Cyclic stretch induces cyclooxygenase-2 gene expression in vascular endothelial cells via activation of nuclear factor kappa-beta

Mechanical wall stress and wall shear stress are associated with atherosclerosis development in non-calcified coronary segments

BACKGROUND AND AIMS

Atherosclerotic plaque onset and progression are known to be affected by local biomechanical factors. While the role of wall shear stress (WSS) has been studied, the impact of another biomechanical factor, namely mechanical wall stress (MWS), remains poorly understood. In this study, we investigated the association of MWS, independently and combined with WSS, towards atherosclerosis in coronary arteries.

METHODS

Thirty-four human coronary arteries were analyzed using near-infrared spectroscopy intravascular ultrasound (NIRS-IVUS) and optical coherence tomography (OCT) at baseline and after 12 months. Baseline WSS and MWS were calculated using computational models, and wall thickness (ΔWT) and lipid-rich necrotic core size (ΔLRNC) change were measured in non-calcified coronary segments. The arteries were further divided into 1.5 mm/45° sectors and categorized as plaque-free or plaque sectors. For each category, associations between biomechanical factors (WSS & MWS) and changes in coronary wall (ΔWT & ΔLRNC) were studied using linear mixed models.

RESULTS

In plaque-free sectors, higher MWS (p < 0.001) was associated with greater vessel wall growth. Plaque sectors demonstrated wall thickness reduction over time, likely due to medical therapy, where higher levels of WSS and WMS, individually and combined, (p < 0.05) were associated with a greater reduction. Sectors with low MWS combined with high WSS demonstrated the highest LRNC increase (p < 0.01).

CONCLUSIONS

In this study, we investigated the association of the (largely-overlooked) biomechanical factor MWS with coronary atherosclerosis, individually and combined with WSS. Our results demonstrated that both MWS and WSS significantly correlate with atherosclerotic plaque initiation and development.

Non-platelet thromboxane generation is associated with impaired cardiovascular performance and mortality in heart failure

BACKGROUND

Non-platelet thromboxane generation, stimulated largely by oxidative stress, is a novel mortality risk factor in individuals with coronary artery disease. Though inversely associated with left ventricular ejection fraction (LVEF), a potential role in the pathobiology of heart failure (HF) remains poorly defined.

METHODS

Non-platelet thromboxane generation and oxidative stress were assessed by measuring urine thromboxane B₂ metabolites (TXB₂-M) and 8-isoPGF_2α by ELISA in 105 subjects taking aspirin undergoing right heart catheterization for evaluation of HF, valve disease or after transplantation. Multivariable logistic regression and survival analyses were used to define associations of TXB₂-M to invasive measures of cardiovascular performance and 4-year clinical outcome.

RESULTS

TXB₂-M was elevated (>1500 pg/mg creatinine) in 46% of subjects and correlated with HF severity by NYHA functional class and brain natriuretic peptide level, modestly with LVEF, but not with HF etiology. There was no association of oxidative stress to HF type or etiology but a trend with NYHA functional class. Multiple invasive hemodynamic parameters independently associated with TXB₂-M after adjustment for oxidative stress, age, sex and race with pulmonary effective arterial elastance (E_{a (pulmonary)})_, reflective of right ventricular afterload, being the most robust on hierarchical analysis. Similar to E_{a (pulmonary),} elevated urinary TXB₂-M associated with increased risk of death (adjusted HR 2.15, P=0.037) and combination of death, transplant, or mechanical support initiation (adjusted HR 2.0, P=0.042).

CONCLUSIONS

Non-platelet TXA₂ thromboxane generation independently associated with HF severity reflected by invasive measures of cardiovascular performance, particularly right ventricular afterload, and independently predicted long-term mortal.

Identification and analysis of novel endometriosis biomarkers via integrative bioinformatics

