Perspective Endothelial Cell Proteoglycans: Possible Mediators of Vascular Responses to Injury William E. Benitz and Merton Bernfield Department of Pediatrics, Stanford University School of Medicine, Stanford, California and the Joint Program in Neonatology, Harvard Medical School, Boston, Massachusetts
New roles for "old" molecules are being described with increasing frequency. One such class of molecules are the proteoglycans, formerly known as the mucopolysaccharides. These specialized glycoproteins contain a protein core covalently linked to one or more chains of a highly sulfated linear polysaccharide known as a glycosaminoglycan or GAG. Proteoglycans are classified chemically according to the nature of their GAG chains, and there are few structural features in common among the core protein molecules. These molecules were once thought merely to fill space and provide. turgor to tissues, especially cartilage, by binding water via their highly anionic chains. As knowledge has grown, they have emerged to represent a diverse group of complex, often multidomain proteins, bearing GAG chains that bind a wide spectrum of biologic effector molecules. The cell biology of the proteoglycans has only recently become the subject of intense inquiry. The core protein likely targets proteoglycans to their correct size within or outside the cell, while the GAG chains bind various proteins and influence the ionic and water composition of tissues. In addition to filling extracellular spaces, newly appreciated roles for proteoglycans in the extracellular matrix include organization of matrix molecules, as in tendons or in the basement membrane. Proteoglycans intercalated in the plasma membrane bind to peptide growth factors, matrix components, anticoagulant and other serum proteins. Moreover, proteoglycans are sequestered in secretory granules within the cell with a variety of other molecules (1, 2). Every cell type produces multiple proteoglycans, but the details oftheir function in cellular economy are still obscure. Vascular endothelial cells have been a focus for proteoglycan biochemists, possibly because the luminal surface of these cells bears proteoglycans containing antithrombogenic heparan sulfate chains that are largely responsible for the nonthrombotic properties of the vascular wall (3). Fetal pulmonary artery endothelial cells produce a heparan sul-
(Received for publication February 23, 1990) Address correspondence to: Merton Bernfield, M.D., Joint Program in Neonatology, Harvard Medical School, 300 Longwood Avenue, Boston, MA 02115. Abbreviation: glycosaminoglycan, GAG. Am. J. Respir. Cell Mol. BioI. Vol. 2. pp. 407-408, 1990
fate-containing basement membrane proteoglycan that is a potent inhibitor of smooth muscle cell proliferation in vitro. In vivo, injury of the vascular intima leads to smooth muscle cell migration and hyperplasia only if the basement membrane is disrupted. These behaviors are blocked by the administration of heparin, suggesting that endogenous proteoglycan growth inhibitor(s) act to tonically inhibit vascular smooth muscle cell migration and proliferation. Proteoglycan synthesis by cultured endothel ial cells can be modified by a variety of stimuli. Wounding of confluent monolayers of bovine aortic endothelial cells induces a marked increase in proteoglycan production along with a shift from predominantly heparan sulfate proteoglycans in confluent monolayers to predominantly chondroitin/dermatan sulfate in wounded cultures. Interleukins induce the accumulation of a chondroitin sulfate proteoglycan-rich pericellular matrix by human umbilical vein endothelial cells. Hypoxic conditions cause bovine pulmonary artery endothelial cells to reduce their release of heparan sulfate proteoglycan into the culture medium. Thus, endothelial cells appear to modify their proteoglycan production in response to a variety of stimuli (4). In this issue of the journal, Kaner and associates (5) describe a marked reduction in the production of chondroitin sulfate and heparan sulfate proteoglycans by umbilical vein endothelial cells following infection with herpes simplex virus. They suggest that virus-induced changes in proteoglycan metabolism may be involved in the pathogenesis of diseases such as atherosclerosis. The role of viral infection in human atherosclerotic disease is speculative, although Benditt and colleagues