Thrombin induced cytoskeletal change in cultured bovine corneal endothelial cells mediated via protein kinase C pathway

Microtubule disassembly breaks down the barrier integrity of corneal endothelium

Increased contractility of the peri-junctional actomyosin ring (PAMR) breaks down the barrier integrity of corneal endothelium. This study has examined the effects of microtubule disassembly on Myosin Light Chain (MLC) phosphorylation, a biochemical marker of actomyosin contraction, and barrier integrity in monolayers of cultured bovine corneal endothelial cells (BCEC). Exposure to nocodazole, which readily induced microtubule disassembly, led to disruption of the characteristically dense assembly of cortical actin cytoskeleton at the apical junctional complex (i.e., PAMR) and dispersion of ZO-1 from its normal locus. Nocodazole also led to an increase in phosphorylation of MLC. Concomitant with these changes, nocodazole caused an increase in permeability to HRP and FITC dextran (10 kDa) and a decrease in trans-endothelial electrical resistance (TER). Y-27632 (a Rho kinase inhibitor) and forskolin (known to inhibit activation of RhoA through direct elevation of cAMP) opposed the nocodazole-induced MLC phosphorylation, decrease in TER, and dispersion of ZO-1. Thrombin, which breaks down the barrier integrity of BCEC monolayers, also induced microtubule disassembly and MLC phosphorylation. Pre-treatment with paclitaxel to stabilize microtubules opposed the thrombin effects. These results suggest that microtubule disassembly breaks down the barrier integrity of BCEC through activation of RhoA and subsequent disruption of the PAMR. The thrombin effect also highlights that signaling downstream of GPCRs can also influence the organization of microtubules.

Functional characteristics of connective tissue growth factor on vitreoretinal cells

Connective tissue growth factor (CTGF) level is elevated in eyes with proliferative vitreoretinal diseases, such as proliferative diabetic retinopathy and proliferative vitreoretinopathy (PVR), as we previously reported, but its functional characteristics on vitreoretinal cells are yet to be clarified. In this study, we demonstrated a growth-promoting effect of CTGF on cultured hyalocytes and bovine retinal pigment epithelial cells (BRPEs) with the induction of p44/p42 mitogen-activated protein kinase phosphorylation and [(3)H]thymidine incorporation. CTGF also stimulated the synthesis of fibronectin by hyalocytes and BRPEs without significant effect on collagen gel contraction by these cells. On the other hand, CTGF had no direct effects on the proliferation, migration, or in vitro tube formation by vascular endothelial cells. Nevertheless, CTGF promoted vascular endothelial growth factor (VEGF) gene expression by hyalocytes and BRPEs. Although the concentrations of both CTGF and VEGF in the human vitreous samples with proliferative vitreoretinal diseases were elevated, there was no significant correlation between these concentrations. These findings indicate that CTGF appears to be involved in the formation of proliferative membranes without direct regulation of their cicatricial contraction in the pathogenesis of proliferative vitreoretinal diseases. Whereas CTGF might have no direct effects or minimal effects, if any, on retinal neovascularization, it is possible that CTGF has indirect effects by modulating the expression of VEGF.

Transforming growth factor-beta2 and connective tissue growth factor in proliferative vitreoretinal diseases: possible involvement of hyalocytes and therapeutic potential of Rho kinase inhibitor

The critical association of connective tissue growth factor (CTGF), which is thought to be one of the downstream mediators of transforming growth factor-beta (TGF-beta), with vitreoretinal diseases remains to be clarified. In the current study, we first demonstrated the correlation between the concentrations of TGF-beta2 as well as CTGF in the vitreous and CTGF gene regulation in cultured hyalocytes. Concentrations of TGF-beta2 and CTGF in the vitreous from patients with proliferative vitreoretinal diseases were significantly higher than in those with nonproliferative diseases, and there was a positive correlation between their concentrations (r = 0.320, P < 0.01). Cultured hyalocytes expressed CTGF mRNA, which was enhanced in the presence of TGF-beta2, associated with nuclear accumulation of Smad4. TGF-beta2-dependent Smad4 translocation and CTGF gene expression were mediated through Rho kinase and at least partially via p38 mitogen-activated protein kinase. Finally, fasudil, a Rho kinase inhibitor already in clinical use, inhibited both Smad4 translocation and CTGF gene expression. In conclusion, combined effects of TGF-beta2 and CTGF appear to be involved in the pathogenesis of proliferative vitreoretinal diseases. Hyalocytes may be a possible source of CTGF and thus might play a role in vitreoretinal interface diseases. Furthermore, Rho kinase inhibitors might have therapeutic potential to control fibrotic disorders in the eye.