Endometriosis is a gynecological disease prevalent in women of reproductive age, and it is characterized by the ectopic presence and growth of the eutopic endometrium. The pathophysiology and diagnostic biomarkers of endometriosis have not yet been comprehensively determined. To discover molecular markers and pathways underlying the pathogenesis of endometriosis, we identified differentially expressed genes (DEGs) in three Gene Expression Omnibus microarray datasets (GSE11691, GSE23339, and GSE7305) and performed gene set enrichment analysis (GSEA) and protein-protein interaction (PPI) network analyses. We also validated the identified genes via immunohistochemical analysis of tissues obtained from patients with endometriosis or healthy volunteers. A total of 118 DEGs (79 upregulated and 39 downregulated) were detected in each dataset with a lower (fold change) FC cutoff (log2|FC| > 1), and 17 DEGs (11 upregulated and six downregulated) with a higher FC cutoff (log2|FC| > 2). KEGG and GO functional analyses revealed enrichment of signaling pathways associated with inflammation, complement activation, cell adhesion, and extracellular matrix in endometriotic tissues. Upregulation of seven genes (C7, CFH, FZD7, LY96, PDLIM3, PTGIS, and WISP2) out of 17 was validated via comparison with external gene sets, and protein expression of four genes (LY96, PDLIM3, PTGIS, and WISP2) was further analyzed by immunohistochemistry and western blot analysis. Based on these results, we suggest that TLR4/NF-κB and Wnt/frizzled signaling pathways, as well as estrogen receptors, regulate the progression of endometriosis. These pathways may be therapeutic and diagnostic targets for endometriosis.

Abnormal Pressure Stress Reduces Interleukin-1β-Induced Cyclooxygenase-2 Expression in Cultured Rat Vascular Smooth Muscle Cells

Elevated mechanical stress on blood vessels associated with hypertension has a direct effect on the function of vascular endothelial cells and vascular smooth muscle cells (VSMCs). In the present study, we have identified the effect of pulsatile pressure stress on cyclooxygenase-2 (COX-2) expression induced by interleukin (IL)-1β in cultured rat VSMCs. VSMCs were isolated from aortic media of Wistar rats and cultured. Pulsatile pressure applied to VSMCs was repeatedly given between either 80 and 160 mmHg, which simulates systolic hypertension, or 80 and 120 mmHg, which simulates normal blood pressure, at a frequency of 4 cycles per min using our original apparatus. Pressure loading that simulates systolic hypertension reduced IL-1β-induced COX-2 expression. The pressure also inhibited the rapid and transient phosphorylation of extracellular signal-regulated kinase (ERK) induced by IL-1β. IL-1β-induced COX-2 expression was significantly inhibited by a specific conventional protein kinase C (PKC) inhibitor. Pressure loading that simulates systolic hypertension also reduced phorbol myristate 13-acetate (PMA) (a PKC activator)-induced COX-2 expression and the rapid and transient phosphorylation of ERK. Pressure loading that simulates normal blood pressure had no effect on IL-1β- and PMA-induced COX-2 expression. The present study shows that pressure stress between 80 and 160 mmHg, which simulates systolic hypertension reduces IL-1β-induced COX-2 expression by affecting a mechanism involving PKC and ERK signaling pathways. Downregulation of COX-2 expression in VSMCs by abnormal pressure stress may further worsen local vascular injury associated with hypertension.

Mechanical loading mediates human nucleus pulposus cell viability and extracellular matrix metabolism by activating of NF-κB

Lower back pain is one of the most frequent complaints in US orthopedic outpatient departments. Intervertebral disc degeneration (IDD) is an important cause of lower back pain. Previous studies have found that mechanical loading was associated with IDD, but the underlying mechanism remains unclear. In the present study, a human nucleus pulposus cell line was used to establish an in vitro mechanical loading model. Mechanical loading, western blot analysis, quantitative PCR, ELISA, cell viability assay and IHC staining were used in the current study. It was found that a short loading time of 4 h followed by a long period of rest (20 h) exerted protective effects against matrix degradation in nucleus pulposus cells, whilst a longer loading time of 20 h followed by a shorter period of rest (4 h) resulted in cell apoptosis and extracellular matrix (ECM) degradation. Excessive mechanical loading may induce ECM degradation by activation of the NF-κB signaling pathway. Taken together, these findings demonstrated that whilst moderate mechanical loading exerted beneficial effects on nucleus pulposus cells, excessive mechanical loading inhibited human nucleus pulposus cell viability and promoted ECM degradation by activating NF-κB.

