Fibrocytes are an indicator for disease activity of IPF and might be useful as a clinical marker for disease progression. This study suggests that quantification of circulating fibrocytes may allow prediction of early mortality in patients with IPF.

Severe forms of pulmonary arterial hypertension (PAH) are characterized by various degrees of remodeling of the pulmonary arterial vessels, which increases the pulmonary vascular resistance and right ventricular afterload, thus contributing to the development of right ventricle dysfunction and failure. Recent years have seen advances in the understanding of the pathobiology of PAH; however, many important questions remain unanswered. Elucidating the pathobiology of PAH continues to be critical to design new effective therapeutic strategies, and appropriate animal models of PAH are necessary to achieve the task. Although the monocrotaline rat model of PAH has contributed to a better understanding of vascular remodeling in pulmonary hypertension, we question the validity of this model as a preclinically relevant model of severe plexogenic PAH. Here we review pertinent publications that either have been forgotten or ignored, and we reexamine the monocrotaline model in the context of human forms of PAH.

Pulmonary hypertension (PH) is a serious condition that affects mainly young and middle-aged women, and its etiology is poorly understood. A prominent pathological feature of PH is accumulation of macrophages near the arterioles of the lung. In both clinical tissue and the SU5416 (SU)/athymic rat model of severe PH, we found that the accumulated macrophages expressed high levels of leukotriene A4 hydrolase (LTA4H), the biosynthetic enzyme for leukotriene B4 (LTB4). Moreover, macrophage-derived LTB4 directly induced apoptosis in pulmonary artery endothelial cells (PAECs). Further, LTB4 induced proliferation and hypertrophy of human pulmonary artery smooth muscle cells. We found that LTB4 acted through its receptor, BLT1, to induce PAEC apoptosis by inhibiting the protective endothelial sphingosine kinase 1 (Sphk1)–endothelial nitric oxide synthase (eNOS) pathway. Blocking LTA4H decreased in vivo LTB4 levels, prevented PAEC apoptosis, restored Sphk1-eNOS signaling, and reversed fulminant PH in the SU/athymic rat model of PH. Antagonizing BLT1 similarly reversed established PH. Inhibition of LTB4 biosynthesis or signal transduction in SU-treated athymic rats with established disease also improved cardiac function and reopened obstructed arterioles; this approach was also effective in the monocrotaline model of severe PH. Human plexiform lesions, one hallmark of PH, showed increased numbers of macrophages, which expressed LTA4H, and patients with connective tissue disease–associated pulmonary arterial hypertension exhibited significantly higher LTB4 concentrations in the systemic circulation than did healthy subjects. These results uncover a possible role for macrophage-derived LTB4 in PH pathogenesis and identify a pathway that may be amenable to therapeutic targeting.

Background Right ventricular dysfunction (RVD) is the most frequent cause of death in patients with pulmonary arterial hypertension. Whereas abnormal energy substrate utilization has been implicated in the development of chronic left heart failure, data describing such metabolic remodeling in RVD remain incomplete. Thus, we sought to characterize metabolic gene expression changes and mitochondrial dysfunction in functional and dysfunctional RV hypertrophy. Methods and Results Two different rat models of RV hypertrophy were studied. The model of RVD (SU5416/hypoxia) exhibited a significantly decreased gene expression of PPAR-gamma coactivator-1 alpha (PGC-1α), PPAR-α and ERR-α. The expression of multiple PCG-1α target genes required for fatty acid oxidation (FAO) was similarly decreased. Decreased PGC-1α expression was also associated with a net loss of mitochondrial protein and oxidative capacity. Reduced mitochondrial number was associated with a downregulation of TFAM and other genes required for mitochondrial biogenesis. Electron microscopy demonstrated that in RVD tissue, mitochondria had abnormal shape and size. Lastly, respirometric analysis demonstrated that mitochondria isolated from RVD-tissue had a significantly reduced ADP-stimulated (state 3) rate for complex I. Conversely, functional RV hypertrophy in the pulmonary artery banding (PAB) model showed normal expression of PGC-1α, whereas the expression of FAO genes was either preserved or unregulated. Moreover, PAB-RV tissue exhibited preserved TFAM expression and mitochondrial respiration despite elevated RV pressure-overload. Conclusions Right ventricular dysfunction, but not functional RV hypertrophy in rats, demonstrates a gene expression profile compatible with a multilevel impairment of fatty acid metabolism and significant mitochondrial dysfunction, partially independent of chronic pressure-overload.

Idiopathic pulmonary fibrosis (IPF) can lead to the development of secondary pulmonary hypertension (PH) and ultimately death. Despite this known association, the precise mechanism of disease remains unknown. Using a rat model of IPF, we explored the role of the proangiogenic and antiapoptotic growth factor VEGF in the vascular remodeling that underlies PH. In this model, adenoviral delivery of active TGF-β1 induces pulmonary arterial remodeling, loss of the microvasculature in fibrotic areas, and increased pulmonary arterial pressure (PAP). Immunohistochemistry and mRNA analysis revealed decreased levels of VEGF and its receptor, which were inversely correlated with PAP and endothelial cell apoptosis in both the micro-and macrovasculature. Treatment of IPF rats with adenoviral delivery of VEGF resulted in reduced endothelial apoptosis, increased vascularization, and improved PAP due to reduced remodeling but worsened PF. These data show that experimental pulmonary fibrosis (PF) leads to loss of the microvasculature through increased apoptosis and to remodeling of the pulmonary arteries, with both processes resulting in PH. As administration of VEGF ameliorated the PH in this model but concomitantly aggravated the fibrogenic process, VEGF-based therapies should be used with caution.

Lipopolysaccharide (LPS) as a major component of the outer membrane of gram‐negative bacteria stimulates various cells to initiate a signalling cascade which ultimately leads to cell activation and expression of immunoregulatory or inflammatory cytokines. The human respiratory epithelium is an important environmental interface, but differences in LPS‐induced cell activation between bronchial and alveolar epithelial cells have not yet been investigated in detail. First, the expression of Toll‐like receptors (TLRs), as pattern‐recognition receptors, was investigated for the bronchial epithelial cells and type II‐like pneumocytes, demonstrating that they fulfil the prerequisites for LPS signalling. Thereafter, the effects of LPS, soluble CD14 (sCD14) and LPS‐binding protein (LBP) on the release of interleukin‐6 (IL‐6) and IL‐8 were studied. In the presence of LPS, sCD14 induced a significant and concentration‐dependent cytokine release in type II‐like pneumocytes, whereas the response of bronchial epithelial cells to sCD14 stimulation was low, implicating sCD14‐independent activation mechanisms. Furthermore, LBP revealed inhibitory effects on the activation of alveolar epithelial cells, which may represent a novel local defence mechanism during gram‐negative infection. We conclude that distinct pathways exist for LPS‐induced activation of bronchial and alveolar epithelial cells.

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Our work identifies a novel role for TLR3 in PAH based on the findings that reduced expression of TLR3 contributes to endothelial apoptosis and pulmonary vascular remodeling.