Hypoxia induces discoidin domain receptor-2 expression via the p38 pathway in vascular smooth muscle cells to increase their migration

Receptor Tyrosine Kinase: Still an Interesting Target to Inhibit the Proliferation of Vascular Smooth Muscle Cells

Vascular smooth muscle cells (VSMCs) proliferation is a critical event that contributes to the pathogenesis of vascular remodeling such as hypertension, restenosis, and pulmonary hypertension. Increasing evidences have revealed that VSMCs proliferation is associated with the activation of receptor tyrosine kinases (RTKs) by their ligands, including the insulin-like growth factor receptor (IGFR), fibroblast growth factor receptor (FGFR), epidermal growth factor receptor (EGFR), vascular endothelial growth factor receptor (VEGFR), and platelet-derived growth factor receptor (PDGFR). Moreover, some receptor tyrosinase inhibitors (TKIs) have been found and can prevent VSMCs proliferation to attenuate vascular remodeling. Therefore, this review will describe recent research progress on the role of RTKs and their inhibitors in controlling VSMCs proliferation, which helps to better understand the function of VSMCs proliferation in cardiovascular events and is beneficial for the prevention and treatment of vascular disease.

Mechanisms of pulmonary vascular dysfunction in pulmonary hypertension and implications for novel therapies

Pulmonary hypertension (PH) is a serious disease characterized by various degrees of pulmonary vasoconstriction and progressive fibroproliferative remodeling and inflammation of the pulmonary arterioles that lead to increased pulmonary vascular resistance, right ventricular hypertrophy, and failure. Pulmonary vascular tone is regulated by a balance between vasoconstrictor and vasodilator mediators, and a shift in this balance to vasoconstriction is an important component of PH pathology, Therefore, the mainstay of current pharmacological therapies centers on pulmonary vasodilation methodologies that either enhance vasodilator mechanisms such as the NO-cGMP and prostacyclin-cAMP pathways and/or inhibit vasoconstrictor mechanisms such as the endothelin-1, cytosolic Ca2+, and Rho-kinase pathways. However, in addition to the increased vascular tone, many patients have a "fixed" component in their disease that involves altered biology of various cells in the pulmonary vascular wall, excessive pulmonary artery remodeling, and perivascular fibrosis and inflammation. Pulmonary arterial smooth muscle cell (PASMC) phenotypic switch from a contractile to a synthetic and proliferative phenotype is an important factor in pulmonary artery remodeling. Although current vasodilator therapies also have some antiproliferative effects on PASMCs, they are not universally successful in halting PH progression and increasing survival. Mild acidification and other novel approaches that aim to reverse the resident pulmonary vascular pathology and structural remodeling and restore a contractile PASMC phenotype could ameliorate vascular remodeling and enhance the responsiveness of PH to vasodilator therapies.

The Yin and Yang of Discoidin Domain Receptors (DDRs): Implications in Tumor Growth and Metastasis Development

Simple Summary

The tumor microenvironment plays an important role in tumor development and metastasis. Collagens are major components of the extracellular matrix and can influence tumor development and metastasis by activating discoidin domain receptors (DDRs). This work shows the different roles of DDRs in various cancers and highlights the complexity of anti-DDR therapies in cancer treatment.

Abstract

The tumor microenvironment is a complex structure composed of the extracellular matrix (ECM) and nontumoral cells (notably cancer-associated fibroblasts (CAFs) and immune cells). Collagens are the main components of the ECM and they are extensively remodeled during tumor progression. Some collagens are ligands for the discoidin domain receptor tyrosine kinases, DDR1 and DDR2. DDRs are involved in different stages of tumor development and metastasis formation. In this review, we present the different roles of DDRs in these processes and discuss controversial findings. We conclude by describing emerging DDR inhibitory strategies, which could be used as new alternatives for the treatment of patients.

