Patients were consecutively recruited during routine visits to the monographic COPD outpatient clinic.

Inclusion criteria: Adult patients with COPD according to GOLD Guidelines7 who were older than 40 years of age.

Exclusion criteria: 1) Patients using treatments interfering with bone resorption (vitamin D metabolites, oral calcium, estrogens, PTH, other antiosteoporotic agents, systemic glucocorticoids, etc.); 2) treatment with pulmonary rehabilitation during the study or 6 months before the inclusion period; 3) COPD exacerbation 8 weeks before the study; 4) other conditions that could alter bone resorption (primary hyperparathyroidism or any other cause of hyper- or hypocalcaemia, recent osteoporotic or traumatic fractures, sarcoidosis, cancer, acute or chronic kidney failure, or rheumatoid arthritis).

The methodologic aspects of this study are similar to those in a previously published study.15 In summary, trained staff recorded clinical information and performed lung function tests. Spirometry was performed according to the Spanish Society of Pulmonology and Thoracic Surgery (SEPAR) Guidelines.16 Diffusing capacity of the lung for carbon monoxide (DLCO) was measured in accordance with ATS/ERS Guidelines.17​ Six-minute walking tests were performed according to the SEPAR protocol.18 We estimated body composition with a bioelectrical impedance device (OMROM BF511, Omrom, Japan).

Blood samples were collected from all participants after they had signed the consent form. To avoid a possible effect of circadian variability on SOST levels, we obtained all samples between 10:00 and 12:00 pm. Serum total calcium, phosphorus and albumin were measured with a Siemens traceable enzymatic method assay (ADVIA 2400 Analyzer, Siemens Healthcare Diagnostics, Germany). Total calcium levels were corrected with albumin. C-reactive protein (CRP) was measured with immunoturbidimetry (ADVIA 2400 Analyzer, Siemens Healthcare Diagnostics, Germany), and 25(OH)D concentrations were measured with an automated competitive chemiluminescence assay (Liaison XL, DiaSorin Inc, Stillwater MN USA). We measured serum intact PTH (iPTH) levels with the automated chemiluminescence competitive assay IDS iSYS (Immunodiagnostic Systems Ltd, Boldon, UK), P1NP (an established marker of osteoblastic function) and β-Crosslaps (a marker of osteoclastic function) with a specific chemiluminescence immunoassay in iSYS (IDS-iSYS Multi-Discipline Automated Analyser, Pouilly-en Auxois, France). Serum SOST levels were measured with a highly sensitive specific immunoassay (Sclerostin HS ELISA, TECO Medical Sissach Switzerland).

After entering the study, patients were followed up for 12 months. We prospectively recorded moderate COPD exacerbations (exacerbations in patients treated with antibiotics and/or systemic corticosteroids) and hospitalizations due to severe COPD exacerbations, by asking the patients during follow-up visits (6 and 12 months after entering the study) and checking the medical records from the hospital and primary care. General or emergency physicians unaffiliated with this study made the diagnosis of exacerbation or the decision to hospitalize the patients.

Data are presented as mean±SD for normally distributed data or median (interquartile range) for nonparametric data. Sample size calculation was performed in Stata Statistical Software: Release 15. College Station, TX: StataCorp LLC.), with an α risk of 0.05 and a β risk of 0.2. Differences between groups were analysed with unpaired t tests for parametric data or Mann-Whitney tests for nonparametric data. Normal distributions were evaluated with Kolmogorov-Smirnov tests. Correlations between data sets were examined with the Pearson (r) correlation coefficient for parametric data or the Spearman rank (rs) correlation coefficient for nonparametric data. Multivariate linear regression was performed to assess factors explaining the variance in SOST. Kaplan-Meier estimates were used to calculate the proportion of participants experiencing an event over time. Univariate and multivariate analyses with Cox proportional risk analysis were performed in SPSS Software version 25.00 for PC to identify risk factors associated with moderate COPD exacerbations and hospitalizations due to COPD exacerbations. Differences were considered significant if p values were less than 0.05. All reported p values are two-sided.

A total of 139 patients were finally included in the study (flowchart for patient selection is shown in Fig. 1). Demographic, clinical and biochemical data are shown in Table 1. The mean patient age was 65±8.4 years, and most were men (70.5%). A high prevalence of current smokers (41.7%) was observed. Most patients had moderate obstruction and a moderate decrease in DLCO. The serum 25(OH)D levels were 15 (10–22) ng/mL, and the serum iPTH levels were 37.3±16 pg/mL. The SOST levels were 23.9±7.6 pmol/l and were higher in men 25.5±7.7 pmol/l than in women 20.3±5.9 pmol/l (p < 0.001). The group of patients with COPD and lower SOST levels included more men, and had worse lung function, less exercise capacity, lower FFMI, a trend towards higher dyspnoea, and higher exacerbation and hospitalization rates. However, the two groups had similar GOLD ABCD distribution, comorbidities and levels of molecules associated with phosphocalcic metabolism, with the exception of phosphorus, which was higher in the group of patients with lower SOST levels. SOST levels varied with GOLD obstruction (GOLD I 27.1±9.2 pmol/l, GOLD II 24.3±7.5 pmol/l, GOLD III 21.5±5.1 pmol/l, GOLD IV 19.5±6.5 pmol/l, p = 0.011) (Fig. 2), and were lower in frequent Vs. non-frequent exacerbators (22.4±7.1 pmol/l vs. 25.2±7.7 pmol/l(p = 0.03) (Fig. 3).