Mechanosensing and Mechanoregulation of Endothelial Cell Functions

Vascular endothelial cells (ECs) form a semiselective barrier for macromolecules and cell elements regulated by dynamic interactions between cytoskeletal elements and cell adhesion complexes. ECs also participate in many other vital processes including innate immune reactions, vascular repair, secretion, and metabolism of bioactive molecules. Moreover, vascular ECs represent a unique cell type exposed to continuous, time-dependent mechanical forces: different patterns of shear stress imposed by blood flow in macrovasculature and by rolling blood cells in the microvasculature; circumferential cyclic stretch experienced by the arterial vascular bed caused by heart propulsions; mechanical stretch of lung microvascular endothelium at different magnitudes due to spontaneous respiration or mechanical ventilation in critically ill patients. Accumulating evidence suggests that vascular ECs contain mechanosensory complexes, which rapidly react to changes in mechanical loading, process the signal, and develop context-specific adaptive responses to rebalance the cell homeostatic state. The significance of the interactions between specific mechanical forces in the EC microenvironment together with circulating bioactive molecules in the progression and resolution of vascular pathologies including vascular injury, atherosclerosis, pulmonary edema, and acute respiratory distress syndrome has been only recently recognized. This review will summarize the current understanding of EC mechanosensory mechanisms, modulation of EC responses to humoral factors by surrounding mechanical forces (particularly the cyclic stretch), and discuss recent findings of magnitude-specific regulation of EC functions by transcriptional, posttranscriptional and epigenetic mechanisms using -omics approaches. We also discuss ongoing challenges and future opportunities in developing new therapies targeting dysregulated mechanosensing mechanisms to treat vascular diseases. © 2019 American Physiological Society. Compr Physiol 9:873-904, 2019.

Genetic ablation of NFAT5/TonEBP in smooth muscle cells impairs flow- and pressure-induced arterial remodeling in mice

The arterial wall adapts to alterations in blood flow and pressure by remodeling the cellular and extracellular architecture. Biomechanical stress of vascular smooth muscle cells (VSMCs) in the media is thought to precede this process and promote their activation and subsequent proliferation. However, molecular determinants orchestrating the transcriptional phenotype under these conditions have been insufficiently studied. We identified the transcription factor, nuclear factor of activated T cells 5 (NFAT5; or tonicity enhancer-binding protein) as a crucial regulatory element of mechanical stress responses of VSMCs. Here, the relevance of NFAT5 for arterial growth and thickening is investigated in mice upon inducible smooth muscle cell (SMC)-specific genetic ablation of Nfat5. In cultured mouse VSMCs, loss of Nfat5 inhibits the expression of gene sets involved in the control of the cell cycle and the interaction with the extracellular matrix and cytoskeletal dynamics. In vivo, SMC-specific knockout of Nfat5 did not affect the general vascular architecture and blood pressure levels under baseline conditions. However, proliferation of VSMCs and the thickening of the arterial wall were inhibited during both flow-induced collateral remodeling and hypertension-mediated arterial hypertrophy. Whereas originally described as a hypertonicity-responsive transcription factor, these findings identify NFAT5 as a novel molecular determinant of biomechanically induced phenotype changes of VSMCs and wall stress-induced arterial remodeling processes.-Arnold, C., Feldner, A., Zappe, M., Komljenovic, D., De La Torre, C., Ruzicka, P., Hecker, M., Neuhofer, W., Korff, T. Genetic ablation of NFAT5/TonEBP in smooth muscle cells impairs flow- and pressure-induced arterial remodeling in mice.

Glycyrrhetinic Acid Antagonizes Pressure-Induced Venous Remodeling in Mice

Development of spider veins is caused by the remodeling of veins located in the upper dermis and promoted by risk factors such as obesity or pregnancy that chronically increase venous pressure. We have repeatedly shown that the pressure-induced increase in biomechanical wall stress is sufficient to evoke the formation of enlarged corkscrew-like superficial veins in mice. Subsequent experimental approaches revealed that interference with endothelial- and/or smooth muscle cell (SMC) activation counteracts this remodeling process. Here, we investigate whether the herbal agent glycyrrhetinic acid (GA) is a suitable candidate for that purpose given its anti-proliferative as well as anti-oxidative properties. While basic abilities of cultured venous SMCs such as migration and proliferation were not influenced by GA, it inhibited proliferation but not angiogenic sprouting of human venous endothelial cells (ECs). Further analyses of biomechanically stimulated ECs revealed that GA inhibits the DNA binding capacity of the mechanosensitive transcription factor activator protein-1 (AP-1) which, however, had only a minor impact on the endothelial transcriptome. Nevertheless, by decreasing gelatinase activity in ECs or mouse veins exposed to biomechanical stress, GA diminished a crucial cellular response in the context of venous remodeling. In line with the observed inhibitory effects, local transdermal application of GA attenuated pressure-mediated enlargement of veins in the mouse auricle. In summary, our data identifies GA as an inhibitor of EC proliferation, gelatinase activity and venous remodeling. It may thus have the capacity to attenuate spider vein formation and remodeling in humans.

