1. Introduction

Rieske iron-sulfur protein (RISP) in complex III of the mitochondrial electron transport chain is a critical molecule for reactive oxygen species (ROS) generation in pulmonary artery (PA) smooth muscle cells (SMCs)^[1][2][3][4]. ROS signaling may play a significant role in the development of chronic obstructive pulmonary disease (COPD) and its associated pulmonary hypertension (PH). Studies have shown that RISP-dependent mitochondrial ROS can mediate important cellular processes such as PA vasoremodeling and vasoconstriction that lead to the development and progression of PH in COPD. The role of RISP-dependent ROS in both COPD and PH was summarized and the aim is to establish RISP and its associated signaling molecules as potential specific and novel targets for these devastating lung diseases compared to current non-specific medications.

2. COPD and PH

COPD, a term which encompasses chronic bronchitis and emphysema, is a chronic inflammatory disease characterized by obstructed airways, often leading to reduced airflow. This devastating disease is currently the third leading cause of death and affects more than 16 million Americans^[5][6][7][8]. Overproduction of mucus (seen in bronchitis), inflammation of the airway smooth muscle cells (ASMCs), and disruption of alveolar attachments (seen in emphysema) contributes to reduced airflow and narrowing airways in COPD^[5][6][7][8].

It has been reported that up to 91% of COPD patients will develop PH, which is characterized by an increase in pulmonary arterial pressure (P_PA, >20 mmHg at rest)^{[9][10][11][12]}. Prolonged PH may account for cardiac dysfunction including arrhythmias, myocardial infarction, and congestive heart failure^[13]. Two-thirds of COPD patients display right ventricular hypertrophy, resulting from increases in P_PA, as assessed right heart catheterization. Thus, pulmonary manifestations of COPD (i.e., hypoxemia, inflammation and arterial remodeling) may cause and exacerbate extra-pulmonary (cardiac) manifestations^[6][12], worsening COPD symptoms, and ultimately increasing risk of death. Although PH in COPD is common, the underlying mechanisms are still largely unknown.

Cigarette smoke (CS) is recognized as a key factor in the development and progression of COPD, where up to 90% of COPD cases can be attributed to CS or e-cigarette smoke ^[5][7][14]. Exposure to CS or nicotine causes, exacerbates, and prolongs the symptoms and features of COPD and associated PH^{[15][16][17][18]}. Nicotine is the major active component of cigarettes, e-cigarettes, and nearly all other tobacco-derived products. More importantly, nicotine inhalation may replicate almost all major features of COPD^{[19][20][21][22]} and also cause PH^[23][24].

Chronic exposure to CS and nicotine is detrimental for the cellular environment, including cell injury, infiltration of inflammatory cells in the lung, ROS, oxidative stress, and pro-inflammatory cytokines. These cellular responses to CS and nicotine call for investigation as they promote the development and progression of PH in COPD in both the airway and pulmonary vasculature.

3. Treatments

There is no cure for COPD or PH. The primary approach for slowing disease progression and improving quality of life relies on limiting exposure to COPD-exacerbating triggers and management of COPD symptoms^[5][7][8]. Current treatments use both non-pharmacological and pharmacological interventions. The former includes lifestyle changes, such as smoking cessation, and avoiding airborne irritants (i.e., secondhand smoke and air pollutants). Avoiding respiratory infections through vaccination against flu and pneumococcus are very important for those diagnosed with COPD.

Pharmacological intervention for treatment of COPD and PH involve using various drugs depending on severity of symptoms, triggers, patient condition and comorbidities, to avoid complications and risk of death. These treatments (1) increase airflow using airway or vasodilators and (2) reduce inflammation using anti-inflammatories or corticosteroids^[5][7].

Current COPD treatments primarily focus on symptom- and trigger-management to slow disease progression. Because there is still no cure for COPD, it is necessary to develop specific treatments that target the underlying causes and mechanisms that lead to the development and progression of COPD and PH.

