Mirna Saker

Pulmonary Artery Smooth Muscle Cell Senescence Is a Pathogenic Mechanism for Pulmonary Hypertension in Chronic Lung Disease

Rationale: Senescence of pulmonary artery smooth muscle cells (PA-SMCs) caused by telomere shortening or oxidative stress may contribute to pulmonary hypertension associated with chronic lung diseases. Objective: To investigate whether cell senescence contributes to pulmonary vessel remodeling and pulmonary hypertension in chronic obstructive pulmonary disease (COPD). Methods and Results: In 124 patients with COPD investigated by right heart catheterization, we found a negative correlation between leukocyte telomere length and pulmonary hypertension severity. In-depth investigations of lung vessels and derived cultured PA-SMCs showed greater severity of remodeling and increases in senescent p16-positive and p21-positive PA-SMCs and proliferating Ki67-stained cells in 14 patients with COPD compared to 13 age-matched and sex-matched control subjects who smoke. Cultured PA-SMCs from COPD patients displayed accelerated senescence, with fewer cell population doublings, an increased percentage of &bgr;-galactosidase–positive cells, shorter telomeres, and higher p16 protein levels at an early cell passage compared to PA-SMCs from controls. Both in situ and in vitro PA-SMC senescence criteria correlated closely with the degree of pulmonary vessel wall hypertrophy. Because senescent PA-SMCs stained for p16 and p21 were virtually confined to the media near the Ki67-positive cells, which predominated in the neointima and hypertrophied media, we evaluated whether senescent cells affected normal PA-SMC functions. We found that senescent PA-SMCs stimulated the growth and migration of normal target PA-SMCs through the production and release of paracrine soluble and insoluble factors. Conclusion: PA-SMC senescence is an important contributor to the process of pulmonary vascular remodeling that underlies pulmonary hypertension in chronic lung disease.

Rapamycin Reverses Pulmonary Artery Smooth Muscle Cell Proliferation in Pulmonary Hypertension

Pulmonary artery (PA) smooth muscle cell (SMC) proliferation in pulmonary hypertension (PH) may be linked to dysregulated mammalian target of rapamycin (mTOR) signaling. The mTOR pathway involves two independent complexes, mTORC1 and mTORC2, which phosphorylate S6 kinase (S6K) and serine/threonine kinase (Akt), respectively, and differ in their sensitivity to rapamycin. Here, we evaluated rapamycin-sensitive mTOR substrates and PA-SMC proliferation in rats with monocrotaline (MCT)-induced PH (MCT-PH). Compared with cells from control rats, cultured PA-SMCs from MCT-PH rats exhibited increased growth responses to platelet-derived growth factor, serotonin (5-hydroxytryptophan), IL-1β, insulin-like growth factor-1, or fetal calf serum (FCS), with increases in phosphorylated (Ser-473)Akt, (Thr-308)Akt, glycogen synthase kinase (GSK)3, and S6K reflecting activated mTORC1 and mTORC2 signaling. Treatment with rapamycin (0.5 μM) or the Akt inhibitor, A-443654 (0.5 μM), reduced FCS-stimulated growth of PA-SMCs from MCT-PH rats to the level in control rats while inhibiting Akt, GSK3, and S6K activation. Neither the tyrosine kinase inhibitor, imatinib (0.1 μM), nor the 5-hydroxytryptophan transporter inhibitor, fluoxetine (5 μM), normalized the increased PA-SMC growth response to FCS. Rapamycin treatment (5 mg/kg/d) of MCT-PH rats from Day 21 to Day 28 markedly reduced phoshop (p)-Aky, p-GSK3, and p-S6K in PAs, and normalized growth of derived PA-SMCs. This effect was not observed after 1 week of imatinib (100 mg/kg/d) or fluoxetine (20 mg/kg/d). Rapamycin given preventively (Days 1-21) or curatively (Days 21-42) inhibited MCT-PH to a greater extent than did imatinib or fluoxetine. Experimental PH in rats is associated with a sustained proliferative PA-SMC phenotype linked to activation of both mTORC1 and mTORC2 signaling and is suppressed by rapamycin treatment.

