CCN3 is a novel endogenous PDGF-regulated inhibitor of glomerular cell proliferation

CCN proteins affect cell proliferation, migration, attachment, and differentiation. We identified CCN3 as a suppressed gene following platelet-derived growth factor (PDGF)-BB or -DD stimulation in a cDNA-array analysis of mesangial cells. In vitro growth-arrested mesangial cells overexpressed and secreted CCN3, whereas the addition of the recombinant protein inhibited cell growth. Induction of mesangial cell proliferation by PDGF-BB or the specific PDGF beta-receptor ligand PDGF-DD led to downregulation of CCN3 mRNA, confirming the array study. Specific PDGF alpha-receptor ligands had no effect. CCN3 protein was found in arterial smooth muscle cells, the medullary interstitium, and occasional podocytes in the healthy rat kidney. Glomerular CCN3 was low prior to mesangial proliferation but increased as glomerular cell proliferation subsided during mesangioproliferative glomerulonephritis (GN). Inhibition of PDGF-B in mesangioproliferative disease led to overexpression of glomerular CCN3 mRNA. CCN3 localized mostly to podocytes in human glomeruli, but this expression varied widely in different human glomerulonephritides. Glomerular cell proliferation negatively correlated with CCN3 expression in necrotizing GN. Our study identifies CCN3 as an endogenous inhibitor of mesangial cell growth and a modulator of PDGF-induced mitogenesis.

Tissue plasminogen activator is a potent activator of PDGF-CC

Tissue plasminogen activator (tPA) is a serine protease involved in the degradation of blood clots through the activation of plasminogen to plasmin. Here we report on the identification of tPA as a specific protease able to activate platelet-derived growth factor C (PDGF-C). The newly identified PDGF-C is secreted as a latent dimeric factor (PDGF-CC) that upon proteolytic removal of the N-terminal CUB domains becomes a PDGF receptor alpha agonist. The CUB domains in PDGF-CC directly interact with tPA, and fibroblasts from tPA-deficient mice fail to activate latent PDGF-CC. We further demonstrate that growth of primary fibroblasts in culture is dependent on a tPA-mediated cleavage of latent PDGF-CC, generating a growth stimulatory loop. Immunohistochemical analysis showed similar expression patterns of PDGF-C and tPA in developing mouse embryos and in tumors, indicating both autocrine and paracrine modes of activation of PDGF receptor-mediated signaling pathways. The identification of tPA as an activator of PDGF signaling establishes a novel role for the protease in normal and pathological tissue growth and maintenance, distinct from its well-known role in plasminogen activation and fibrinolysis.

TGF-beta and PDGF act synergistically in affecting the growth of human osteoblast-enriched cultures

Transforming growth factor beta (TGF-beta) is a polypeptide found in high concentrations in bone and is produced by and acts on primary adult human derived osteoblast-enriched cultures (PHO cells). Receptors for TGF-beta are present on PHO cells and TGF-beta is mitogenic for these cells. Results of these studies in conjunction with those of others suggest that TGF-beta may have an important therapeutic role in orthopaedic surgery; however, with respect to its mitogenic actions, further studies were needed to establish whether TGF-beta was acting directly to stimulate the growth of PHO cells. TGF-beta has been found in other systems to act as an indirect mitogen, stimulating growth via secretion of another growth factor, platelet-derived growth factor (PDGF). In an effort to determine whether the TGF-beta growth stimulation was mediated directly or indirectly, we have examined the growth stimulation of PHO cells by PDGF alone and in combination with TGF-beta. These studies revealed that TGF-beta in combination with either PDGF-AA or BB led to stimulation greater than that observed with either growth factor alone. TGF-beta in combination with PDGF-BB led to a synergistic stimulatory response while that observed with the AA isoform was more nearly additive. Further studies demonstrated that TGF-beta was capable of up-regulating the protein levels of the PDGF alpha (alpha) receptor within thirty minutes of TGF-beta pretreatment. Thus, TGF-beta appears to have both direct and indirect mechanisms of action as a mitogen in the PHO system. Finally, we showed that both the positive and negative alkaline phosphatase staining PHO cells were responsive to the mitogenic effects of both growth factor singly and in combination.