SPARC-null mice display abnormalities in the dermis characterized by decreased collagen fibril diameter and reduced tensile strength

Although collagen and elastic fibers are among the major structural constituents responsible for the mechanical properties of skin, proteins that associate with these components are also important for directing formation and maintaining the stability of these fibers. We present evidence that SPARC (secreted protein acidic and rich in cysteine) contributes to collagen fibril formation in the dermis. The skin of SPARC-null adult mice had approximately half the tensile strength as that of wild-type skin. Moreover, the collagen content of SPARC-null skin, as measured by hydroxyproline analysis, was substantially reduced in adult mice. At 2 weeks of age, no differences in collagen content were observed; within 2 months, however, the dermis of SPARC-null mice displayed a reduced collagen content that persisted through adulthood until approximately 20 months, when collagen levels of SPARC-null skin approximated those of wild-type controls. The collagen fibrils present in SPARC-null skin were smaller and more uniform in diameter, in comparison with those of wild-type skin. At 5 months of age, the average fibril diameter in SPARC-null versus wild-type skin was 60.2 nm versus 87.9 nm, respectively. Extraction of soluble dermal collagen revealed a relative increase in collagen VI, accompanied by a decrease in collagen I, in SPARC-null mice. A reduction in the relative amounts of higher-molecular weight collagen complexes was also observed in extracts of dermis from SPARC-null animals. Thus the absence of SPARC compromises the mechanical properties of the dermis, an effect that we attribute, at least in part, to the changes in the structure and composition of its collagenous extracellular matrix.

SPARC-null mice exhibit increased adiposity without significant differences in overall body weight

Secreted protein acidic and rich in cysteine/osteonectin/BM-40 (SPARC) is a matrix-associated protein that elicits changes in cell shape, inhibits cell-cycle progression, and influences the synthesis of extracellular matrix (ECM). The absence of SPARC in mice gives rise to aberrations in the structure and composition of the ECM that result in generation of cataracts, development of severe osteopenia, and accelerated closure of dermal wounds. In this report we show that SPARC-null mice have greater deposits of s.c. fat and larger epididymal fat pads in comparison with wild-type mice. Similar to earlier studies of SPARC-null dermis, we observed a reduction in collagen I in SPARC-null fat pads in comparison with wild-type. Although elevated levels of serum leptin were observed in SPARC-null mice, their overall body weights were not significantly different from those of wild-type counterparts. The diameters of adipocytes from SPARC-null versus wild-type epididymal fat pads were 252 +/- 61 and 161 +/- 33 microm (means +/- SD), respectively, and there was an increase in adipocyte number within SPARC-null fat pads in comparison with wild-type pads. Thus the absence of SPARC appears to result in an increase in the size of individual adipocytes as well as an increase in the number of adipocytes per fat pad. In fat pads isolated from wild-type mice, SPARC mRNA was associated with both the stromal/vascular and adipocyte fractions. We propose that SPARC limits the accumulation of adipose tissue in mice in part through its demonstrated effects on the regulation of cell shape and production of ECM.

Expression of the matricellular protein SPARC in murine lens: SPARC is necessary for the structural integrity of the capsular basement membrane

SPARC (Secreted Protein, Acidic and Rich in Cysteine) is a matricellular glycoprotein that modulates cell proliferation, adhesion, migration, and extracellular matrix (ECM) production. Although SPARC is generally abundant in embryonic tissues and is diminished in adults, we have found that the expression of SPARC in murine lens persists throughout embryogenesis and adulthood. Our previous studies showed that targeted ablation of the SPARC gene in mice results in cataract formation, a pathology attributed partially to an abnormal lens capsule. Here we provide evidence that SPARC is not a structural component of the lens capsule. In contrast, SPARC is abundant in lens epithelial cells, and newly differentiated fiber cells, with stable expression in wild-type mice up to 2 years of age. Pertubation of the lens capsule in animals lacking SPARC appears to be a consequence of the invasion of the lens cells situated beneath the capsule. Immunoreactivity for SPARC in the lens cells was uneven, with minimal reactivity in the epithelial cells immediately anterior to the equator. These epithelial cells appeared essentially noninvasive in SPARC-null mice, in comparison to the centrally located anterior epithelial cells, in which strong labeling by anti-SPARC IgG was observed. The posterior lens fibers exhibited cytoplasmic extensions into the posterior lens capsule, which was severely damaged in SPARC-null lenses. The expression of SPARC in wild-type lens cells, together with the abnormal lens capsule in SPARC-null mice, indicated that the structural integrity of the lens capsule is dependent on the matricellular protein SPARC. The effects of SPARC in the lens appear to involve regulation of lens epithelial and fiber cell morphology and functions rather than deposition as a structural component of the lens capsule.