Activation of Lung p53 by Nutlin-3a Prevents and Reverses Experimental Pulmonary Hypertension

Background— Induction of cellular senescence through activation of the p53 tumor suppressor protein is a new option for treating proliferative disorders. Nutlins prevent the ubiquitin ligase MDM2 (murine double minute 2), a negative p53 regulator, from interacting with p53. We hypothesized that cell senescence induced by Nutlin-3a exerted therapeutic effects in pulmonary hypertension (PH) by limiting the proliferation of pulmonary artery smooth muscle cells (PA-SMCs). Methods and Results— Nutlin-3a treatment of cultured human PA-SMCs resulted in cell growth arrest with the induction of senescence but not apoptosis; increased phosphorylated p53 protein levels; and expression of p53 target genes including p21, Bax, BTG2, and MDM2. Daily intraperitoneal Nutlin-3a treatment for 3 weeks dose-dependently reduced PH, right ventricular hypertrophy, and distal pulmonary artery muscularization in mice exposed to chronic hypoxia or SU5416/hypoxia. Nutlin-3a treatment also partially reversed PH in chronically hypoxic or transgenic mice overexpressing the serotonin-transporter in SMCs (SM22-5HTT+ mice). In these mouse models of PH, Nutlin-3a markedly increased senescent p21-stained PA-SMCs; lung p53, p21, and MDM2 protein levels; and p21, Bax, PUMA, BTG2, and MDM2 mRNA levels; but induced only minor changes in control mice without PH. Marked MDM2 immunostaining was seen in both mouse and human remodeled pulmonary vessels, supporting the use of Nutlins as a PH-targeted therapy. PH prevention or reversal by Nutlin-3a required lung p53 stabilization and increased p21 expression, as indicated by the absence of Nutlin-3a effects in hypoxia-exposed p53−/− and p21−/− mice. Conclusions— Nutlin-3a may hold promise as a prosenescence treatment targeting PA-SMCs in PH.

CCR5 as a Treatment Target in Pulmonary Arterial Hypertension

Background— Pulmonary arterial hypertension (PH), whether idiopathic or related to underlying diseases such as HIV infection, results from complex vessel remodeling involving both pulmonary artery smooth muscle cell (PA-SMC) proliferation and inflammation. CCR5, a coreceptor for cellular HIV-1 entry expressed on macrophages and vascular cells, may be involved in the pathogenesis of PH. Maraviroc is a new CCR5 antagonist designed to block HIV entry. Methods and Results— Marked CCR5 expression was found in lungs from patients with idiopathic PH, in mice with hypoxia-induced PH, and in Simian immunodeficiency virus–infected macaques, in which it was localized chiefly in the PA-SMCs. To assess the role for CCR5 in experimental PH, we used both gene disruption and pharmacological CCR5 inactivation in mice. Because maraviroc does not bind to murine CCR5, we used human-CCR5ki mice for pharmacological and immunohistochemical studies. Compared with wild-type mice, CCR5−/− mice or human-CCR5ki mice treated with maraviroc exhibited decreased PA-SMC proliferation and recruitment of perivascular and alveolar macrophages during hypoxia exposure. CCR5−/− mice reconstituted with wild-type bone marrow cells and wild-type mice reconstituted with CCR5−/− bone marrow cells were protected against PH, suggesting CCR5-mediated effects on PA-SMCs and macrophage involvement. The CCR5 ligands CCL5 and the HIV-1 gp120 protein increased intracellular calcium and induced growth of human and human-CCR5ki mouse PA-SMCs; maraviroc inhibited both effects. Maraviroc also reduced the growth-promoting effects of conditioned media from CCL5-activated macrophages derived from human-CCR5ki mice on PA-SMCs from wild-type mice. Conclusion— The CCL5-CCR5 pathway represents a new therapeutic target in PH associated with HIV or with other conditions.

