Our study revealed several novel and important findings: First, patients with stable COPD, compared with smokers without COPD, had significantly higher serum levels of HN and GDF-15, and showed a trend toward elevated FGF21 levels. Second, the levels of these mitokines did not correlate with each other in COPD, thus suggesting that their regulation and/or metabolism are different. Finally, the mitokines were associated with outcomes in patients with COPD, although a different relationship was observed for each mitokine.
The differences among mitokines were consistent with the known metabolic characteristics of each molecule. Whereas the HN gene is under the direct control of mitochondrial DNA (and is indicative of mitochondrial stress when elevated), nuclear DNA encodes the GDF15 and FGF21 genes, and they are only partially under the control of mitochondria through mitochondrial-to-nuclear signaling (ATF3/4, etc.) (35). In fact, many other stimuli—such as inflammatory cytokines, hypoxia, PPAR-alpha ligands, carcinogens, diet, exercise, lipids and amino acids—modulate GDF15 and FGF21 through specific transcription factors (36–38).
HN had not previously been studied in the context of COPD. According to our data, HN is associated with malnutrition and 6MWD. Both characteristics indicate that skeletal muscle—a tissue affected by COPD that accounts for half the body weight and is rich in highly active mitochondria—is the most important source of excessive circulating HN, although immune, airway, parenchymal lung cells and pulmonary vasculature cells may also contribute. The increased levels of HN in COPD appear to be a compensatory reaction to protect mitochondria, and hence cells, against generalized oxidative stress. High HN levels indicate not only lower 6MWD but also oxygen desaturation, thus suggesting a relationship with exercise capacity as well as ventilation/perfusion mismatch during exercise. Furthermore, mitochondrial dysfunction associated with COPD-induced low-grade inflammation may also be responsible for the increased HN levels, thus making HN an interesting prognostic biomarker that provides information on malnutrition, skeletal muscle dysfunction and chronic inflammation. In fact, our data show that high circulating HN levels are a prognostic factor for moderate and severe exacerbation in the next year.
Previous studies have shown that GDF15 is a promising, albeit unspecific, biomarker in COPD (33). GDF15 levels were initially described to be highly elevated in septic patients in critical care and have also been described to be elevated in patients with COPD (39–42). Our data confirmed these findings. Various explanations have been suggested regarding the factors associated with high GDF15 levels in COPD. Wu et al. and Verhamme et al. (43, 44) have demonstrated that cigarette smoke induces GDF15 in human tracheobronchial epithelial cells. Mutlu et al. (40) have suggested that generalized inflammation is a factor, because GDF15 levels correlate with levels of CRP, a well-known systemic inflammatory marker. Moreover, GDF15 levels have been found to be lower in stable patients than in patients with exacerbation in at least two studies (39, 40). In another study, higher GDF15 levels have been associated with higher coronary artery scores in patients with COPD, whereas no correlation has been found with common markers of COPD severity (45). The authors have proposed that high GDF15 may be mediated by asymptomatic atherosclerosis, another cause of chronic low-grade inflammation. Our data also showed that comorbidities were associated with high levels of GDF15, thus suggesting that GDF15 may be elevated in many diseases. In contrast, Patel et al. have shown that in COPD, circulating GDF15 is inversely correlated with exercise capacity (42), but not with BMI or FFMI. Regarding the prognostic value of GDF15, the longitudinal Bergen COPD study (33) has importantly found that high concentrations of GDF15 at the time of entry into the study were associated with a higher annual exacerbation rate, mortality, and a faster decline in lung function over 3 years of follow up. In our study, the number of patients was smaller, the follow-up period was shorter, and patients with important comorbidities (heart disease, kidney failure, etc.) were excluded; therefore, our negative results might have been due to less statistical power. Nonetheless, our data suggested that high HN levels may be more specific prognostic factors than high GDF15 levels. Clearly, more studies are needed to clarify these aspects.
FGF21 had not previously been studied in COPD. FGF21 levels were higher in patients with HRE, and were predictive of hospitalization, but the strength of the association was weak, probably because of the many factors that regulate FGF21 levels, some of which are not fully known. Nonetheless, the relationship between FGF21 levels and COPD outcomes is less clear than that observed with HN herein.
Our study has several limitations. Because this was a single center study, these results should be replicated in larger multicenter studies, which should examine other sociodemographic characteristics and diseases known to alter mitokine levels. Further studies are required to demonstrate or exclude a potential role of HN, FGF21 and other “new” mitokines not studied herein (46). We used stringent criteria to exclude patients with altered pulmonary function and other conditions known to increase mitokine levels (active exacerbation, sepsis, severe inflammation, renal insufficiency, clinical coronary artery disease, etc.); therefore, the results are not generalizable to all patients with COPD, but they add new pathophysiological information. States of low-grade inflammation and clinical asymptomatic atherosclerosis or other asymptomatic diseases were not excluded, although we do not believe that these conditions could have strongly influenced our results. However, our study reveals only associations but not causality.
The main strength of our study is that it was prospective and was specifically designed to evaluate the possible utility of measuring mitokines in a group of well characterized COPD patients differing in obstruction severity and clinical characteristics.