Senescent endothelial progenitor cells from dogs with pulmonary arterial hypertension: a before-after self-controlled study

Stem cell therapy in pulmonary hypertension: current practice and future opportunities

Pulmonary hypertension (PH) is a progressive disease characterised by elevated pulmonary arterial pressure and right-sided heart failure. While conventional drug therapies, including prostacyclin analogues, endothelin receptor antagonists and phosphodiesterase type 5 inhibitors, have been shown to improve the haemodynamic abnormalities of patients with PH, the 5-year mortality rate remains high. Thus, novel therapies are urgently required to prolong the survival of patients with PH. Stem cell therapies, including mesenchymal stem cells, endothelial progenitor cells and induced pluripotent stem cells, have shown therapeutic potential for the treatment of PH and clinical trials on stem cell therapies for PH are ongoing. This review aims to present the latest preclinical achievements of stem cell therapies, focusing on the therapeutic effects of clinical trials and discussing the challenges and future perspectives of large-scale applications.

Endothelial Senescence: A New Age in Pulmonary Hypertension

Pulmonary hypertension is an enigmatic, deleterious disease driven by multiple heterogeneous causes with a burgeoning proportion of older patients with complex, chronic comorbidities without adequate treatment options. The underlying endothelial pathophenotypes that direct vasoconstriction and panvascular remodeling remain both controversial and incompletely defined. This review discusses emerging concepts centered on endothelial senescence in pulmonary vascular disease. This principle proposes a more heterogeneous, dynamic pulmonary endothelium in disease; it provides a potentially unifying feature of endothelial dysfunction in pulmonary hypertension irrespective of cause; and it supports a clinically relevant link between aging and pulmonary hypertension like other chronic illnesses. Thus, taking cues from studies on aging and age-related diseases, we present possible opportunities and barriers to diagnostic and therapeutic targeting of senescence in pulmonary hypertension.

Pluripotent hematopoietic stem cells augment α-adrenergic receptor-mediated contraction of pulmonary artery and contribute to the pathogenesis of pulmonary hypertension

Pulmonary hypertension (PH) is a multicellular and progressive disease with a high mortality rate. Among many cell types, hematopoietic stem cells (HSCs) are incriminated in the pathogenesis of PH. However, our understanding of the mechanisms that increase HSCs in blood and lungs of hypertensive animals or patients and the role played by HSCs in the pathogenesis of PH remains elusive. Studies suggest that glycolysis is critical for the survival and growth of HSCs. In various cell types from hypertensive lungs of animals and patients, glycolysis and the glucose-6-phosphate dehydrogenase (G6PD) activity are increased. Herein, we demonstrated in mice that chronic hypoxia increased HSCs (CD34+, CD117+, CD133+, CD34+/CD117+, and CD34+/CD133+) in bone marrow and blood and around hypertensive pulmonary arteries in a time-dependent manner. Intriguingly, we found fewer CD133+ cells in the bone marrow of C57BL/6 mice compared with Sv129J mice, and C57BL mice developed less severe chronic hypoxia-elicited PH and heart failure than Sv129J mice. Similarly, the numbers of CD34+ and CD117+ cells in blood of patients with pulmonary arterial hypertension (PAH) were higher (>3-fold) compared with healthy individuals. By allogeneic bone marrow transplantation, we found that GFP+ bone marrow cells infiltrated the lungs and accumulated around the pulmonary arteries in lungs of hypoxic mice, and these cells contributed to increased α-adrenergic receptor-mediated contraction of the pulmonary artery cultured in hypoxia. Inhibition of G6PD activity with (3β,5α)-3,21-dihydroxypregnan-20-one, a novel and potent G6PD inhibitor, decreased HSCs in bone marrow, blood, and lungs of hypoxic mice and reduced α-agonist-induced contraction of the pulmonary artery and established hypoxia-induced PH. We did not observe CD133+ cells around the pulmonary arteries in the lungs of chronically hypoxic G6PD-deficient mice. Furthermore, knockdown of G6PD and inhibition of G6PD activity: 1) downregulated canonical and noncanonical Wnt and Fzd receptors genes; 2) upregulated Bmpr1a; 3) decreased Cxcl12, and 4) reduced HSC (CD117+ and CD133+) numbers. In all, our findings demonstrate unexpected function for bone marrow-derived HSCs in augmenting α-adrenergic receptor-mediated contraction of pulmonary arteries and remodeling of pulmonary arteries that contribute to increase pulmonary vascular resistance in PAH patients and hypoxic mice and suggest that G6PD, by regulating expression of genes in the WNT and BMPR signaling, contributed to increase and release of HSCs from the bone marrow in response to hypoxic stimuli.

