The strongest finding in the present study is to identify of sex-specific metals related to lipid changes among the participants from MEHWC. The consistent findings from sPLS and BKMR analysis indicated a significantly positive association of manganese with elevated TC and LDL-C among overall participants and men. Furthermore, manganese was associated with increased risk of incident hyper-cholesterolemia and hyper-LDL cholesterolemia (odds ratio: 1.849 vs. 2.354). The BKMR model showed manganese dominated the positive cumulative effect of ten metal mixtures on elevated TC and LDL-C, though the effects including the null. Interestingly, we found a significantly negative association of copper with elevated TC and LDL-C among women.
In addition to sex-difference in metal level, previous researches have also revealed sex differences in lipid levels or lipid patterns. Higher risk of cardiovascular disease was observed in men compared with women, even if with comparable serum lipid concentrations (Johnson et al. 2004). It was reported higher TC and HDL-C were observed in women from the Multi-Ethnic Study of Atherosclerosis study (Goff et al. 2006). Moreover, higher large HDL-C, large HDL-C to total HDL-C ratio and less small HDL-C were found in women in comparison to men (Johnson et al. 2004). Whilst men have a higher fraction of small dense LDL-C and larger VLDL particles, and small dense LDL-C has been implicated as a major cardiovascular disease risk factor. Based on the sex-difference in the metals and lipids, we performed the stratified analysis by sex. Interestingly, we found some associations between metal exposure and change in lipid profile with sex-specific heterogeneity.
By selecting the predictive metals associated with lipid changes, our study may provide additional clinical value to identify individuals with elevated lipid profile. The use of sPLS model helped to identify the associations between metals and lipid changes, which might be covered up by high correlation or data dimension in traditional statistical models, such as copper and rubidium for LDL-C in women and cobalt for TC in overall participants. Furthermore, the importance of each metal on the lipid change was quantified accounting for inter-metal interactions. As showed in PIP analysis, manganese might play the most important role in association of metal mixture exposures with change in TC in men, given the highest PIP (0.559) among metals. It is interesting given that the association of manganese with lipid change was independent, suggesting the complexity in relationship may not be noticed in conventional analysis strategy. However, manganese serves as both essential metals and neurotoxins depending on doses (Li &Yang 2018). The variation in the shape of relationship may weaken the overall association with change of lipid profile. Moreover, MEWHC was performed among occupational workers in China, the harmful effects of heavy metals might be less profound among general population.
Moreover, our study assessed the potential sexual heterogeneity in the association of manganese with lipid changes. For instance, positive association was found between manganese and change in lipid in men but not women. Oxidative stress leading to lipid peroxidation is a well-known mechanism for manganese toxicity, and manganese imbalance may promote more reactive oxygen species (ROS) producing, leading to oxidative stress, inflammation and endothelial dysfunction (Bornhorst et al. 2013). An animal experiment found that manganese enhanced cholesterol biosynthesis in the rats’ liver microsome, and stimulated farnesyl pyrophosphate synthase activity, which was an important synthesis pathway for regulating cholesterol biosynthesis and metabolism (Bell &Hurley 1973). Differences in response to hormones between men and women might account for the association of manganese with lipids changes. Manganese was positively associated with sex hormone binding globulin (SHBG) (Rotter et al. 2016). Previous researchers indicated higher SHBG level showed significant relation to an elevated risk of metabolism syndrome among men (Bhasin et al. 2011, Haring et al. 2013). The interactions in manganese and SOD, SHBG and other sex hormones may involve in this. Sex difference in smoking habit may be a potential explanation. Higher serum manganese levels were observed in smokers in comparison to non-smokers (Ates Alkan et al. 2019). In this study, current smokers accounted for a higher proportion in men, which may provide an interpretation for association with sex specific. The presence of sex-specific associations and the physiological mechanisms behind warrants further investigation.
Interestingly, a negative association between copper and change in TC (beta = -0.359) and LDL-C (beta = -0.267) was observed only in women. In line with the previous research, higher copper concentration were observed in women in comparison to men (Helgeland et al. 1982). Differences in diet and copper absorption between men and women as well as the effects of estrogen on copper metabolism (Songchitsomboon &Komindr 1996) might provide an explanation. Furthermore, evidence emerged from cross-sectional epidemiological study and showed serum copper of subjects with normal levels of low-density lipoprotein cholesterol was significantly lower in comparison to those who having high levels of low-density lipoprotein cholesterol in women (Ghayour-Mobarhan et al. 2009). In addition, copper ions showed activity to activate cholesterogenic genes in macrophages, underlying the potential mechanism of atherosclerosis related with copper (Svensson et al. 2003). Furthermore, a recent cross-sectional study of our team observed copper in serum was inversely associated with estradiol (Zan et al. 2021). Estrogen-related receptor alpha (ERRα) acts downstream of substantial sex differences in lipid metabolism, and endogenous estrogen plays a estrogen/ERα signaling in contributing to the sex-difference in hepatic VLDL secretion affecting hepatic lipid homeostasis. Moreover, hepatocyte-specific ERα-knock-out mice has lost the ability of estrogen to diminish liver fatty degeneration, which indicated hepatic estrogen directly decrease lipid accumulation via ERα. Lack of hepatocyte ERα leads to lack of estrogen regulation of target genes, increased expression of lipid synthesis genes, and impaired estrogen-regulation of other lipid metabolic target genes.
The study is the first to evaluate the associations of metal mixture exposures with lipid change by a prospective longitudinal study. In addition, we used different methods (sPLS and BKMR models) to remedy the gaps in traditional approaches, and we observed consistent findings. However, there were several limitations in our study. First, the sample size was relatively small, limiting the power for interpretation of results, particularly in women. Second, baseline levels of metals might not completely represent the exposure level in a long term. However, metals levels could be considered to be relatively stable because the occupation type of the participants were fixed; Moreover, metals in blood cells showed fair reproducibility (ICC > 0.4). Last but not the least, this study was based on an occupational population who were exposed to relatively high level of metals, and the findings need to be further confirmed in the general population.