Fig. 1.

Flowchart of patient selection.

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Fig. 2.

Sclerostin levels in overall cohort distributed by GOLD obstruction.

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Fig. 3.

Sclerostin levels in frequent and non-frequent exacerbators.

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Serum SOST levels showed a positive correlation with age (r = 0.361, p < 0.001), BMI (r = 0.187, p = 0.03), FFMI (r = 0.382, p < 0.001), 6MWD (r = 0.192, p = 0.024), FVC (%) (r = 0.223, p = 0.011), FEV1 (%) (r = 0.265, p = 0.002) and DLCO (%) (r = 0.385 p < 0.001), and a negative correlation with CAT (r = -0.235, p = 0.005). Otherwise, SOST did not correlate with C-reactive protein. In multivariate analysis, only age (beta=0.264; p = 0.009), and FFMI (beta=1.241; p = 0.005) remained significant, although we also found a weak, positive, trend towards significance with DLCO (beta=0.074; p = 0.084).

Serum SOST levels showed a significant negative correlation with serum phosphorus (r = -0.291, p < 0.001) and β-Crosslaps (r = -0.167, p = 0.05), and a nearly significant correlation with P1NP (r = -0.165, p = 0.052). Serum SOST levels did not correlate with 25(OH) vitamin D, iPTH, total calcium, ionic calcium, alkaline phosphatase, albumin or magnesium levels. Finally, serum β-Crosslaps correlated positively with P1NP (r = 0.575, p < 0.001).

Univariate logistic regression indicated that low levels of SOST were associated with age (p < 0.001), sex (p = 0.002), DLCO (p = 0.042), FFMI (p < 0.001) and 6MWD (p = 0.014) at baseline, but not with smoking status, the Charlson index, mMRC dyspnoea score, FEV1 or the frequent exacerbator phenotype.

In addition, multivariate logistic regression (Table 2) revealed that low levels of SOST were independently associated with FFMI (OR 1.936 (IC95% 1.238–3.028) p = 0.004) but not with the other variables.

A total of 139 patients were followed up for 12 months: 55 patients were in the low SOST group (SOST levels in the lower tertile <20 pmol/l) and 84 patients were in the high SOST group (SOST levels higher than the lower tertile). Table 1 shows the clinical characteristics of both groups.

During the 12-month follow-up period, 57 patients presented moderate COPD exacerbations (31 in the low SOST group), and 15 patients were hospitalized (11 in the low SOST group).

Univariate Cox proportional risk analysis indicated that low SOST (p = 0.001), FEV1 (p = 0.011), previous moderate COPD exacerbations (p = 0.002) and male sex (p = 0.007) were risk factors for COPD exacerbations, whereas age, BMI, use of inhaled glucocorticosteroids, Charlson index or smoking status were not. Multivariate Cox proportional risk analysis revealed that low SOST (HR 2.015, CI95% 1.136–3.577, p = 0.017) was a risk factor for moderate COPD exacerbation (Fig. 4; Table 3).

Fig. 4.

The curve representation for COPD exacerbation events as “cumulative survival from COPD exacerbation events” (on the y axis) during 365 days of follow-up (on x axis) comparing low Sclerostin patients versus the rest of the patients. Cox regression adjusted for age, sex, smoking status, Charlson index, modified Medical Research Council dyspnea score, FEV1 (predicted %), and high risk of exacerbation.

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Univariate Cox proportional risk analysis showed that low SOST (p = 0.005), FEV1 (p = 0.001), previous admission due to COPD exacerbation (p = 0.001) and mMRC dyspnoea score (p = 0.001) were risk factors for hospitalization, whereas age, sex, use of inhaled glucocorticosteroids, the Charlson index and smoking status were not. Multivariate Cox proportional risk analysis indicated that previous admission (HR 4.99, CI95% 1.472–16.911, p = 0.01) and low SOST (HR 5.142, CI95% 1.380–19.158, p = 0.015) were risk factors for hospital admission (Fig. 5; Table 4).

Fig. 5.

The curve representation for COPD hospitalization events as “cumulative survival from COPD exacerbation events” (on the y axis) during 365 days of follow-up (on x axis) comparing low Sclerostin patients versus the rest of the patients. Cox regression adjusted for age, sex, smoking status, Charlson index, modified Medical Research Council dyspnea score, FEV1 (predicted %), and previous admission.

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