Amelioration of diabetic nephropathy in SPARC-null mice

SPARC (Secreted Protein, Acidic and Rich in Cysteine) is a matricellular protein that inhibits mesangial cell proliferation and also affects production of extracellular matrix (ECM) by regulating transforming growth factor-beta1 (TGF-beta1) and type I collagen in mesangial cells. This study is an investigation of the role of SPARC in streptozotocin (STZ)-induced diabetic nephropathy (DN) of 6-mo duration in wild type (WT) and SPARC-null mice. SPARC expression was evaluated by immunohistochemistry (IHC) and by in situ hybridization (ISH). Deposition of type I and IV collagen and laminin was evaluated by IHC, and TGF-beta 1 mRNA was assessed by ISH. Renal function studies revealed no significant difference in BUN between diabetic SPARC-null mice and diabetic WT mice, whereas a significant increase in albumin excretion was detected in diabetic WT relative to diabetic SPARC-null mice. Diabetic WT animals exhibited increased levels of SPARC mRNA and protein in glomerular epithelial cells and in interstitial cells, in comparison with nondiabetic WT mice. Neither SPARC mRNA nor protein was detected in SPARC-null mice. Morphometry revealed a significant increase in the percentage of the glomerular tufts occupied by ECM in diabetic WT compared with nondiabetic WT mice, although there was no difference in the mean glomerular tuft area among groups. In contrast, diabetic SPARC-null mice did not show a significant difference in the percentage of the glomerular tufts occupied by ECM relative to nondiabetic null mice. Tubulointerstitial fibrosis was ameliorated in diabetic SPARC-null mice compared with diabetic WT animals. Further characterization of diabetic SPARC-null mice revealed diminished glomerular deposition of type IV collagen and laminin, and diminished interstitial deposition of type I and type IV collagen correlated with decreases in TGF-beta 1 mRNA compared with WT diabetic mice. These observations suggest that SPARC contributes to glomerulosclerosis and tubulointerstitial damage in response to hyperglycemia through increasing TGF-beta 1 expression in this model of chronic DN.

SPARC regulates cell cycle progression in mesangial cells via its inhibition of IGF-dependent signaling

Glomerular mesangial cells both synthesize and respond to insulin-like growth factor-1 (IGF-1). Increased activity of the IGF signaling pathway has been implicated as a major contributor to renal enlargement and subsequent development of diabetic nephropathy. Secreted protein acidic and rich in cysteine (SPARC), a matricellular protein, has been shown to modulate the interaction of cells with growth factors and extracellular matrix. We have reported that primary glomerular mesangial cells derived from SPARC-null mice exhibit an accelerated rate of proliferation and produce substantially decreased levels of transforming growth factor beta1 (TGF-beta1) in comparison to their wild-type counterparts (Francki et al. [1999] J. Biol. Chem. 274: 32145-32152). Herein we present evidence that SPARC modulates IGF-dependent signaling in glomerular mesangial cells. SPARC-null mesangial cells produce increased amounts of IGF-1 and -2, as well as IGF-1 receptor (IGF-1R) in comparison to wild-type cells. Addition of recombinant SPARC to SPARC-null cells inhibited IGF-1-stimulated mitogen activated protein kinase (MAPK) activation and DNA synthesis. We also show that the observed accelerated rate of basal and IGF-1-stimulated proliferation in mesangial cells derived from SPARC-null animals is due, at least in part, to markedly diminished levels of cyclin D1 and the cyclin-dependent kinase (cdk) inhibitors p21 and p27. Since expression of SPARC in the glomerulus is especially prominent during renal injury, our findings substantiate previous claims that SPARC is involved in glomerular remodeling and repair, a process commonly associated with mesangioproliferative glomerulonephritis and diabetic nephropathy.

