Liver fibrogenesis: a new role for the renin-angiotensin system

Potential cellular and molecular causes of hypertrophic scar formation

A scar is an expected result of wound healing. However, in some individuals, and particularly in burn victims, the wound healing processes may lead to a fibrotic hypertrophic scar, which is raised, red, inflexible and responsible for serious functional and cosmetic problems. It seems that a wide array of subsequent processes are involved in hypertrophic scar formation, like an affected haemostasis, exaggerated inflammation, prolonged reepithelialization, overabundant extracellular matrix production, augmented neovascularization, atypical extracellular matrix remodeling and reduced apoptosis. Platelets, macrophages, T-lymphocytes, mast cells, Langerhans cells and keratinocytes are directly and indirectly involved in the activation of fibroblasts, which in turn produce excess extracellular matrix. Following the chronology of normal wound healing, we unravel, clarify and reorganize the complex molecular and cellular key processes that may be responsible for hypertrophic scars. It remains unclear whether these processes are a cause or a consequence of unusual scar tissue formation, but raising evidence exists that immunological responses early following wounding play an important role. Therefore, when developing preventive treatment modalities, one should aim to put the early affected wound healing processes back on track as quickly as possible.

Genesis of hepatic fibrosis and its biochemical markers

Liver fibrosis is characterized by an abnormal hepatic accumulation of extracellular matrix (ECM) that results from both increased deposition and reduced degradation of collagen fibres. Fibrotic liver injury results in activation of the hepatic stellate cell (HSC). Surrogate markers are gradually being substituted for biomarkers that reflect the complex balance between synthesis and degradation of the extracellular matrix. Once the hepatic stellate cell is activated, the preceding matrix changes and recurrent injurious stimuli will perpetuate the activated state. The ECM directs cellular differentiation, migration, proliferation and fibrogenic activation or deactivation. The metabolism of the extracellular matrix is closely regulated by matrix metalloproteinases (MMP) and their specific tissue inhibitors (TIMP). Although liver biopsy combined with connective tissue stains has been a mainstay of diagnosis, there is a need for less invasive methods. These diagnostic markers should be considered in combination with liver function tests, ultrasonography and clinical manifestations.

The renin-angiotensin system and malignancy

The renin-angiotensin system (RAS) is usually associated with its systemic action on cardiovascular homoeostasis. However, recent studies suggest that at a local tissue level, the RAS influences tumour growth. The potential of the RAS as a target for cancer treatment and the suggested underlying mechanisms of its paracrine effects are reviewed here. These include modulation of angiogenesis, cellular proliferation, immune responses and extracellular matrix formation. Knowledge of the RAS has increased dramatically in recent years with the discovery of new enzymes, peptides and feedback mechanisms. The local RAS appears to influence tumour growth and metastases and there is evidence of tissue- and tumour-specific differences. Recent experimental studies provide strong evidence that drugs that inhibit the RAS have the potential to reduce cancer risk or retard tumour growth and metastases. Manipulation of the RAS may, therefore, provide a safe and inexpensive anticancer strategy.

Role of nicotinamide adenine dinucleotide phosphate-oxidase family in liver fibrogenesis

NADPH oxidase (nicotinamide adenine dinucleotide phosphate-oxidase, NOX) is a multi-protein complex producing reactive oxygen species (ROS), present both in phagocytes, being essential in host defense and in non-phagocytic cells, regulating intracellular signaling. In liver, NADPH oxidase plays a central role in fibrogenesis. A functionally active form of NADPH oxidase is expressed not only in Kupffer cells (phagocytic cell type) but also in hepatic stellate cells (HSCs) (non-phagocytic cell type), suggestive of its role the non-phagocytic NADPH oxidase in HSCs activation. This paper reviewed effects of NOX in liver fibrogenesis.

Impact of chewing betel-nut (Areca catechu) on liver cirrhosis and hepatocellular carcinoma: a population-based study from an area with a high prevalence of hepatitis B and C infections

BACKGROUND

Chewing betel-nuts (Areca catechu) is carcinogenic but the risk for hepatocellular carcinoma (HCC) and liver cirrhosis (LC) is little considered. Worldwide 600 million people chew betel, including emigrants from palm-growing countries.

OBJECTIVE

We aimed to assess the relationships and dose-response effects of betel chewing on LC and HCC risks, since habit cessation could reduce the increased risks of HCC and LC found in such communities.

