Hypoxia-driven proliferation of human pulmonary artery fibroblasts: cross-talk between HIF-1alpha and an autocrine angiotensin system

Urotensin II is an autocrine/paracrine growth factor for aortic adventitia of rat

Urotensin II (UII) is a potent vasoconstrictive peptide; however, its significance in vascular adventitia has not been clearly elucidated. In this study, rat aortic adventitia showed mRNA expression and immunoreactivity of UII and its receptor (UT). Moreover, radioligand-binding assay showed that maximum binding capacity (Bmax) of [(125)I]-UII was higher in adventitia than in media (28.60+/-1.94 vs. 20.21+/-1.11 fmol/mg, P<0.01), with no difference in binding affinity (dissociation constant [Kd] 4.27+/-0.49 vs. 4.60+/-0.40 nM, P>0.05). Furthermore, in cultured adventitial fibroblasts, UII stimulated DNA synthesis, collagen synthesis and secretion in a concentration-dependent manner. These effects were inhibited by the UII receptor antagonist urantide (10(-6) mol/l), Ca(2+) channel blocker nicardipine (10(-5) mol/l), protein kinase C inhibitor H7 (10(-6) mol/l), and mitogen-activated protein kinase inhibitor PD98059 (10(-6) mol/l) but not the phosphatidyl inositol-3 kinase inhibitor wortmannin (10(-7) mol/l). UII may act as an autocrine/paracrine factor through its receptor and the Ca(2+) channel, protein kinase C, and mitogen-activated protein kinase signal transduction pathways, in the pathogenesis of vascular remodeling by activating vascular adventitia.

Hypoxia-driven proliferation of embryonic neural stem/progenitor cells--role of hypoxia-inducible transcription factor-1alpha

We recently reported that intermittent hypoxia facilitated the proliferation of neural stem/progenitor cells (NPCs) in the subventricule zone and hippocampus in vivo. Here, we demonstrate that hypoxia promoted the proliferation of NPCs in vitro and that hypoxia-inducible factor (HIF)-1alpha, which is one of the key molecules in the response to hypoxia, was critical in this process. NPCs were isolated from the rat embryonic mesencephalon (E13.5), and exposed to different oxygen concentrations (20% O(2), 10% O(2), and 3% O(2)) for 3 days. The results showed that hypoxia, especially 10% O(2), promoted the proliferation of NPCs as assayed by bromodeoxyuridine incorporation, neurosphere formation, and proliferation index. The level of HIF-1alpha mRNA and protein expression detected by RT-PCR and western blot significantly increased in NPCs subjected to 10% O(2). To further elucidate the potential role of HIF-1alpha in the proliferation of NPCs induced by hypoxia, an adenovirus construct was used to overexpress HIF-1alpha, and the pSilencer 1.0-U6 plasmid as RNA interference vector targeting HIF-1alpha mRNA was used to knock down HIF-1alpha. We found that overexpression of HIF-1alpha caused the same proliferative effect on NPCs under 20% O(2) as under 10% O(2). In contrast, knockdown of HIF-1alpha inhibited NPC proliferation induced by 10% O(2). These results demonstrated that moderate hypoxia was more beneficial to NPC proliferation and that HIF-1alpha was critical in this process.

Effect of hypoxia and Beraprost sodium on human pulmonary arterial smooth muscle cell proliferation: the role of p27kip1

BACKGROUND

Hypoxia induces the proliferation of pulmonary arterial smooth muscle cell (PASMC) in vivo and in vitro, and prostacyclin analogues are thought to inhibit the growth of PASMC. Previous studies suggest that p27kip1, a kind of cyclin-dependent kinase inhibitor, play an important role in the smooth muscle cell proliferation. However, the mechanism of hypoxia and the subcellular interactions between p27kip1 and prostacyclin analogues in human pulmonary arterial smooth muscle cell (HPASMC) are not fully understood.

