Tyrosine kinase dependent expression of TGF-beta induced by stretch in mesangial cells

Nitric oxide modulates stretch activation of mitogen-activated protein kinases in mesangial cells

BACKGROUND

In vivo, intraglomerular hypertension results in resident cell hypertrophy, proliferation and matrix protein production, leading to glomerulosclerosis. Mesangial cells (MCs) exposed to in vitro stretch also proliferate and produce matrix. We have shown activation of Jun N-terminal kinase/stress-activated protein kinase (SAPK) and p42/44 mitogen-activated protein kinase (MAPK) in stretched MCs and have also demonstrated that L-arginine decreases resident cell proliferation and protects against glomerulosclerosis in remnant kidney glomeruli, presumably by increasing nitric oxide (NO) production. Consequently, we studied whether NO could affect SAPK and p42/44 MAPK activation in stretched MCs.

METHODS

MCs (passages 5 to 10) cultured on type 1 collagen-coated, flexible-bottom plates were exposed to 0 to 30 minutes of cyclic strain (60 cycles per minute) by computer-driven generation of vacuum of -27 kPa, inducing 28% elongation in the diameter of the surface. Control MCs were grown on coated, flexible-bottom plates. Protein levels (by Western blot) and activity assays for SAPK/JNK and p42/44 MAPK were performed under these conditions. As maximal activation was at 10 minutes, with decay by 30 minutes, the effect of NO on kinase activation was studied at 0, 2, 5, and 10 minutes by preincubation with 70 micromol/L s-nitroso-n-acetylpenicillamine (SNAP; an NO donor) or 1 mmol/L 8-bromo cyclic guanosine monophosphate (8-bromo-cGMP). Downstream events in response to stretch and NO were studied at the time of maximal response (10 minutes) by examining nuclear translocation of SAPK with immunofluorescence microscopy and transcription factor activator protein-1 nuclear protein binding by gel mobility shift assay. The effect of kinase inhibition by NO donors on MC proliferation was studied by Western blotting for proliferating cell nuclear antigen (PCNA).

RESULTS

Cyclic MC stretch led to prompt SAPK and p42/44 MAPK activation, which was maximal at 10 minutes. Preincubation with either SNAP or 8-bromo-cGMP decreased this by 50 and 70%, respectively (N = 4), suggesting that the effect of NO was through cGMP generation. Nuclear translocation of both phosphorylated kinases was seen after 10 minutes of stretch and was largely prevented by 8-bromo-cGMP. Increased DNA binding of activator protein-1 proteins was observed in the nuclei of stretched MCs at 10 minutes by mobility shift assay (N = 4), which was also largely prevented by 8-bromo-cGMP. Stretch increased PCNA expression by MCs, and this was inhibited by 8-bromo-cGMP.

CONCLUSIONS

Stretch-induced activation of SAPK and p42/44 MAPK in MCs can be inhibited by NO. The effect of NO is mediated by the generation of cGMP. These mechanisms may be responsible, at least in part, for the protective effect of NO in animal models of glomerular injury characterized by glomerular capillary hypertension.

Nitric oxide modulates mechanical strain-induced activation of p38 MAPK in mesangial cells

Mesangial cells (MC), grown on extracellular matrix (ECM) protein-coated plates and stretched, proliferate and produce ECM, recapitulating in vivo responses to increased glomerular capillary pressure (Pgc). Transduction of strain involves mitogen-activated protein kinases (MAPK), and we have shown that p38 MAPK is activated by strain in MC. Because in vivo studies show that nitric oxide (NO) in the remnant kidney limits glomerular injury without reducing Pgc, we studied whether NO attenuated stretch-induced p38 activation in MC. Increasing p38 activation occurred with increasing stretch, maximally at 10 min at -27-kPa vacuum. Cyclic strain increased nuclear translocation of phosphorylated p38 by immunofluorescent microscopy and nuclear protein binding to nuclear factor-kappaB (NF-kappaB) consensus sequences by mobility shift assay. Both events were largely abrogated by the p38 inhibitor SB-203580. The NO donors 3-morpholinosydnonimine, S-nitroso-N-acetylpenicillamine, and 8-bromoguanosine 3',5'-cyclic monophosphate, a stable cGMP analog, prevented p38 activation and nuclear translocation. Thus strain induces p38 activity and translocation to the nucleus and p38-dependent increases in nuclear protein binding to NF-kappaB. This pathway is attenuated by the NO donors or a cGMP analog.

