Ofelia M. Martínez-Estrada

The transcription factors Slug and Snail act as repressors of Claudin-1 expression in epithelial cells

Claudin-1 is an integral membrane protein component of tight junctions. The Snail family of transcription factors are repressors that play a central role in the epithelial-mesenchymal transition, a process that occurs during cancer progression. Snail and Slug members are direct repressors of E-cadherin and act by binding to the specific E-boxes of its proximal promoter. In the present study, we demonstrate that overexpression of Slug or Snail causes a decrease in transepithelial electrical resistance. Overexpression of Slug and Snail in MDCK (Madin-Darby canine kidney) cells down-regulated Claudin-1 at protein and mRNA levels. In addition, Snail and Slug are able to effectively repress human Claudin-1-driven reporter gene constructs containing the wild-type promoter sequence, but not those with mutations in two proximal E-box elements. We also demonstrate by band-shift assay that Snail and Slug bind to the E-box motifs present in the human Claudin-1 promoter. Moreover, an inverse correlation in the levels of Claudin-1 and Slug transcripts were observed in breast cancer cell lines. E-box elements in the Claudin-1 promoter were found to play a critical negative regulatory role in breast cancer cell lines that expressed low levels of Claudin-1 transcript. Significantly, in invasive human breast tumours, high levels of Snail and Slug correlated with low levels of Claudin-1 expression. Taken together, these results support the hypothesis that Claudin-1 is a direct downstream target gene of Snail family factors in epithelial cells.

Visceral and subcutaneous fat have different origins and evidence supports a mesothelial source

Fuelled by the obesity epidemic, there is considerable interest in the developmental origins of white adipose tissue (WAT) and the stem and progenitor cells from which it arises. Whereas increased visceral fat mass is associated with metabolic dysfunction, increased subcutaneous WAT is protective. There are six visceral fat depots: perirenal, gonadal, epicardial, retroperitoneal, omental and mesenteric, and it is a subject of much debate whether these have a common developmental origin and whether this differs from that for subcutaneous WAT. Here we show that all six visceral WAT depots receive a significant contribution from cells expressing Wt1 late in gestation. Conversely, no subcutaneous WAT or brown adipose tissue arises from Wt1-expressing cells. Postnatally, a subset of visceral WAT continues to arise from Wt1-expressing cells, consistent with the finding that Wt1 marks a proportion of cell populations enriched in WAT progenitors. We show that all visceral fat depots have a mesothelial layer like the visceral organs with which they are associated, and provide several lines of evidence that Wt1-expressing mesothelium can produce adipocytes. These results reveal a major ontogenetic difference between visceral and subcutaneous WAT, and pinpoint the lateral plate mesoderm as a major source of visceral WAT. They also support the notion that visceral WAT progenitors are heterogeneous, and suggest that mesothelium is a source of adipocytes.

Erythropoietin protects the in vitro blood-brain barrier against VEGF-induced permeability.

The blood–brain barrier (BBB) ensures the homeostasis of the brain microenvironment, mostly through complex tight junctions between brain endothelial cells that prevent the passage of hydrophilic molecules from blood to brain and vice versa. A recent study has shown in vivo that systemic administration of erythropoietin (Epo) protects against brain injury. Using an in vitro model of the bovine BBB, we observed that the expression of the Epo receptor is modulated by its ligand and hypoxic stimuli such as vascular endothelial growth factor (VEGF) treatment. In addition, Epo protects against the VEGF‐induced permeability of the BBB, decreases the levels of endothelial nitric oxide synthase and restores junction proteins. The kinetic transport experiments revealed the capacity of Epo to cross the in vitro BBB in a saturable and specific way. Our results suggest a new mechanism for Epo‐induced neuroprotection, in which circulating Epo controls and maintains the BBB through an Epo receptor signalling pathway and the re‐establishment of cell junctions.

Wt1 controls retinoic acid signalling in embryonic epicardium through transcriptional activation of Raldh2

