The adhesion receptor CD44 promotes atherosclerosis by mediating inflammatory cell recruitment and vascular cell activation

Atherosclerosis causes most acute coronary syndromes and strokes. The pathogenesis of atherosclerosis includes recruitment of inflammatory cells to the vessel wall and activation of vascular cells. CD44 is an adhesion protein expressed on inflammatory and vascular cells. CD44 supports the adhesion of activated lymphocytes to endothelium and smooth muscle cells. Furthermore, ligation of CD44 induces activation of both inflammatory and vascular cells. To assess the potential contribution of CD44 to atherosclerosis, we bred CD44-null mice to atherosclerosis-prone apoE-deficient mice. We found a 50-70% reduction in aortic lesions in CD44-null mice compared with CD44 heterozygote and wild-type littermates. We demonstrate that CD44 promotes the recruitment of macrophages to atherosclerotic lesions. Furthermore, we show that CD44 is required for phenotypic dedifferentiation of medial smooth muscle cells to the "synthetic" state as measured by expression of VCAM-1. Finally, we demonstrate that hyaluronan, the principal ligand for CD44, is upregulated in atherosclerotic lesions of apoE-deficient mice and that the low-molecular-weight proinflammatory forms of hyaluronan stimulate VCAM-1 expression and proliferation of cultured primary aortic smooth muscle cells, whereas high-molecular-weight forms of hyaluronan inhibit smooth muscle cell proliferation. We conclude that CD44 plays a critical role in the progression of atherosclerosis through multiple mechanisms.

CTGF modulates cell cycle progression in cAMP-arrested NRK fibroblasts

Connective tissue growth factor (CTGF) is a 38-kDa cysteine-rich peptide, whose synthesis and secretion are selectively induced by transforming growth factor beta (TGF-beta) in connective tissue cells. Previous studies have demonstrated that CTGF functions as a downstream mediator of TGF-beta mitogenic activity, where it controls cell cycle progression through late G1 and S-phase entry of NRK fibroblast suspension cultures. Here we report that CTGF induces this S-phase entry by upregulating cyclin A levels. The molecular mechanism for cyclin A induction appears to be via reduction of p27(Kip1) levels, which results in hyperphosphorylation of pRb and release of E2F, a known modulator of cyclin A gene transcription. These data indicate that CTGF acts as a mediator of TGF-beta-induced fibroblast proliferation in suspension cultures by regulating cdk activities.

CTGF modulates cell cycle progression in cAMP-arrested NRK fibroblasts

Connective tissue growth factor (CTGF) is a 38-kDa cysteine-rich peptide, whose synthesis and secretion are selectively induced by transforming growth factor beta (TGF-beta) in connective tissue cells. Previous studies have demonstrated that CTGF functions as a downstream mediator of TGF-beta mitogenic activity, where it controls cell cycle progression through late G1 and S-phase entry of NRK fibroblast suspension cultures. Here we report that CTGF induces this S-phase entry by upregulating cyclin A levels. The molecular mechanism for cyclin A induction appears to be via reduction of p27(Kip1) levels, which results in hyperphosphorylation of pRb and release of E2F, a known modulator of cyclin A gene transcription. These data indicate that CTGF acts as a mediator of TGF-beta-induced fibroblast proliferation in suspension cultures by regulating cdk activities.

Inhibition of TGF-beta-stimulated CTGF gene expression and anchorage-independent growth by cAMP identifies a CTGF-dependent restriction point in the cell cycle

