αv integrins on mesenchymal cells regulate skeletal and cardiac muscle fibrosis.

Mesenchymal cells expressing platelet-derived growth factor receptor beta (PDGFRß) are known to be important in fibrosis of organs such as the liver and kidney. Here we show that PDGFRß+ cells contribute to skeletal muscle and cardiac fibrosis via a mechanism that depends on αv integrins. Mice in which αv integrin is depleted in PDGFRß+ cells are protected from cardiotoxin and laceration-induced skeletal muscle fibrosis and angiotensin II-induced cardiac fibrosis. In addition, a small-molecule inhibitor of αv integrins attenuates fibrosis, even when pre-established, in both skeletal and cardiac muscle, and improves skeletal muscle function. αv integrin blockade also reduces TGFß activation in primary human skeletal muscle and cardiac PDGFRß+ cells, suggesting that αv integrin inhibitors may be effective for the treatment and prevention of a broad range of muscle fibroses.

Peptidomimetic antagonists of alphavbeta3 inhibit bone resorption by inhibiting osteoclast bone resorptive activity, not osteoclast adhesion to bone.

Osteoclasts are actively motile on bone surfaces and undergo alternating cycles of migration and resorption. Osteoclast interaction with the extracellular matrix plays a key role in the osteoclast resorptive process and a substantial body of evidence suggests that integrin receptors are important in osteoclast function. These integrin receptors bind to the Arg-Gly-Asp (RGD) sequence found in a variety of extracellular matrix proteins and it is well established that the interaction of osteoclast alpha v beta 3 integrin with the RGD motif within bone matrix proteins is important in osteoclast-mediated bone resorption. In this study, we characterized the effects of two synthetic peptidomimetic antagonists of alpha v beta 3, SC-56631 and SC-65811, on rabbit osteoclast adhesion to purified matrix proteins and bone, and on bone resorption in vitro. SC-56631 and SC-65811 are potent inhibitors of vitronectin binding to purified alpha v beta 3. Both SC-56631 and SC-65811 inhibited osteoclast adhesion to osteopontin- and vitronectin-coated surfaces and time-lapse video microscopy showed that osteoclasts rapidly retract from osteopontin-coated surfaces when exposed to SC-56631 and SC-65811. SC-56631 and SC-65811 blocked osteoclast-mediated bone resorption in a dose-responsive manner. Further analysis showed that SC-65811 and SC-56631 reduced the number of resorption pits produced per osteoclast and the average pit size. SC-65811 was a more potent inhibitor of bone resorption and the combination of reduced pit number and size led to a 90% inhibition of bone resorption. Surprisingly, however, osteoclasts treated with SC-65811, SC-56631 or the disintegrin echistatin, at concentrations that inhibit bone resorption did not inhibit osteoclast adhesion to bone. These results suggest that alphavbeta3 antagonists inhibited bone resorption by decreasing osteoclast bone resorptive activity or efficiency but not by inhibiting osteoclast adhesion to bone per se.

A peptidomimetic antagonist of the integrin alpha(v)beta3 inhibits Leydig cell tumor growth and the development of hypercalcemia of malignancy.

The integrin alpha(v)beta3 interacts with the arginine-glycine-aspartic acid (RGD) tripeptide recognition sequence of a variety of extracellular matrix proteins. Recent studies show that alpha(v)beta3 plays an important role in tumor-induced angiogenesis and tumor growth and that antagonists of alpha(v)beta3 inhibit angiogenic processes that include endothelial cell adhesion and migration. Consequently, we reasoned that an RGD-based peptidomimetic antagonist of alpha(v)beta3 might inhibit tumor angiogenesis and tumor growth in vivo. An RGD-peptidomimetic library was screened to identify antagonists of vitronectin binding to alpha(v)beta3, and the compounds chosen were modified to produce selective and potent inhibitors of alpha(v)beta3. One of these compounds, beta-[[2-2-[[[3-[(aminoiminomethyl)amino]-phenyl]carbonyl]amino]ac etyl]amino]-3,5-dichlorobenzenepropanoic acid (SC-68448), inhibited vitronectin binding to both alpha(v)beta3 and the closely related platelet receptor, alpha(IIb)beta3, in a dose-responsive manner. SC-68448 inhibited vitronectin binding to alpha(v)beta3 (IC50, 1 nM) and fibrinogen binding to the platelet receptor alpha(IIb)beta3 (IC50, >100 nM), demonstrating that SC-68448 was 100-fold more potent as an inhibitor of alpha(v)beta3 versus alpha(IIb)beta3. In cell-based studies, SC-68448 inhibited alpha(v)beta3-mediated endothelial cell proliferation in a dose-dependent manner but did not inhibit tumor cell proliferation, suggesting that effects on endothelial cell proliferation were not due to SC-68448-induced cytotoxicity. In accord with these results, SC-68448 inhibited angiogenesis in vivo in a basic fibroblast growth factor-induced rat corneal neovascularization model. A xenogeneic severe combined immune deficiency mouse/rat Leydig cell tumor model was developed for testing SC-68448 as an inhibitor of tumor growth in vivo. Rat Leydig cell tumors grew rapidly in severe combined immune deficiency mice and produced humoral hypercalcemia of malignancy. SC-68448 inhibited the growth of the tumors in mice by up to 80% and completely blocked the development of hypercalcemia. Together, these results demonstrate the feasibility of antitumor therapies based upon the development of nontoxic small molecule pharmacological antagonists of integrin alpha(v)beta3.

A peptidomimetic antagonist of the alpha(v)beta3 integrin inhibits bone resorption in vitro and prevents osteoporosis in vivo.

Osteoclastic bone degradation requires intimacy between the matrix and the resorptive cell. While the precise role the integrin alpha(v)beta3 plays in the process is not yet understood, occupancy of the heterodimer by soluble ligand or by blocking antibody effectively inhibits bone resorption in vitro and in vivo, suggesting that alpha(v)beta3 blockade may prevent postmenopausal osteoporosis. Thus, we identified a synthetic chemical peptide mimetic, beta-[2-[[5-[(aminoiminomethyl)amino]-1-oxopentyl]amino]-1-+ ++oxoethyl]amino-3-pyridinepropanoic acid, bistrifluoroacetate (SC56631) based upon the alpha(v)beta3 ligand, Arg-Gly-Asp (RGD), which recognizes the isolated integrin, and its relative, alpha(v)beta5, as effectively as does the natural peptide. The mimetic dampens osteoclastic bone resorption in vitro and in vivo. Most importantly, intravenous administration of the mimetic prevents the 55% loss of trabecular bone sustained by rats within 6 wk of oophorectomy. Histological examination of bones taken from SC56631-treated, oophorectomized animals also demonstrates the compound's bone sparing properties and its capacity to decrease osteoclast number. Thus, an RGD mimetic prevents the rapid bone loss that accompanies estrogen withdrawal.