Enhanced growth of tumors in SPARC null mice is associated with changes in the ECM

SPARC, a 32-kDa glycoprotein, participates in the regulation of morphogenesis and cellular differentiation through its modulation of cell-matrix interactions. Major functions defined for SPARC in vitro are de-adhesion and antiproliferation. In vivo, SPARC is restricted in its expression to remodeling tissues, including pathologies such as cancer. However, the function of endogenous SPARC in tumor growth and progression is not known. Here, we report that implanted tumors grew more rapidly in mice lacking SPARC. We observed that tumors grown in SPARC null mice showed alterations in the production and organization of ECM components and a decrease in the infiltration of macrophages. However, there was no change in the levels of angiogenic growth factors in comparison to tumors grown in wild-type mice, although there was a statistically significant difference in total vascular area. Whereas SPARC did inhibit the growth of tumor cells in vitro, it did not have a demonstrable effect on the proliferation or apoptosis of tumor cells in vivo. These data indicate that host-derived SPARC is important for the appropriate organization of the ECM in response to implanted tumors and highlight the importance of the ECM in regulating tumor growth.

Compromised production of extracellular matrix in mice lacking secreted protein, acidic and rich in cysteine (SPARC) leads to a reduced foreign body reaction to implanted biomaterials

SPARC (secreted protein, acidic and rich in cysteine), a matricellular glycoprotein, modulates the interaction of cells with the extracellular matrix (ECM). Recently, accelerated cutaneous wound closure and altered deposition of collagen were reported in SPARC-null mice. Herein we asked whether SPARC might influence the foreign body reaction to biomaterial implants. Polydimethylsiloxane (silicone rubber) disks and cellulose Millipore filters were implanted into wild-type and SPARC-null mice. In wild-type animals, significant levels of SPARC were observed in the cells and the ECM comprising the capsules around the implants. After 4 weeks, SPARC-null mice exhibited a significant decrease in the thickness of the foreign body capsule, as compared to that observed in wild-type mice. A significant reduction in capsular vascular density was also associated with the silicone implants in the SPARC-null animals. Electron microscopy revealed that collagen fibers in the capsules produced by SPARC-null mice were smaller and more uniform in size than those in wild-type animals. Furthermore, staining with picrosirius-red showed that the collagen fibers were less mature in SPARC-null than in wild-type mice. The altered ECM resulting in decreased capsular thickness, indicative of an altered foreign body reaction in SPARC-null mice, implicates SPARC as an important modulator of the encapsulation of implanted biomaterials.

Inhibition of TIMP1 enhances angiogenesis in vivo and cell migration in vitro

Neovascular invasion into a 3-dimensional matrix is controlled, in part, by matrix metalloproteinases (MMPs) and their inhibitors, tissue inhibitors of metalloproteinases (TIMPs). We tested the hypothesis that increasing MMP activity, via a specific blocking antibody to TIMP1, would enhance fibrovascular invasion into a PVA sponge. In vivo, inhibition of TIMP1 doubled the amount of angiogenic invasion (percentage area of invasion 33.5 +/- 3.5 vs 16.9 +/- 9.5, P = 0.003). The blocking antibody to TIMP1 did not increase the proportion of cells that were proliferating in the sponge implants, underscoring the importance of migration. In vitro, human microvascular endothelial cells (hmEC) and dermal fibroblasts treated with the antibody did not secrete greater amounts of collagenase but migrated significantly farther on collagen I (increase in distance migrated 26.6 +/- 9.4%, P = 0.003). Human dermal microvascular endothelial cells exposed to the TIMP1 blocking antibody exhibited a significant change in cell shape to a more elongated morphology. In conclusion, inhibition of TIMP1 increased angiogenesis into a PVA sponge in vivo and enhanced the migration of dermal hmEC and fibroblasts on collagen I in vitro. We propose that blocking TIMP1 improves angiogenesis by increasing cell motility during fibrovascular invasion.