Discoidin domain Receptor 2: A determinant of metabolic syndrome-associated arterial fibrosis in non-human primates

Collagen accumulation and remodeling in the vascular wall is a cardinal feature of vascular fibrosis that exacerbates the complications of hypertension, aging, diabetes and atherosclerosis. With no specific therapy available to date, identification of mechanisms underlying vascular fibrogenesis is an important clinical goal. Here, we tested the hypothesis that Discoidin Domain Receptor 2 (DDR2), a collagen-specific receptor tyrosine kinase, is a determinant of arterial fibrosis. We report a significant increase in collagen type 1 levels along with collagen and ECM remodeling, degradation of elastic laminae, enhanced fat deposition and calcification in the abdominal aorta in a non-human primate model of high-fat, high-sucrose diet (HFS)-induced metabolic syndrome. These changes were associated with a marked increase in DDR2. Resveratrol attenuated collagen type I deposition and remodeling induced by the HFS diet, with a concomintant reduction in DDR2. Further, in isolated rat vascular adventitial fibroblasts and VSMCs, hyperglycemia increased DDR2 and collagen type I expression via TGF-β1/SMAD2/3, which was attenuated by resveratrol. Notably, gene knockdown and overexpression approaches demonstrated an obligate role for DDR2 in hyperglycemia-induced increase in collagen type I expression in these cells. Together, our observations point to DDR2 as a hitherto unrecognized molecular link between metabolic syndrome and arterial fibrosis, and hence a therapeutic target.

Multitasking discoidin domain receptors are involved in several and specific hallmarks of cancer

Discoidin domain receptors, DDR1 and DDR2, are two members of collagen receptor family that belong to tyrosine kinase receptor subgroup. Unlike other matrix receptor-like integrins, these collagen receptors have not been extensively studied. However, more and more studies are focusing on their involvement in cancer. These two receptors are present in several subcellular localizations such as intercellular junction or along type I collagen fibers. Consequently, they are involved in multiple cellular functions, for instance, cell cohesion, proliferation, adhesion, migration and invasion. Furthermore, various signaling pathways are associated with these multiple functions. In this review, we highlight and characterize hallmarks of cancer in which DDRs play crucial roles. We discuss recent data from studies that demonstrate the involvement of DDRs in tumor proliferation, cancer mutations, drug resistance, inflammation, neo-angiogenesis and metastasis. DDRs could be potential targets in cancer and we conclude this review by discussing the different ways to inhibits them.

Discoidin domain receptors: Microenvironment sensors that promote cellular migration and invasion

Extracellular matrix (ECM) provides cells scaffolding for cell migration and microenvironment for various cellular functions. Collagens are major ECM components in tissue and discoidin domain receptors (DDRs) are receptor tyrosine kinases (RTK) that recognise fibrillar collagens. Unlike other RTK, their ligands are solid ECM the that are abundantly present in the pericellular environment in various tissue, and thus its activation and regulations are unique amongst RTK family. It is emerging that DDRs may be the sensors that monitor and detects changes in ECM microenvironment and determines the cellular fates upon tissue injuries. In this mini-review, recent findings on the role of DDRs as microenvironment sensor and their roles in cell migration and invasion are discussed.

Molecular basis and functional significance of Angiotensin II-induced increase in Discoidin Domain Receptor 2 gene expression in cardiac fibroblasts

Delineation of mechanisms underlying the regulation of fibrosis-related genes in the heart is an important clinical goal as cardiac fibrosis is a major cause of myocardial dysfunction. This study probed the regulation of Discoidin Domain Receptor 2 (DDR2) gene expression and the regulatory links between Angiotensin II, DDR2 and collagen in Angiotensin II-stimulated cardiac fibroblasts. Real-time PCR and western blot analyses showed that Angiotensin II enhances DDR2 mRNA and protein expression in rat cardiac fibroblasts via NADPH oxidase-dependent reactive oxygen species induction. NF-κB activation, demonstrated by gel shift assay, abolition of DDR2 expression upon NF-κB inhibition, and luciferase and chromatin immunoprecipitation assays confirmed transcriptional control of DDR2 by NF-κB in Angiotensin II-treated cells. Inhibitors of Phospholipase C and Protein kinase C prevented Angiotensin II-dependent p38 MAPK phosphorylation that in turn blocked NF-κB activation. Angiotensin II also enhanced collagen gene expression. Importantly, the stimulatory effects of Angiotensin II on DDR2 and collagen were inter-dependent as siRNA-mediated silencing of one abolished the other. Angiotensin II promoted ERK1/2 phosphorylation whose inhibition attenuated Angiotensin II-stimulation of collagen but not DDR2. Furthermore, DDR2 knockdown prevented Angiotensin II-induced ERK1/2 phosphorylation, indicating that DDR2-dependent ERK1/2 activation enhances collagen expression in cells exposed to Angiotensin II. DDR2 knockdown was also associated with compromised wound healing response to Angiotensin II. To conclude, Angiotensin II promotes NF-κB activation that up-regulates DDR2 transcription. A reciprocal regulatory relationship between DDR2 and collagen, involving cross-talk between the GPCR and RTK pathways, is central to Angiotensin II-induced increase in collagen expression in cardiac fibroblasts.