Pediatric acute respiratory distress syndrome - current views

Acute respiratory distress syndrome (ARDS) mainly involves acute respiratory failure. In addition to this affected patients feel progressive arterial hypoxemia, dyspnea, and a marked increase in the work of breathing. The only clinical solution for the above pathological state is ventilation. Mechanical ventilation is necessary to support life in ARDs but it itself worsen lung injury and the term is known clinically as ‘ventilation induced lung injury’ (VILI). At the cellular level, respiratory epithelial cells are subjected to cyclic stretch, i.e. repeated cycles of positive and negative strain, during normal tidal ventilation. In aerated areas of diseased lungs, or even normal lungs subjected to injurious positive pressure mechanical ventilation, the cells are at risk of being over distended, and worsening injury by disrupting the alveolar epithelial barrier. Further, hypercapnic acidosis (HCA) in itself confers protection from stretch injury, potentially via a mechanisms involving inhibition of nuclear factor κB (NF-κB), a transcription factor central to inflammation, injury and repair. Mesenchymal stem cells are the latest in the field and are being investigated as a possible therapy for ARDS.

Endothelial cell signaling and ventilator-induced lung injury: molecular mechanisms, genomic analyses, and therapeutic targets

Mechanical ventilation is a life-saving intervention in critically ill patients with respiratory failure due to acute respiratory distress syndrome (ARDS). Paradoxically, mechanical ventilation also creates excessive mechanical stress that directly augments lung injury, a syndrome known as ventilator-induced lung injury (VILI). The pathobiology of VILI and ARDS shares many inflammatory features including increases in lung vascular permeability due to loss of endothelial cell barrier integrity resulting in alveolar flooding. While there have been advances in the understanding of certain elements of VILI and ARDS pathobiology, such as defining the importance of lung inflammatory leukocyte infiltration and highly induced cytokine expression, a deep understanding of the initiating and regulatory pathways involved in these inflammatory responses remains poorly understood. Prevailing evidence indicates that loss of endothelial barrier function plays a primary role in the development of VILI and ARDS. Thus this review will focus on the latest knowledge related to 1) the key role of the endothelium in the pathogenesis of VILI; 2) the transcription factors that relay the effects of excessive mechanical stress in the endothelium; 3) the mechanical stress-induced posttranslational modifications that influence key signaling pathways involved in VILI responses in the endothelium; 4) the genetic and epigenetic regulation of key target genes in the endothelium that are involved in VILI responses; and 5) the need for novel therapeutic strategies for VILI that can preserve endothelial barrier function.

Blood Brothers: Hemodynamics and Cell-Matrix Interactions in Endothelial Function

SIGNIFICANCE

Alterations in endothelial function contribute to a variety of vascular diseases. In pathological conditions, the endothelium shows a reduced ability to regulate vasodilation (endothelial dysfunction) and a conversion toward a proinflammatory and leaky phenotype (endothelial activation). At the interface between the vessel wall and blood, the endothelium exists in a complex microenvironment and must translate changes in these environmental signals to alterations in vessel function. Mechanical stimulation and endothelial cell interactions with the vascular matrix, as well as a host of soluble factors, coordinately contribute to this dynamic regulation.

RECENT ADVANCES

Blood hemodynamics play an established role in the regulation of endothelial function. However, a growing body of work suggests that subendothelial matrix composition similarly and coordinately regulates endothelial cell phenotype such that blood flow affects matrix remodeling, which affects the endothelial response to flow.

CRITICAL ISSUES

Hemodynamics and soluble factors likely affect endothelial matrix remodeling through multiple mechanisms, including transforming growth factor β signaling and alterations in cell-matrix receptors, such as the integrins. Likewise, differential integrin signaling following matrix remodeling appears to regulate several key flow-induced responses, including nitric oxide production, regulation of oxidant stress, and activation of proinflammatory signaling and gene expression. Microvascular remodeling responses, such as angiogenesis and arteriogenesis, may also show coordinated regulation by flow and matrix.

FUTURE DIRECTIONS

Identifying the mechanisms regulating the dynamic interplay between hemodynamics and matrix remodeling and their contribution to the pathogenesis of cardiovascular disease remains an important research area with therapeutic implications across a variety of conditions. Antioxid. Redox Signal. 25, 415-434.