Localization of SPARC in developing, mature, and chronically injured human allograft kidneys

BACKGROUND

The matricellular protein SPARC (secreted protein acidic and rich in cysteine) is expressed during development, tissue remodeling and repair. It functions as an endogenous inhibitor of cell proliferation, regulates angiogenesis, regulates cell adhesion to extracellular matrix, binds cytokines such as platelet derived growth factor and stimulates transforming growth factor-beta (TGF-beta) production. This study describes the expression of SPARC during human renal development, in normal kidneys and during renal allograft rejection.

METHODS

A total of 60 renal specimens, including normal areas from tumor nephrectomies (N = 24), fetal kidneys (N = 27) and explanted renal allografts (N = 9), were included in the study. SPARC protein was localized by immunohistochemistry using two different antibodies. On consecutive sections SPARC mRNA was detected by in situ hybridization.

RESULTS

In the normal adult kidney SPARC protein was expressed by visceral and parietal epithelial cells, collecting duct epithelium (CD), urothelium, smooth muscle cells of muscular arteries and focally in interstitial cells. During renal development immature glomeruli demonstrated a polarized SPARC expression in visceral epithelial cells at their surface abutting the capillary basement membranes. In the fully differentiated glomeruli the expression pattern mirrored that of the adult kidney. Furthermore, SPARC was abundantly expressed by derivatives of the ureteric bud, and smooth muscle cells of arterial walls. During chronic allograft rejection SPARC is expressed in neointimal arterial smooth muscle cells, infiltrating inflammatory cells as well as by interstitial myofibroblasts in areas of interstitial fibrosis. SPARC mRNA synthesis detected by in situ hybridization mirrored these protein expression patterns.

CONCLUSION

These studies co-localize SPARC to several sites of renal injury previously shown to be sites of PDGF B-chain expression and/or activity. We speculate that SPARC could function as an accessory molecule in chronic PDGF-mediated sclerosing interstitial and vascular injury. SPARC localization to glomerular epithelial cells corresponds to similar findings in rodents, and may reflect its role in cell adhesion and /or regulation of cell shape.

Matricellular proteins: extracellular modulators of cell function

The term 'matricellular' has been applied to a group of extracellular proteins that do not contribute directly to the formation of structural elements in vertebrates but serve to modulate cell-matrix interactions and cell function. Our understanding of the mode of action of matricellular proteins has been advanced considerably by the recent elucidation of the phenotypes of mice that are deficient in these proteins. In many cases, aspects of these phenotypes have illuminated previously unsuspected consequences of the lack of appropriate interactions of cells with their environment.

Alterations in the lens capsule contribute to cataractogenesis in SPARC-null mice

The lens capsule, which is also called the lens basement membrane, is a specialized extracellular matrix produced anteriorly by the lens epithelium and posteriorly by newly differentiated fiber cells. SPARC (secreted protein,acidic and rich in cysteine) is a matricellular glycoprotein that regulates cell-cell and cell-matrix interactions, cellular proliferation and differentiation, and the expression of genes encoding extracellular matrix components. SPARC-null mice exhibit lens opacity 1 month after birth and mature cataract and capsular rupture at 5-7 months. In this study, we report disruption of the structural integrity of the lens capsule in mice lacking SPARC. The major structural protein of basement membrane, collagen type IV, in the lens capsule was substantially altered in the absence of SPARC. The lens cells immediately beneath the capsule showed aberrant morphology, with numerous protrusions into the lens basement membrane. SPARC-null lenses at 1 month of age exhibited an increased penetration of dye or radioactive tracer through the capsule, as well as a higher content of water than their wild-type counterparts. Moreover, SPARC-null fibers exhibited swelling as early as 1 month of age; by 3 months, all the fiber cells appeared swollen to a marked degree. By contrast, the absence of SPARC had no apparent morphological effect on the early stages of lens formation, cell proliferation or fiber cell differentiation. Degradation of crystallins and MIP 26, or changes in the levels of these proteins, were not detected. These results underscore the importance of the capsular extracellular matrix in the maintenance of lens transparency and indicate that SPARC participates in the synthesis, assembly and/or stabilization of the lens basement membrane.

