A structural approach to the role of CCN (CYR61/CTGF/NOV) proteins in tumourigenesis

Abstract

The CCN (CYR61 [Cystein-rich61]/CTGF [connective tissue growth factor]/NOV [Nephroblastoma overexpressed]) proteins constitute a family of regulatory factors involved in many aspects of cell proliferation and differentiation. An increasing body of evidence indicates that abnormal expression of the CCN proteins is associated to tumourgenesis. The multimodular architecture of the CCN proteins, and the production of truncated isoforms in tumours, raise interesting questions regarding the participation of each individual module to the various biological properties of these proteins. In this article, we review the current data regarding the involvement of CCN proteins in tumourigenesis. We also attempt to provide structural basis for the stimulatory and inhibitory functions of the full length and truncated CCN proteins that are expressed in various tumour tissues.

Figure 1

Multimodular structure of the CCN proteins. SP, Signal Peptide ; IGFBP, Insulin-like Growth Factor Binding Protein-like module ; VWC, Von Willebrand Factor-like module ; TSP1, Thrombospondin-like module ; CT, cystein knot containing family of growth regulator-like module. HO : homology ; ID : identity.

Figure 2

Alignment of CCN protein sequences. Alignments were performed with the " interalign " program. Conserved residues have been indicated on a black box. Grey boxes indicate positions where more than one amino acid residue is conserved among the CCN proteins. Conserved cysteins are indicated in the yellow boxes.

Figure 3

Alignment of CCN protein sequences. Alignments were performed with the " interalign " program. Conserved residues have been indicated on a black box. Grey boxes indicate positions where more than one amino acid residue is conserved among the CCN proteins. Conserved cysteins are indicated in the yellow boxes.

Figure 4

Insertional mutagenesis of MAV. The organisation of the chicken CCN3 gene is represented at the top. The CCN3 chicken protein encode a putative secreted 36 KDa protein. Following post-translational modifications, a full length 41 KDa CCN3 protein is released in the culture medium. In one MAV-induced nephroblastoma, proviral sequences were integrated within exon 2 of the CCN3 gene. As a consequence, MAV LTR was driving the expression of a chimaeric mRNA species encoding an amino truncated CCN3 protein deprived of module I and signal peptide. ORF : open reading frame ; (th) : theoretical ; (s) :secreted. The signal peptide (sp) is encoded by exon 1. E1–E5 : exons encoding the CCN3 protein. n201, n318 indicate the boundaries of the RNA stretch corresponding to the remaining sequences from exon 2. 91 n : 91 nucleotides of viral origin ; 117 n exon 2 sequences which are out of frame in the chimaeric mRNA species.

Figure 5

Subcellular localisation of the CCN3 isoforms is critical for biological activity. The cDNA sequences encoding the truncated and the full length CCN3 protein were inserted into RCAS non defective Rous sarcoma virus (RSV)-derived proviral vector. The amino-truncated CCN3 protein which is deprived of signal peptide (top) is not secreted and induces morphological transformation of chicken embryo fibroblasts, whereas the secreted full length CCN3 protein induces cell growth arrest.