Immunoelectron microscopic study on type I, II and III TGF-beta receptors on visceral glomerular epithelial cells in relation to glomerular basement membrane alterations in proteinuric rats

BACKGROUND

Transforming growth factor (TGF)-beta is a regulator of extracellular matrix accumulation. Both TGF-beta receptors, type I (TbetaRI) and type II (TbetaRII), may be required for signal transduction in the TGF-beta pathway. The aim of this study was to investigate the relationship between the TGF-beta pathways and glomerular basement membrane (GBM) accumulation in vivo.

METHODS

We examined TbetaRI, II, and III protein expression on visceral glomerular epithelial cells (GEP) in relation to GBM alterations in passive Heymann nephritis (PHN), anti-GBM nephritis and anti-thymocyte serum (ATS) nephritis. Renal tissues were examined by pre-embedding immunoelectron microscopy 3, 7 and 14 days after induction of nephritis in rats.

RESULTS

In normal control rats TbetaRI was not detected on GEP, TbetaRII expression was very occasionally found on GEP and TbetaRIII was seen in the cytoplasm of the GEP. TbetaRI, TbetaRII, and TbetaRIII were constitutively expressed on glomerular endothelial cells. By day 3 of anti-GBM nephritis and PHN, expression of TbetaRI, TbetaRII, and TbetaRIII was still similar to that of normal control rats, and GBM alterations in both models were not prominent except for deposit formation in PHN. From day 7 onwards, in both models, expression of TbetaRI and TbetaRII on GEP increased in association with GBM thickening. Expression of TbetaRIII in the cytoplasm of the GEP was increased, with occasional positive staining being seen on the urinary surface of the GEP from day 7 onwards. On the other hand, at day 3 of ATS nephritis, increased expression of TbetaRI and TbetaRII on GEP was noted, but from day 7 onwards, expression of TbetaR II on GEP dramatically decreased. Expression of TbetaRIII in the cytoplasm of the GEP also transiently increased at day 3. GBM thickening was not noted in ATS nephritis.

CONCLUSIONS

The results suggest that persistent upregulation of expression of TbetaRI, TbetaRII and possibly TbetaRIII on GEP may contribute to GBM matrix accumulation in vivo.

Stress-responsive signal transduction mechanisms in glomerular cells

Mechanical stresses appear to play a key role in the progression of glomerular diseases that are characterized by increased transcapillary hydraulic pressure. Glomerular mesangial cells proliferate and produce extracellular matrix proteins in vivo in such diseases. Mesangial cell responses to pulsatile mechanical stimuli have been studied extensively in vitro during the past few years. Mechanical signals are sensed at the cell membrane and propagated through the cytoplasm, and result in the activation of transcription factors that elicit production of prosclerotic cytokines and matrix proteins, and cell proliferation. Endothelial cells are exposed to shear and pulsatile stress and show some similar responses in other vascular beds.

Modelling the effects of vascular stress in mesangial cells

It has recently been shown that mesangial cells are subjected to multiple forms of mechanical strain (fluid shear, hydrostatic pressure, and triaxial stretch) as a result of forces exerted by the vasculature. Nevertheless, the exact nature and the relative response to these stimuli have not been clarified. Although it is now well established that cyclic stretching of mesangial cells in culture results in the overproduction of extracellular matrix, indicating how intraglomerular hypertension may lead to glomerular scar formation, the contribution of different intracellular signalling mechanisms and extracellular mediators of the response are only now being identified. Recent studies point to a role for high glucose concentrations, transforming growth factor beta and its receptors, vascular endothelial growth factor, and connective tissue growth factor as important mediators, or modifiers of the response to mechanical strain. Although evidence exists for a role for protein kinase C, recent studies also implicate the mitogen-activated protein kinases along with enhanced DNA-binding activity of AP-1 as part of the signalling cascade altering matrix synthesis and cell proliferation in response to stretch. Finally, recent studies examining the effects of oscillating hyperbaric pressure demonstrate similarities, as well as differences, in comparison to those of cyclic stretch.