have presented evidence for herpesvirus infection of medial smooth muscle cells in early atherosclerotic lesions in the ascending aorta (6). Whether viral infection in vivo duplicates this response is not known, and the biologic consequences of this change have not been examined. Indeed, the reduced synthesis may not be quickly reflected in lower local concentrations of these products if they turn over slowly in vivo. Thus, assessment of the role of the proteoglycans as mediators of vascular responses to viral infection will require information about the kinetics and mechanisms of proteoglycan degradation. The distribution of proteoglycans among the various tissue compartments (i .e., intracellular, cell surface, basement membrane, and interstitial matrix) needs to be elucidated in greater detail. Future work must also focus upon which specific molec-
408
AMERICAN JOURNAL OF RESPIRATORY CELL AND MOLECULAR BIOLOGY VOL. 2 t 990
ular species of proteoglycans are involved to be able to correlate any viral-induced changes with the defined functional properties of these molecules. The hypothesis of Kaner and associates that proteoglycans may be involved in the pathogenesis of vascular diseases is part of a more global notion that proteoglycans are critically involved in normal and abnormal vascular biology (7). This notion is consistent with the data that various stimuli alter endothelial production and metabolism of proteoglycans and that these molecules mediate a variety of significant molecular and cellular interactions (4, 7). These data suggest a number of mechanisms by which changes in proteoglycan production by endothelial cells may contribute to the vascular response to injuries. Loss of antithrombogenic heparan sulfate proteoglycan from the cell surface may permit local activation of the coagulation cascade and platelet adhesion and degranulation, inducing additional proteolytic injury and releasing chemoattractants that elicit the involvement of macrophages, granulocytes, and smooth muscle cells. Removal of antiproliferative heparan sulfate from the endothelial basement membrane may permit local proliferation and migration of smooth muscle cells, leading to neointimal muscularization, as in early atherosclerosis, or to medial hypertrophy, as occurs in pulmonary hypertension. Altered binding of growth factors, such as bFGF, may contribute to pathologic proliferation of smooth muscle cells, or impair endothelial healing at sites where these materials are no longer bound in the extracellular matrix.
To date, most of the data have been derived from in vitro experiments and model systems. Meticulous investigation of injury-induced changes in metabolism of individual proteoglycans, elucidation of the biologic functions of these molecules, and correlation with in vivo observations will be required to ascertain the significance of these molecules in normal and abnormal vascular biology. As these data unravel the potential role of these molecules, it may be possible to determine whether proteoglycans are protagonists in the drama of vascular development and disease or are merely innocent bystanders whose metabolism is profoundly affected by vascular injuries.
References I. Gallagher, 1. T. 1989. Extended family of proteoglycans: social residents of the pericellular zone. Current Opinion in Cell Biology 1:1201-1218. 2. Bernfield, M., and R. D. Sanderson. 1990. Syndecan, a morphogenetically regulated cell surface proteoglycan that binds extracellular matrix and growth factors. Philos. Trans. R. Soc. Lond. [Bioi.] 327:171-186. 3. Marcum, 1. A., and R. D. Rosenberg. 1989. Role of endothelial cell surface heparin-like polysaccharides. Ann. NY Acad. Sci. 556:81-94. 4. Wight, T. N. 1989. Cell biology of arterial proteoglycans. Arteriosclerosis
9:1-20. 5. Kaner, R. 1., R. V. Iozzo, Z. Ziaie, and N. A. Kefalides. 1990. Inhibition of proteoglycan synthesis in human endothelial cells after infection with herpes simplex virus type I in vitro. Am. J. Respir. Cell Mol. Bioi. 2:423-431. 6. Benditt, E. P., T. Varrett, and 1. K. McDougall. 1983. Viruses in the etiology of atherosclerosis. Proc. Natl. Acad. Sci. USA 80:6386-6389. 7. Berenson, G. S., B. Radhakrishnamurthy, S. R. Srinivasan, P. Vijayagopal, and E. 1. Dalferes. 1988. Arterial wall injury and proteoglycan changes in atherosclerosis. Arch. Pathol. Lab. Med. 112:1002-1010.
