Cyp2c44-mediated decrease of 15-HETE exacerbates pulmonary hypertension

pulmonary hypertension (PH) is a progressive disease characterized by elevated pulmonary arterial pressure and pulmonary vascular resistance (19). According to the latest estimations, the PH incidence accounts for 1% of the total global population (up to 5 million people), and its rate increases up to 10% in individuals above 65 yr of age (24). The median survival rate for patients diagnosed with PH is predicted to be 2.8 yr (8).

PH develops as a result of vasoconstriction and remodeling in the intra-alveolar pulmonary arteries and leads to progressive right ventricular hypertrophy and cardiac failure (1, 11, 27, 29, 31, 34, 37). Pulmonary vasoconstriction is thought to be an early component of PH and has been primarily attributed to vascular endothelial dysfunction that leads to impaired production of vasodilators such as nitric oxide and prostacyclin as well as overexpression of vasoconstrictors such as endothelin-1 (25). According to recently published data, there are currently 10 drugs from 5 different substance classes available and licensed for the treatment of PH, including endothelin receptor antagonists, phosphodiesterase-5 inhibitors, stimulators of soluble guanylate cyclase, and arachidonic acid pathway-targeted drugs such as prostacyclin analogs and prostacyclin receptor agonists (23). These medications are typically prescribed singly or in combination, and their primary focus is to induce vasodilation of the pulmonary arteries. A recent randomized trial that used a combination of these classes of medications has reported a 40% success rate in patients with PH (18). In addition, the use of some medications, such as endothelin-1 receptor antagonists, remains limited due to their known hepatotoxicity, and several drugs from this class have been already withdrawn from clinical practice (6). These data suggest that other options should be investigated to develop new therapeutic targets to improve outcomes in patients with PH.

While the cyclooxygenase and lipoxygenase metabolites of arachidonic acid metabolism have been well studied and clinically implicated in the prevention of endothelium-targeted vasoconstrictor effects during PH, the cytochrome P-450 (CYP) pathway of arachidonic acid metabolism remains understudied. Because CYP proteins are sensitive to changes in O₂ tension and are widely expressed in lung tissues (38), there is an increasing need to further study the effects of CYP on the progression and development of PH. For example, an intriguing recent study (26) by Joshi et al. reported the importance of the Cyp2c44 gene in the progression of PH and showed that Cyp2c44 gene deletion results in increased hypoxia-induced pulmonary artery remodeling and hypertension. The Cyp2c44 gene is broadly expressed, and studies have shown that it may be found in the kidney, liver, and adrenal glands (12, 26, 35). In addition, original studies on arachidonic acid metabolism proposed a primary role for CYP in the generation of EETs; however, more recent studies have reported that Cyp2c44, in particular, is capable of generating 15-hydroxyeicosatetraenoic acid (15-HETE) and that the amount of 15-HETE produced by Cyp2c44 accounts for up to 30% of total 15-HETE production (12).

In a recent study in the American Journal of Physiology-Heart and Circulatory Physiology, Hashimoto et al. (21) proposed a new role for the effects of CYP family proteins on hematopoetic progenitor stem cells in mice. The authors showed that Cyp2c44 gene deletion leads to a decrease in 15-HETE production and to an increase in the numbers of CD133-positive hematopoetic progenitor stem cells. The evidence of the relationship between PH and CD133-positive progenitor cells has been accumulating for the past decade, although the reported relationship was only correlative. As such, increased numbers of circulating and tissue levels of CD133-positive cells have been reported in both preclinical models and patients with PH (2, 14, 28, 30, 32, 36). In addition, transplantation of bone marrow CD133-positive cells from patients with PH into mice resulted in angioproliferative pulmonary vascular remodeling, right ventricular failure, and death, suggesting a pathophysiological mechanism for CD133-positive cells (3). In this study, Hashimoto et al. provided the mechanistic link between the CYP pathway of arachidonic acid metabolism and showed a decrease in 15-HETE production and an increase in th eaccumulation of CD133-positive cells in the adventitia surrounding the medial layer of the remodeled pulmonary arteries in the lungs of Cyp2c44-null mice with PH. These data suggest that Cyp2c44 gene deletion and the subsequent reduction in 15-HETE levels further worsen and actively participate in the remodeling processes during PH. The data produced by Hashimoto et al. support previously published data of the involvement of CD133-positive cells in the development of plexiform lesions in the arteries of patients with PH (7, 33). These findings indicate a possible beneficial effect of 15-HETE in the remodeling of arteries during PH. However, the existing knowledge of the diverse biological actions of 15-HETE (10) indicates a necessity for future studies on the compensatory mechanisms that are activated due to Cyp2c44 gene deletion.