SUBJECTS

Screening 60 326 subjects aged 30-79 years in a population-based study in Taiwan identified LC in 588 and HCC in 131 subjects. Demographic features, hepatitis B/C infections, other risk factors and betel chewing were noted. Multiple Cox regression models were used to assess independent relationships, interactions and synergisms between age, betel chewing and hepatitis B/C.

RESULTS

Betel chewing increased LC and HCC risk 4.25-fold (95 % CI 2.9, 6.2) in current chewers and 1.89-fold (95 % CI 1.13, 3.16) in ex-chewers v. never-chewers, with dose effects for quantity, duration and cumulative exposure in chewers. Subjects without hepatitis B/C infections had 5.0-fold (95 % CI 2.87, 9.03) increased risk of LC/HCC v. never-chewers, and betel chewing had an additive synergistic effect on hepatitis B/C-related risks. Risk reduction with betel habit cessation could exceed that expected from immunization programmes for hepatitis B and C.

CONCLUSION

Increased risks of cirrhosis and hepatocellular cancer were found in betel chewers free of hepatitis B/C infection, and these risks were synergistically additive to those of hepatitis B/C infections. Estimated risk reduction from effective anti-betel chewing programmes would be sizeable.

Effect of angiotensin II and angiotensin II type 1 receptor antagonist on the proliferation, contraction and collagen synthesis in rat hepatic stellate cells

BACKGROUND

Angiotensin II (Ang II) is a very important vasoactive peptide that acts upon hepatic stellate cells (HSCs), which are major effector cells in hepatic cirrhosis and portal hypertension. The present study was aimed to investigate the effects of Ang II and angiotensin II type 1 receptor antagonist (AT(1)RA) on the proliferation, contraction and collagen synthesis in HSCs.

METHODS

HSC-T6 rat hepatic stellate cell line was studied. The proliferation of the HSC cells was evaluated by MTT colorimetric assay while HSC DNA synthesis was measured by (3)H-thymidine incorporation. The effects of angiotensin II and AT(1)RA on HSCs contraction were studied by analysis of the contraction of the collagen lattice. Cell culture media were analyzed by RT-PCR to detect secretion of collagen I (Col I), collagen III (Col III) and transforming growth factor beta1 (TGF-beta1) by enzyme linked immunosorbent assay. HSC was harvested to measure collagen I, collagen III and tissue inhibitor of metalloproteinase-1 (TIMP-1) mRNA expression.

RESULTS

Ang II ((1 x 10(-10) - 1 x 10(-4)) mol/L) stimulated DNA synthesis and proliferation in HSCs compared with untreated control cells. AT(1)RA inhibited angiotensin II induced proliferation of HSCs. A linear increase in the contractive area of collagen lattice correlated with the concentration of angiotensin II (1 x 10(-9) - 1 x 10(-5) mol/L) and with time over 48 hours. AT(1)RA blocks angiotensin II induced contraction of collagen lattice. Col I, Col III and TGF-beta1 levels of the Ang II group were higher than those of control group and this increase was downregulated by AT(1)RA. The mRNA expressions of Col I, Col III and TIMP-1 were higher in HSCs from the Ang II group than the control group and downregulated by AT(1)RA.

CONCLUSIONS

Angiotensin II increased DNA synthesis and proliferation of HSCs in a dose-dependent manner, stimulated the contraction of HSCs dose- and time-dependently. Angiotensin also promoted excretion of Col I, Col III and TGF-beta1 levels and stimulated Col I, Col III and TIMP-1 expression in HSCs. Angiotensin acts via the angiotensin II receptor because all of these effects are blocked by angiotensin II type 1 receptor antagonist.

ACE inhibition and AT1 receptor blockade prevent fatty liver and fibrosis in obese Zucker rats

OBJECTIVE

Non-alcoholic steatohepatitis (NASH), which is a common liver disease in industrialized countries, is associated with obesity, hypertension, and type-2 diabetes (metabolic syndrome). Since angiotensin II (ANG II) has been suggested to play an important role in liver inflammation and fibrosis, the purpose of this study was to investigate whether therapy against renin-angiotensin system (RAS) may provide some beneficial effect in liver of an animal model of metabolic syndrome.

METHODS AND PROCEDURES

For 6 months, obese Zucker rats (OZRs) were treated as follows: OZR-group, OZR + Perindopril (P) group, OZR + Irbesartan (IRB) group, OZR + Amlodipine (AML) group, and lean Zucker rats (LZRs) group as a control. Livers were evaluated by immunohistochemistry techniques using corresponding antibodies.