Inhibition of PDGF-stimulated and matrix-mediated proliferation of human vascular smooth muscle cells by SPARC is independent of changes in cell shape or cyclin-dependent kinase inhibitors

Interactions among growth factors, cells, and extracellular matrix regulate proliferation during normal development and in pathologies such as atherosclerosis. SPARC (secreted protein, acidic, and rich in cysteine) is a matrix-associated glycoprotein that modulates the adhesion and proliferation of vascular cells. In this study, we demonstrate that SPARC inhibits human arterial smooth muscle cell proliferation stimulated by platelet-derived growth factor or by adhesion to monomeric type I collagen. Binding studies with SPARC and SPARC peptides indicate specific and saturable interaction with smooth muscle cells that involves the C-terminal Ca2+-binding region of the protein. We also report that SPARC arrests monomeric collagen-supported smooth muscle cell proliferation in the late G1-phase of the cell cycle in the absence of an effect on cell shape or on levels of cyclin-dependent kinase inhibitors. Cyclin-dependent kinase-2 activity, p107 and cyclin A levels, and retinoblastoma protein phosphorylation are markedly reduced in response to the addition of exogenous SPARC and/or peptides derived from specific domains of SPARC. Thus, SPARC, previously characterized as an inhibitor of platelet-derived growth factor binding to its receptor, also antagonizes smooth muscle cell proliferation mediated by monomeric collagen at the level of cyclin-dependent kinase-2 activity.

Altered phosphodiesterase 3-mediated cAMP hydrolysis contributes to a hypermotile phenotype in obese JCR:LA-cp rat aortic vascular smooth muscle cells: implications for diabetes-associated cardiovascular disease

Cardiovascular diseases represent a significant cause of morbidity and mortality in diabetes. Of the many animal models used in the study of non-insulin-dependent (type 2) diabetes, the JCR:LA-cp rat is unique in that it develops insulin resistance in the presence of obesity and manifests both peripheral and coronary vasculopathies. In this animal model, arterial vascular smooth muscle cells (VSMCs) from homozygous obese (cp/cp) rats, but not from age-matched healthy (+/+ or + /cp, collectively defined +/?) littermates, display an " activated" phenotype in vitro and in vivo and have an elevated level of cAMP phosphodiesterase (PDE) activity. In this report, we confirm that cp/cp rat aortic VSMCs have an elevated level of PDE3 activity and show that only particulate PDE3 (PDE3B) activity is elevated. In marked contrast to results obtained in + /? VSMCs, simultaneous activation of adenylyl cyclase and inhibition of PDE3 activity in cp/cp VSMCs synergistically increased cAMP. Although PDE3 inhibition did not potentiate the antimigratory effects of forskolin on +/? VSMCs, PDE3 inhibition did markedly potentiate the forskolin-induced inhibition of migration of cp/cp-derived VSMCs. Although PDE3 activity was elevated in cp/cp rat aortic VSMCs, levels of expression of cytosolic PDE3 (PDE3A) and PDE3B in +/? and cp/cp VSMCs, as well as activation of these enzymes following activation of the cAMP-protein kinase A signaling cascade, were not different. Our data are consistent with an increased role for PDE3 in regulating cAMP-dependent signaling in cp/cp VSMCs and identify PDE3 as a cellular activity potentially responsible for the phenotype of cp/cp VSMCs.

Fractal analysis of region-based vascular change in the normal and non-proliferative diabetic retina

PURPOSE

Evaluation of normal and abnormal vascular pattern in the human retina using a novel method: quantitative region-based fractal analysis.

METHODS

Binary (black/white) vascular patterns of the human retina originating at the optic disc were obtained by semi-automatic computer processing of digital images from 60-degree fundus fluorescein angiography of 5 normal eyes and 5 eyes with non-proliferative diabetic retinopathy (NPDR). As determined by image resolution, vascular patterns included vessels with diameters >or=50 microm and excluded small vessels and capillaries. The density of linearized (i.e., skeletonized) vascular patterns in the macular region versus paramacular region (termed "region-based" linearized vascular pattern) was quantified with the fractal dimension (D(f)) and confirmed by grid intersection (rho(v)).