Discoidin domain receptor functions in physiological and pathological conditions

The discoidin domain receptors, DDR1 and DDR2, are nonintegrin collagen receptors that are members of the receptor tyrosine kinase family. Both DDRs bind a number of different collagen types and play important roles in embryo development. Dysregulated DDR function is associated with progression of various human diseases, including fibrosis, arthritis, and cancer. By interacting with key components of the extracellular matrix and displaying distinct activation kinetics, the DDRs form a unique subfamily of receptor tyrosine kinases. DDR-facilitated cellular functions include cell migration, cell survival, proliferation, and differentiation, as well as remodeling of extracellular matrices. This review summarizes the current knowledge of DDR-ligand interactions, DDR-initiated signal pathways and the molecular mechanisms that regulate receptor function. Also discussed are the roles of DDRs in development and disease progression.

The anti-angiogenic role of discoidin domain receptor 2 (DDR2) in laser-induced choroidal neovascularization

Choroidal neovascularization (CNV), an aberrant growth of blood vessels in the choroid layer of the eye, is a major cause of vision loss. In view of our recent finding that discoidin domain receptor 2 (DDR2), a collagen-binding receptor tyrosine kinase, is involved in control of vascular endothelial activity and tumor angiogenesis, the present study aims to investigate whether and how DDR2 affects the pathogenesis of CNV. We initially found that a spontaneous DDR2 mutant mouse colony (slie) exhibited enhanced amplitude of laser-induced CNV. The inhibitory role of DDR2 in CNV development was further confirmed by experiments through intravitreous injection of DDR2 small interference RNA (siRNA) or DDR2-expressing adenovirus. Quantitative real-time polymerase chain reaction (qPCR) and immunoblot analysis showed that DDR2 regulates the expression of several major pro-angiogenic factors in the laser-injured choroid as well as in retinal pigment epithelium (RPE) cells. In addition, it was demonstrated that the CNV-induced increases in the phosphorylation levels of Akt and mTOR were affected by the upregulation or downregulation of DDR2. Thus, the data from this study for the first time revealed that DDR2 negatively regulates the development of experimental CNV in vivo, which may provide a novel target for preventing human pathological ocular neovascularization. Key messages: DDR2 does not affect retinal development. DDR2 inhibits laser-induced CNV. DDR2 regulates angiogenic factor expression in CNV lesion as well as in RPE cells. DDR2 is involved in modulation of CNV-induced activation of PI3K pathway.

Discoidin domain receptor 2 (DDR2) promotes breast cancer cell metastasis and the mechanism implicates epithelial-mesenchymal transition programme under hypoxia

A wide range of genes involved in breast cancer metastasis have been reported to be related to the microenvironment. We studied the role of discoidin domain receptor 2 (DDR2), a collagen-binding receptor, in breast cancer progression under hypoxic conditions. We showed that DDR2 protein expression closely correlated with the expression of hypoxic marker HIF-1α in clinical breast cancer specimens. The in vitro data demonstrated that hypoxia treatment increased the levels of both expression and phosphorylation of DDR2 in human breast cancer cell lines. In vivo, orthotopic breast tumour xenografts with DDR2 knockdown displayed reduced dissemination and significant prevention in pulmonary and lymphatic metastasis; conversely, these processes were significantly facilitated by the enforced expression of the activated form of DDR2. Further mechanism studies indicated that DDR2 plays an indispensable role in a series of hypoxia-induced behaviours of breast cancer cells, including migration, invasion, and epithelial-mesenchymal transition (EMT). The transcription factor Snail was found to mediate DDR2-induced down-regulation of the cell-cell adhesion molecule E-cadherin. It was also documented that there is a correlation between DDR2 and E-cadherin expression with the presence of lymph node metastases in 160 cases of invasive human breast carcinoma. In addition, we provided evidence that DDR2 silencing in breast cancer cells prevents the hypoxia-induced activation of ERK MAPK, suggesting its potential involvement in mediating the effect of DDR2 on hypoxia-induced signalling. Based on the results of this study, we conclude that DDR2 participates in hypoxia-induced breast cancer metastasis through the regulation of cell migration, invasion, and EMT, and thus may serve as an accessible therapeutic target for the treatment of breast cancer.