Muscarinic cholinoceptor-stimulated phosphatidyl inositol pathway in corneal epithelial and endothelial cells

BACKGROUND

Muscarinic cholinoceptors are distributed widely in both the central and peripheral nervous system. The presence of muscarinic cholinoceptors in corneal tissue is well established. Previous reports have shown that corneal muscarinic cholinoceptors are of the m2 or m4 subtype. However, recent studies have indicated the presence of the m5 muscarinic cholinoceptor subtype in human corneal epithelium and endothelium. The aim of the study was to confirm the presence of the m5 cholinoceptor subtype in bovine corneal epithelium and endothelium and the activation of phosphatidyl inositol pathway by its stimulation.

METHODS

Muscarinic m5 cholinoceptor sites, phosphatidyl inositol 4,5-biphosphate, inositol 1,4,5-triphosphate and protein kinase C, were studied using immunocytochemistry and immunofluorescence. Activation of protein kinase C after stimulation of the m5 muscarinic cholinoceptor subtype was measured using the HTS protein kinase C assay kit.

RESULTS

Immunocytochemistry/immunofluorescence revealed the presence of the m5 muscarinic cholinoceptor subtype, phosphatidyl inositol 4,5-biphosphate and protein kinase C in bovine corneal epithelial and endothelial cells. In bovine corneal epithelium and endothelium, protein kinase C activity was stimulated by acetylcholine in a dose-dependent manner (P<0.0001).

CONCLUSIONS

Our findings indicate that acetylcholine-induced stimulation of muscarinic m5 cholinoceptors activates the phosphatidyl inositol pathway in corneal epithelial and endothelial cells, resulting in increased protein kinase C activity. Further work will be needed to clear the physiologic role of this signaling pathway in corneal epithelium and endothelium.

Thrombin-induced phosphorylation of the regulatory light chain of myosin II in cultured bovine corneal endothelial cells

PURPOSE

Phosphorylation of the regulatory light chain of myosin II (referred to as myosin light chain or MLC) leads to a loss of barrier integrity in cellular monolayers by an increase in the contractility of the cortical actin cytoskeleton. This effect has been examined in corneal endothelial (CE) cells.

METHODS

Experiments were performed using cultured bovine CE cells (BCEC). MLC phosphorylation was induced by a thrombin-mediated activation of the proteinase-activated receptor-1 (PAR-1). Expression of MLC kinase (MLCK), a Ca2+/calmodulin-dependent protein kinase that phosphorylates MLC at its Ser-19 and Thr-18 residues, was determined by RT-PCR and Western blotting. Expression of PAR-1, RhoA, and Rho kinase-1 (effector of RhoA) was ascertained by RT-PCR. MLC phosphorylation was assessed by urea-glycerol gel electrophoresis followed by immunoblotting. The effects of Rho kinase-1 and PKC were characterized by using their selective inhibitors, Y-27632 and chelerythrine, respectively. Reorganization of the cytoskeleton was evaluated by the phalloidin staining of actin. [Ca2+]i was measured using Fura-2. The barrier integrity was assayed as permeability of BCEC monolayers to horseradish peroxidase (HRP; 44 kDa).

RESULTS

RT-PCR showed expression of MLCK, PAR-1, Rho kinase-1, and RhoA. Western blotting indicated expression of the non-muscle and smooth muscle isoforms of MLCK. Exposure to thrombin induced an increase in [Ca2+]i with the peak unaffected by an absence of extracellular Ca2+. Pre-exposure to thrombin (2 U ml(-1); 2 min) led to mono- and di-phosphorylation of MLC. Under both basal conditions and in the presence of thrombin, MLC phosphorylation was prevented by chelerythrine (10 microm) and Y-27632 (<25 microm). Thrombin led to inter-endothelial gaps secondary to the disruption of the cortical actin cytoskeleton, which under resting conditions was organized as a perijunctional actomyosin ring (PAMR). These responses were blocked by pre-treatment with Y-27632. Thrombin also increased permeability to HRP, which was abolished by pre-treatment with Y-27632.

CONCLUSIONS

Thrombin induces MLC phosphorylation in BCEC. The consequent increase in the contractility of the actin cytoskeleton produces a centripetal force resulting in inter-endothelial gaps and a breakdown of barrier integrity. These responses are PKC- and Rho kinase-dependent. [Ca2+]i increase, as well as sensitivity of the thrombin response to PKC and Rho kinase inhibitors, are consistent with the expression of PAR-1 receptors in BCEC. Thrombin-induced hyperpermeability is a model to investigate barrier dysfunction induced by MLC phosphorylation.

Depletion of protein kinase Cepsilon in normal and scleroderma lung fibroblasts has opposite effects on tenascin expression

OBJECTIVE

To determine whether the extracellular matrix protein tenascin-C (TN-C) is overexpressed in lung fibroblasts from systemic sclerosis (SSc) patients, the molecular mechanisms regulating TN-C secretion in SSc and normal lung fibroblasts, and how these processes might contribute to lung fibrosis in SSc patients.

