tsukada T, Ikawa Y, Aizawa S, Nagayoshi
M. 1996 A follistatin-like gene, mac25, may act as a growth
suppressor of osteosarcoma cells. Oncogene. 12:13611364.[Medline]