Discoidin domain receptor 2 germline gene deletion leads to altered heart structure and function in the mouse

Discoidin domain receptor 2 (DDR2) is a fibrillar collagen receptor that is expressed in mesenchymal cells throughout the body. In the heart, DDR2 is selectively expressed on cardiac fibroblasts. We generated a germline DDR2 knockout mouse and used this mouse to examine the role of DDR2 deletion on heart structure and function. Echocardiographic measurements from null mice were consistent with those from a smaller heart, with reduced left ventricular chamber dimensions and little change in wall thickness. Fractional shortening appeared normal. Left ventricular pressure measurements revealed mild inotropic and lusitropic abnormalities that were accentuated by dobutamine infusion. Both body and heart weights from 10-wk-old male mice were ~20% smaller in null mice. The reduced heart size was not simply due to reduced body weight, since cardiomyocyte lengths were atypically shorter in null mice. Although normalized cardiac collagen mass (assayed by hydroxyproline content) was not different in null mice, the collagen area fraction was statistically higher, suggesting a reduced collagen density from altered collagen deposition and cross-linking. Cultured cardiac fibroblasts from null mice deposited collagen at a slower rate than wild-type littermates, possibly due to the expression of lower prolyl 4-hydroxylase α-isoform 1 enzyme levels. We conclude that genetic deletion of the DDR2 collagen receptor alters cardiac fibroblast function. The resulting perturbations in collagen deposition can influence the structure and function of mature cardiomyocytes.

A host deficiency of discoidin domain receptor 2 (DDR2) inhibits both tumour angiogenesis and metastasis

Discoidin domain receptor 2 (DDR2) is a unique receptor tyrosine kinase (RTK) that signals in response to collagen binding and is implicated in tumour malignant phenotypes such as invasion and metastasis. Although it has been reported that DDR2 expression is up-regulated in activated endothelial cells (ECs), functional studies are lacking. Herein, we found that enforced expression of DDR2 promoted proliferation, migration and tube formation of primary human umbilical vein endothelial cells (HUVECs). The results of immunohistochemical analysis showed a strikingly high level of DDR2 in human tumour ECs. Most significantly, we discovered that a host deficiency of DDR2 inhibits subcutaneous angiogenesis induced by either VEGF or tumour cells. In addition, the remaining tumour vessels in DDR2-deficient mice exhibit some normalized properties. These vascular phenotypes are accompanied by the up-regulation of anti-angiogenic genes and down-regulation of pro-angiogenic genes, as well as by alleviated tumour hypoxia. By use of a tail vein metastasis model of melanoma, we uncovered that loss of stromal DDR2 also suppresses tumour metastasis to the lung. Hence, our current data disclose a new mechanism by which DDR2 affects tumour progression, and may strengthen the feasibility of targeting DDR2 as an anticancer strategy.

Discoidin domain receptor tyrosine kinases: new players in cancer progression

Almost all human cancers display dysregulated expression and/or function of one or more receptor tyrosine kinases (RTKs). The strong causative association between altered RTK function and cancer progression has been translated into novel therapeutic strategies that target these cell surface receptors in cancer. Yet, the full spectrum of RTKs that may alter the oncogenic process is not completely understood. Accumulating evidence suggests that a unique set of RTKs known as the discoidin domain receptors (DDRs) play a key role in cancer progression by regulating the interactions of tumor cells with their surrounding collagen matrix. The DDRs are the only RTKs that specifically bind to and are activated by collagen. DDRs control cell and tissue homeostasis by acting as collagen sensors, transducing signals that regulate cell polarity, tissue morphogenesis, and cell differentiation. In cancer, DDRs are hijacked by tumor cells to disrupt normal cell-matrix communication and initiate pro-migratory and pro-invasive programs. Importantly, several cancer types exhibit DDR mutations, which are thought to alter receptor function and contribute to cancer progression. Other evidence suggests that the actions of DDRs in cancer are complex, either promoting or suppressing tumor cell behavior in a DDR type/isoform specific- and context-dependent manner. Thus, there is still a considerable gap in our knowledge of DDR actions in cancer tissues. This review summarizes and discusses the current knowledge on DDR expression and function in cancer. It is hoped that this effort will encourage more research into these poorly understood but unique RTKs, which have the potential of becoming novel therapeutic targets in cancer.