Epicardial-derived signals are key regulators of cardiac embryonic development. An important part of these signals is known to relate to a retinoic acid (RA) receptor-dependent mechanism. RA is a potent morphogen synthesised by Raldh enzymes, Raldh2 being the predominant one in mesodermal tissues. Despite the importance of epicardial retinoid signalling in the heart, the molecular mechanisms controlling cardiac Raldh2 transcription remain unknown. In the current study, we show that Wt1-null epicardial cells display decreased expression of Raldh2 both in vivo and in vitro. Using a RA-responsive reporter, we have confirmed that Wt1-null epicardial cells actually show reduced synthesis of RA. We also demonstrate that Raldh2 is a direct transcriptional target of Wt1 in epicardial cells. A secondary objective of this study was to identify the status of RA-related receptors previously reported to be critical to epicardial biology (PDGFRα,β; RXRα). PDGFRα and PDGFRβ mRNA and protein levels are downregulated in the absence of Wt1, but only Pdgfra expression is rescued by the addition of RA to Wt1-null epicardial cells. RXRα mRNA levels are not affected in Wt1-null epicardial cells. Taken together, our results indicate that Wt1 critically regulates epicardial RA signalling via direct activation of the Raldh2 gene, and identify a role for Wt1 in the regulation of morphogen receptors involved in the proliferation, migration, and differentiation of epicardial and epicardially-derived cells (EPDC).

Circulating endothelial progenitor cells after kidney transplantation

Circulating endothelial progenitor cells (EPCs) promote vascular repair and maintain integrity of the endothelial monolayer. Reduced EPCs number has been associated with endothelial dysfunction in various cardiovascular diseases. Cardiovascular disease risk is higher in renal transplant patients (RT) than the general population. We studied EPCs number and proliferation in RT, and examined the association with other cardiovascular risk factors such as reduced glomerular filtration rate (GFR) and LDL cholesterol. EPCs concentration was determined in 94 RT and 39 control subjects (C) by flow cytometry. EPCs proliferation was also studied after 7 days in culture. EPCs concentration was significantly reduced in RT versus C (median 33.5 [5–177] vs. 53 [9–257] EPCs/105 PMN cells, p = 0.006). EPCs proliferation was also reduced in RT versus C (mean ± SD; 372.7 ± 229.3 vs. 539.8 ± 291.3 EPCs × field, p = 0.003). In multiple regression analysis, GFR, HDL, LDL and body weight were independent predictors of EPCs concentration in RT (r2= 0.25, p < 0.001). EPCs number is reduced in RT, particularly in patients with reduced GFR. Moreover, EPCs from RT studied in vitro, showed reduced proliferation, which is a sign of functional impairment. These alterations may be involved in increased cardiovascular risk of RT.

cAMP inhibits TGFβ1-induced in vitro angiogenesis

Transforming growth factor‐β (TGFβ1) is a proangiogenic factor both, in vitro and in vivo, that is mainly involved in the later phases of angiogenesis. In an attempt to identify genes that participate in this effect, we found that TGFβ1 down‐regulates expression of adenylate cyclase VI. In addition, cAMP analogs (8‐Bromo‐cAMP) and forskolin (an adenylate cyclase activator) also reduced TGFβ1‐induced in vitro angiogenesis in mouse endothelial cell lines and in primary cultures of human umbilical vein endothelial cells on collagen gels. Induction of Ets‐1 and plasminogen activator inhibitor‐1 (PAI‐1) by TGFβ1 was blocked by these cAMP agonists and activators, in the absence of effects on endothelial cell viability. Moreover, the signal transduction pathways stimulated by TGFβ1 were unaffected. Thus, Smad2 was normally phosphorylated and translocated to the nucleus in the presence of forskolin. In contrast, transfection studies using the PAI‐1‐promoter indicated that these cAMP analogues inhibit transcriptional stimulation by TGFβ1. Electrophoretic mobility shift assay showed that Smad2/3 were bound normally to a TGFβ1‐response region in the presence of the cAMP analogs. In all, these data suggest that the cAMP pathway inhibits the transcriptional activity of Smads, that could be responsible for the block of the TGFβ1‐induced in vitro angiogenesis caused by this second messenger.

Putative endothelial progenitor cells are associated with flow‐mediated dilation in refractory hypertensives

Background. Hypertension has been related to endothelial dysfunction. Patients with refractory hypertension (RH) have a reduced number of endothelial progenitor cells (EPCs). Aim. To evaluate if blood EPC levels relate to endothelium‐dependent vasodilation (ED‐VD) in RH. Methods. We analyzed 29 RH confirmed by 24‐h ambulatory blood pressure monitoring and assessed complete clinical and laboratory evaluation. EPCs were isolated from peripheral mononuclear cells (MNC) by flow cytometry. ED‐VD was determined measuring flow‐mediated dilation (FMD) by venous occlusion plethysmography. Results. Circulating EPCs/105 MNC (median [Q1–Q3]): 23.0 [4.5–53.8]. FMD (median [Q1–Q3]): 211.7 [79.5–365.8]%. Significant correlations with log‐FMD: EPCs (r = 0.469; p = 0.018) and homocysteine (r = −0.414; p = 0.045). There was no collinearity between EPCs and homocysteine. FMD did not correlate with age, gender, office BP, 24‐h systolic blood pressure or 24‐h diastolic blood pressure, laboratory parameters, C‐reactive‐protein, left ventricular‐mass index, dyslipidaemia, smoking habit and statin or angiotensin system blockers treatment. Multiple linear regression analysis showed that after age‐adjustment, EPC (p = 0.027) and homocysteine (p = 0.004) were the only variables that predicted FMD (R = 0.740). After dividing patients according to EPC number, patients in the lower tertile showed a significantly reduced FMD compared with those in the group of the two upper tertiles of EPC: log‐FMD (mean±SD): 4.7±0.9 vs 5.6±0.8, respectively (p = 0.031). Conclusions. ED‐VD independently correlates with circulating EPCs in RH. Homocysteine is also an independent predictor of lower FMD in such patients.