RESULTS

All treated groups showed a similar reduction in blood pressure compared to untreated OZR. Therapy either with IRB or P improves insulin sensitivity and reduces hepatic enzyme level with respect to untreated OZR. Conversely, AML failed to modify both parameters. Untreated OZR displayed higher hepatic ANG II levels and steatosis together with a marked increase in tumor necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6) and transforming growth factor-beta1 (TGF-beta1) level compared to LZR. Following RAS inhibition either by P or IRB, a significant reduction (P < 0.01) in the immunostaining of TNF-alpha, IL-6 and TGF-beta1 compared to untreated OZR was observed.

DISCUSSION

These results indicate that ANG II expression is increased in the liver of these animals with steatohepatitis. Furthermore, RAS control by either angiotensin-converting enzyme inhibition or AT1 receptor blockade seems to provide a beneficial modulation concerning the inflammatory response to liver injury in this model. Consequently, blockade of RAS could be a new approach to prevent or to treat patients with NASH.

Bioconjugation of oligonucleotides for treating liver fibrosis

Liver fibrosis results from chronic liver injury due to hepatitis B and C, excessive alcohol ingestion, and metal ion overload. Fibrosis culminates in cirrhosis and results in liver failure. Therefore, a potent antifibrotic therapy is urgently needed to reverse scarring and eliminate progression to cirrhosis. Although activated hepatic stellate cells (HSCs) remain the principle cell type responsible for liver fibrosis, perivascular fibroblasts of portal and central veins as well as periductular fibroblasts are other sources of fibrogenic cells. This review will critically discuss various treatment strategies for liver fibrosis, including prevention of liver injury, reduction of inflammation, inhibition of HSC activation, degradation of scar matrix, and inhibition of aberrant collagen synthesis. Oligonucleotides (ODNs) are short, single-stranded nucleic acids, which disrupt expression of target protein by binding to complementary mRNA or forming triplex with genomic DNA. Triplex forming oligonucleotides (TFOs) provide an attractive strategy for treating liver fibrosis. A series of TFOs have been developed for inhibiting the transcription of alpha1(I) collagen gene, which opens a new area for antifibrotic drugs. There will be in-depth discussion on the use of TFOs and how different bioconjugation strategies can be utilized for their site-specific delivery to HSCs or hepatocytes for enhanced antifibrotic activities. Various insights developed in individual strategy and the need for multipronged approaches will also be discussed.

Hepatic stellate cells: protean, multifunctional, and enigmatic cells of the liver

The hepatic stellate cell has surprised and engaged physiologists, pathologists, and hepatologists for over 130 years, yet clear evidence of its role in hepatic injury and fibrosis only emerged following the refinement of methods for its isolation and characterization. The paradigm in liver injury of activation of quiescent vitamin A-rich stellate cells into proliferative, contractile, and fibrogenic myofibroblasts has launched an era of astonishing progress in understanding the mechanistic basis of hepatic fibrosis progression and regression. But this simple paradigm has now yielded to a remarkably broad appreciation of the cell's functions not only in liver injury, but also in hepatic development, regeneration, xenobiotic responses, intermediary metabolism, and immunoregulation. Among the most exciting prospects is that stellate cells are essential for hepatic progenitor cell amplification and differentiation. Equally intriguing is the remarkable plasticity of stellate cells, not only in their variable intermediate filament phenotype, but also in their functions. Stellate cells can be viewed as the nexus in a complex sinusoidal milieu that requires tightly regulated autocrine and paracrine cross-talk, rapid responses to evolving extracellular matrix content, and exquisite responsiveness to the metabolic needs imposed by liver growth and repair. Moreover, roles vital to systemic homeostasis include their storage and mobilization of retinoids, their emerging capacity for antigen presentation and induction of tolerance, as well as their emerging relationship to bone marrow-derived cells. As interest in this cell type intensifies, more surprises and mysteries are sure to unfold that will ultimately benefit our understanding of liver physiology and the diagnosis and treatment of liver disease.