Hashimoto et al. showed that 15-HETE is capable of directly affecting the CD117-positive precursor of endothelial cells. In the in vitro experiments, 15-HETE significantly reduced numbers of CD117-positive cells when they were incubated with bone marrow cells. The authors linked the increased circulating levels of CD117-positive cells and increased expression of von Willibrand factor-positive cells in the lungs of Cyp2c44-null mice, indicating that in addition to active remodeling of the arteries during PH, 15-HETE may also have a strong effect on the formation of neointimal lesions. These data suggest that 15-HETE also plays an important role in altering the proangiogenic responses during PH. In addition, the presence of CD117-positive cells in hypertrophied hearts of Cyp2c44-null mice also indicates possible roles of 15-HETE in heart failure during PH. In 2015, the Global Burden of Disease Study reported >60 million heart failure patients worldwide, and it has been predicted that up to 50% of these patients could develop PH (20). Thus, finding a pathway that plays a role in both the pathology of failing hearts and PH might serve as a potentially effective therapeutic target. However, further studies to dissect the specific roles of 15-HETE in the failing heart and during the development of PH are warranted.

Inflammation is associated with most forms of PH (4, 22). Mononuclear cells activate fibroblasts in the adventitia and pulmonary artery smooth muscle cells in the media, leading to remodeling of the pulmonary arteries in both preclinical models and patients with PH (2, 3, 9, 13–17). The Hashimoto et al. study (21) also showed that Cyp2c44 gene deletion, and the subsequent decrease in 15-HETE production, resulted in an increase of monocytes and macrophages in both the bone marrow and circulation. Following previously published material that indicated that CD133-positive cells can give rise to all hematopoietic lineages (including erythroid cells, myeloid cells, monocytes/macrophages, and megakaryocytes) and the data that granulocyte/monocyte colony formation is increased in PH patients (3, 14), the Hashimoto et al. study suggests a new role for 15-HETE in the mediation of inflammatory responses, the subsequent arteriole wall thickening, and the progression of PH.

Collectively, these findings suggest that the Cyp2c44 pathway, particularly through regulation of the production of 15-HETE, may have strong implications for the amelioration of PH pathophysiology (Fig. 1). These findings also suggest that targeting the Cyp2c44 pathway may be beneficial for the development of new therapeutic targets to improve the success rate of the currently available treatments for PH.

Fig. 1.

Schematic representation of Cyp2c44 and 15-HETE roles in pulmonary hypertension (PH). Upon activation, Cyp2c44 produces 15-HETE, which accounts for ~30% of total 15-HETE production. During PH, 15-HETE is known to induce vasoconstriction; however, recent findings suggest that 15-HETE may also have beneficial effects in the pathophysiology of PH through the attenuation of hematopoetic stem cell accumulation and the inflammatory response. Future studies are needed to investigate the effects of 15-HETE on oxidative stress and vascular remodeling in PH.

Among future studies to investigate the effects of the Cyp2c44 and 15-HETE in the pathophysiology of PH, special focus should be paid to oxidative stress. A previous study (5) has demonstrated that lung tissue from patients with severe PH exhibits significant oxidative stress and is associated with increased pulmonary production of several HETEs, including 12- and 15-HETE, as well as 5-oxo-eicosatetraenoic acid, suggesting possible roles in Cyp2c44 gene signaling. It is also important to consider how different cells within the vessel wall use Cyp2c44-dependent pathways and how genetic depletion of these pathways affects these cell types individually. Finally, it is important to further study how vascular remodeling processes (i.e., smooth muscle proliferation as well as the synthesis and deposition of extracellular matrix components) and inflammatory processes are controlled at a cellular level by Cyp2c44-dependent pathways.

GRANTS

This work was supported by National Institutes of Health (NIH) Grants R01-AT-006526, R01-AG047879, R01-AG038747, and R01-NS056218, the Geroscience Training Program in Oklahoma (NIH Grant T32-AG-052363), the Oklahoma Nathan Shock Center (NIH Grant 3-P30-AG050911-02S1), Oklahoma Shared Clinical and Translational Resources (NIH Grant U54-GM-104938, to A. Yabluchanskiy), the Oklahoma Center for the Advancement of Science and Technology (to A. Yabluchanskiy), the Reynolds Foundation (to A. Yabluchanskiy), and the Presbyterian Health Foundation (to A. Yabluchanskiy).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the author(s).

AUTHOR CONTRIBUTIONS

G.Á.F. and A.Y. interpreted results of experiments; G.Á.F. and A.Y. drafted manuscript; G.Á.F. and A.Y. edited and revised manuscript; A.Y. approved final version of manuscript.