VEGF production by primary human renal proximal tubular cells: requirement of HIF-1, PI3-kinase and MAPKK-1 signaling

EGF regulation of proximal tubule cell proliferation and VEGF-A secretion

Proximal tubule cell (PTC) proliferation is critical for tubular regeneration and recovery from acute kidney injury. Epidermal growth factor (EGF) and vascular endothelial growth factor (VEGF‐A) are important for the maintenance of tubulointerstitial integrity and can stimulate PTC proliferation. We utilized HK‐2 cells, an immortalized human PTC line, to characterize the EGF‐dependent regulation of VEGF‐A secretion and proliferation in PTCs. We demonstrate that EGF stimulates VEGF‐A secretion via the EGF receptor (EGFR) and stimulates cell proliferation via activation of the VEGF receptor, VEGFR‐2. EGFR activation promotes MAPK (ERK1/2) activation and HIF‐1α expression, which are required for basal and EGF‐stimulated VEGF‐A secretion. EGF also stimulates the phosphorylation of P70S6 kinase (P70S6K), the downstream target of mTORC1. Rapamycin decreased basal and EGF stimulated HIF‐1α and enhanced MAPK (ERK1/2) activation, while MAPK (ERK/12) inhibition downregulated HIF‐1α expression and the phosphorylation of p70S6K. EGF stimulation of p70S6K was also independent of p‐AKT. Inhibition of the mTORC1 pathway with rapamycin abolished phosphorylation of p70S6K but had no effect on VEGF‐A secretion, indicating that EGF‐stimulated VEGF‐A secretion did not require mTORC1 pathway activation. We demonstrate evidence of a complex crosstalk between the MAPK/ERK and mTORC1 pathways, wherein MAPK (ERK1/2) activation stimulates p‐P70S6K, while p‐P70S6K activation seems to inhibit MAPK (ERK1/2) in EGF‐treated HK‐2 cells. Our results suggest that EGF stimulates MAPK (ERK1/2) in HK‐2 cells, which in turn increases HIF‐1α expression and VEGF‐A secretion, indicating that VEGF‐A mediates EGF‐stimulated cell proliferation as an autocrine proximal tubular epithelial cell growth factor.

Ethanol extract of gardenia fruit alleviates renal interstitial fibrosis induced by unilateral ureteral obstruction in rats

Gardenia fruit has been used in traditional Chinese medicine for thousands of years. A previous study by the present authors indicated that the ethanol extract of gardenia fruits (EEG) primarily contains eight constituents. In the present study, the potential effects of EEG on unilateral ureteral obstruction (UUO)-induced renal interstitial fibrosis were observed in rats. A total of 30 rats were randomly divided into three groups (n=10 each): Sham group, UUO group, and EEG group, which were administered with EEG (200 mg/kg/day) or the same volume of distilled water as a vehicle. UUO were established by ligating left ureter at two points and cut between the ligatures. All rats were sacrificed at 14 days after UUO operation. the present results demonstrated that EEG significantly elevated the expressions of vascular endothelial growth factor and E-cadherin induced by UUO (both P<0.05), and reduced levels of hypoxia-inducible factor-1α, transforming growth factor-β1, connective tissue growth factor and α-smooth muscle actin (all P<0.05). The present findings suggest that EEG is a potential novel renoprotective compound for renal fibrosis through inhibiting epithelial-to-mesenchymal transition.

Hypoxia-induced vasculogenic mimicry formation via VE-cadherin regulation by Bcl-2

Vasculogenic mimicry (VM) refers to the unique ability of highly aggressive tumor cells to mimic the pattern of embryonic vasculogenic networks. Hypoxia plays a pivotal role in the formation of VM. Hypoxia-induced Bcl-2 overexpression is observed in many types of tumors including melanoma, in which it is associated with tumorigenicity and angiogenesis. VE-cadherin, the major endothelial adhesion molecule controlling cellular junctions and blood vessel formation, is also overexpressed in melanoma. Despite these connections, whether hypoxia induces VM formation via VE-cadherin regulation by Bcl-2 is not confirmed. We used human melanoma cells to upregulate or knockdown the expression of Bcl-2 to investigate the possible molecular mechanism of VM formation under hypoxia. Bcl-2 overexpression increased VE-cadherin expression and VM formation under normoxia, whereas Bcl-2 siRNA significantly decreased VE-cadherin expression and VM formation under hypoxia. We then demonstrated that Bcl-2 regulated VE-cadherin transcription activity by Western blot, three-dimensional cultures, reporter gene assay, and clinical analysis. Therefore, Bcl-2-dependent VE-cadherin overexpression may be an important mechanism by which hypoxia induces VM.