Senescent Cells Contribute to the Physiological Remodeling of Aged Lungs

Age-associated decline in organ function governs life span. We determined the effect of aging on lung function and cellular/molecular changes of 8- to 32-month old mice. Proteomic analysis of lung matrix indicated significant compositional changes with advanced age consistent with a profibrotic environment that leads to a significant increase in dynamic compliance and airway resistance. The excess of matrix proteins deposition was associated modestly with the activation of myofibroblasts and transforming growth factor-beta signaling pathway. More importantly, detection of senescent cells in the lungs increased with age and these cells contributed toward the excess extracellular matrix deposition observed in our aged mouse model and in elderly human samples. Mechanistic target of rapamycin (mTOR)/AKT activity was enhanced in aged mouse lungs compared with those from younger mice associated with the increased expression of the histone variant protein, MH2A, a marker for aging and potentially for senescence. Introduction in the mouse diet of rapamycin, significantly blocked the mTOR activity and limited the activation of myofibroblasts but did not result in a reduction in lung collagen deposition unless it was associated with prevention of cellular senescence. Together these data indicate that cellular senescence significantly contributes to the extracellular matrix changes associated with aging in a mTOR 1-dependent mechanism.

Telomere Dysfunction and Cell Senescence in Chronic Lung Diseases: Therapeutic Potential.

Cellular senescence--defined as a stable cell-cycle arrest combined with stereotyped phenotypic changes--might play a causal role in various lung diseases, including chronic obstructive pulmonary disease (COPD), which is predicted to become the third leading cause of death worldwide by 2020. COPD is characterized by slowly progressive airflow obstruction and emphysema due to destruction of the lung parenchyma and is often complicated by pulmonary hypertension (PH). No curative treatment is available. Senescent cells, which accumulate with age, are increased in lungs from patients with COPD and express a robust senescence-associated secretory phenotype (SASP), which is proinflammatory. The aim of this review is to show how senescent cells can drive the lung alterations seen in COPD, which mechanisms may be involved, and whether therapeutic interventions may slow or delay the in vitro cell-senescence process and in vivo lung alterations.

Osteopontin, a Key Mediator Expressed by Senescent Pulmonary Vascular Cells in Pulmonary Hypertension

Objective—Senescent pulmonary artery smooth muscle cells (PA-SMCs) may contribute to the pathogenesis of pulmonary hypertension by producing secreted factors. The aim of this study was to explore the role in pulmonary hypertension of extracellular matrix proteins released by senescent PA-SMCs. Approach and Results—Polymerase chain reaction array analysis of human PA-SMCs undergoing replicative senescence revealed osteopontin upregulation, which mediated the stimulatory effect of senescent PA-SMC media and matrix on PA-SMC growth and migration. Osteopontin was upregulated in lungs from patients with chronic obstructive pulmonary disease or idiopathic pulmonary arterial hypertension. Prominent osteopontin immunostaining was noted in PA-SMCs that also stained for p16 at sites of vascular hypertrophy, and lung osteopontin levels correlated closely with age. Compared with younger mice, 1-year-old mice displayed higher lung osteopontin levels, right ventricular systolic pressure, pulmonary vessel muscularization, and numbers of PA-SMCs stained for p16 or p21 and also for osteopontin. No such changes with age were observed in osteopontin−/− mice, which developed attenuated pulmonary hypertension during hypoxia. Compared with cultured PA-SMCs from young mice, PA-SMCs from 1-year-old mice grew faster; a similar fast growth rate was seen with PA-SMCs from young mice stimulated by matrix or media from old mice. Differences between old/young mouse PA-SMC growth rates were suppressed by antiosteopontin antibodies. PA-SMCs from osteopontin−/− mice grew more slowly than did wild-type PA-SMCs; they were stimulated by wild-type PA-SMCs media and matrix, and this effect was stronger with PA-SMCs from older versus younger mice. Conclusions—Osteopontin is a key mediator released by senescent PA-SMCs and contributing to pulmonary hypertension progression.