Changes associated with tyrosine phosphorylation during short-term hypoxia in retinal microvascular endothelial cells in vitro

Herbimycin A inhibits angiogenic activity in endothelial cells and reduces neovascularization in a rat model of retinopathy of prematurity

The pathogenesis of retinopathy of prematurity involves dysregulated angiogenesis resulting in pre-retinal growth of new vessels. Inhibition of tyrosine kinase-dependent pro-angiogenic signals may provide a rational therapeutic approach to the reduction of pre-retinal neovascularization. Vascular endothelial growth factor stimulates endothelial cell mitogenesis, differentiation and migration, by binding and activating the receptor tyrosine kinases vascular endothelial growth factor receptor-1 and vascular endothelial growth factor receptor-2. One of the vascular endothelial growth factor receptor substrates implicated in vascular endothelial growth factor signal transduction is c-Src. The ability of herbimycin A, a c-Src-selective tyrosine kinase inhibitor, to inhibit vascular endothelial growth factor-induced bovine retinal microvascular endothelial cell proliferation and tube formation was investigated. The ability of the compound to inhibit pathologic angiogenesis was tested in a rat model of retinopathy of prematurity. Exposure of neonatal rats to oxygen concentrations cycling between 10 and 50% induced severe pre-retinal neovascularization in all rats. Some of the eyes of these variable oxygen-exposed rats were herbimycin A-injected or vehicle-injected 1 or 3 days post-oxygen exposure while some eyes were non-injected. All rats were sacrificed for assessment 6 days post-exposure. Herbimycin A inhibited both vascular endothelial growth factor-induced bovine retinal microvascular endothelial cell proliferation and capillary tube formation in a dose-dependent manner. Injection of herbimycin A into oxygen-treated rats 1 day post-oxygen exposure produced a 63% decrease in pre-retinal neovascularization relative to vehicle (P = 0.0029). There was a 41% decrease in pre-retinal neovascularization in herbimycin-injected eyes relative to vehicle-injected eyes 3 days post-oxygen (P = 0.031). Pre-retinal neovascularization was reduced in vehicle-injected eyes relative to non-injected eyes at both injection times. There were no significant differences in retinal vascular area between any of the experimental groups. Based on the results of this study, herbimycin A inhibits endothelial cell proliferation and tube formation at non-toxic concentrations and reduces pre-retinal neovascularization in a rat model of retinopathy of prematurity. Reduction of angiogenesis by the inhibition of tyrosine kinase activity may be a viable route to the development of effective chemotherapies applicable to eye disease.

Cytoprotection against thermal injury: evaluation of herbimycin A by cell viability and cDNA arrays

Herbimycin A (HA), a known inducer of the heat shock response, was investigated for its ability to increase survival of a human cell line following thermal injury. Its effect on transcriptional activity was also assessed with cDNA arrays to provide new targets for cytoprotection. Pretreatment with at least 0.75 microg/ml HA significantly increased the fraction of cells surviving thermal injury by up to 50% (based on 8s exposure) compared to untreated controls. HA also significantly induced transcription of mRNA for HSP90 and HSP70, and protein production for HSP40 and HSP70. Gene expression profiling demonstrated that the most highly elevated genes included growth factors and transcription factors, while prominently suppressed genes included transcription factors and kinases. These results suggest that cytoprotection may be due to the contribution of the products of a significant number of genes in addition to the classic stress response genes, suggesting that modulation of these genes might induce thermotolerance and amelioration of thermal injury.

Genistein inhibits herbimycin A-induced over-expression of inducible heat shock protein 70 kDa

It has been shown that expression of heat shock proteins (HSPs) can interfere with the effectiveness of therapeutic cytotoxic drugs. In this study, we investigated the regulation of expression of HSPs in human epidermoid A-431 cells. Two known protein tyrosine kinase inhibitors were studied. Treatment of cells with herbimycin A increased production of inducible HSP 70 kDa (HSP-70i) in a concentration-dependent manner, whereas genistein did not. The increase induced by herbimycin A was observed within 2 h, reached a peak at 6 h, and remained above the basal level 3 days later. Pretreatment with genistein inhibited the herbimycin A-induced increase in HSP-70i. Herbimycin A treatment increased levels of HSP-70i mRNA in cells, suggesting that herbimycin A increases HSP-70i by promoting transcription. Treatment with genistein or genistein combined with herbimycin A did not increase HSP-70i mRNA, suggesting that the inhibitory effect of genistein also occurs at the level of mRNA production. Herbimycin A increased intracellular Ca2+ concentration ([Ca2+]i), but treatment with genistein decreased it. Chelation of [Ca2+]i with BAPTA blocked the herbimycin A-induced increase in HSP-70i mRNA and HSP-70i protein. Herbimycin A induced the phosphorylation of heat shock factor 1 (HSF1), while genistein reduced HSF1 production. The ability of genistein to inhibit the herbimycin A-induced increase in HSP-70i is not associated with genistein's capacity to decrease basal [Ca2+]i, but because it decreases HSFI production. The herbimycin A-induced increase in HSP-70i protected cells from hypoxia injury.

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.