Transcriptional profile of mechanically induced genes in human vascular smooth muscle cells

Vascular smooth muscle cells must monitor and respond to their mechanical environment; however, the molecular response of these cells to mechanical stimuli remains incompletely defined. By applying a highly uniform biaxial cyclic strain to cultured cells, we used DNA microarray technology to describe the transcriptional profile of mechanically induced genes in human aortic smooth muscle cells. We first identified vascular endothelial growth factor (VEGF) as a mechanically induced gene in these cells; VEGF served as a positive control for these experiments. We then used a DNA microarray with 5000 genes with putative functions to identify additional mechanically induced genes. Surprisingly, relatively few genes are mechanically induced in human aortic smooth muscle cells. Only 3 transcripts of 5000 were induced >2.5-fold: cyclooxygenase-1, tenascin-C, and plasminogen activator inhibitor-1. Downregulated transcripts included matrix metalloproteinase-1 and thrombomodulin. The transcriptional profile of mechanically induced genes in human aortic smooth muscle cells suggests a response of defense against excessive deformation. These data also demonstrate that in addition to identifying large clusters of genes that respond to a given stimulus, DNA microarray technology may be used to identify a small subset of genes that comprise a highly specific molecular response.

Fetal partial urethral obstruction causes renal fibrosis and is associated with proteolytic imbalance

PURPOSE

We determine whether fetal bladder outlet obstruction induces renal fibrosis, and is associated with an alteration in the regulation of connective tissue degradation and the presence of fibrogenic interstitial cells.

MATERIALS AND METHODS

Partial bladder outlet obstruction was surgically induced in 33 fetal sheep at 95 days of gestation. These animals and 24 normal age matched controls were sacrificed at 109, 116 and 135 (term) days of gestation, and the kidneys were rapidly retrieved, drained and weighed. Representative whole kidney samples were snap frozen for assessment of deoxyribonucleic acid, protein and collagen content. Morphometric analysis and alpha-smooth muscle actin immunohistochemistry were performed on histological specimens from formalin fixed kidneys. Tissue extract from fresh kidney specimens were analyzed for metalloproteinase and tissue inhibitor of metalloproteinase activity. Urine samples obtained at the time of sacrifice were analyzed for electrolyte, creatinine and N-acetyl glucosaminidase excretion.

RESULTS

All obstructed kidneys were hydronephrotic and larger than age matched controls. Obstructed kidneys at term showed interstitial fibrosis, as measured by increased extracellular matrix volume fraction (45% in male obstructed kidneys versus 2.5% in normal male kidneys, p = 0.0004), increased total collagen content (120 mg./kidney in male obstructed versus 20 mg. in normal male animals, p = 0.016) and collagen/deoxyribonucleic acid content per kidney (2.78 versus 0.53 mg./mg., p = 0.016). Metalloproteinase-1 activity was significantly lower in obstructed kidneys (210 versus 380 U./mg. protein in normal kidneys). Tissue inhibitor of metalloproteinase activity was undetectable in both groups. The presence of an increased population of myofibroblasts often associated with fibrotic processes was seen by alpha-smooth muscle actin staining which was localized to interstitial cells throughout the cortex in obstructed kidneys.

CONCLUSIONS

Fetal partial bladder outlet obstruction induces renal interstitial fibrosis as early as 2 weeks after obstruction. A possible mechanism for this process is a shift in proteolytic activity to reduce matrix degradation in obstructed kidneys. These changes might be mediated by the increased number of fibrogenic interstitial cells. The observations suggest several potential approaches to developing an understanding of congenital obstructive uropathy.

LDL stimulates collagen mRNA synthesis in mesangial cells through induction of PKC and TGF-beta expression

Abnormal lipid accumulation in glomeruli could be implicated in the pathogenesis of glomerulosclerosis. Low-density lipoprotein (LDL) stimulates collagen mRNA expression in cultured human mesangial cells (HMC). To explore the possible molecular mechanisms by which LDL promotes collagen gene expression, we examined the effects of LDL on protein kinase C (PKC) activity and transforming growth factor-beta (TGF-beta) expression in relation to collagen gene regulation in HMC. LDL (200 microg/ml) induced an acute increase in PKC activity, particularly PKC-alpha and -delta, within 15 min, which decreased to control value at 2 h. LDL stimulated TGF-beta1, and alpha1(I) and alpha1(IV) collagen mRNA expression within 30 min of incubation with HMC, and levels remained elevated until hour 4. LDL induced the secretion of TGF-beta by HMC. This TGF-beta was shown by CCL-64 mink lung cell assay to be, in part, bioactive. The stimulatory effects of LDL on collagen gene regulation in HMC were blocked by the inhibition of PKC using GF-109203X (GFX) or the downregulation of PKC using phorbol myristate acetate. Neutralizing antibody to TGF-beta inhibited the increased collagen mRNA expression by HMC exposed to LDL. The downregulation or inhibition of PKC did not affect the stimulatory effect of LDL on TGF-beta mRNA or protein expression. These results suggest that in HMC, LDL stimulates collagen mRNA expression through the rapid activation of PKC-alpha and -delta and transcriptional upregulation of TGF-beta. Thus PKC and TGF-beta may function as independent key signaling intermediaries in the pathway by which LDL upregulates collagen gene expression in HMC.