Gene expression changes induced by the tumorigenic pyrrolizidine alkaloid riddelliine in liver of Big Blue rats

Background

Pyrrolizidine alkaloids (PAs) are probably the most common plant constituents that poison livestock, wildlife, and humans worldwide. Riddelliine is isolated from plants grown in the western United States and is a prototype of genotoxic PAs. Riddelliine was used to investigate the genotoxic effects of PAs via analysis of gene expression in the target tissue of rats in this study. Previously we observed that the mutant frequency in the liver of rats gavaged with riddelliine was 3-fold higher than that in the control group. Molecular analysis of the mutants indicated that there was a statistically significant difference between the mutational spectra from riddelliine-treated and control rats.

Results

Riddelliine-induced gene expression profiles in livers of Big Blue transgenic rats were determined. The female rats were gavaged with riddelliine at a dose of 1 mg/kg body weight 5 days a week for 12 weeks. Rat whole genome microarray was used to perform genome-wide gene expression studies. When a cutoff value of a two-fold change and a P-value less than 0.01 were used as gene selection criteria, 919 genes were identified as differentially expressed in riddelliine-treated rats compared to the control animals. By analysis with the Ingenuity Pathway Analysis Network, we found that these significantly changed genes were mainly involved in cancer, cell death, tissue development, cellular movement, tissue morphology, cell-to-cell signaling and interaction, and cellular growth and proliferation. We further analyzed the genes involved in metabolism, injury of endothelial cells, liver abnormalities, and cancer development in detail.

Conclusion

The alterations in gene expression were directly related to the pathological outcomes reported previously. These results provided further insight into the mechanisms involved in toxicity and carcinogenesis after exposure to riddelliine, and permitted us to investigate the interaction of gene products inside the signaling networks.

Pathogenesis and management issues for non-alcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) has, although it is a very common disorder, only relatively recently gained broader interest among physicians and scientists. Fatty liver has been documented in up to 10 to 15 percent of normal individuals and 70 to 80 percent of obese individuals. Although the pathophysiology of NAFLD is still subject to intensive research, several players and mechanisms have been suggested based on the substantial evidence. Excessive hepatocyte triglyceride accumulation resulting from insulin resistance is the first step in the proposed 'two hit' model of the pathogenesis of NAFLD. Oxidative stress resulting from mitochondrial fatty acids oxidation, NF-kappaB-dependent inflammatory cytokine expression and adipocytokines are all considered to be the potential factors causing second hits which lead to hepatocyte injury, inflammation and fibrosis. Although it was initially believed that NAFLD is a completely benign disorder, histologic follow-up studies have showed that fibrosis progression occurs in about a third of patients. A small number of patients with NAFLD eventually ends up with end-stage liver disease and even hepatocellular carcinoma. Although liver biopsy is currently the only way to confirm the NAFLD diagnosis and distinguish between fatty liver alone and NASH, no guidelines or firm recommendations can still be made as for when and in whom it is necessary. Increased physical activity, gradual weight reduction and in selected cases bariatric surgery remain the mainstay of NAFLD therapy. Studies with pharmacologic agents are showing promising results, but available data are still insufficient to make specific recommendations; their use therefore remains highly individual.

Hepatic conversion of angiotensin I and the portal hypertensive response to angiotensin II in normal and regenerating liver

BACKGROUND AND AIM

Angiotensin I (AI) and angiotensin II (AII) induce a portal hypertensive response (PHR) and the liver is able to convert AI into AII to trough the action of the angiotensin-converting enzyme (ACE). Our purpose was to characterize angiotensin I liver conversion.

METHODS

AI, AII or angiotensin (1-7) were used in monovascular or bivascular perfusions.

RESULTS

The maximum gain in portal pressure induced by AII took place significantly earlier (P = 0.031) than that occurring after an equimolar AI infusion. The AI-induced PHR was abolished both by captopril or losartan, whereas the AII-induced PHR was not affected by captopril, but was abolished by losartan. Angiotensin (1-7) has no hemodynamic effect in the perfused liver. After partial hepatectomy, the AII-PHR pattern changes from a rapid return to baseline values to a pattern where there was no return to baseline values (3-7 days ex-surgery). In the bivascular perfusion system when AII was infused in the arterial branch in the retrograde mode of perfusion (peptide available only to the periportal zone), the PHR was at least 50% of that obtained when the prograde mode was used (peptide available to the periportal and perivenous zones).

CONCLUSION

AI does not induce PHR; this effect is a result of its mandatory conversion into AII by the ACE and the sequential action of AII on the AII receptor type 1 located in the hepatic periportal zone. AII induced PHR pattern changes during liver regeneration.