Hypoxia induces Wee1 expression and attenuates hydrogen peroxide-induced endothelial damage in MS1 cells

In an oxygen-depleted environment, endothelial cells initiate an adaptive pattern of synthesis, which may enable them to survive hypoxic crises. Using high-resolution two-dimensional gel electrophoresis in conjunction with mass spectroscopy, we obtained a 24 differential display of proteins in the pancreatic endothelial cell line, MS-1, at four time points following induction of hypoxia. The induction of Wee1 under hypoxia was confirmed both at the mRNA and protein levels. The phosphorylation of cell division cycle 2, which is downstream of Wee1, was also increased after hypoxic exposure. In addition, pre-exposure to hypoxia attenuated a decrease in hydrogen peroxide-induced cell number. The induction of bax (a pro-apoptotic protein) and reduction of bcl (an anti-apoptotic protein) after hypoxia stimulus were also attenuated by hypoxic pre-exposure. Moreover, hydrogen peroxide-induced morphologic damage did not appear in the wild-type Wee1-expressing cells. Taken together, our results suggest that Wee1 may have important role in hypoxia- induced pathophysiological situations in endothelial cells.

Fetal hypoxia and programming of matrix metalloproteinases

Fetal hypoxia adversely affects the brain and heart development, yet the mechanisms responsible remain elusive. Recent studies indicate an important role of the extracellular matrix in fetal development and tissue remodeling. The matrix metalloproteinases (MMPs) and their endogenous inhibitors, tissue inhibitors of metalloproteinases (TIMPs) have been implicated in a variety of physiological and pathological processes in the cardiovascular and central nervous systems. This review summarizes current knowledge of the mechanisms by which fetal hypoxia induces the imbalance of MMPs, TIMPs and collagen expression patterns, resulting in growth restriction and aberrant tissue remodeling in the developing heart and brain. Collectively, this information could lead to the development of preventive diagnoses and therapeutic strategies in the fetal programming of cardiovascular and neurological disorders.

Deletion of discoidin domain receptor 2 does not affect smooth muscle cell adhesion, migration, or proliferation in response to type I collagen

Collagen receptors expressed on vascular smooth muscle cells include the discoidin domain receptors (DDR1 and DDR2). DDR1 is known to play important roles in mediating smooth muscle cell responses to vascular injury, including neointimal hyperplasia, but much less is known about the function of DDR2. In this study, we harvested smooth muscle cells from DDR2 wild-type and knockout mice and studied the cells using in vitro models of migration and growth. There were no significant differences in the ability of Ddr2(+/+) or Ddr2(-/-) smooth muscle cells to attach to, migrate, or proliferate on type I collagen. Furthermore, neither matrix metalloproteinase (MMP) 2 nor MMP-9 activity nor type I collagen expression was different between the cell types. We conclude that in vitro, endogenous DDR2 is not required for smooth muscle cell hyperplastic responses to collagen.

Collagen I induces discoidin domain receptor (DDR) 1 expression through DDR2 and a JAK2-ERK1/2-mediated mechanism in primary human lung fibroblasts

Discoidin domain receptors (DDRs) DDR1 and DDR2 are receptor tyrosine kinases with the unique ability among receptor tyrosine kinases to respond to collagen. Several signaling molecules have been implicated in DDR signaling, including Shp-2, Src, and MAPK pathways, but a detailed understanding of these pathways and their transcriptional targets is still lacking. Similarly, the regulation of the expression of DDRs is poorly characterized with only a few inflammatory mediators, such as lipopolysaccharide and interleukin-1β identified as playing a role in DDR1 expression. DDRs have been reported to induce the expression of various genes including matrix metalloproteinases and bone morphogenetic proteins, but the regulatory mechanisms underlying DDR-induced gene expression remain to be determined. The aim of the present work was to elucidate the molecular mechanisms implicated in the expression of DDRs and to identify DDR-induced signaling pathways and target genes. Our data show that collagen I induces the expression of DDR1 in a dose- and time-dependent manner in primary human lung fibroblasts. Furthermore, activation of DDR2, JAK2, and ERK1/2 MAPK signaling pathways was essential for collagen I-induced DDR1 and matrix metalloproteinase 10 expression. Finally, inhibition of the ERK1/2 pathway abrogated DDR1 expression by blocking the recruitment of the transcription factor polyoma enhancer A-binding protein 3 to the DDR1 promoter. Our data provide new insights into the molecular mechanisms of collagen I-induced DDR1 expression and demonstrate an important role for ERK1/2 activation and the recruitment of polyoma enhancer-A binding protein 3 to the DDR1 promoter.