Molecular mechanisms of diabetic renal hypertrophy

Altered growth of renal cells is one of the early abnormalities detected after the onset of diabetes. Cell culture studies whereby renal cells are exposed to high glucose concentrations have provided a considerable amount of insight into mechanisms of growth. In the glomerular compartment, there is a very early and self-limited proliferation of mesangial cells with subsequent hypertrophy, whereas proximal tubular cells primarily undergo hypertrophy. There is overwhelming evidence from in vivo and cell culture studies that induction of the transforming growth factor-beta (TGF-beta) system mediates the actions of high ambient glucose and that this system is pivotal for the hypertrophy of mesangial and tubular cells. Other factors such as hemodynamic forces, protein glycation products, and several mediators (for example, angiotensin II, endothelin-1, thromboxane, and platelet-derived growth factor) may further amplify the synthesis of TGF-beta and/or the expression of its receptors in the diabetic state. Cellular hypertrophy can be characterized by cell cycle arrest in the G1 phase. The molecular mechanism arresting mesangial cells in the G1 phase of the cell cycle is the induction of cyclin-dependent kinase (CdK) inhibitors such as p27Kip1 and p21, which bind to and inactivate cyclin-CdK complexes responsible for G1-phase exit. High-glucose-induced activation of protein kinase C and stimulated TGF-beta expression appear to be essential for stimulated expression of p27Kip1. In addition, a decreased turnover of protein caused by the inhibition of proteases contributes to hypertrophy. The development of irreversible renal changes in diabetes mellitus such as glomerulosclerosis and tubulointerstitial fibrosis is always preceded by the early hypertrophic processes in the glomerular and the tubular compartments. It may still be debated whether diabetic renal hypertrophy will inevitably lead to irreversible fibrotic changes in the absence of other factors such as altered intraglomerular hemodynamics and genetic predisposition. Nevertheless, understanding cellular growth on a molecular level may help design a novel therapeutic approach to prevent or treat diabetic nephropathy effectively.

Pertussis toxin-sensitive G proteins as mediators of stretch-induced decrease in nitric-oxide release of osteoblast-like cells

Mechanical loading plays an important role in regulating bone remodeling, and nitric oxide may be one regulator of this process. To determine how mechanical stress modulates osteoblast function, we loaded cyclic tensile stretch on osteoblast-like cells and measured levels of nitric oxide in the medium. High frequency of stretch at any magnitude inhibited release of nitric oxide; however, low frequency of stretch enhanced its release from the static control. To examine the involvement of G protein (guanine nucleotide-binding regulatory protein) in stress-inhibited release of nitric oxide, we added pertussis toxin, a specific inhibitor of the Gi class, and found that it completely reversed the stress-inhibited release. These data support the idea that pertussis toxin-sensitive G protein is activated in the presence of cyclic tensile stretch.

Pulsatile stretch activates mitogen-activated protein kinase (MAPK) family members and focal adhesion kinase (p125(FAK)) in cultured rat cardiac myocytes

Recently, we demonstrated that pulsatile mechanical stretch induced rapid secretion of vascular endothelial growth factor (VEGF) by cultured rat cardiac myocytes in vitro. To investigate whether pulsatile stretch activates intracellular signaling in cardiac myocytes, we examined the activation of mitogen-activated protein kinase (MAPK) family members and focal adhesion kinase (p125(FAK)) in cultured rat cardiac myocytes. We found that pulsatile stretch rapidly phosphorylated p44/p42 MAPKs (extracellular signal-regulated protein kinase [ERK] 1/2), stress-activated protein kinase (SAPK), p38MAPK, and p125(FAK). The stretch-induced activation of ERKs was at least partly mediated by VEGF, which was shown to be induced by transforming growth factor (TGF)-beta, and was also partly dependent on tyrosine kinases as well as protein kinase C (PKC). These data provide the direct evidence that pulsatile stretch can activate intracellular signaling in cardiac myocytes and that this was at least partly mediated by VEGF, which may play a role in cardiac adaptation to mechanical overload.