Reversal of renal dysfunction by targeted administration of VEGF into the stenotic kidney: a novel potential therapeutic approach

Renal microvascular (MV) damage and loss contribute to the progression of renal injury in renovascular disease (RVD). Whether a targeted intervention in renal microcirculation could reverse renal damage is unknown. We hypothesized that intrarenal vascular endothelial growth factor (VEGF) therapy will reverse renal dysfunction and decrease renal injury in experimental RVD. Unilateral renal artery stenosis (RAS) was induced in 14 pigs, as a surrogate of chronic RVD. Six weeks later, renal blood flow (RBF) and glomerular filtration rate (GFR) were quantified in vivo in the stenotic kidney using multidetector computed tomography (CT). Then, intrarenal rhVEGF-165 or vehicle was randomly administered into the stenotic kidneys (n = 7/group), they were observed for 4 additional wk, in vivo studies were repeated, and then renal MV density was quantified by 3D micro-CT, and expression of angiogenic factors and fibrosis was determined. RBF and GFR, MV density, and renal expression of VEGF and downstream mediators such as p-ERK 1/2, Akt, and eNOS were significantly reduced after 6 and at 10 wk of untreated RAS compared with normal controls. Remarkably, administration of VEGF at 6 wk normalized RBF (from 393.6 ± 50.3 to 607.0 ± 45.33 ml/min, P < 0.05 vs. RAS) and GFR (from 43.4 ± 3.4 to 66.6 ± 10.3 ml/min, P < 0.05 vs. RAS) at 10 wk, accompanied by increased angiogenic signaling, augmented renal MV density, and attenuated renal scarring. This study shows promising therapeutic effects of a targeted renal intervention, using an established clinically relevant large-animal model of chronic RAS. It also implies that disruption of renal MV integrity and function plays a pivotal role in the progression of renal injury in the stenotic kidney. Furthermore, it shows a high level of plasticity of renal microvessels to a single-dose VEGF-targeted intervention after established renal injury, supporting promising renoprotective effects of a novel potential therapeutic intervention to treat chronic RVD.

Cigarette smoke and α,β-unsaturated aldehydes elicit VEGF release through the p38 MAPK pathway in human airway smooth muscle cells and lung fibroblasts

BACKGROUND AND PURPOSE

Vascular endothelial growth factor (VEGF) is an angiogenic factor known to be elevated in the sputum of asymptomatic smokers as well as smokers with bronchitis type of chronic obstructive pulmonary disease. The aim of this study was to investigate whether acute exposure to cigarette smoke extract altered VEGF production in lung parenchymal cells.

EXPERIMENTAL APPROACH

We exposed human airway smooth muscle cells (ASMC), normal human lung fibroblasts (NHLF) and small airways epithelial cells (SAEC) to aqueous cigarette smoke extract (CSE) in order to investigate the effect of cigarette smoke on VEGF expression and release.

KEY RESULTS

Vascular endothelial growth factor release was elevated by sub-toxic concentrations of CSE in both ASMC and NHLF, but not in SAEC. CSE-evoked VEGF release was mimicked by its component acrolein at concentrations (10-100 µM) found in CSE, and prevented by the antioxidant and α,β-unsaturated aldehyde scavenger, N-acetylcysteine (NAC). Both CSE and acrolein (30 µM) induced VEGF mRNA expression in ASMC cultures, suggesting an effect at transcriptional level. Crotonaldehyde and 4-hydroxy-2-nonenal, an endogenous α,β-unsaturated aldehyde, stimulated VEGF release, as did H(2)O(2). CSE-evoked VEGF release was accompanied by rapid and lasting phosphorylation of p38 MAPK (mitogen-activated protein kinase), which was abolished by NAC and mimicked by acrolein. Both CSE- and acrolein-evoked VEGF release were blocked by selective inhibition of p38 MAPK signalling.

CONCLUSIONS AND IMPLICATIONS

α,β-Unsaturated aldehydes and possibly reactive oxygen species contained in cigarette smoke stimulate VEGF expression and release from pulmonary cells through p38 MAPK signalling.

