Association between elevated serum bilirubin levels with preserved lung function under conditions of exposure to air pollution

Chronic respiratory diseases are among the leading causes of morbidity and mortality worldwide [1]. Short- and long-term exposures to elevated air pollution levels lead to deterioration of lung function in individuals of all ages [2‐4]. In adults it has been shown that exposure to air pollution cause a decline in pulmonary function by generating oxidative stress in the lung tissue [5, 6]. Bilirubin is the end product of heme breakdown. The majority of bilirubin originates from degradation of erythrocyte hemoglobin in the reticuloendothelial system, while the remaining ~ 20% comes from inefficient erythropoiesis in bone marrow and other heme proteins degradation [7, 8]. Bilirubin was shown to exhibit anti-oxidative and anti-inflammatory properties in vitro and in vivo [9]. In a Swiss study on Air Pollution and Lung Disease in adults (SAPALDIA), increased concentrations of serum bilirubin were associated with improved parameters of forced expiratory volume in 1 s (FEV1) and forced vital capacity (FVC) ratio (FEV1/FVC) and forced expiratory flow at 25–75% of FVC (FEF25–75%), after adjusting for the effects of sex, age, education, height, and weight and tobacco smoke exposure [10]. One longitudinal study demonstrated a significant negative relationship between baseline bilirubin levels, incidence of coronary heart disease (CHD), and mortality from cardiovascular disease (CVD) [11]. Another study showed inverse association between total bilirubin level and CVD risk, independent of other risk factors [12]. The inclusion of total bilirubin in the standard established risk factors panel, however, provided no significant improvement in CVD risk prediction. A large prospective Korean study revealed that a higher basal bilirubin level within the normal range was associated with a low risk of lung cancer. Smoking and low bilirubin levels have been cumulatively associated with a higher risk of lung cancer [13]. Moreover, bilirubin concentrations were negatively associated with the history, duration, and intensity of smoking, suggesting a possible role of inhaled tobacco in the pathogenesis of lung function deterioration related to oxidative stress [14].

To the best of our knowledge, there are no published data on any correlations between bilirubin levels and pulmonary function test results under conditions of air pollution. In the present study, we assessed this correlation and the interaction between functional parameters, levels of bilirubin and air pollution in a large cohort of apparently healthy Israeli individuals who participated in an annual health survey.

The study population consisted of 4646 (61.2%) men and 2641 (38.8%) women, with a mean age of 44.84 (± 11.27) years. Among them were 1338 (17.8%) current smokers. Individuals with lower FEV1 values (< 80% of predictive values) were more likely to be men, older, and have a higher BMI. The study population’s characteristics are presented in Table 1.

As expected, current smokers had lower FEV1 and FEV1/FVC values compared to past smokers and participants who never smoked. Additionally, women demonstrated higher FEV1, FVC, and FEV1/FVC values compared to men (Table 2).

To evaluate the effect of total serum bilirubin levels on FEV1, we divided the cohort into low, medium, and high serum bilirubin levels. The results showed that FEV1, but not FVC or FEV1/FVC, was significantly higher in participants with high blood bilirubin compared to low or medium levels (p < 0.001 and p = 0.018, respectively) (Fig. 2).

Next, we explored the possible effect of bilirubin on FEV1 under conditions of exposure to major air pollutants. Importantly, participants with high levels of bilirubin presented with preserved lung function (FEV1 values) under conditions of exposure to high and medium pollution levels of both NOx and CO pollutants (p = 0.003 and p = 0.022, respectively, Fig. 3a, b). FEV1, under moderate and high level of NOx with high bilirubin, compared to low bilirubin level and moderate bilirubin level is significantly high, p = 0.014, and p < 0.001 respectively (Fig. 3a). Under moderate and high level of CO with high bilirubin, compared to low bilirubin level and moderate bilirubin level, FEV1 is also significantly high, p = 0.027 and p = (Fig. 3b). The apparently protective effect of elevated serum bilirubin concentrations on FEV1 under conditions of moderate and high NOx and CO pollution levels also remained significant after adjusting for age, BMI, sex, smoking status, and diabetes diagnosis (Fig. 4a, b). The interactions between air pollutants (NOx or CO) with bilirubin on PFT results were not significant.

Our results of elevated serum bilirubin levels being associated with improved lung function are compatible with the findings in Swiss [10] and South Korean [14] populations that also described positive associations between serum bilirubin levels and lung function in healthy populations. Moreover, there was an association between a lower risk of respiratory disease and all-cause mortality in patients with normal-range bilirubin levels, while relatively higher levels of bilirubin were associated with a lower risk of respiratory diseases [17]. In another study, bilirubin levels were associated with a longer 6-min walk distance and a better quality of life [18].

Bilirubin is known as a potential antioxidant with anti-inflammatory properties. Elevated bilirubin levels have cytoprotective properties, including antioxidant, anti-inflammatory, and antiproliferative effects [19]. These effects seem to be mediated by an elevated heme oxygenase 1 (HO-1) activity which may be moderated by genetic variations or environmental factors. The role of HO-1 in correlation with the environment was shown in some other studies as well. Moreover, environmental cadmium moderates diabetes type 2 through HO-1 activity [20] and the effect of environmental toxins, such as cigarette smoke, silica, and asbestos on different lung diseases seem to be mediated by bilirubin derivate HO-1 [21]. In contrast to all of these studies that correlated bilirubin with the environment or the effect of bilirubin on functional parameters of respiration, we extended these investigations and looked at how bilirubin levels and pulmonary function test findings were correlated to environmental pollutants.

Our novel finding is that the positive association between pulmonary function tests and bilirubin was significantly stronger among participants exposed to elevated levels of major air pollutants, such as CO and NOx, suggesting that high levels of circulating bilirubin may have a protective effect against lung impairment under conditions of exposure to ordinary urban air pollution. This led us to focus our attention upon a possible mechanism of this protection looking at the metabolism of HO-1 and its therapeutic effects. The use of pharmacological agents that augment expression of HO-1 has been recently investigated, and the results showed protection against a variety of oxidative stress and inflammatory conditions [22]. Induction of HO-1 by a triterpenoid protected neurons against ischemic injury [23], and HO-1 was expressed more abundantly in the lesions of synovial tissue from rheumatoid arthritis patients than in those from the other patient groups. Hemin, auranofin, and HO-1-expression vector induced HO-1, and reduced expression of Tumor Necrosis Factor may mediate less inflammation in those patients [23]. Consequently, its metabolites, such CO, biliverdin and bilirubin, could become parts of a therapeutic strategy for the treatment of various inflammatory illnesses [24, 25].

We are aware that this study has several limitations that bear mention, one of which is its retrospective design. We do not have follow-up data on major respiratory or cardiovascular events, thus precluding any prognostic application of our findings. In addition, measurements of air pollution levels were made according to the subjects’ residential areas and did not take into account their places of work. Finally, we did not perform any mechanistic-oriented studies.

The key finding of this study was that elevated serum bilirubin concentrations are associated with preserved lung function in healthy individuals exposed to elevated levels of air pollution. Bilirubin should be further studied as a surrogate marker for lung function in respiratory diseases in which there are elevated levels of oxidative stress.