Heavy metal mixtures exposure and breast cancer risk: synergistic effects with SHBG identified by mediation and moderation models.

BACKGROUND: Environmental heavy metal exposure and endogenous sex hormones are implicated in breast cancer (BC) etiology, but their combined effects remain incompletely characterized. This study aimed to investigate the associations between blood heavy metals and BC risk, while examining the potential mediating role of sex hormones and their interaction effects. METHODS: We analyzed data from 2,939 female participants in the National Health and Nutrition Examination Survey (NHANES) 2013-2020. Multivariable logistic regression, weighted quantile sum (WQS) regression, quantile g-computation (Qgcomp), and Bayesian kernel machine regression (BKMR) models were employed to examine the independent and combined associations of 5 blood metals-lead (Pb), cadmium (Cd), mercury (Hg), selenium (Se), and manganese (Mn)-with BC risk. Nonlinear relationships were evaluated using restricted cubic splines (RCS). Mediation and moderation analyses were conducted to elucidate the mechanistic role of sex hormones in heavy metal-associated BC risk. RESULTS: In the single-exposure models, the fully adjusted analysis results revealed significant positive associations for blood Pb (OR = 2.71, 95% CI = 2.22-3.30) and Hg (OR = 1.34, 95% CI = 1.12-1.61), a negative association for Mn (OR = 0.83, 95% CI = 0.70-0.99), while non-significant associations for Cd (OR = 1.17, 95% CI = 0.98-1.39) and Se (OR = 1.07, 95% CI = 0.90-1.27). RCS analyses revealed linear dose-response relationships for Pb and Hg (P non-linear > 0.05), but significant nonlinear relationships for Cd and Mn (P over < 0.001, P non-linear < 0.05). In mixture analyses, multi-metal exposure significantly increased BC risk (WQS: OR = 1.43; Qgcomp: OR = 1.28; both P < 0.001), with BKMR models confirming consistent associations. Pb emerged as the primary contributor across all mixture models. Both single and mixture metal exposures showed significant positive associations with sex hormone-binding globulin (SHBG). Each unit increase in SHBG elevated BC risk by 1.65-fold (P < 0.001), demonstrating a linear dose-response. Mediation analysis indicated significant masking effects for SHBG in the metal-BC risk pathway, and moderation analysis further revealed that the interaction between blood heavy metals and SHBG synergistically amplified BC risk. CONCLUSION: Heavy metal exposure (particularly Pb) is associated with increased BC risk, potentially through SHBG upregulation and synergistic interactions, while Mn examines inverse associations. These findings highlight a potential link between environmental metals and hormone-related BC risk.