Effect of losartan on early liver fibrosis development in a rat model of nonalcoholic steatohepatitis

BACKGROUND AND AIM

Nonalcoholic steatohepatitis (NASH) is a metabolic disorder of the liver that may evolve into fibrosis or cirrhosis. Recent studies have shown reduction of experimental liver fibrosis with the use of angiotensin-converting-enzyme inhibitors or angiotensin-receptor antagonists. The aim of this study was to determine whether losartan can influence the early phase of fibrogenesis in an animal model of NASH.

METHODS

To induce NASH, a choline-deficient diet (CDD) was given to Sprague-Dawley rats for 12 weeks. These animals were then compared with a control group receiving choline-supplemented diet (CSD) and a group fed a CDD plus losartan (10 mg/kg/day). Biochemical (serum levels of alanine aminotransferase and aspartate aminotransferase) and histological evaluation of fatty liver was performed by conventional techniques. Hydroxyproline content in liver tissue was assayed by spectrophotometry. In addition, mRNA levels of procollagen I and transforming growth factor (TGF)-beta were assessed by semiquantitative RT-PCR and stellate cell activation by alpha-actin immunofluorescence stain.

RESULTS

After 12 weeks CDD induced a marked elevation of serum aminotranferases, a severe fatty liver infiltration with mild histological inflammation and fibrosis. These findings correlated with a significant increase in mRNA levels of both procollagen I and TGF-beta and significant increased liver hydroxyproline content. No differences were seen between rats receiving CDD alone and rats receiving CDD plus losartan with regard to the biochemical, morphological or molecular alterations induced by the CDD.

CONCLUSION

Losartan does not seem to influence liver injury and fibrogenic events in the CDD model of NASH.

NOX in liver fibrosis

NADPH oxidase is a multi-protein complex producing reactive oxygen species (ROS) both in phagocytic cells, being essential in host defense, and in non-phagocytic cells, regulating intracellular signalling. In the liver, NADPH oxidase plays a central role in fibrogenesis. A functionally active form of the NADPH oxidase is expressed not only in Kupffer cells (phagocytic cell type) but also in hepatic stellate cells (HSCs) (non-phagocytic cell type), suggesting a role of the non-phagocytic NADPH oxidase in HSC activation. Consistent with this concept, profibrogenic agonists such as Angiotensin II (Ang II) and platelet derived growth factor (PDGF), or apoptotic bodies exert their activity through NADPH oxidase-activation in HSCs. Both pharmacological inhibition with DPI and genetic studies using p47(phox) knockout mice provided evidence for a central role of NADPH oxidase in the regulation of HSC-activity and liver fibrosis. In addition to the p47(phox) component, only Rac1 has been identified as a functional active component of the NADPH oxidase complex in HSCs.

Signal transduction and activation of hepatic stellate cells

Liver fibrosis, which leads to cirrhosis, occurs as a result of various injurious processes and it is the common pathologic basis of all the chronic hepatic diseases. At present, a good many researches demonstrate that the activation of hepatic stellate cells play a critical role in fibrogenesis. Prolonged liver injury results in hepatocyte damages and secretion of many fibrogenic cytokines such as transforming growth factor-beta 1, angiotensin, and leptin, which triggers the activation of hepatic stellate cells through different intracellular signal transduction pathways. In this article, we reviewed the research advancement in the signal transduction pathway of nuclear receptor and membrane receptor during the activation of hepatic stellate cells.

Distinct cellular responses differentiating alcohol- and hepatitis C virus-induced liver cirrhosis

BACKGROUND

Little is known at the molecular level concerning the differences and/or similarities between alcohol and hepatitis C virus induced liver disease. Global transcriptional profiling using oligonucleotide microarrays was therefore performed on liver biopsies from patients with cirrhosis caused by either chronic alcohol consumption or chronic hepatitis C virus (HCV).

RESULTS

Global gene expression patterns varied significantly depending upon etiology of liver disease, with a greater number of differentially regulated genes seen in HCV-infected patients. Many of the gene expression changes specifically observed in HCV-infected cirrhotic livers were expectedly associated with activation of the innate antiviral immune response. We also compared severity (CTP class) of cirrhosis for each etiology and identified gene expression patterns that differentiated ethanol-induced cirrhosis by class. CTP class A ethanol-cirrhotic livers showed unique expression patterns for genes implicated in the inflammatory response, including those related to macrophage activation and migration, as well as lipid metabolism and oxidative stress genes.