METHODS

We investigated the role of p27kip1 in the ability of Beraprost sodium (BPS; a stable prostacyclin analogue) to inhibit the proliferation of HPASMC during hypoxia. To clarify the biological effects of hypoxic air exposure and BPS on HPASMC, the cells were cultured in a hypoxic chamber under various oxygen concentrations (0.1-21%). Thereafter, DNA synthesis was measured as bromodeoxyuridine (BrdU) incorporation, the cell cycle was analyzed by flow cytometry with propidium iodide staining. The p27kip1 mRNA and protein expression and it's stability was measured by real-time RT-PCR and Western blotting. Further, we assessed the role of p27kip1 in HPASMC proliferation using p27kip1 gene knockdown using small interfering RNA (siRNA) transfection.

RESULTS

Although severe hypoxia (0.1% oxygen) suppressed the proliferation of serum-stimulated HPASMC, moderate hypoxia (2% oxygen) enhanced proliferation in accordance with enhanced p27kip1 protein degradation, whereas BPS suppressed HPASMC proliferation under both hypoxic and normoxic conditions by suppressing p27kip1 degradation with intracellular cAMP-elevation. The 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP), a cAMP analogue, had similar action as BPS in the regulation of p27kip1. Moderate hypoxia did not affect the stability of p27kip1 protein expression, but PDGF, known as major hypoxia-induced growth factors, significantly decreased p27kip1 protein stability. We also demonstrated that BPS and 8-Br-cAMP suppressed HPASMC proliferation under both hypoxic and normoxic conditions by blocking p27kip1 mRNA degradation. Furthermore, p27kip1 gene silencing partially attenuated the effects of BPS and partially restored hypoxia-induced proliferation.

CONCLUSION

Our study suggests that moderate hypoxia induces HPASMC proliferation, which is partially dependent of p27kip1 down-regulation probably via the induction of growth factors such as PDGF, and BPS inhibits both the cell proliferation and p27kip1 mRNA degradation through cAMP pathway.

Hypoxia influences the cellular cross-talk of human dermal fibroblasts. A proteomic approach

The ability of cells to respond to changes in oxygen availability is critical for many physiological and pathological processes (i.e. development, aging, wound healing, hypertension, cancer). Changes in the protein profile of normal human dermal fibroblasts were investigated in vitro after 96 h in 5% CO(2) and 21% O(2) (pO(2) = 140 mm Hg) or 2% O(2) (pO(2) = 14 mm Hg), these parameters representing a mild chronic hypoxic exposure which fibroblasts may undergo in vivo. The proliferation rate and the protein content were not significantly modified by hypoxia, whereas proteome analysis demonstrated changes in the expression of 56 proteins. Protein identification was performed by mass spectrometry. Data demonstrate that human fibroblasts respond to mild hypoxia increasing the expression of hypoxia inducible factor (HIF1a) and of the 150-kDa oxygen-regulated protein. Other differentially expressed proteins appeared to be related to stress response, transcriptional control, metabolism, cytoskeleton, matrix remodelling and angiogenesis. Furthermore, some of them, like galectin 1, 40S ribosomal protein SA, N-myc-downstream regulated gene-1 protein, that have been described in the literature as possible cancer markers, significantly changed their expression also in normal hypoxic fibroblasts. Interestingly, a bovine fetuin was also identified that appeared significantly less internalised by hypoxic fibroblasts. In conclusion, results indicate that human dermal fibroblasts respond to an in vitro mild chronic hypoxic exposure by modifying a number of multifunctional proteins. Furthermore, data highlight the importance of stromal cells in modulating the intercellular cross-talk occurring in physiological and in pathologic conditions.