Hypoxia stimulates the expression of macrophage migration inhibitory factor in human vascular smooth muscle cells via HIF-1alpha dependent pathway

Background

Hypoxia plays an important role in vascular remodeling and directly affects vascular smooth muscle cells (VSMC) functions. Macrophage migration inhibitory factor (MIF) is a well known proinflammatory factor, and recent evidence suggests an important role of MIF in the progression of atherosclerosis and restenosis. However, the potential link between hypoxia and MIF in VSMC has not been investigated. The current study was designed to test whether hypoxia could regulate MIF expression in human VSMC. The effect of modulating MIF expression on hypoxia-induced VSMC proliferation and migration was also investigated at the same time.

Results

Expression of MIF mRNA and protein was up-regulated as early as 2 hours in cultured human VSMCs after exposed to moderate hypoxia condition (3% O₂). The up-regulation of MIF expression appears to be dependent on hypoxia-inducible transcription factor-1α(HIF-1α) since knockdown of HIF-1α inhibits the hypoxia induction of MIF gene and protein expression. The hypoxia induced expression of MIF was attenuated by antioxidant treatment as well as by inhibition of extracellular signal-regulated kinase (ERK). Under moderate hypoxia conditions (3% O₂), both cell proliferation and cell migration were increased in VSMC cells. Blocking the MIF by specific small interference RNA to MIF (MIF-shRNA) resulted in the suppression of proliferation and migration of VSMCs.

Conclusion

Our results demonstrated that in VSMCs, hypoxia increased MIF gene expression and protein production. The hypoxia-induced HIF-1α activation, reactive oxygen species (ROS) generation and ERK activation might be involved in this response. Both MIF and HIF-1α mediated the hypoxia response of vascular smooth muscle cells, including cell migration and proliferation.

Impaired vasoconstriction and nitric oxide-mediated relaxation in pulmonary arteries of hypoxia- and monocrotaline-induced pulmonary hypertensive rats

Pulmonary hypertension (PH) is a life-threatening disease with unclear vascular mechanisms. We tested whether PH involves abnormal pulmonary vasoconstriction and impaired vasodilation. Male Sprague-Dawley rats were exposed to hypoxia (9% O(2)) for 2 weeks or injected with single dose of monocrotaline (MCT, 60 mg/kg s.c.). Control rats were normoxic or injected with saline. After the hemodynamic measurements were performed, pulmonary and mesenteric arteries were isolated for measurement of vascular function. Hematocrit was elevated in hypoxic rats. Right ventricular systolic pressure and Fulton's Index [right/(left + septum) ventricular weight] were greater in hypoxic and MCT-treated rats than in normoxic rats. Pulmonary artery contraction by phenylephrine and 96 mM KCl was less in hypoxic and MCT-treated rats than in normoxic rats. Acetylcholine-induced relaxation was less in the pulmonary arteries of hypoxic and MCT-treated rats than of normoxic rats, suggesting reduced effects of endothelium-derived vasodilators. The nitric oxide synthase inhibitor, N(omega)-nitro-l-arginine methyl ester, and the guanylate cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, inhibited acetylcholine relaxation, suggesting that it was mediated by nitric oxide (NO)-cGMP. The NO donor sodium nitroprusside caused less relaxation in the pulmonary arteries of hypoxic and MCT-treated than of normoxic rats, suggesting decreased responsiveness of vascular smooth muscle cells (VSMCs) to vasodilators. Phenylephrine and KCl contraction and acetylcholine and sodium nitroprusside relaxation were not different in the mesenteric arteries from all groups. In lung tissue sections, the wall thickness of pulmonary arterioles was greater in hypoxic and MCT-treated rats than in normoxic rats. The specific reductions in pulmonary, but not systemic, arterial vasoconstriction and vasodilation in hypoxia- and MCT-induced PH are consistent with the possibility of de-differentiation of pulmonary VSMCs to a more proliferative/synthetic and less contractile phenotype in PH.