Biglycan expression in vascular smooth muscle cells is increased by TGF-β, which also markedly increases the size of the GAG chains on the biglycan, and accordingly there is a strong association with biglycan, TGF-β, and lipid as a causal step in the development of atherosclerosis, and this is a therapeutic target [28, 115, 116].

The most investigated area of the potential of therapeutic agents directed at proteoglycan synthesis is the signaling pathways that mediate elongation of CS/DS GAG chains on biglycan because of the direct association with increased lipid binding [26]. Many growth factors and hormones stimulate vascular smooth muscle cells to synthesize and secrete biglycan with increased GAG length [117], a phenomenon that we have termed hyperelongation, because it represents a further elongation of GAG than that seen under basal conditions [26]. Vasoactive factors that stimulate GAG elongation include seven-transmembrane G protein-coupled receptor agonists, such as angiotensin II, endothelin-1, and thrombin; protein tyrosine kinase receptor agonists such as PDGF and epidermal growth factor (EGF); and serine/threonine kinase agonists such as TGF-β (Fig. 11.1) [117]. Taking the serine/threonine kinase receptor for TGF-β, transforming growth factor-β receptor I (TβRI), as an example, there has been considerable progress in characterizing the pathways through which this receptor leads to GAG hyperelongation and in distinguishing these pathways from those that mediate increased expression of the proteoglycan core protein [32, 118]. TGF-β also stimulates a pathway, known as non-Smad, which involves the activation of the MAP kinase pathway leading to stimulation of Erk1/2 and phosphorylation of Smad in the linker region. The linker region is located between receptor Smads Mad homology domains 1 and 2. Although phosphorylation in the linker region was originally shown to inhibit Smad function [119, 120], most recent studies show that linker region phosphorylation is a positive driver of Smad responses [116, 121, 122]. There is a small group of serine and threonine residues in the linker region of Smad transcription factors, and these are linked to specific downstream pathways that regulate gene transcription, including that controlling the expression of genes which mediate GAG elongation [121]. For vasoactive agents which are mitogens, such as PDGF, which also stimulate proteoglycan core protein expression, the core protein expression and mitogenic pathways appear to be common or at least similar, including PI3K and Akt, whereas GAG elongation pathways are mediated via MAP kinases, especially Erk1/2 [32]. The highly specific biochemical targets that have been identified in this work have the potential to be used for the development of drugs to prevent GAG elongation and lipid deposition in atherogenesis.

The potential for inhibiting GAG elongation and preventing lipid deposition and atherosclerosis has been demonstrated in animal models [32, 34]. The antitumor agent imatinib (STI 571, Gleevec, Glivec) was developed for the inhibition of Abl tyrosine kinase in chronic myeloid leukemia, in which this kinase is constitutively activated [123]. Imatinib has been characterized as a tyrosine kinase inhibitor and it inhibits only a small number of kinases. In the vascular context, imatinib inhibits Abl as well as Kit and PDGF receptor kinase [124]. Imatinib is a potent inhibitor of PDGF-stimulated GAG elongation [32, 34]. GAG chains isolated from vascular smooth muscle cells treated with imatinib show reduced binding to normal human LDL, and high-fat-fed atherosclerosis-prone mice treated with imatinib show reduce lipid deposition ex vivo and in vivo [32, 34]. Atherosclerosis is associated with many growth factors and hormones, and intriguingly we have found that imatinib inhibits the GAG elongation action of not only tyrosine kinase agonists but also of seven-transmembrane G protein-coupled receptor and serine/threonine kinase agonists (Little, unpublished observation). These latter agonists have distinct signaling pathways (Fig. 11.1), so identifying the precise target of the action of imatinib might produce a therapeutically useful GAG inhibitor. Such a drug would be used in combination with a HMG CoA reductase inhibitor (statin), such that the statin would reduce circulating levels of atherogenic lipoproteins and the GAG inhibitor would render the vessel wall less sticky, with the overall effect of a reduced rate of progression of atherosclerosis [125].