RESULTS

By region-based quantification, D(f) and rho( v) were significantly higher in the normal macular region than in the NPDR macular region (p = 0.008 and p = 0.019, respectively). However, differences in D(f) and rho(v) between the normal and NPDR paramacular regions were not strongly significant (p = 0.168 and p = 0.337, respectively).

CONCLUSIONS

Results from the retrospective analytical study demonstrate the feasibility of using quantitative region-based fractal analysis of early-stage vascular disease in the human retina. The results are encouraging for a broader study of diverse patient populations.

SPARC-null mice exhibit accelerated cutaneous wound closure

Expression of SPARC (secreted protein acidic and rich in cysteine; osteonectin, BM-40), an extracellular matrix (ECM) associated protein, is coincident with matrix remodeling. To further identify the functions of SPARC in vivo, we have made excisional wounds on the dorsa of SPARC-null and wild-type mice and monitored closure over time. A significant decrease in the size of the SPARC-null wounds, in comparison to that of wild-type, was observed at Day 4 and was maximal at Day 7. Although substantial differences in the percentage of proliferating cells were not apparent in SPARC-null relative to wild-type wounds, primary cultures of SPARC-null dermal fibroblasts displayed accelerated migration, relative to wild-type fibroblasts, in wound assays in vitro. Although the expression of collagen I mRNA in wounds, as measured by in situ hybridization (ISH), was not significantly different in SPARC-null vs wild-type mice, the collagen content of unwounded skin appeared to be substantially lower in the SPARC-null animals. By hydroxyproline analysis, the concentration of collagen in SPARC-null skin was found to be half that of wild-type skin. Moreover, we found an inverse correlation between the efficiency of collagen gel contraction by dermal fibroblasts and the concentration of collagen within the gel itself. We propose that the accelerated wound closure seen in SPARC-null dermis results from its decreased collagen content, a condition contributing to enhanced contractibility.

Increased fibrovascular invasion of subcutaneous polyvinyl alcohol sponges in SPARC-null mice

The expression of SPARC (secreted protein acidic and rich in cysteine/osteonectin/BM-40) is elevated in endothelial cells participating in angiogenesis in vitro and in vivo. SPARC acts on endothelial cells to elicit changes in cell shape and to inhibit cell cycle progression. In addition, SPARC binds to and diminishes the mitotic activity of vascular endothelial growth factor. To determine the effect(s) of SPARC on angiogenic responses in vivo, we implanted polyvinyl alcohol sponges subcutaneously into wild-type and SPARC-null mice. On days 12 and 20 following implantation, SPARC-null mice showed increased cellular invasion of the sponges in comparison to wild-type mice. Areas of the sponge with the highest cell density exhibited the highest numbers of vascular profiles in both wild-type and SPARC-null animals. The endothelial component of the vessels was substantiated by immunoreactivity with three different markers specific for endothelial cells. Although sponges from SPARC-null relative to wild-type mice were populated by significantly more cells and blood vessels, an increase in the ratio of vascular to nonvascular cells was not apparent. No differences in the percentage of proliferating cells within the sponge were detected between wild-type and SPARC-null sections. However, elevated levels of vascular endothelial growth factor were associated with sponges from SPARC-null versus wild-type mice. An increase in vascular endothelial growth factor production was also observed in SPARC-null primary dermal fibroblasts relative to those of wild-type cells. In conclusion, we have shown that the fibrovascular invasion of polyvinyl alcohol sponges is enhanced in mice lacking SPARC, and we propose that increased levels of vascular endothelial growth factor account, at least in part, for this response.