High water intake ameliorates tubulointerstitial injury in rats with subtotal nephrectomy: possible role of TGF-beta

BACKGROUND

It has been shown that tubulointerstitial injury correlates well with a decline of renal function. In this study, we investigated the effect of high water intake (HWI) on functional and structural parameters in rats with subtotal nephrectomy.

METHODS

Two weeks after the ablative procedure, rats were divided into two groups. One group received the treatment with HWI (3% sucrose added to drinking water) for eight weeks. Functional parameters were compared with sham-operated control (CONT) or nephrectomized rats without treatment (NX). Remnant kidneys were then assessed histologically for evidence of interstitial fibrosis and glomerulosclerosis.

RESULTS

Creatinine clearance was significantly improved in HWI rats compared with NX rats. Simultaneously, urinary protein was also significantly reduced in HWI rats. HWI predominantly ameliorated interstitial lesions and, to a lesser extent, glomerular lesions. Northern blot analysis demonstrated that transforming growth factor-beta (TGF-beta) mRNA expression was significantly suppressed in HWI rats. In situ hybridization revealed that HWI suppressed TGF-beta mRNA expression mainly in the outer medulla. Fibronectin mRNA was also reduced by the HWI treatment. The changes in TGF-beta and fibronectin mRNA were in parallel with Na+/myo-inositol cotransporter (SMIT) mRNA, which is regulated by extracellular osmolarity. Immunohistochemistry demonstrated that protein expression of TGF-beta and fibronectin coincided with the mRNA expression.

CONCLUSION

These results suggest that HWI reduces TGF-beta mRNA expression in medullary interstitium and ameliorates tubulointerstitial injury in rats with reduced renal mass.

Interaction of angiotensin II and mechanical stretch on vascular endothelial growth factor production by human mesangial cells

The antiproteinuric effect of angiotensin-converting enzyme inhibitors underscores the importance of a hemodynamic injury and the renin-angiotensin system in the proteinuria of various glomerular diseases. Vascular endothelial growth factor (VEGF), a potent promoter of vascular permeability, is induced in mesangial cells by both mechanical stretch and TGF-beta1. This study investigates the effect of TGF-beta blockade, angiotensin II (AngII), and the interaction between AngII and stretch on human mesangial cell VEGF production. Exposure to AngII (1 microM) induced a significant increase in VEGF mRNA and protein levels (1.5+/-0.1 and 1.7+/-0.3, respectively, fold increase over control, P<0.05). The AngII receptor (AT1) antagonist Losartan (10 microM) prevented AngII-induced, but not stretch-induced, VEGF protein secretion (AngII 1.7+/-0.3, AngII + Losartan 1.0+/-0.1, P<0.05; stretch 2.4+/-0.4, stretch + Losartan 2.6+/-0.5). Stretch-induced VEGF production was also unaffected by the addition of an anti-TGF-beta neutralizing antibody (stretch 2.85+/-0.82 versus stretch + anti-TGF-beta 2.84+/-0.01, fold increase over control). Simultaneous exposure to both AngII and stretch for 12 h had an additive effect on VEGF production (AngII 1.6+/-0.1, stretch 2.6+/-0.27, AngII + stretch 3.1+/-0.35). Conversely, preexposure to stretch magnified AngII-induced VEGF protein secretion (unstretched + AngII 1.3+/-0.0, stretched + AngII 1.9+/-0.1, P<0.01) with a parallel 1.5-fold increase in AT1 receptor levels. AngII and stretch can both independently induce VEGF production; in addition, mechanical stretch upregulates the AT1 receptor, enhancing the cellular response to AngII.

Collagen secretion and growth of mesangial cells require geranylgeranylpyrophosphate

BACKGROUND

The mevalonate pathway is important for the biosynthesis of isoprenoids such as geranylgeranylpyrophosphate (GGPP) and farnesylpyrophosphate, as well as cholesterol. It has been reported that treatment with 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor ameliorates glomerular injury in several experimental models of progressive glomerular disease. However, the effect of HMG-CoA reductase inhibitor on mesangial cell function has not been fully understood. This investigation was performed to elucidate the role of a mevalonate metabolite(s) in mesangial cell proliferation and extracellular matrix accumulation.