CTGF is a 38 kDa cysteine-rich peptide whose synthesis and secretion are selectively induced by transforming growth factor beta (TGF-beta) in connective tissue cells. We have investigated the signaling pathways controlling the TGF-beta induction of connective tissue growth factor (CTGF) gene expression. Our studies indicate that inhibitors of tyrosine kinases and protein kinase C do not block the signaling pathway used by TGF-beta to induce CTGF gene expression. In contrast, elevation of cAMP levels within the target cells by a variety of methods blocked the induction of CTGF by TGF-beta. Furthermore, agents that elevate cAMP blocked the induction of anchorage-independent growth (AIG) by TGF-beta. Inhibition of AIG could be overcome by the addition of CTGF, indicating that it was not a general inhibition of growth but a selective inhibition of CTGF synthesis that is responsible for the inhibition of TGF-beta-induced AIG by cAMP. Kinetic studies of the induction of DNA synthesis by CTGF in cells arrested by cAMP indicate that the block occurs in very late G1. These and other studies in monolayer cultures suggest that the CTGF restriction point in the cell cycle is distinct from the adhesion-dependent arrest point.

Transforming growth factor beta induces anchorage-independent growth of NRK fibroblasts via a connective tissue growth factor-dependent signaling pathway

Connective tissue growth factor (CTGF) is a M(r)38,000 cysteine-rich peptide, the synthesis and secretion of which are selectively induced by transforming growth factor beta (TGF-beta). The relationship of CTGF to TGF-beta action on fibroblastic cells is not well understood. TGF-beta has the unique ability to stimulate the growth of normal fibroblasts in soft agar, a property of transformed cells. We have investigated whether CTGF can substitute for TGF-beta or whether CTGF action is essential for TGF-beta to stimulate anchorage-independent growth (AIG) of NRK fibroblasts. Our studies demonstrate that CTGF cannot induce AIG of NRK fibroblasts. However, CTGF synthesis and action are essential for the TGF-beta-induced AIG of NRK fibroblasts. Anti-CTGF antibodies specifically block TGF-beta-induced AIG but have no effect on platelet-derived growth factor or epidermal growth factor-induced growth in monolayer cultures and do not cross-react with platelet-derived growth factor or TGF-beta. Clones of NRK fibroblasts that express an antisense CTGF gene (NRK-ASCTGF), which blocks the expression of the endogenous CTGF gene, do not respond to TGF-beta in the AIG assay. The growth and morphology of the cells (NRK-ASCTGF) in monolayer culture are unaltered from the parent NRK cell line. The addition of recombinant CTGF to the NRK-ASCTGF clones in the presence of TGF-beta restores the AIG response of the cells. These studies demonstrate that the TGF-beta stimulation of NRK fibroblast AIG is dependent on events induced via the synergistic action of CTGF-dependent and CTGF-independent signaling pathways.

Stimulation of fibroblast cell growth, matrix production, and granulation tissue formation by connective tissue growth factor

Connective tissue growth factor (CTGF) is a 36-to 38-kDa peptide that is selectively induced by transforming growth factor-beta (TGF-beta) in fibroblastic cell types. We compared the biologic activities of CTGF with TGF-beta on fibroblasts in culture and in animal models of fibroplasia. CTGF was active as a mitogen in monolayer cultures of normal rat kidney fibroblasts. CTGF did not stimulate anchorage-independent growth of NRK fibroblasts, however, or inhibit the growth of mink lung epithelial cells, distinguishing CTGF's growth-regulatory activities from those of TGF-beta. In NRK fibroblasts, both TGF-beta and CTGF significantly increased the transcripts encoding alpha 1 type I collagen, alpha 5 integrin, and fibronectin. Stimulation of type I collagen and fibronectin protein synthesis by TGF-beta and CTGF was confirmed by pulse labeling of cells with [35S]methionine. Subcutaneous injection of TGF-beta and CTGF into neonatal NIH Swiss mice resulted in a large stimulation of granulation tissue and fibrosis at the site of injection. In situ hybridization studies revealed that TGF-beta injection induced high levels of CTGF mRNA in the dermal fibroblasts at the injection site, demonstrating that TGF-beta can induce the expression of CTGF in connective tissue cells in vivo. No CTGF transcripts were detected in the epidermal cells in either control or TGF-beta-injected skin or in fibroblasts in control (saline-injected) skin. These results demonstrate that, like TGF-beta, CTGF can induce connective tissue cell proliferation and extracellular matrix synthesis.