A growth factor mixture that significantly enhances angiogenesis in vivo

Studies of therapeutic angiogenesis have generally focused on single growth factor strategies. However, multiple factors participate in angiogenesis. We evaluated the angiogenic potential of a growth factor mixture (GFm) derived from bovine bone. The major components of GFm (SDS-polyacrylamide gel electrophoresis, mass spectrometry, and Western blot) include transforming growth factor-beta1-3, bone morphogenic protein-2-7, and fibroblast growth factor-1. GFm was first shown to induce an angiogenic response in chorioallantoic membranes. Next, myocardial ischemia was induced in 21 dogs (ameroid) that were randomized 3 weeks later to received GFm 1 mg/ml (I), GFm 10 mg/ml (II), or placebo (P) (with investigators blinded to conditions) injected in and adjacent to ischemic myocardium. Dogs were assessed 6 weeks later using quantitative and semiquantitative measures. There were GFm concentration-dependent improvements in distal left anterior descending artery (LAD) opacification by angiography (P: 0.4 +/- 0.2, I: 1.1 +/- 0.14, II: 1.6 +/- 0.3, angiographic score p = 0.014). Histologically, there was also concentration-dependent vascular growth response of relatively large vessels (P: 0.21 +/- 0.15, I: 1.00 +/- 0.22, II: 1.71 +/- 0.18, vascular growth score p = 0.001). Resting myocardial blood flow (colored microspheres) was not significantly impaired in any group. However, maximum blood flow (adenosine) was reduced in ischemic territories and did not improve in GFm-treated hearts. GFm, a multiple growth factor mixture, is a potent angiogenic agent that stimulates large vessel growth. Although blood flow did not improve during maximal vasodilatory stress, large intramyocardial collateral vessels developed and angiographic visualization of the occluded distal LAD improved significantly. The use of multiple growth factors may be an effective strategy for therapeutic angiogenesis provided a more effective delivery strategy is devised that can achieve improved maximum blood flow potential.

Regulation of retinal capillary cells by basic fibroblast growth factor, vascular endothelial growth factor, and hypoxia

Vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) feature prominently in retinal neovascular diseases. Although the role of VEGF in retinal angiogenesis is well established, the importance of bFGF in this process requires further clarification. This study was undertaken to investigate the responses of retinal capillary cells (endothelial cells and pericytes) to bFGF under hypoxic conditions, as well as the potentially synergistic effects of bFGF and VEGF on the proliferation and cord formation of retinal endothelial cells. Cell proliferation was determined by cell number and by 3H-thymidine incorporation. Cord formation was assessed in three-dimensional gels of collagen type I. VEGF and bFGF increased 3H-thymidine incorporation by both cell types, an effect that was more pronounced in a hypoxic environment. Moreover, the proliferation of pericytes was stimulated to a greater extent by bFGF relative to VEGF. Endothelial migration in collagen gels, however, was induced more effectively by VEGF than by bFGF. A synergistic effect of VEGF and bFGF on cell invasion was observed in the collagen gel assay. VEGF and bFGF each augment proliferation of these cells, especially under hypoxia. We thus propose that these two cytokines have a synergistic effect at several stages of angiogenesis in the retina.

SPARC and the kidney glomerulus: matricellular proteins exhibit diverse functions under normal and pathological conditions

In the last decade, numerous studies have emphasized the important functions that matricellular proteins subserve during angiogenesis, wound healing, and the maintenance of organ and tissue integrity. Matricellular proteins are defined as a group of secreted regulatory macromolecules that are not structural components of the extracellular matrix (ECM) but rather mediate interactions between the ECM and cells. One of these matricellular proteins, termed SPARC (secreted protein acidic and rich in cysteine), is produced during the process of wound healing and is prominent in several types of injury. An excessive deposition of glomerular matrix and an elevated proliferation of certain glomerular cells characterize a variety of kidney diseases. The proliferation of these cells is associated typically with the remodeling process that occurs after kidney injury, and is, at least in part, modulated by the altered expression of ECM, various growth factors, and the elevated production of matricellular proteins (e.g., SPARC). The secretion of one or more of the matricellular proteins can lead to expansion of the glomerular basement membrane, infiltration of immunocompetent cells, and, in some cases, to a reversal of the pathological condition. However, these proteins can also contribute collectively to renal fibrosis, glomerulosclerosis, glomerulonephritis, and the eventual loss of renal function. The purpose of this review is to evaluate the multiple functions of SPARC in the kidney glomerulus under normal and pathological conditions.