New roles for "old" molecules are being described with increasing frequency. One such class of molecules are the proteoglycans, formerly known as the mucopolysaccharides. These specialized glycoproteins contain a protein core covalently linked to one or more chains of a highly sulfated linear polysaccharide known as a glycosaminoglycan or GAG. Proteoglycans are classified chemically according to the nature of their GAG chains, and there are few structural features in common among the core protein molecules. These molecules were once thought merely to fill space and provide. turgor to tissues, especially cartilage, by binding water via their highly anionic chains. As knowledge has grown, they have emerged to represent a diverse group of complex, often multidomain proteins, bearing GAG chains that bind a wide spectrum of biologic effector molecules. The cell biology of the proteoglycans has only recently become the subject of intense inquiry. The core protein likely targets proteoglycans to their correct size within or outside the cell, while the GAG chains bind various proteins and influence the ionic and water composition of tissues. In addition to filling extracellular spaces, newly appreciated roles for proteoglycans in the extracellular matrix include organization of matrix molecules, as in tendons or in the basement membrane. Proteoglycans intercalated in the plasma membrane bind to peptide growth factors, matrix components, anticoagulant and other serum proteins. Moreover, proteoglycans are sequestered in secretory granules within the cell with a variety of other molecules (1, 2). Every cell type produces multiple proteoglycans, but the details oftheir function in cellular economy are still obscure. Vascular endothelial cells have been a focus for proteoglycan biochemists, possibly because the luminal surface of these cells bears proteoglycans containing antithrombogenic heparan sulfate chains that are largely responsible for the nonthrombotic properties of the vascular wall (3). Fetal pulmonary artery endothelial cells produce a heparan sul-
(Received for publication February 23, 1990) Address correspondence to: Merton Bernfield, M.D., Joint Program in Neonatology, Harvard Medical School, 300 Longwood Avenue, Boston, MA 02115. Abbreviation: glycosaminoglycan, GAG. Am. J. Respir. Cell Mol. BioI. Vol. 2. pp. 407-408, 1990
fate-containing basement membrane proteoglycan that is a potent inhibitor of smooth muscle cell proliferation in vitro. In vivo, injury of the vascular intima leads to smooth muscle cell migration and hyperplasia only if the basement membrane is disrupted. These behaviors are blocked by the administration of heparin, suggesting that endogenous proteoglycan growth inhibitor(s) act to tonically inhibit vascular smooth muscle cell migration and proliferation. Proteoglycan synthesis by cultured endothel ial cells can be modified by a variety of stimuli. Wounding of confluent monolayers of bovine aortic endothelial cells induces a marked increase in proteoglycan production along with a shift from predominantly heparan sulfate proteoglycans in confluent monolayers to predominantly chondroitin/dermatan sulfate in wounded cultures. Interleukins induce the accumulation of a chondroitin sulfate proteoglycan-rich pericellular matrix by human umbilical vein endothelial cells. Hypoxic conditions cause bovine pulmonary artery endothelial cells to reduce their release of heparan sulfate proteoglycan into the culture medium. Thus, endothelial cells appear to modify their proteoglycan production in response to a variety of stimuli (4). In this issue of the journal, Kaner and associates (5) describe a marked reduction in the production of chondroitin sulfate and heparan sulfate proteoglycans by umbilical vein endothelial cells following infection with herpes simplex virus. They suggest that virus-induced changes in proteoglycan metabolism may be involved in the pathogenesis of diseases such as atherosclerosis. The role of viral infection in human atherosclerotic disease is speculative, although Benditt and colleagues have presented evidence for herpesvirus infection of medial smooth muscle cells in early atherosclerotic lesions in the ascending aorta (6). Whether