Phosphatidylinositol-3-kinase/akt regulates bleomycin-induced fibroblast proliferation and collagen production

Abnormal repair and dysregulated angiogenesis have been implicated in the pathogenesis of pulmonary fibrosis, but the underlying mechanisms of regulation are not well understood. The present study investigated the role of phosphatidylinositol-3-kinase (PI3K)/Akt in fibrogenesis of human lung fibroblasts and its regulation by reactive oxygen species (ROS). Exposure of lung fibroblasts to bleomycin, a known inducer of fibrosis, resulted in rapid activation of PI3K/Akt and a parallel increase in fibroblast proliferation and collagen production, characteristics of lung fibrosis. Bleomycin had no significant effect on total Akt protein expression but induced phosphorylation of the protein at threonine 308 and serine 473 positions. Inhibition of this phosphorylation by PI3K inhibitors or by dominant-negative Akt (T308A/S473A) expression abrogated the effects of bleomycin on fibroblast proliferation and collagen production, suggesting the role of PI3K/Akt in the fibrogenic process. Activation of PI3K/Akt by bleomycin also led to transcriptional activation and protein expression of hypoxia-inducible factor-1alpha (HIF-1alpha) and vascular endothelial growth factor, which contributed to the fibroproliferative and collagen-inducing effects of bleomycin. The fibrogenic effects of bleomycin were dependent on ROS generation, particularly superoxide anion and hydrogen peroxide, which were induced by bleomycin. Inhibition of ROS generation by antioxidant enzymes, catalase and superoxide dismutase mimetic MnTBAP, abrogated the fibrogenic effects of bleomycin as well as its induction of PI3K/Akt and HIF-1alpha activation. Together, our results indicate a novel role of PI3K/Akt in fibrogenesis of human lung fibroblasts and its regulation by ROS, which could be exploited for the treatment of pulmonary fibrosis and related disorders.

Inhibition of tubulointerstitial fibrosis by pentoxifylline is associated with improvement of vascular endothelial growth factor expression

AIM

Recent information indicates that pentoxifylline (PTX) has the ability to suppress inflammation and profibrotic cell proliferation. In this study, we investigated the effect of PTX on tubulointerstitial fibrosis and the expression of vascular endothelial growth factor (VEGF) in a rat model of obstructive nephropathy.

METHODS

Wistar rats with left ureteral ligation were divided into control and PTX-treated groups. The histopathologic degree of tubulointerstitial fibrosis was scored with PAS and Masson-stained sections. The protein and mRNA for vascular endothelial growth factor (VEGF) were semiquantitatively measured with immunohistochemistry and RT-PCR. The protein for transforming growth factor beta1 (TGFbeta1) and hypoxia-induced factor 1 alpha (HIF-1alpha) was determined by Western blot.

RESULTS

Compared with the control group, PTX treatment reduced fibrosis scores at d 7 and d 14 (P<0.05). The reduction was accompanied by inhibited expression of transforming growth factor-beta 1 (TGFbeta1), a key cytokine in tubulointerstitial fibrogenesis (P<0.01). Meanwhile, VEGF protein and mRNA in the kidney were increased in the PTX-treated group compared with the control group (P<0.01). PTX up-regulated expression of VEGF mRNA in a dose- and time-dependent manner in cultured HK-2 cells (P<0.01). However, expression of HIF-1alpha (a key transcription factor for VEGF gene expression) was unchanged by PTX treatment. PTX prolonged the half-life of VEGF mRNA by a 1.07-fold increase.

CONCLUSIONS

PTX inhibited tubulointerstitial fibrosis in a rat model of obstructive nephropathy while preventing loss of VEGF. PTX up-regulated expression of VEGF mRNA through stabilization of its mRNA in cultured renal tubular epithelial cells.

Telmisartan-enhanced hypercholesterolaemic serum-induced vascular endothelial growth factor expression in immortalized human umbilical vascular endothelial cells

OBJECTIVE

To clarify whether hypercholesterolaemia can increase vascular endothelial growth factor (VEGF) expression in human umbilical vascular endothelial cells (HUVECs) through the phosphatidylinositol 3-kinase (PI3K) pathway, and whether a special angiotensin II receptor blocker, telmisartan, can attenuate VEGF expression induced by hypercholesterolaemia.

METHODS

Levels of VEGF expression, PI3K activity and angiogenesis in vitro were determined by various methods after HUVECs were incubated with hypercholesterolaemic serum or combined with telmisartan and/or wortmannin.

RESULTS

We found that hypercholesterolaemic serum (cholesterol > or = 0.08 mmol/L) can increase VEGF expression in HUVECs and that telmisartan cooperates with hypercholesterolaemic serum in promoting VEGF expression. The increased VEGF expression was associated with enhanced PI3K activity and could be significantly inhibited by wortmannin, a potent PI3K inhibitor. Likewise, hypercholesterolaemic serum significantly promoted angiogenesis in vitro, which could be inhibited when PI3K activity was suppressed.