4. Airway and Vascular Cellular Responses

Airway remodeling, primarily mediated by ASMCs, contributes to narrowing airways in COPD, increasing structural respiratory resistance^[25][26][27]. Increased proliferation and functional response of ASMCs may cause airway hyperresponsiveness (AHR) in response to irritants and agonists^[28][29][30]. AHR is a key feature in COPD and is reversible with medication (bronchodilators). Inflammation, in response to irritants such as CS or nicotine, also plays a dominant role in airway remodeling. Pro-inflammatory cytokines such as interleukin (IL)-1β and tumor necrosis factor-β are upregulated in airways and cause ASMC proliferation, remodeling, AHR and airflow restriction in COPD. These ASMC responses may be secondary to increased ROS.

The airway remodeling, AHR, and airflow restriction in COPD may contribute to the cellular responses in PASMCs in PH. CS, nicotine inhalation, and other factors may also directly cause cellular responses in PASMCs, leading to PA vasoconstriction, vasoremodeling, and PH^[9][10][21]. In COPD patients, PH is primarily characterized by increased pulmonary vascular resistance (PVR), leading to increased P_PA, which occurs due to increased PA vasoconstriction and remodeling. These major cellular responses are highly determined by hyperresponsiveness, hyperproliferation, and decreased apoptosis of PASMCs. Hypoxic pulmonary vasoconstriction (HPV) is an adaptive mechanism in response to alveolar hypoxia, which matches perfusion to ventilation in small resistant PAs^[31]. Vasoconstriction is highly controlled by Ca²⁺ signaling in PASMCs, specifically the release of Ca²⁺ from intracellular stores through ryanodine receptor (RyR) channels in the sarcoplasmic reticulum (SR).

Vascular remodeling, or vasoremodeling, often involves hyperproliferation of key cells (PASMCs), basement membrane thickening, fibrosis, and deposition of collagen^[32]. This process plays a significant role and contributes to COPD-associated PH. Cigarette or tobacco smoke-induced ROS generation may contribute to airway and pulmonary vascular remodeling in COPD and PH^[21][25]. However, the mechanisms of how cigarette smoke leads to airway or vasoremodeling are still largely unknown.

5. The Important Role of Rieske Iron-Sulfur Protein in Mitochondrial Complex III

Mitochondria are the primary and most important source of ROS generation in PASMCs. Due to their oxygen-rich environment and involvement of the mitochondrial electron transport chain (ETC), electron reduction of oxygen to superoxide is common and a major reaction producing ROS. Although there are 4 complexes in the ETC, complexes I and III are highly regarded as the main contributors of mitochondrial ROS generation^{[1][2][33][34][35]}. OuThe researchers' laboratory and others have shown that RISP, a catalytic subunit of complex III, is a major contributor to ROS generation. Specifically, knockdown of RISP in PASMCs inhibits hypoxia-induced increase in mitochondrial ROS generation, whereas overexpression promotes the hypoxic response^{[1][2][3][4][36]}.

ROS-regulated Ca²⁺ signaling through processes such as ROS-induced Ca²⁺ release (RICR), contributes to this disease development. The Ourresearchers' laboratory has previously shown that RyR2 is the major subtype of RyRs and plays a dominant role in PASMCs^[3]. RISP knockdown blocks hypoxia-induced RyR2 oxidation and subsequent PH [36]^[37]. Importantly, RISP KD using lentiviral shRNAs significantly decreased RyR2 activity in PASMCs from hypoxic mice. RyR2 oxidation was assessed by immunoprecipitation and immunoblotting of 2,4-dinitrophenyl (DNP) on carbonyl-bound RyR2. Increased RyR2 oxidation (marked by DNP) was detected in PASMCs in hypoxic mice, whereas RyR2 oxidation was blocked by RISP KD^[36]. This restudyearch suggests that RISP plays a critical role in regulating RyR2-mediated Ca²⁺ signaling.