METHODS

Cycling or quiescent human mesangial cells were incubated in RPMI 1640 containing 10% heat-inactivated fetal calf serum (FCS) in the absence or presence of pravastatin, an inhibitor of HMG-CoA reductase, and mevalonate metabolites. Type IV collagen secretion, mRNA expression, and [3H]thymidine incorporation were measured. Cell cycle phases were monitored by flow cytometry.

RESULTS

Pravastatin inhibited FCS-stimulated type IV collagen secretion (IC50 = 210 microM) and mRNA expression. Pravastatin also inhibited FCS-stimulated [3H]thymidine incorporation (IC50 = 430 microM). Analysis with flow cytometry revealed that pravastatin inhibited the G1 to S phase transition of FCS-stimulated mesangial cells. Mevalonate reversed these inhibitory effects of pravastatin completely. Among two major metabolites of mevalonate, GGPP and farnesylpyrophosphate, only GGPP reversed pravastatin-induced inhibition of type IV collagen secretion, DNA synthesis, and the G1 to S phase progression.

CONCLUSIONS

These results suggest that GGPP plays critical roles for the type IV collagen secretion and G1 to S phase transition in FCS-stimulated human mesangial cells.

Activation of mesangial cell signaling cascades in response to mechanical strain

BACKGROUND

Mesangial cells (MCs) are constantly exposed to pulsatile stretch and relaxation in their role as architectural support for the glomerulus. There is no cell proliferation in normal glomeruli. In contrast, animal models of increased glomerular capillary pressure are characterized by resident glomerular cell proliferation and elaboration of extracellular matrix (ECM) protein, resulting in glomerulosclerosis. This process can be ameliorated by maneuvers, such as angiotensin converting enzyme inhibition, that reduce glomerular capillary pressure. MCs grown on ECM-coated plates and exposed to cyclic stretch/relaxation proliferate and produce ECM protein, suggesting that this may be a useful in vitro model for MC behavior in response to increased physical forces. Previous work has shown induction of c-fos in response to application of mechanical strain to MCs, which may induce increases in AP-1 transcription factor activity, which, in turn, may augment ECM protein and transforming growth factor beta transcription and cell proliferation. Stimuli that lead to c-fos induction pass through mitogen-activated protein kinase (MAPK) pathways. Three MAPK cascades have been characterized in mammalian cells--p44/42 (classic MAPK), the stress-activated protein kinase/Jun terminal kinase (SAPK/JNK) pathway, and p38/HOG--and mechanical strain activates p44/42 and SAPK/JNK in cardiac fibroblasts. However, in contrast to MCs, these cells do not proliferate in response to physical force. Accordingly, we studied activation of the MAPK pathways in MCs exposed to mechanical strain.

METHODS

MCs (passages 5 to 10) cultured on type 1 collagen-coated, flexible-bottom plates were exposed to 30, 60, or 120 minutes of cyclic strain (60 cycles/min) by computer-driven generation of vacuums of -14 and -28 kPa, inducing 20% and 29% elongations in the diameter of the surfaces, respectively. Control MCs were grown on coated rigid bottom plates. Proliferation was assessed at 24 hours by 3H-thymidine incorporation. Protein levels (by Western blot) and activity assays for all three kinase cascades were performed at 30, 60, and 120 minutes.

RESULTS

Cyclic strain/relaxation lead to an approximate doubling of 3H-thymidine incorporation at 24 hours (N = 3, P < 0.05) only in cultures stretched 29%, but not in cultures stretched 20%. At -29% elongation, the increase in 3H-thymidine incorporation was preceded by early activation of MAPK signaling pathways. p44/42 activity increased to a maximum of eightfold greater than control at 60 minutes. p38/HOG activity was not measurable at baseline but was increased markedly at 30 minutes, which was sustained through to 120 minutes. SAPK/JNK activity was present at a very low level in MCs and was not changed by stretch. However, it was markedly increased by sorbitol. In MCs stretched to 20% elongation, lesser increases in p44/42 were seen with a similar time course, whereas no increases in p38/HOG or SAPK could be detected at the time points studied. No increase in any kinase pathway activity was seen at any time in static cultures.

CONCLUSIONS

High-pressure cyclic stretch leads to MC proliferation, preceded by marked activation of p44/42 and p38/HOG MAPKs. Cell proliferation is not seen with low-pressure stretch, and there is only modest p44/42 MAPK activation, suggesting that glomerular capillary hypertension may lead to cell proliferation and injury partly through differential activation of kinase cascades.