Pentose Shunt, Glucose-6-Phosphate Dehydrogenase, NADPH Redox, and Stem Cells in Pulmonary Hypertension

Redox signaling plays a critical role in the pathophysiology of cardiovascular diseases. The pentose phosphate pathway is a major source of NADPH redox in the cell. The activities of glucose-6-phosphate dehydrogenase (the rate-limiting enzyme in the pentose shunt) and glucose flux through the shunt pathway is increased in various lung cells including, the stem cells, in pulmonary hypertension. This chapter discusses the importance of the shunt pathway and glucose-6-phosphate dehydrogenase in the pathogenesis of pulmonary artery remodeling and occlusive lesion formation within the hypertensive lungs.

Monoclonal Antibodies 13A4 and AC133 Do Not Recognize the Canine Ortholog of Mouse and Human Stem Cell Antigen Prominin-1 (CD133)

The pentaspan membrane glycoprotein prominin-1 (CD133) is widely used in medicine as a cell surface marker of stem and cancer stem cells. It has opened new avenues in stem cell-based regenerative therapy and oncology. This molecule is largely used with human samples or the mouse model, and consequently most biological tools including antibodies are directed against human and murine prominin-1. Although the general structure of prominin-1 including its membrane topology is conserved throughout the animal kingdom, its primary sequence is poorly conserved. Thus, it is unclear if anti-human and -mouse prominin-1 antibodies cross-react with their orthologs in other species, especially dog. Answering this issue is imperative in light of the growing number of studies using canine prominin-1 as an antigenic marker. Here, we address this issue by cloning the canine prominin-1 and use its overexpression as a green fluorescent protein fusion protein in Madin-Darby canine kidney cells to determine its immunoreactivity with antibodies against human or mouse prominin-1. We used immunocytochemistry, flow cytometry and immunoblotting techniques and surprisingly found no cross-species immunoreactivity. These results raise some caution in data interpretation when anti-prominin-1 antibodies are used in interspecies studies.

Non-muscle myosin light chain promotes endothelial progenitor cells senescence and dysfunction in pulmonary hypertensive rats through up-regulation of NADPH oxidase

Non-muscle myosin regulatory light chain (nmMLC20) is reported to exert transcriptional function in regulation of gene expression, and NADPH oxidase (NOX)-derived reactive oxygen species contribute to vascular remodeling of pulmonary artery hypertension (PAH). This study aims to determine if nmMLC20 can promote endothelial progenitor cells (EPCs) senescence and dysfunction through up-regulation of NOX in PAH rats. The rats were exposed to10% hypoxia for 3 weeks to establish a PAH model, which showed an increase in right ventricle systolic pressure, right ventricular and pulmonary vascular remodeling, and the accelerated senescence and impaired functions in EPCs, accompanied by an increase in Rho-kinase (ROCK) and NOX activities, p-nmMLC20 level, NOX expression and H2O2 content; these phenomena were reversed by fasudil, a selective inhibitor of ROCK. Next, normal EPCs were cultured under hypoxia to induce senescence in vitro. Consistent with the in vivo findings, hypoxia increased the senescence and dysfunction of EPCs concomitant with an increase in ROCK and NOX activities, p-nmMLC20 level, NOX expression and H2O2 content; these phenomena were reversed by fasudil. Knockdown of nmMLC20 showed similar results to that of fasudil except no effect on ROCK activity. Based on these observations, we conclude that nmMLC20 could promote the senescence and dysfunctions of EPCs in PAH through up-regulation of NOX in a phosphorylation-dependent manner.

Endothelial progenitor cells and pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease characterised by lung endothelial cell dysfunction and vascular remodelling. A number of studies now suggest that endothelial progenitor cells (EPCs) may induce neovascularisation and could be a promising approach for cell based therapy for PAH. On the contrary EPCs may contribute to pulmonary vascular remodelling, particularly in end-stage pulmonary disease. This review article will provide a brief summary of the relationship between PAH and EPCs, the application of the EPCs to PAH and highlight the potential clinical application of the EPCs cell therapy to PAH.

Endothelial progenitor cell-based therapy for pulmonary arterial hypertension

A growing body of evidence in animal models and clinical studies supports the concept that endothelial progenitor cell (EPC)-mediated therapy ameliorates pulmonary arterial hypertension (PAH) and thus may represent a novel approach to treat it. Conversely, several experimental findings suggest that EPCs may be involved in PAH pathogenesis and disease progression. These discrepant results confuse the application of EPC transplantation as an effective treatment strategy for PAH. To improve the study of EPC transplantation in PAH therapy, it is high time that we resolve this dilemma. In this review, we examine the pathobiological changes of PAH, the characteristics of EPCs, and the underlying mechanisms of EPC effects on PAH.