Sildenafil in hypoxic pulmonary hypertension potentiates a compensatory up‐regulation of NO‐cGMP signaling

The availability of inhibitors of cGMP-specific phosphodiesterase 5 (PDE 5), such as sildenafil, has revolutionized the treatment of pulmonary hypertension (PH). Sildenafil may exert its protective effects in a mechanism-based fashion by targeting a pathophysiologically attenuated NO-cGMP signaling pathway. To elucidate this, we analyzed changes in the pulmonary expression and activity of key enzymes of NO-cGMP signaling as well as the functional pulmonary responses to sildenafil in the 5 or 21 day hypoxia mouse model of PH. Surprisingly, we found doubled NO synthase (NOS) II and III levels, no evidence for attenuated NO bioavailability as evidenced by the nitrosative/oxidative stress marker protein nitro tyrosine, and no changes in the expression and activity of the NO receptor, soluble guanylyl cyclase (sGC). PDE 5 was either unchanged at day 5 or, after 21 days of hypoxia, even significantly decreased along with unchanged activity. Biochemically, these changes were mirrored by increased cGMP spillover into the lung perfusate and cGMP-dependent phosphorylation of the vasodilator-stimulated phosphoprotein, VASP. Sildenafil further augmented cGMP and phospho-VASP levels in lungs of mice exposed for 5 or 21 days and decreased pulmonary arterial pressure in mice after 5 days but not 21 days of hypoxia. In conclusion, NO-cGMP signaling is compensatorily up-regulated in the hypoxic mouse model of PH, and sildenafil further augments this pathway to functionally alleviate pulmonary vasoconstriction.

Polymorphism C242T of the gene of the p22phox subunit for nicotinamide adenine dinucleotide phosphate oxidase, and erythrocytic antioxidant enzymes, in patients with tetralogy of Fallot

BACKGROUND

Nicotinamide adenine dinucleotide phosphate oxidase of the vascular cell membrane is an important source of reactive oxygen species. The aim of our study was to evaluate the possible influence of the p22phox C242T gene polymorphism on blood pressure and some markers of oxidative stress in children with tetralogy of Fallot.

METHODS

After surgical repair in early life, we recruited 38 children, aged 11.7 plus or minus 3.2 years, including 185 healthy individuals as controls for the purposes of establishing frequencies of alleles and genotypes. From this latter group, we matched a sub-sample of 53 healthy caucasian children, aged 11.0 plus or minus 1.0 years, in order to compare enzymic activities.

RESULTS

The children with tetralogy of Fallot showed significantly lower values of low-molecular-weight protein tyrosine phosphatase, particularly in carriers of CC genotype for the p22phox gene, with values of 145.2 plus or minus 77.4 micromol/g Hb/h, compared to controls, at 344.4 plus or minus 100.4 micromol/g Hb/h (p less than 0.001). Methemoglobin reductase activity in the patients with tetralogy was also lower in those with the CC genotype, at 9.8 plus or minus 3.2 micromol/g Hb-1 min(-1) compared to 24.2 plus or minus 11.8 micromol/g Hb(-1) min(-1) as measured in the controls (p less than 0.01). Lower systolic (p less than 0.05) and diastolic (p less than 0.01) blood pressures were also observed in the patients with tetralogy of Fallot.

CONCLUSIONS

Patients with tetralogy of Fallot having the CC genotype may be at a higher state of oxidative stress than T allele carriers, a finding which could have prognostic implications. Long term follow-up of these patients, however, may be necessary in order to draw definite conclusions.

Transient middle cerebral artery occlusion causes different structural, mechanical, and myogenic alterations in normotensive and hypertensive rats