Mechanical stretch: physiological and pathological implications for human vascular endothelial cells

Vascular endothelial cells are subjected to hemodynamic forces such as mechanical stretch due to the pulsatile nature of blood flow. Mechanical stretch of different intensities is detected by mechanoreceptors on the cell surface which enables the conversion of external mechanical stimuli to biochemical signals in the cell, activating downstream signaling pathways. This activation may vary depending on whether the cell is exposed to physiological or pathological stretch intensities. Substantial stretch associated with normal physiological functioning is important in maintaining vascular homeostasis as it is involved in the regulation of cell structure, vascular angiogenesis, proliferation and control of vascular tone. However, the elevated pressure that occurs with hypertension exposes cells to excessive mechanical load, and this may lead to pathological consequences through the formation of reactive oxygen species, inflammation and/or apoptosis. These processes are activated by downstream signaling through various pathways that determine the fate of cells. Identification of the proteins involved in these processes may help elucidate novel mechanisms involved in vascular disease associated with pathological mechanical stretch and could provide new insight into therapeutic strategies aimed at countering the mechanisms’ negative effects.

Endovascular biopsy: Strategy for analyzing gene expression profiles of individual endothelial cells obtained from human vessels✩

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The combination of guide wire sampling, FACS and high throughput microfluidic single-cell quantitative RT-PCR, is an effective strategy for analyzing molecular changes of ECs in vascular lesions.

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Although heterogeneous, the ECs in normal iliac artery fall into two classes.

Purpose

To develop a strategy of achieving targeted collection of endothelial cells (ECs) by endovascular methods and analyzing the gene expression profiles of collected single ECs.

Methods and results

134 ECs and 37 leukocytes were collected from four patients' intra-iliac artery endovascular guide wires by fluorescence activated cell sorting (FACS) and analyzed by single-cell quantitative RT-PCR for expression profile of 48 genes. Compared to CD45⁺ leukocytes, the ECs expressed higher levels (p < 0.05) of EC surface markers used on FACS and other EC related genes. The gene expression profile showed that these isolated ECs fell into two clusters, A and B, that differentially expressed 19 genes related to angiogenesis, inflammation and extracellular matrix remodeling, with cluster B ECs have demonstrating similarities to senescent or aging ECs.

Conclusion

Combination of endovascular device sampling, FACS and single-cell quantitative RT-PCR is a feasible method for analyzing EC gene expression profile in vascular lesions.

Lung-derived mediators induce cytokine production in downstream organs via an NF-κB-dependent mechanism

In the setting of acute lung injury, levels of circulating inflammatory mediators have been correlated with adverse outcomes. Previous studies have demonstrated that injured, mechanically ventilated lungs represent the origin of the host inflammatory response; however, mechanisms which perpetuate systemic inflammation remain uncharacterized. We hypothesized that lung-derived mediators generated by mechanical ventilation (MV) are amplified by peripheral organs in a "feed forward" mechanism of systemic inflammation. Herein, lung-derived mediators were collected from 129X1/SVJ mice after 2 hours of MV while connected to the isolated perfused mouse lung model setup. Exposure of liver endothelial cells to lung-derived mediators resulted in a significant increase in G-CSF, IL-6, CXCL-1, CXCL-2, and MCP-1 production compared to noncirculated control perfusate media (P < 0.05). Furthermore, inhibition of the NF-κB pathway significantly mitigated this response. Changes in gene transcription were confirmed using qPCR for IL-6, CXCL-1, and CXCL-2. Additionally, liver tissue obtained from mice subjected to 2 hours of in vivo MV demonstrated significant increases in hepatic gene transcription of IL-6, CXCL-1, and CXCL-2 compared to nonventilated controls. Collectively, this data demonstrates that lung-derived mediators, generated in the setting of MV, are amplified by downstream organs in a feed forward mechanism of systemic inflammation.

Stretch magnitude- and frequency-dependent cyclooxygenase 2 and prostaglandin E2 up-regulation in human endometrial stromal cells: Possible implications in endometriosis