Mechanisms of dysfunction in senescent pulmonary endothelium

Age-dependent changes in pulmonary endothelium contribute to worsened clinical outcomes in elderly individuals. Due to altered pulmonary endothelial responses, older participants have increased vulnerability to infection-related sequelae, higher prevalence of pulmonary hypertension, mitigated DNA repair mechanisms, and attenuated parenchymal healing. Aberrant signaling in pulmonary endothelium undergird these clinical processes. In this review, we provide an overview of the work that has elucidated age-related molecular derangements in pulmonary endothelial cells. In particular, we summarize studies describing mishandling of intracellular reactive oxygen species, pathological nitric oxide signaling, and deficient recruitment of endothelial stem cell precursors. We conclude with a summary of potential future avenues of investigation. The signaling pathways associated with pulmonary endothelial senescence reviewed herein suggest a number of putative therapeutic drug targets. Further elucidation of the cellular processes associated with aging in the pulmonary endothelium may provide critical insights into the rational design of therapies that may subvert or even reverse the effects of aging on a molecular level.

Hypoxia does neither stimulate pulmonary artery endothelial cell proliferation in mice and rats with pulmonary hypertension and vascular remodeling nor in human pulmonary artery endothelial cells

BACKGROUND

Hypoxia results in pulmonary hypertension and vascular remodeling due to induction of pulmonary artery cell proliferation. Besides pulmonary artery smooth muscle cells, pulmonary artery endothelial cells (PAECs) are also involved in the development of pulmonary hypertension, but the effect of hypoxia on PAEC proliferation has not been completely understood.

METHODS

We investigated PAEC proliferation in mice and rats with hypoxia-induced pulmonary hypertension and vascular remodeling as well as in human PAECs under hypoxia.

RESULTS AND CONCLUSION

We did not find significant PAEC proliferation in chronically hypoxic rats or mice. There was a slight decrease in proliferation in mice and rats with pulmonary hypertension and vascular remodeling. We also did not find significant human PAEC proliferation and cell cycle progression under different levels of oxygen (1, 2, 3, 5 and 10%) for one day, although the same conditions of hypoxia induced significant proliferation and cell cycle progression in pulmonary artery smooth muscle cells and pulmonary artery fibroblasts. Exposure to hypoxia for 7 days also did not increase PAEC proliferation. These results demonstrated that hypoxia alone is not a stimulus to PAEC proliferation in vivo and in vitro. The present study provides a novel role for PAECs in hypoxia-induced pulmonary hypertension and vascular remodeling.

Treatment of pulmonary arterial hypertension with circulating angiogenic cells

Despite advances in the treatment of pulmonary arterial hypertension, a truly restorative therapy has not been achieved. Attention has been given to circulating angiogenic cells (CACs, also termed early endothelial progenitor cells) because of their ability to home to sites of vascular injury and regenerate blood vessels. We studied the efficacy of human CAC therapy in the treatment of pulmonary arterial hypertension at two different stages of disease severity. Cells were isolated from peripheral blood and administered to nude rats on day 14 ("early") or day 21 ("late") after monocrotaline injection. The control group received monocrotaline but no cell treatment. Disease progression was assessed using right heart catheterization and echocardiography at multiple time points. Survival differences, right ventricular hypertrophy (RVH), and vascular hypertrophy were analyzed at the study endpoint. Quantitative PCR was performed to evaluate cell engraftment. Treatment with human CACs either at the early or late time points did not result in increased survival, and therapy did not prevent or reduce the severity of disease compared with control. Histological analysis of RVH and vascular muscularization showed no benefit with therapy compared with control. No detectable signal was seen of human transcript in transplanted lungs at 14 or 21 days after cell transplant. In conclusion, CAC therapy was not associated with increased survival and did not result in either clinical or histological benefits. Future studies should be geared toward either earlier therapeutic time points with varying doses of unmodified CACs or genetically modified cells as a means of delivery of factors to the pulmonary arterial circulation.

Current perspective of pathophysiological and interventional effects on endothelial progenitor cell biology: focus on PI3K/AKT/eNOS pathway

For more than a decade, endothelial progenitor cells (EPCs) have been implicated in cardiovascular homeostasis. EPCs are believed to reside within the bone marrow in close contact with surrounding stromal cells, and, under stimulation of pro-inflammatory cytokines, EPCs are mobilized out of the bone marrow. Hereafter circulating EPCs home to peripheral tissues, undergoing further proliferation and differentiation. Under certain pathophysiologic conditions this process seems to be blunted, resulting in a reduced capacity of EPCs to engage in vasculogenesis at sites of endothelial injury or tissue ischemia. In this review, we focus on the effects of traditional cardiovascular risk factors on EPC biology and we explore whether pharmacological, dietary and lifestyle interventions can favorably restore EPC mobilization, differentiation, homing and angiogenic properties. Because the PI3K/Akt/eNOS pathway plays a pivotal role in the process of EPC mobilization, migration and homing, we specifically emphasize the involvement of PI3K, Akt and eNOS in EPC biology under these different (patho)physiologic conditions. (Pre)clinically used drugs or lifestyle interventions that have been shown to ameliorate EPC biology are reviewed. These treatment strategies remain attractive targets to restore the regenerative capacity of EPCs in cardiovascular diseases.