Transient focal cerebral ischemia in the rat alters vessel properties, and spontaneously hypertensive rats (SHR) show a poorer outcome after ischemia. In the present study we examined the role of hypertension on vessel properties after ischemia-reperfusion. The right middle cerebral artery (MCA) was occluded (90 min) and reperfused (24 h) in SHR (n = 12) and Wistar-Kyoto rats (WKY; n = 11). Sham-operated rats (SHR, n = 10; WKY, n = 10) were used as controls. The structural, mechanical, and myogenic properties of the MCA were assessed by pressure myography. Nuclei distribution and elastin content and organization were analyzed by confocal microscopy. Infarct volume was larger in SHR than in WKY rats. Ischemia-reperfusion induced adventitial hypertrophy associated with an increase in the total number of adventitial cells. In addition, fenestrae area and arterial distensibility increased and myogenic tone decreased in the MCA of WKY rats after ischemia-reperfusion. Hypertension per se induced hypertrophic inward remodeling. Ischemia-reperfusion decreased the cross-sectional area of the MCA in SHR, without significant changes in distensibility, despite an increase in fenestrae area. In addition, MCA myogenic properties were not altered after ischemia-reperfusion in SHR. Our results indicate that in normotensive rats, MCA develops a compensatory mechanism (i.e., enhanced distensibility and decreased myogenic tone) that counteracts the effect of ischemia-reperfusion and ensures correct cerebral irrigation. These compensatory mechanisms are lost in hypertension, thereby explaining, at least in part, the greater infarct volume observed in SHR.

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.

Hypoxia-induced pulmonary vascular remodeling: cellular and molecular mechanisms

Chronic hypoxic exposure induces changes in the structure of pulmonary arteries, as well as in the biochemical and functional phenotypes of each of the vascular cell types, from the hilum of the lung to the most peripheral vessels in the alveolar wall. The magnitude and the specific profile of the changes depend on the species, sex, and the developmental stage at which the exposure to hypoxia occurred. Further, hypoxia-induced changes are site specific, such that the remodeling process in the large vessels differs from that in the smallest vessels. The cellular and molecular mechanisms vary and depend on the cellular composition of vessels at particular sites along the longitudinal axis of the pulmonary vasculature, as well as on local environmental factors. Each of the resident vascular cell types (ie, endothelial, smooth muscle, adventitial fibroblast) undergo site- and time-dependent alterations in proliferation, matrix protein production, expression of growth factors, cytokines, and receptors, and each resident cell type plays a specific role in the overall remodeling response. In addition, hypoxic exposure induces an inflammatory response within the vessel wall, and the recruited circulating progenitor cells contribute significantly to the structural remodeling and persistent vasoconstriction of the pulmonary circulation. The possibility exists that the lung or lung vessels also contain resident progenitor cells that participate in the remodeling process. Thus the hypoxia-induced remodeling of the pulmonary circulation is a highly complex process where numerous interactive events must be taken into account as we search for newer, more effective therapeutic interventions. This review provides perspectives on each of the aforementioned areas.

p38 MAP kinase: essential role in hypoxia-mediated human pulmonary artery fibroblast proliferation

Pulmonary arterial hypertension (PAH) is a disease that results in thickening of the vascular wall. Some of the most prominent changes are seen in the adventitia as a result of fibroblast proliferation and increased extracellular matrix deposition. Previous work from this laboratory using animal models has shown that pulmonary but not systemic artery fibroblasts proliferate to hypoxic exposure and that this response is dependent on activation of p38 mitogen-activated protein kinase (p38MAPK). In this study, we wished to determine whether human pulmonary artery fibroblasts (HPAFs) behaved similarly under conditions of acute hypoxic exposure (35 mmHg for 24 h). Fibroblast proliferation was assessed by [(3)H]thymidine uptake and protein assays performed using Western blotting techniques. HPAFs proliferated in response to acute hypoxic exposure, human systemic artery fibroblasts did not. This hypoxia-mediated proliferation was p38 MAPK dependent and could be blocked using a specific p38 MAPK inhibitor. Hypoxia-inducible factor-1 (HIF-1) expression was increased in hypoxic pulmonary but not systemic cells and could be partially abrogated with the p38 inhibitor. This work in man confirmed our previous findings in animals that significant differences exist between the pulmonary and systemic circulations in response to hypoxic exposure. This study highlights the importance of p38 MAPK and HIF-1 in hypoxia-mediated proliferation of pulmonary artery adventitial fibroblasts.