WT1 regulates the expression of inhibitory chemokines during heart development

The embryonic epicardium is an important source of cardiovascular precursor cells and paracrine factors that are required for adequate heart formation. Signaling pathways regulated by WT1 that promote heart development have started to be described; however, there is little information on signaling pathways regulated by WT1 that could act in a negative manner. Transcriptome analysis of Wt1KO epicardial cells reveals an unexpected role for WT1 in repressing the expression of interferon-regulated genes that could be involved in a negative regulation of heart morphogenesis. Here, we showed that WT1 is required to repress the expression of the chemokines Ccl5 and Cxcl10 in epicardial cells. We observed an inverse correlation of Wt1 and the expression of Cxcl10 and Ccl5 during epicardium development. Chemokine receptor analyses of hearts from Wt1(gfp/+) mice demonstrate the differential expression of their chemokine receptors in GFP(+) epicardial enriched cells and GFP(-) cells. Functional assays demonstrate that CXCL10 and CCL5 inhibit epicardial cells migration and the proliferation of cardiomyocytes respectively. WT1 regulates the expression levels of Cxcl10 and Ccl5 in epicardial cells directly and indirectly through increasing the levels of IRF7. As epicardial cell reactivation after a myocardial damage is linked with WT1 expression, the present work has potential implications in adult heart repair.

Mitochondrial fragmentation in excitotoxicity requires ROCK activation

Mitochondria morphology constantly changes through fission and fusion processes that regulate mitochondrial function, and it therefore plays a prominent role in cellular homeostasis. Cell death progression is associated with mitochondrial fission. Fission is mediated by the mainly cytoplasmic Drp1, which is activated by different post-translational modifications and recruited to mitochondria to perform its function. Our research and other studies have shown that in the early moments of excitotoxic insult Drp1 must be nitrosylated to mediate mitochondrial fragmentation in neurons. Nonetheless, mitochondrial fission is a multistep process in which filamentous actin assembly/disassembly and myosin-mediated mitochondrial constriction play prominent roles. Here we establish that in addition to nitric oxide production, excitotoxicity-induced mitochondrial fragmentation also requires activation of the actomyosin regulator ROCK. Although ROCK1 has been shown to phosphorylate and activate Drp1, experiments using phosphor-mutant forms of Drp1 in primary cortical neurons indicate that in excitotoxic conditions, ROCK does not act directly on Drp1 to mediate fission, but may act on the actomyosin complex. Thus, these data indicate that a wider range of signaling pathways than those that target Drp1 are amenable to be inhibited to prevent mitochondrial fragmentation as therapeutic option.

Synaptic activity‐induced glycolysis facilitates membrane lipid provision and neurite outgrowth

The formation of neurites is an important process affecting the cognitive abilities of an organism. Neurite growth requires the addition of new membranes, but the metabolic remodeling necessary to supply lipids for membrane expansion is poorly understood. Here, we show that synaptic activity, one of the most important inducers of neurite growth, transcriptionally regulates the expression of neuronal glucose transporter Glut3 and rate‐limiting enzymes of glycolysis, resulting in enhanced glucose uptake and metabolism that is partly used for lipid synthesis. Mechanistically, CREB regulates the expression of Glut3 and Siah2, the latter and LDH activity promoting the normoxic stabilization of HIF‐1α that regulates the expression of rate‐limiting genes of glycolysis. The expression of dominant‐negative HIF‐1α or Glut3 knockdown blocks activity‐dependent neurite growth in vitro while pharmacological inhibition of the glycolysis and specific ablation of HIF‐1α in early postnatal mice impairs the neurite architecture. These results suggest that the manipulation of neuronal glucose metabolism could be used to treat some brain developmental disorders.