Regulation of endothelin-1 gene expression in human microvascular endothelial cells by hypoxia and cobalt: role of hypoxia responsive element

Endothelin-1 (Et-1) is a vasoconstrictor peptide that plays an important role in the pathophysiology of hypertension, myocardial ischemia, and other diseases. We examined the mechanism of regulation the Et-1 mRNA expression in human microvascular endothelial cells (HMEC-1) in response to hypoxia and cobalt. To determine whether the 5'-flanking region of Et-1 gene mediate transcriptional responses to cellular hypoxia, we constructed reporter plasmids in which Et-1 5'-flanking sequences of Et-1 gene were fused to luciferase coding sequences. Constructs, which contain native Et-1 sequence 5'-AACGTGCA-3', located between -118 and -125 in the opposite orientation as the transcriptional unit, mediate transcriptional response to hypoxia and cobalt. This responsiveness was inhibited by genistein, a tyrosine kinase selective inhibitor. Both hypoxia and cobalt induced binding of HIF-1 (hypoxia inducible-1 factor) to this Et-1 hypoxia responsive element in gel shift assays. Mutation in this sequence eliminated both the hypoxia-induced HIF-1 binding and luciferase expression. Using the supershift assay we have shown that this hypoxia responsive element binds HIF-1alpha and HIF-1beta proteins. Interestingly, genistein only slightly affected HIF-1 binding. These results indicate that the Et-1 gene contains HIF-1 binding hypoxia responsive elements which mediate transcriptional responses to hypoxia and cobalt in microvascular endothelial cells. Genistein appears to inhibit this response by affecting the transcriptional activity of the HIF-1 complex, without significantly affecting its DNA-binding properties.

Mitogen-activated protein kinase phosphatase-1 (MKP-1) expression is induced by low oxygen conditions found in solid tumor microenvironments. A candidate MKP for the inactivation of hypoxia-inducible stress-activated protein kinase/c-Jun N-terminal protein kinase activity

Pathophysiological hypoxia is an important modulator of gene expression in solid tumors and other pathologic conditions. We observed that transcriptional activation of the c-jun proto-oncogene in hypoxic tumor cells correlates with phosphorylation of the ATF2 transcription factor. This finding suggested that hypoxic signals transmitted to c-jun involve protein kinases that target AP-1 complexes (c-Jun and ATF2) that bind to its promoter region. Stress-inducible protein kinases capable of activating c-jun expression include stress-activated protein kinase/c-Jun N-terminal protein kinase (SAPK/JNK) and p38 members of the mitogen-activated protein kinase (MAPK) superfamily of signaling molecules. To investigate the potential role of MAPKs in the regulation of c-jun by tumor hypoxia, we focused on the activation SAPK/JNKs in SiHa human squamous carcinoma cells. Here, we describe the transient activation of SAPK/JNKs by tumor-like hypoxia, and the concurrent transcriptional activation of MKP-1, a stress-inducible member of the MAPK phosphatase (MKP) family of dual specificity protein-tyrosine phosphatases. MKP-1 antagonizes SAPK/JNK activation in response to diverse environmental stresses. Together, these findings identify MKP-1 as a hypoxia-responsive gene and suggest a critical role in the regulation of SAPK/JNK activity in the tumor microenvironment.

Alterations in protein tyrosine kinase pathways following retinal vein occlusion in the rat

PURPOSE

To determine whether an experimental retinal vein occlusion in the rat activates protein tyrosine kinase pathways and increases angiogenic growth factors in the retina.

METHODS

Retinal vein occlusion (RVO) was induced in the rat retina with argon laser photocoagulation. Retinas were collected at 2 days, 1, 2, and 4 weeks after RVO and divided into halves: one half represented an area within the distribution of the occluded vein [RVO(IN)] and the other half represented an area outside the distribution of the occluded vein [RVO(OUT)]. RVO(IN) and (OUT) were examined by western blot analysis of tyrosine-phosphorylated proteins, vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and 3 signal proteins in the tyrosine kinase pathways: phospholipase Cgamma (PLCgamma), phosphatidylinositol 3-kinase (PI3K), and mitogen-activated protein kinase (MAPK).

RESULTS

RVO caused a severe capillary nonperfusion in RVO(IN). Overall tyrosine-phosphorylated proteins were increased after RVO, especially in RVO(IN) at 2 days and 1 week. VEGF and bFGF were markedly increased in RVO(IN) at 2 days and 1 week. Tyrosine-phosphorylated PLCgamma, PI3K, and MAPK were also increased in RVO(IN) at these time points.

CONCLUSIONS

RVO caused an increase in overall protein tyrosine phosphorylation in the rat retina. This increase was associated with an increase in angiogenic growth factors (VEGF and bFGF). These results suggest that protein tyrosine kinase pathways are activated during retinal ischemia and may play a role in mitogenesis of vascular endothelial cells and other responses in the retina after RVO.

Role of protein tyrosine phosphorylation in rat corneal neovascularization

BACKGROUND

Recent studies have suggested that tyrosine kinase pathways that are activated by angiogenic growth factors may play a role in corneal neovascularization.

METHODS

Corneal neovascularization was induced in rat corneas by chemical cauterization. At 6, 24, 48, 96, and 168 h after chemical cauterization the rat corneas without the corneal epithelium were prepared for gel electrophoresis. Total protein profiles of the corneal samples were examined by staining gels with Coomassie brilliant blue. Tyrosine-phosphorylated proteins, three angiogenic growth factors (basic fibroblast growth factor, vascular endothelial growth factor, and platelet-derived growth factor-B chain), and three intracellular signal proteins in the tyrosine kinase pathways (phospholipase C gamma, SHC, and mitogen-activated protein kinase) in the corneal samples were examined by western blot analysis. A topical treatment of genistein eye drop (5 mg/ml) was used for inhibition of corneal neovascularization after chemical cauterization in rats.

RESULTS

In total protein profiles, three bands in the corneal samples were increased after cauterization. Overall tyrosine-phosphorylated proteins and all three angiogenic growth factors increased with progression of corneal neovascularization. The tyrosine-phosphorylated forms of three intracellular signal proteins were also increased after cauterization. Treatment with topical genistein was effective in inhibiting corneal neovascularization in rats.

CONCLUSION

Protein tyrosine phosphorylation was involved in inflammation-induced corneal neovascularization. Tyrosine kinase inhibitors may have utility as inhibitors of corneal neovascularization.