Pulsatile stretch stimulates vascular endothelial growth factor (VEGF) secretion by cultured rat cardiac myocytes

Evidence has accumulated that vascular endothelial growth factor (VEGF) is expressed in the heart, and its expression is markedly increased in response to hypoxia. Recently, it was shown that pulsatile myocardial stretch in vivo markedly enhanced VEGF mRNA level in the heart. To investigate whether pulsatile mechanical stretch really stimulates VEGF expression by cardiac myocytes, using an in vitro preparation, we examined the secretion of VEGF into the culture media from cardiac myocytes subjected to pulsatile stretch. We found that pulsatile mechanical stretch induced rapid secretion of VEGF by cultured rat cardiac myocytes and mRNA expression of VEGF and VEGF receptors in the cardiac myocytes. We also found that the stretch-induced secretion of VEGF was at least in part mediated by TGF-beta. These data provide the direct evidence that mechanical overload itself can induce VEGF secretion by cardiac myocytes, which may play a role in ameliorating the relative myocardial hypoxia.

Signal transduction of mechanical stresses in the vascular wall

The vascular wall is constantly subjected to a variety of mechanical forces in the form of stretch (tensile stress), due to blood pressure, and shear stress, due to blood flow. Alterations in either of these stresses are known to result in vascular remodeling, an adaptation characterized by modified morphology and function of the blood vessels, allowing the vessels to cope with physiological or pathological conditions. The processes involved in vascular remodeling include cellular hypertrophy and hyperplasia, as well as enhanced protein synthesis or extracellular matrix protein reorganization. In vitro studies using vascular cells have attempted to identify the mechanisms behind structural alterations. Possible pathways include ion channels, integrin interaction between cells and the extracellular matrix, activation of various tyrosine kinases (such as c-Src, focal adhesion kinase, and mitogen-activated protein kinases), and autocrine production and release of growth factors. These pathways lie upstream of de novo synthesis of immediate response genes and total protein synthesis, both of which are likely to be involved in the process of vascular remodeling.

Effects of hypertonic stress on transforming growth factor-beta activity in normal rat kidney cells

Hypertonicity is known to modulate the expression of some genes and the action of several cytokines. We evaluated whether hypertonicity would increase the expression and/or activity of transforming growth factor-beta (TGF-beta) in normal rat kidney (NRK) cells. The bioassay for TGF-beta showed that mature TGF-beta activity was significantly increased when the cells were cultured in a hypertonic medium (500 mOsm/kg). Comparing to the isotonic medium, hypertonicity accelerated the increase in TGF-beta activity during the initial 24 hours after changing the medium. The activity was increased as the medium osmolality increased from 300 to 500 mOsm/kg. Raffinose was found to be the most effective in increasing TGF-beta activity. NaCl, glucose, and mannitol also increased TGF-beta activity. In contrast, total TGF-beta (mature and latent) activity and TGF-beta mRNA abundance did not change significantly, suggesting that hypertonicity activated TGF-beta without affecting the synthesis of TGF-beta. To determine whether collagen synthesis was increased by hypertonicity, we examined [3H] proline incorporation into NRK cells cultured in hypertonic medium. Proline incorporation increased in an osmolality-dependent manner. Raffinose was also the most effective solute at increasing the proline incorporation. Furthermore, anti-TGF-beta antibody prevented the increase in proline incorporation induced by hypertonicity. These results suggest that hypertonicity promotes the processing of latent TGF-beta to the biologically active form, resulting in the stimulation of collagen synthesis in NRK cells.

Anti-latent TGF-beta binding protein-1 antibody or synthetic oligopeptides inhibit extracellular matrix expression induced by stretch in cultured rat mesangial cells

