They showed positive staining for intracellular myosin and vimentin and were negative for α-smooth muscle actin, cytokeratin, and factor VIII.
MCs were established in culture from rat kidney as previously reported by us 23, 24 and used at passage 5 to 10. To increase our understanding of the role of HA in affecting MC function, the present study examined the kinetics of HA synthesis by MCs and the subsequent effect of HA on the accumulation of proteoglycans and other matrix constituents. 14, 22 The subsequent role of HA in controlling MC function, however, has received little attention. 22 A HA-based matrix is also produced in response to high concentrations of glucose and is assembled by MCs into structures adhesive to monocytes. 15 In vitro MCs synthesize HA in response to low-density lipoprotein, 18, 19, 20, 21 fibronectin, or growth factors such as platelet-derived growth factor (PDGF). 17 HA is also expressed at high levels in the glomerulus of rejecting kidneys 18, 19, 20 and in lupus nephritis. 12, 13, 14, 15 It is an abundant extracellular component of crescents in rat autoimmune glomerulonephritis 12, 16 and is elevated in proliferating MCs in the Thy-1 model of glomerular injury. Although in the normal kidney high levels of HA are found only in the renal papilla, there is greatly increased interstitial expression of both HA and its receptor, CD44, in the cortex after acute ischemic injury, interstitial inflammation, or during progressive fibrosis. It consists of repeating disaccharide units of glucuronic acid and N-acetylglucosamine and is extruded from the plasma membrane as a chain, which may reach a molecular mass of as much as 10 7 Da. HA is a water-soluble, nonsulfated glycosaminoglycan that is a key constituent of the pericellular matrix and has important structural functions in the extracellular matrix of most tissues. Furthermore, some matrix molecules that have low expression levels or are not present at all in the normal glomerulus may become expressed at high levels in disease. 6, 7, 8, 9, 10, 11 This may adversely affect glomerular function leading to glomerulosclerosis and ultimately organ failure. 5 In a variety of glomerular diseases, this equilibrium is shifted leading to changes in the amount and composition of the mesangial matrix. 3, 4 In the normal kidney these components are maintained at constant levels by the establishment of an equilibrium between their synthesis and degradation.
2 The mesangial matrix is synthesized by mesangial cells (MCs) and is a complex mix of glycoproteins such as collagen type IV, laminin, fibronectin, and a number of different proteoglycans. 1 The glomerular basement membrane forms part of the filtration mechanism, whereas the mesangial matrix forms the structural matrix of the glomerulus. In the glomerulus, two separate extracellular matrices have been described, the glomerular basement membrane and the mesangial matrix. The extracellular matrix plays a key role in homeostasis and in maintaining the structural architecture of tissues. HA may thus be an attractive target for therapeutic intervention. These results suggest that high levels of HA in the mesangium in disease is a mechanism controlling the accumulation of specific mesangial matrix components. Adding exogenous HA to unstimulated cells that had undetectable pericellular coats of HA selectively reduced perlecan and versican turnover, whereas other proteoglycans were unaffected. HA was both released into the medium and incorporated into extensive pericellular coats. When incubated with interleukin-1, platelet-derived growth factor, or fetal calf serum, MCs initiated rapid HA synthesis associated with the up-regulation of HA synthase-2 and increased the synthesis of versican, perlecan, and decorin/biglycan. Metabolic labeling, ion exchange and size exclusion chromatography, Western blotting, and immunocytochemistry were used to identify changes in matrix accumulation. This study examines proteoglycan and hyaluronan (HA) synthesis by MCs triggered by proinflammatory agents and investigates the effect of an exogenous HA matrix on matrix synthesis by MCs. In many renal diseases this matrix is reorganized in response to a variety of cytokines and growth factors. Mesangial cells (MCs) are essential for normal renal function through the synthesis of their own extracellular matrix, which forms the structural support of the renal glomerulus.
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