METHODS

TN-C secretion by SSc and normal fibroblasts was compared in vivo (in bronchoalveolar lavage [BAL] fluid) and in vitro (in culture medium). The ability of thrombin to induce TN-C was confirmed at both the protein and the messenger RNA (mRNA) level. The role of protein kinase Cepsilon (PKCepsilon) in the expression of TN-C was evaluated by determining the effects of thrombin on PKCepsilon levels and by directly manipulating PKCepsilon levels via the use of antisense oligonucleotides.

RESULTS

BAL fluid from SSc patients contained high levels of TN-C, whereas that from normal subjects contained little or no TN-C. In vitro, SSc lung fibroblasts expressed much higher amounts of TN-C than did normal lung fibroblasts. Consistent with the idea that thrombin is a physiologic inducer of TN-C, thrombin stimulated TN-C mRNA and protein expression in both SSc and normal lung fibroblasts by a mechanism that required proteolytic cleavage of the thrombin receptor. Surprisingly, thrombin treatment and antisense oligonucleotide-mediated depletion of PKCepsilon indicated that TN-C expression is regulated via opposite signaling mechanisms in SSc and normal cells. In SSc lung fibroblasts, thrombin decreased PKCepsilon levels, and the decreased PKCepsilon induced TN-C secretion; in normal fibroblasts, thrombin increased PKCepsilon levels, and the increased PKCepsilon induced TN-C secretion. Normal and SSc lung fibroblasts also differed in the subcellular localization of PKCepsilon, both before and after thrombin treatment.

CONCLUSION

These studies are the first to demonstrate that thrombin is a potent simulator of TN-C in lung fibroblasts and that PKCepsilon is a critical regulator of TN-C protein levels in these cells. Furthermore, our results indicate that both the regulation of PKCepsilon levels by thrombin and the regulation of TN-C levels by PKCepsilon are defective in SSc lung fibroblasts.

Mesangial cell actin disassembly in high glucose mediated by protein kinase C and the polyol pathway

High glucose alters mesangial cell cytoskeletal structure and function. We postulated that high glucose causes mesangial cell filamentous (F) actin disassembly through a protein kinase C (PKC) mechanism involving the polyol pathway. Rat mesangial cells (passage < 10, N = 60/group) were growth-arrested and then cultured in glucose 5.6 mM (NG), 15 mM (MG) or 30 mM (HG) for 48 hours, with or without the aldose reductase inhibitor Tolrestat 0.3 mM. F and globular (G) actin were labeled with rhodamine-phalloidin and FTTC-DNase-1, respectively. Both fluorescence probes were imaged simultaneously in each cell using dual-channel confocal laser microscopy. In HG, F-actin disassembly was observed and measured by a 40% decrease in F-/G-actin fluorescence intensity ratio (no change in NG + mannitol 24.4 mM). In separate experiments, cells were labeled with BODIPY FL-bisindolylmaleimide, specific for most PKC isoforms, and fluorescence intensity/cell was measured. In NG, exposure to phorbol 12-myristate 13-acetate (PMA) 0.1 microM for 15 minutes caused perinuclear and nuclear translocation of PKC, and F-actin disassembly identical to observations in HG alone. In HG, total PKC fluorescence increased by 50% and PMA exposure for 24 hours normalized both the total PKC and F-/G-actin fluorescence ratio. In NG and HG, exposure (15 min) to PMA 0.1 microM increased PKC activity three to four times, measured by in situ 32P-phosphorylation of EGF-receptor substrate. By immunofluorescence and confocal imaging, diacylglycerol-sensitive PKC-delta was localized to the cytosol in NG, and after 15 minutes exposure to PMA, translocated to the perinuclear region and plasma membrane. In HG. PKC-delta immunofluorescence was significantly increased/cell, distributed in a cytoskeletal pattern and the intensity was glucose-concentration dependent (30 > 15 > 5.6 mM). In HG, exposure to PMA for 24 hours returned the PKC-delta fluorescence to the intensity and cytosolic pattern observed in NG, and simultaneously prevented F-actin disassembly. Tolrestat significantly reduced the total PKC and PKC-delta fluorescence intensity and F-actin disassembly observed in HG. Immunoblot confirmed increased PKC-delta in HG, which was normalized by Tolrestat. The immunofluorescence pattern of diacylglycerol-insensitive PKC-delta was unchanged in HG, with PMA or Tolrestat. We conclude that mesangial cell F-actin disassembly in high glucose is likely mediated through diacylglycerol-sensitive PKC isoforms, including PKC-delta and involves the polyol pathway.