Transforming growth factor-beta (TGF-beta) is usually secreted as a large latent complex associated with latent TGF-beta binding protein-1 (LTBP-1), which is known to bind to extracellular matrix (ECM) components. Although the LTBP-ECM interaction has been suggested to play a role in the activation and biological action of TGF-beta, the precise mechanism is still unclear. In glomerular hypertension mesangial cells are believed to perceive the increased cyclic strain and we have recently reported that cyclic mechanical stretch in vitro enhances the expression of ECM components via an autocrine/paracrine secretion of TGF-beta in cultured rat mesangial cells. Therefore, in this study we examined the role of LTBP-1 in the stretch-induced, TGF-beta-mediated ECM expression. Mesangial cells expressed mRNA for short and long forms of LTBP-1 (LTBP-1S and LTBP-1L, respectively). Mesangial cells were subjected to cyclic stretch to provide a maximal elongation of 20% at a rate of 60 cycles/min for 24 to 36 hours in the presence of polyclonal antibody raised against human LTBP-1 or synthetic oligopeptides corresponding to the N-terminal portions of human LTBP-1, which may work as competitive inhibitors against the LTBP-ECM association. Both anti-LTBP-1 antibody (Ab39) and synthetic oligopeptides inhibited the stretch-induced mRNA expression of type I collagen and fibronectin in a dose-dependent manner, but the inhibition by Ab39 or the oligopeptides was recovered by adding recombinant TGF-beta. Ab39 or the oligopeptides did not change the effect of exogenously added TGF-beta, such as growth-inhibition in mink lung epithelial cells. These results suggest that mesangial cells secrete TGF-beta as a large latent complex, and the LTBP-ECM interaction may be a pivotal step in TGF-beta action and ECM accumulation, providing a new therapeutic strategy against progression of glomerulosclerosis and other fibrotic diseases.

Extracellular signal-regulated kinase and c-Jun NH2-terminal kinase activation by mechanical stretch is integrin-dependent and matrix-specific in rat cardiac fibroblasts

Integrins, which connect the cytoskeleton to the extracellular matrix and mediate a variety of signaling cascades, may transduce mechanical stimuli into biochemical signals. We studied integrin- and matrix-dependent activation of extracellular signal-regulated kinase (ERK2), c-Jun NH2-terminal kinase (JNK1), and p38 in response to 4% static biaxial stretch in rat cardiac fibroblasts. ERK2 and JNK1, but not p38, were rapidly activated by stretch when the fibroblasts were allowed to synthesize their own matrices. When the cells were limited to specific matrix substrates, ERK2 and JNK1 were differentially activated: ERK2 was only activated when the cells were plated on fibronectin, while JNK1 was activated when the cells were plated on fibronectin, vitronectin, or laminin. Plating cells on collagen before stretching did not activate either kinase. Adhesion to all matrices was integrin-dependent because it could be blocked by inhibitors of specific integrins. ERK2 activation could be blocked with a combination of anti-alpha4 and -alpha5 antibodies and an arginine-glycine-aspartic acid (RGD) peptide, while the antibodies or peptide used separately failed to block ERK2 activation. This result suggests that at least two integrins, alpha4beta1 and an RGD-directed, non-alpha5beta1 integrin, activate ERK2 in response to mechanical stimulation. Activation of JNK1 could not be blocked with the inhibitors, suggesting that an RGD-independent integrin or integrins other than alpha4beta1 can activate JNK1 in cells adherent to fibronectin. This study demonstrates that integrins act as mechanotransducers, providing insight into potential mechanisms for in vivo responses to mechanical stimuli.

Mechanical stretch induces vascular permeability factor in human mesangial cells: mechanisms of signal transduction

Hemodynamic abnormalities have been implicated in the pathogenesis of the increased glomerular permeability to protein of diabetic and other glomerulopathies. Vascular permeability factor (VPF) is one of the most powerful promoters of vascular permeability. We studied the effect of stretch on VPF production by human mesangial cells and the intracellular signaling pathways involved. The application of mechanical stretch (elongation 10%) for 6 h induced a 2.4-fold increase over control in the VPF mRNA level (P < 0.05). There was a corresponding 3-fold increase in VPF protein level by 12 h (P < 0. 001), returning to the baseline by 24 h. Stretch-induced VPF secretion was partially prevented both by the protein kinase C (PKC) inhibitor H7 (50 microM: 72% inhibition, P < 0.05) and by pretreatment with phorbol ester (phorbol-12-myristate-13 acetate 10(-)7 M: 77% inhibition, P < 0.05). A variety of protein tyrosine kinase (PTK) inhibitors, genistein (20 microg/ml), herbimycin A (3.4 microM), and a specific pp60(src) peptide inhibitor (21 microM) also significantly reduced, but did not entirely prevent, stretch-induced VPF protein secretion (respectively 63%, 80%, and 75% inhibition; P < 0.05 for all). The combination of both PKC and PTK inhibition completely abolished the VPF response to mechanical stretch (100% inhibition, P < 0.05). Stretch induces VPF gene expression and protein secretion in human mesangial cells via PKC- and PTK-dependent mechanisms.