Endometriosis, with a prevalence rate ranging from 6% to 10%, is the major contributor to pelvic pain and subfertility, and considerably reduces the quality of life in affected women. However, the pathogenesis of this disease remains largely unknown. The present study aimed to uncover the role of hyperperistalsis in the pathogenesis of endometriosis, by exploring the response of human endometrial stromal cells (ESCs) to the cyclic stretch in vitro. ESCs isolated from 18 different endometrium biopsies undergoing hysterectomy for myoma were subjected to uniaxial cyclic stretches with different magnitude and frequency using the Uniaxial Tension System. Expression of cyclooxygenase-2 (COX-2) and microsomal prostaglandin E2 synthase-1 (mPGES-1) in stretched and unstretched ESCs were assessed by realtime quantitative polymerase chain reaction and Western blot. Production of prostaglandin E2 (PGE2) in the culture medium was measured by enzyme-linked immunosorbent assay. The cyclic stretch mimicking hyperperistalsis in endometriosis (5% elongation at 4 cycles/min) stimulated quick up-regulations of COX-2 and mPGES-1 simultaneously on both transcriptional and translational levels, and delayed PGE2 overproduction was also noted in ESCs. As the stretch magnitude or frequency increased, so did overexpression of COX-2 and PGE2 ( P < 0.05). By contrast, the cyclic stretch mimicking physiological peristalsis (3% elongation at 2 cycles/min) did not induce significant COX-2, mPGES-1 or PGE2production within 12 h. Both COX-2 and mPEGS-1 are PGE2 synthases, and the aberrant COX-2 and PGE2 production play important roles in the pathogenesis of endometriosis. Therefore, the present findings revealed that increased stretch stimuli from the hyperperistalsis of endometriosis were capable of causing the aberrant COX-2 and PGE2 expression in the endometrium by mechanotransduction, in a magnitude and frequency-dependent manner. It implied possible roles of hyperperistalsis in the pathogenesis of endometriosis, particularly in the view of COX-2 and PGE2.

Response of a preosteoblastic cell line to cyclic tensile stress conditioning and growth factors for bone tissue engineering

Bone regeneration can be accelerated by utilizing mechanical stress and growth factors (GFs). However, a limited understanding exists regarding the response of preosteoblasts to tensile stress alone or with GFs. We measured cell proliferation and expression of heat-shock proteins (HSPs) and other bone-related proteins by preosteoblasts following cyclic tensile stress (1%-10% magnitude) alone or in combination with bone morphogenetic protein-2 (BMP-2) and transforming growth factor-β1 (TGF-β1). Tensile stress (3%) with GFs induced greater gene upregulation of osteoprotegerin (3.3 relative fold induction [RFI] compared to sham-treated samples), prostaglandin E synthase 2 (2.1 RFI), and vascular endothelial growth factor (VEGF) (11.5 RFI), compared with samples treated with stimuli alone or sham-treated samples. The most significant increases in messenger RNA expression occurred with GF addition to either static-cultured or tensile-loaded (1% elongation) cells for the following genes: HSP47 (RFI=2.53), cyclooxygenase-2 (RFI=72.52), bone sialoprotein (RFI=11.56), and TGF-β1 (RFI=8.05). Following 5% strain with GFs, VEGF secretion increased 64% (days 3-6) compared with GF alone and cell proliferation increased 23% compared with the sham-treated group. GF addition increased osteocalcin secretion but decreased matrix metalloproteinase-9 significantly (days 3-6). Tensile stress and GFs in combination may enhance bone regeneration by initiating angiogenic and anti-osteoclastic effects and promote cell growth.

Impact of portal branch ligation on tissue regeneration, microcirculatory response and microarchitecture in portal blood-deprived and undeprived liver tissue

Partial ligation of portal branches leads to atrophy of the deprived lobes and hypertrophy of the intact lobes. In this study we investigated the microcirculatory response and their consequences on tissue regeneration after left-sided portal branch ligation (PBL) in Sprague-Dawley rats. At day 1 and 3 after PBL the hepatic microcirculation was assessed by intravital microscopy (IVM). In addition histological, immunohistochemical and biochemical techniques were used to determine alterations of hepatic microarchitecture. IVM analysis of the microcirculation of the ligated hepatic lobes revealed significant alterations with a reduction in sinusoidal perfusion rate, a decrease of red blood cell velocity, an increase of sinusoidal diameter and a marked reduction in shear stress at days 1 and 3 after PBL. On the contrary, the non-ligated lobes presented with higher blood flow velocities, marked sinusoidal vasoconstriction and thus, shear stress elevation. In consequence, ligated liver lobes exhibited marked cell apoptosis and necrosis, being accompanied by massive intrahepatic leukocyte accumulation and a ~30% weight loss. The non-ligated liver tissue showed marked PCNA expression and thereby completely compensated weight loss. Beside full restoration of liver mass, sinusoidal blood flow was comparable in ligated and non-ligated lobes as well as in sham-treated controls. This study shows that the liver aims at constant tissue mass and blood flow, most probably for maintenance of adequate clearance function. In addition, it supports the hypothesis that shear stress plays a pivotal role in triggering liver hypertrophy in the non-ligated lobes.