CONCLUSION

Stages of liver cirrhosis could be differentiated based on gene expression patterns in ethanol-induced, but not HCV-induced, disease. In addition to genes specifically regulating the innate antiviral immune response, mechanisms responsible for differentiating chronic liver damage due to HCV or ethanol may be closely related to regulation of lipid metabolism and to effects of macrophage activation on deposition of extracellular matrix components.

Interferon augments the anti-fibrotic activity of an angiotensin-converting enzyme inhibitor in patients with refractory chronic hepatitis C

AIM: To evaluate the effect of combination treatment with the interferon (IFN) and angiotensin-converting enzyme inhibitor (ACE-I) on several fibrotic indices in patients with refractory chronic hepatitis C (CHC).

METHODS: Perindopril (an ACE-I; 4 mg/d) and/or natural IFN (3 MU/L; 3 times a week) were administered for 12 mo to refractory CHC patients, and several indices of serum fibrosis markers were analyzed.

RESULTS: ACE-Idecreased the serum fibrosis markers, whereas single treatment with IFN did not exert these inhibitory effects. However, IFN significantly augmented the effects of ACE-I, and the combination treatment exerted the most potent inhibitory effects. The serum levels of alanine transaminase and HCV-RNA were not significantly different between the groups, whereas the plasma level of transforming growth factor-β was significantly attenuated almost in parallel with suppression of the serum fibrosis markers.

CONCLUSION: The combination therapy of an ACE-Iand IFN may have a diverse effect on disease progression in patients with CHC refractory to IFN therapy through its anti-fibrotic effect.

CARMA3/Bcl10/MALT1-dependent NF-kappaB activation mediates angiotensin II-responsive inflammatory signaling in nonimmune cells

Angiotensin II (Ang II) is a peptide hormone that, like many cytokines, acts as a proinflammatory agent and growth factor. After injury to the liver, the hormone assists in tissue repair by stimulating hepatocytes and hepatic stellate cells to synthesize extracellular matrix proteins and secrete secondary cytokines and by stimulating myofibroblasts to proliferate. However, under conditions of chronic liver injury, all of these effects conspire to promote pathologic liver fibrosis. Much of this effect of Ang II results from activation of the proinflammatory NF-kappaB transcription factor in response to stimulation of the type 1 Ang II receptor, a G protein-coupled receptor. Here, we characterize a previously undescribed signaling pathway mediating Ang II-dependent activation of NF-kappaB, which is composed of three principal proteins, CARMA3, Bcl10, and MALT1. Blocking the function of any of these proteins, through the use of either dominant-negative mutants, RNAi, or gene targeting, effectively abolishes Ang II-dependent NF-kappaB activation in hepatocytes. In addition, Bcl10(-/-) mice show defective hepatic cytokine production after Ang II treatment. Evidence also is presented that this pathway activates NF-kappaB through ubiquitination of IKKgamma, the regulatory subunit of the IkappaB kinase complex. These results elucidate a concrete series of molecular events that link ligand activation of the type 1 Ang II receptor to stimulation of the NF-kappaB transcription factor. These findings also uncover a function of the CARMA, Bcl10, and MALT1 proteins in cells outside the immune system.

Angiotensin II induces phosphorylation of glucose-regulated protein-75 in WB rat liver cells

Studies in vascular smooth muscle cells suggest that, angiotensin II (Ang II)-mediated cellular response requires transactivation of epidermal growth factor receptor (EGF-R), and involves tyrosine phosphorylation of caveolin-1. Here we demonstrate that, exposure of WB rat liver cells to Ang II does not cause transactivation of EGF-R, but did rapidly activate p42/p44 mitogen-activated protein (MAP) kinases suggesting that it activates MAP kinases independent of EGF-R transactivation. We observed that the phospho-specific anti-caveolin-1 antibody detected a tyrosine phosphorylated, 75kDa protein in Ang II-treated cells which we identified as glucose regulated protein-75 (GRP-75). Phosphoamino acid analysis showed that Ang II induced its phosphorylation at tyrosine, serine and threonine residues and was localized to the cytoplasm. The ability of Ang-II to induce GRP-75 phosphorylation suggests that it may play a role in the protection of cytoplasmic proteins from the damaging effect of oxidative stress known to be produced during Ang-II induced signaling.