CONCLUSIONS

Our study suggests that hypercholesterolaemic serum induces VEGF expression through PI3K in HUVECs and that telmisartan cooperates with hypercholesterolaemia in promoting VEGF expression.

Intrarenal oxygenation in chronic renal failure

In chronic renal failure (CRF), renal impairment correlates with tubulointerstitial fibrosis characterized by inflammation, interstitial expansion with accumulation of extracellular matrix (ECM), tubular atrophy and vascular obliteration. Tubulointerstitial injury subsequent to glomerular sclerosis may be induced by proteinuria, leakage of glomerular filtrate or injury to the post-glomerular peritubular capillaries (hypoxia). In vivo data in animal models suggest that CRF is associated with hypoxia, with the decline in renal Po2 preceding ECM accumulation. Chronic renal failure is characterized by loss of microvascular profiles but, in the absence of microvascular obliteration, hypoxia can occur by a variety of complementary mechanisms, including anaemia, decreased capillary flow, increased vasoconstriction, increased metabolic demand and increased diffusion distances due to ECM deposition. Hypoxia regulates a wide array of genes, including many fibrogenic factors. Hypoxia-inducible factors (HIF) are the major, but not the sole, transcriptional regulators in the hypoxic response. In CRF, hypoxia may play a role in the sustained inflammatory response. In vitro studies in tubulointerstitial cells suggest that hypoxia can induce profibrogenic changes in proximal tubular epithelial cells and interstitial fibroblasts consistent with changes observed in CRF in vivo. The effect of hypoxia on renal microvascular cells warrants investigation. Hypoxia may play a role in the recruitment, retention and differentiation of circulating progenitor cells to the kidney contributing to the disease process and may also affect intrinsic stem cell populations. Chronic hypoxia in CRF fails to induce a sustained angiogenic response. Therapeutic manipulation of the hypoxic response may be of benefit in slowing progression of CRF. Potential therapies include correction of anaemia, inhibition of the renin-angiotensin system, administration of exogenous pro-angiogenic factors to protect the microvasculature, activation of HIF and hypoxia-mediated targeting of engineered progenitor cells.

IL-4 enhances hypoxia induced HIF-1alpha protein levels in human transformed intestinal cells

Hypoxia-inducible factor-1 (HIF-1) is a transcription factor that mediates the adaptive response to hypoxia. Increasing evidence suggests a crucial role for HIF-1 in immune reactions. Here we investigated the effect of the Th2 type cytokines IL-4 and IL-10 on HIF-1alpha mediated response in normoxia (21% O2) and hypoxia (1% O2). Incubation of human transformed intestinal cells (HT-29) with IL-4 significantly increased HIF-1alpha protein levels during hypoxia but not during normoxia. Mechanisms involved are IL-4 induced up-regulation of HIF-1alpha gene transcription and the PI3K signaling pathway. The increase in hypoxia-induced accumulation of HIF-1alpha protein after IL-4 treatment did not result in up-regulation of HIF-1 DNA-binding activity or HIF-1 dependent gene expression. IL-10 did not affect HIF-1alpha protein levels.

Review: hypoxia-inducible factor-1 (HIF-1): a novel transcription factor in immune reactions

Hypoxia-inducible factor-1 (HIF-1) is a dimeric transcriptional complex that has been recognized primarily for its role in the maintenance of oxygen and energy homoeostasis. The HIF-1alpha subunit is O(2) labile and is degraded by the proteasome following prolyl-hydroxylation and ubiquitination in normoxic cells. The present review summarizes evidence that HIF-1 is also involved in immune reactions. Immunomodulatory peptides, including interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha), stimulate HIF-1 dependent gene expression even in normoxic cells. Both the hypoxic and the cytokine-induced activation of HIF-1 involve the phosphatidylinositol- 3-kinase (PI3K) and the mitogen-activated protein kinase (MAPK) signaling pathways. In addition, heat shock proteins (HSP) and other cofactors interact with HIF-1 subunits. HIF-1 increases the transcription of several genes for proteins that promote blood flow and inflammation, including vascular endothelial growth factor (VEGF), heme oxygenase-1, endothelial and inducible nitric oxide synthase (NOS) and cyclooxygenase-2 (COX-2). The pharmacologic activation of the HIF-1 complex can be desirable in ischemic and inflammatory disorders. In contrast, HIF-1 blockade may be beneficial to prevent tumor angiogenesis and tumor growth.