viral infection in vivo duplicates this response is not known, and the biologic consequences of this change have not been examined. Indeed, the reduced synthesis may not be quickly reflected in lower local concentrations of these products if they turn over slowly in vivo. Thus, assessment of the role of the proteoglycans as mediators of vascular responses to viral infection will require information about the kinetics and mechanisms of proteoglycan degradation. The distribution of proteoglycans among the various tissue compartments (i .e., intracellular, cell surface, basement membrane, and interstitial matrix) needs to be elucidated in greater detail. Future work must also focus upon which specific molec-
408
AMERICAN JOURNAL OF RESPIRATORY CELL AND MOLECULAR BIOLOGY VOL. 2 t 990
ular species of proteoglycans are involved to be able to correlate any viral-induced changes with the defined functional properties of these molecules. The hypothesis of Kaner and associates that proteoglycans may be involved in the pathogenesis of vascular diseases is part of a more global notion that proteoglycans are critically involved in normal and abnormal vascular biology (7). This notion is consistent with the data that various stimuli alter endothelial production and metabolism of proteoglycans and that these molecules mediate a variety of significant molecular and cellular interactions (4, 7). These data suggest a number of mechanisms by which changes in proteoglycan production by endothelial cells may contribute to the vascular response to injuries. Loss of antithrombogenic heparan sulfate proteoglycan from the cell surface may permit local activation of the coagulation cascade and platelet adhesion and degranulation, inducing additional proteolytic injury and releasing chemoattractants that elicit the involvement of macrophages, granulocytes, and smooth muscle cells. Removal of antiproliferative heparan sulfate from the endothelial basement membrane may permit local proliferation and migration of smooth muscle cells, leading to neointimal muscularization, as in early atherosclerosis, or to medial hypertrophy, as occurs in pulmonary hypertension. Altered binding of growth factors, such as bFGF, may contribute to pathologic proliferation of smooth muscle cells, or impair endothelial healing at sites where these materials are no longer bound in the extracellular matrix.
To date, most of the data have been derived from in vitro experiments and model systems. Meticulous investigation of injury-induced changes in metabolism of individual proteoglycans, elucidation of the biologic functions of these molecules, and correlation with in vivo observations will be required to ascertain the significance of these molecules in normal and abnormal vascular biology. As these data unravel the potential role of these molecules, it may be possible to determine whether proteoglycans are protagonists in the drama of vascular development and disease or are merely innocent bystanders whose metabolism is profoundly affected by vascular injuries.
References I. Gallagher, 1. T. 1989. Extended family of proteoglycans: social residents of the pericellular zone. Current Opinion in Cell Biology 1:1201-1218. 2. Bernfield, M., and R. D. Sanderson. 1990. Syndecan, a morphogenetically regulated cell surface proteoglycan that binds extracellular matrix and growth factors. Philos. Trans. R. Soc. Lond. [Bioi.] 327:171-186. 3. Marcum, 1. A., and R. D. Rosenberg. 1989. Role of endothelial cell surface heparin-like polysaccharides. Ann. NY Acad. Sci. 556:81-94. 4. Wight, T. N. 1989. Cell biology of arterial proteoglycans. Arteriosclerosis
9:1-20. 5. Kaner, R. 1., R. V. Iozzo, Z. Ziaie, and N. A. Kefalides. 1990. Inhibition of proteoglycan synthesis in human endothelial cells after infection with herpes simplex virus type I in vitro. Am. J. Respir. Cell Mol. Bioi. 2:423-431. 6. Benditt, E. P., T. Varrett, and 1. K. McDougall. 1983. Viruses in the etiology of atherosclerosis. Proc. Natl. Acad. Sci. USA 80:6386-6389. 7. Berenson, G. S., B. Radhakrishnamurthy, S. R. Srinivasan, P. Vijayagopal, and E. 1. Dalferes. 1988. Arterial wall injury and proteoglycan changes in atherosclerosis. Arch. Pathol. Lab. Med. 112:1002-1010.