Role of the adventitia in pulmonary vascular remodeling

An increasing volume of experimental data indicates that the adventitial fibroblast, in both the pulmonary and systemic circulations, is a critical regulator of vascular wall function in health and disease. A rapidly emerging concept is that the vascular adventitia acts as biological processing center for the retrieval, integration, storage, and release of key regulators of vessel wall function. In response to stress or injury, resident adventitial cells can be activated and reprogrammed to exhibit different functional and structural behaviors. In fact, under certain conditions, the adventitial compartment may be considered the principal injury-sensing tissue of the vessel wall. In response to vascular stresses such as overdistension and hypoxia, the adventitial fibroblast is activated and undergoes phenotypic changes, which include proliferation, differentiation, upregulation of contractile and extracellular matrix proteins, and release of factors that directly affect medial smooth muscle cell tone and growth and that stimulate recruitment of inflammatory and progenitor cells to the vessel wall. Each of these changes in fibroblast phenotype modulates either directly or indirectly changes in overall vascular function and structure. The purpose of this review is to present the current evidence demonstrating that the adventitial fibroblast acts as a key regulator of pulmonary vascular function and structure from the "outside-in."

Proliferation and differentiation of bone marrow stromal cells under hypoxic conditions

Low oxygen tension is a potent differentiation inducer of numerous cell types and an effective stimulus of many gene expressions. Here, we described that under 8% O(2), bone marrow stromal cells (MSCs) exhibited proliferative and morphologic changes. The level of differentiated antigen H-2Dd and the number of G(2)/S/M phase cells increased evidently under 8% O(2) condition. Also, the proportion of wide, flattened, and epithelial-like cells (which were alkaline phosphatase staining positive) in MSCs increased significantly. When cultured in adipogenic medium, there was a 5- to 6-fold increase in the number of lipid droplets under hypoxic conditions compared with that in normoxic culture. We also demonstrated the existence of MSC differentiation under hypoxic conditions by electron microscopy. Expression of Oct4 was inhibited under 8% O(2) condition, but after adipocyte differentiation in normoxic culture and hypoxia-mimicking agents cobalt chloride (CoCl(2)) and deferoxamine mesylate (DFX) treatments, Oct4 was still expressed in MSCs. These results indicate hypoxia accelerates MSC differentiation and hypoxia and hypoxia-mimicking agents exert different effects on MSC differentiation.

Impact of HIF‐1α and HIF‐2α on proliferation and migration of human pulmonary artery fibroblasts in hypoxia

Proliferation of adventitial fibroblasts of small intrapulmonary arteries (FBPA) has been disclosed as an early event in the development of pulmonary hypertension and cor pulmonale in response to hypoxia. We investigated the role of hypoxia-inducible transcription factors (HIF) in human FBPA exposed to hypoxia. Primary cultures of FBPA displayed a strong mitogenic response to 24 h hypoxia, whereas the rate of apoptosis was significantly suppressed. In addition, the migration of FBPA was strongly increased under hypoxic conditions but not the expression of alpha-smooth muscle actin. Hypoxia induced a marked up-regulation (protein level) of both HIF-1alpha and HIF-2alpha, alongside with nuclear translocation of these transcription factors. Specific inhibition of either HIF-1alpha or HIF-2alpha was achieved by RNA interference technology, as proven by HIF-1alpha and HIF-2alpha mRNA and protein analysis and expression analysis of HIF downstream target genes. With the use of this approach, the hypoxia-induced proliferative response of the FBPA was found to be solely HIF-2alpha dependent, whereas the migratory response was significantly reduced by both HIF-1alpha and HIF-2alpha interference. In conclusion, HIF up-regulation is essential for hypoxic cellular responses in human pulmonary artery adventitial fibroblasts such as proliferation and migration, mimicking the pulmonary hypertensive phenotype in vivo. Differential HIF subtype dependency was noted, with HIF-2alpha playing a predominant role, which may offer future intervention strategies.