HPV, a Ca²⁺-regulated process initiated by hypoxia, can increase P_PA and leads to PH. Studies have shown that RISP-mediated ROS signaling can also regulate HPV ^{[2][4][35][3738]}. Using a RISP KO animal model, lung slices from these mice demonstrated abolished hypoxia-induced increases in Ca²⁺ signaling. Additionally, in vivo RISP depletion attenuated hypoxia-induced increase in RVSP. Thus, these studies suggest RISP-generated ROS can mediate HPV, an important factor in the development of PH.

It is well known inflammation plays a significant role in the development and progression of COPD and PH. Inflammation in PH in COPD can contribute to vasoconstriction and vascular remodeling through the recruitment of inflammatory cells, such as neutrophils and macrophages^[3839][3940]. Exposure to CS and nicotine can activate these inflammatory cells and cause a cascade of inflammatory signaling. ROS-dependent regulation of inflammatory signaling is interesting because studies have shown that ROS can both inhibit and promote inflammatory signaling. Studies from outhe researchers' laboratory have shown that RISP regulates RyR2-mediated Ca²⁺ release in PASMCs and can lead to increased vasoremodeling and constriction of PAs after hypoxia exposure^{[3][4][36][3738]}. In outhe researchers' recent publication, RyR2 KO attenuated hypoxia-induced activation of NF-κB as shown through decreased p65/p50 nuclear translocation^[36]. Administration of a potent NF-κB inhibitor, PDTC, also blocked hypoxia-induced effects on P_PA and vasoremodeling^[36]. This restudyearch may suggest that RISP-mediated ROS generation can lead to indirect and direct activation of inflammation, influencing vasoremodeling and constriction seen in PH and COPD.

6. Potential Therapeutics for PH in COPD

Current treatments for COPD involve non-specific and normally non-effective therapies, including anti-inflammatories like inhaled corticosteroids. These corticosteroids can control inflammation and swelling in the airways^[4041]. However, these corticosteroids are not the first-line therapies against COPD and are often paired with bronchodilators. Moreover, although targeting specific inflammatory pathways sound enticing, these inflammatory pathways, like NF-κB, are ubiquitously expressed and not ideal in treating pulmonary diseases like COPD and PH. Consequently, it is necessary to determine specific and direct targets that alter NF-κB signaling (i.e., RISP of complex III) in order to develop targeted therapies for COPD and PH.

Considering the central role of ROS in the development of PH in COPD, we must consider the potential role for antioxidants as therapy^[4142]. In several animal studies, compounds with antioxidant properties (such as PDTC) have been used to block the progression of PH in COPD^[4243][4344]. However, there are potential challenges in using antioxidant therapy. Use of general antioxidants may affect the homeostatic balance of ROS production and scavenging. Moreover, directly targeting ROS generators such as RISP may prove more effective and specific in treating PH in COPD.

7. Conclusions and Future Avenues of Research

It is evident that RISP in mitochondrial complex III and its associated ROS signaling can mediate the development and progression of PH in COPD. Here, wthe hresearchers have briefly discussed how RISP-mediated ROS can activate RyR2-regulated Ca²⁺ signaling and NF-κB inflammatory signaling. These signaling pathways are highly involved in hypoxia-induced PA vasoconstriction, vasoremodeling and hypertension in COPD. Although the weresearchers know CS and nicotine are critical factors in the development of COPD and PH, the underlying mechanisms of how CS and nicotine leads to COPD and PH are still largely unknown. We The researchers speculate that CS and nicotine may activate nicotinic receptors on the mitochondria, cause RISP-mediated ROS generation, lead to an onslaught of detrimental signaling pathways such as Ca²⁺ signaling (through RyR2), NF-κB-mediated inflammation, and possibly DNA damage, thereby resulting in PA vasoconstriction, vasoremodeling and hypertension. Thorough assessment of these signaling pathways is necessary in order to establish RISP and its associated key molecules as specific and effective therapeutic targets for COPD and PH.