SPARC, a matricellular protein: at the crossroads of cell-matrix communication

SPARC is a multifunctional glycoprotein that belongs to the matricellular group of proteins. It modulates cellular interaction with the extracellular matrix (ECM) by its binding to structural matrix proteins, such as collagen and vitronectin, and by its abrogation of focal adhesions, features contributing to a counteradhesive effect on cells. SPARC inhibits cellular proliferation by an arrest of cells in the G1 phase of the cell cycle. It also regulates the activity of growth factors, such as platelet-derived growth factor (PDGF), fibroblast growth factor (FGF)-2, and vascular endothelial growth factor (VEGF). The expression of SPARC in adult animals is limited largely to remodeling tissue, such as bone, gut mucosa, and healing wounds, and it is prominent in tumors and in disorders associated with fibrosis. The crystal structure of two of the three domains of the protein has revealed a novel follistatin-like module and an extracellular calcium-binding (EC) module containing two EF-hand motifs. The follistatin-like module and the EC module are shared by at least four other proteins that comprise a family of SPARC-related genes. Targeted disruption of the SPARC locus in mice has shown that SPARC is important for lens transparency, as SPARC-null mice develop cataracts shortly after birth. SPARC is a prototypical matricellular protein that functions to regulate cell-matrix interactions and thereby influences many important physiological and pathological processes.

SPARC, a matricellular protein: at the crossroads of cell-matrix

SPARC is a multifunctional glycoprotein that belongs to the matricellular group of proteins. It modulates cellular interaction with the extracellular matrix (ECM) by its binding to structural matrix proteins, such as collagen and vitronectin, and by its abrogation of focal adhesions, features contributing to a counteradhesive effect on cells. SPARC inhibits cellular proliferation by an arrest of cells in the G1 phase of the cell cycle. It also regulates the activity of growth factors, such as platelet-derived growth factor (PDGF), fibroblast growth factor (FGF)-2, and vascular endothelial growth factor (VEGF). The expression of SPARC in adult animals is limited largely to remodeling tissue, such as bone, gut mucosa, and healing wounds, and it is prominent in tumors and in disorders associated with fibrosis. The crystal structure of two of the three domains of the protein has revealed a novel follistatin-like module and an extracellular calcium-binding (EC) module containing two EF-hand motifs. The follistatin-like module and the EC module are shared by at least four other proteins that comprise a family of SPARC-related genes. Targeted disruption of the SPARC locus in mice has shown that SPARC is important for lens transparency, as SPARC-null mice develop cataracts shortly after birth. SPARC is a prototypical matricellular protein that functions to regulate cell-matrix interactions and thereby influences many important physiological and pathological processes.

Lenses of SPARC-null mice exhibit an abnormal cell surface-basement membrane interface

SPARC (secreted protein acidic and rich in cysteine) is a matricellular protein involved in cell-matrix interactions. We have shown previously that mice deficient in SPARC develop posterior cortical cataract early in life that progresses to a mature opacity and capsule rupture. To evaluate the primary effects of SPARC deficiency in the lens, we examined the lenses of SPARC-null and wild-type mice by electron microscopy and immunohistochemistry to investigate whether ultrastructural abnormalities occur at the basement membrane (capsule)-lens cell interface in SPARC-null mice. The most notable feature in the lenses of SPARC-null mice, relative to wild-type animals, was the modification of the basal surface of the lens epithelial and fiber cells at the basement membrane (capsule) interface. Electron microscopy revealed numerous filopodial projections of the basal surface of the lens epithelial and fiber cells into the extracellular matrix of the anterior, posterior, and equatorial regions of the lens capsule. In 1 week old precataractous lenses, basal invasive filopodia projecting into the capsule were small and infrequent. Both the size and frequency of these filopodia increased in precataractous 3-4 week old lenses and were prominent in the cataractous 5-6 week old lenses. By rhodamine-phalloidin labeling, we confirmed the presence of basal invasive filopodia projecting into the lens capsule and demonstrated that the projections contained actin filaments. In contrast to the obvious abnormal projections at the interface between the basal surface of the lens epithelial and fiber cells and the lens capsule, the apical and lateral plasma membranes of lens epithelial cells and lens fibers in SPARC-null mice were as smooth as those of wild-type mice. We conclude that the absence of SPARC in the murine lens is associated with a filopodial protrusion of the basal surface of the lens epithelium and differentiating fiber cells into the lens capsule. The altered structures appear prior to the opacification of the lens in the SPARC-null model. These observations are consistent with one or more functions previously proposed for SPARC as a modulator of cell shape and cell-matrix interactions.