Participant Characteristics and Air Pollution Exposure
Descriptive characteristics of study participants, outcome measurements, and environmental factors are summarized in Table 1. As expected, large day-to-day variations in levels of size-fractioned particulates (e.g., PM2.5, PNCs in the diameter size range of 5-100 nm [UFPs]) and traffic-related pollutants (e.g., BC, NO2 and NOX) were found across the entire study period, which were reflected by large IQRs and wide ranges for measured pollutants. For instance, the mean 7 MA days of PM2.5 and NOX exposures were 91.8 µg/m3 (range: 30.6 to 236.2 µg/m3) and 124.8 µg/m3 (range: 54.9 to 235.8 μg/m3), respectively.
Association Between Air pollution and Dysfunctional Bone-Vascular Axis
Based on LME models, the adjusted changes in regulators in the core axis of the bone-vascular system in relation to air pollutants are shown in Figure 1. We found significant increases in OPG of 9.7% (95% CI, 3.2-16.2) to 34.1% (95% CI, 22.9-45.2) associated with IQR increases in PM2.5, PNC100-560, PSC100-560, CO, NO2 and NOX at prior 2 to 7 MA days. The estimated effects were greater when the exposure periods were extended, suggesting that accumulative exposure to air pollution may exert prominent effects on activation of the bone-vascular axis. Inverse associations with OPG were also observed for UFPs exposures. Further, elevations of sRANKL /OPG ratio were positively related to BC, CO, NO2 and NOX, with stronger effects observed at prior 3 MA days. With IQR increases in exposure to BC, CO, NO2 and NOX at prior 3 to 7 MA days, BMP-2 levels were significantly increased by 8.8% (95% CI, 2.1-15.4) to 14.6% (95% CI, 5.1-24.1), whereas significant reductions of 9.1% (95% CI, -17.2 to -1.0) to 19.9% (95% CI, -28.6 to -11.2) were found for BMP-4 levels.
Association Between Air pollution and Insurance Resistance
For biomarkers indicative of insulin resistance-related metabolic factors, as shown in Figure 2, we found increases in circulating insulin of 8.9% (95% CI, 0.4-17.3) to 49.6% (95% CI, 20.2-79.1) associated with most air pollutants (except for PSC100-560). For soluble form of the receptor for insulin, significant reductions of sIRα levels were observed in association with a variety of air pollutants (Figure 2). Among the examined exposure periods, the largest reductions in sIRα levels, ranging from 8.9% (95% CI, -17.7 to -0.1) to 13.8% (95% CI, -21.4 to -6.2), were associated with IQR increases in PM2.5, BC and CO at prior 5 MA days. Additionally, positive associations of adiponectin and leptin with air pollutants were found at prior 1 to 7 MA days, but inverse associations were found for resistin (Figure 2 and Figure 3). For growth factors, significant reductions of CNTF levels were associated with air pollutants, with estimate effects ranging from 4.9% (95% CI, -9.3 to -0.5) to 22.2% (95% CI, -39.2 to -5.1; Figure 3). Concomitantly, elevations of BTC and HGF levels were found for exposure to PM2.5, CO, and NOX (Figure 3).
Association Between Air Pollution and Immune Inflammation
In line with the hypothesized mechanism that air pollution exposure may activate immune-inflammatory responses, we found significant changes in a suite of biomarkers indicative of systemic immune and inflammation in relation to air pollutants (Figure 4 and 5). As shown in Figure 4, significant increases in T cell-related cytokines (IL-2, 10, 22) of 7.6% (95% CI, 0.8-14.5) to 42.5% (95% CI, 7.2-77.8) and IL-1 family mediator (sIL1RA) of 13.3% (95% CI, 1.9-24.6) to 32.0% (95% CI, 14.3-49.7), were observed in association with IQR increases in PM2.5, CO, and NOX at prior 1 to 7 MA days. For chemokines, significant increases in CCL-2 were related to PM2.5, PNC100-560, PSC100-560, BC, CO, NO2, and NOX, with the stronger effects observed at prior 5 to 7 MA days of exposure (Figure 5). Similar association patterns and the magnitude of effect estimates were also observed for CCL-5 (Figure 5). As expected, significant elevations of 7.8% (95% CI, 0.9-14.8) to 20.4% (95% CI, 10.3-30.5) in specific macrophage activation marker sCD163 were observed in association with IQR increases in exposure to air pollutants at prior 1 to 7 days. In addition, greater magnitude of elevated CXCL-8 levels was found ranging from 12.4% (95% CI, 0.3-24.6) to 31.5% (95% CI, 12.1-50.9, Figure 5).
In mediation analyses, single-mediator models showed that dysregulated bone-vascular axis (e.g., heightened the biological activity of OPG-RANK-RANKL system) could mediate up to 66% of the effects of selected pollutants (e.g., PM2.5 and CO) on insulin resistance (Additional Table 1). Further, air pollutants-associated activations of immune inflammation could also be mediated via dysregulation of the bone-vascular axis, and elevated products of immune-inflammatory mediators (e.g., CCL-2) might further increase in circulating levels of regulators in the bone-vascular axis such as OPG (Additional Table 2 and 3). Results obtained from multiple-mediator models were in line with those observed in single-mediator models (Additional Table 4). As shown in Additional Figure 1, the main findings and the study conclusions remained unchanged when sensitivity analyses were performed, including repeated analyses with a subject-normalization approach, excluding individuals with urinary cotinine levels >200 ng/mg Cr, and excluding individuals who lived beyond 1 kilometer from the air monitoring station.