Non-Essential Trace Elements Dietary Exposure in French Polynesia: Intake Assessment, Nail Bio Monitoring and Thyroid Cancer Risk

Understanding heavy metal toxicity: Implications on human health, marine ecosystems and bioremediation strategies

Heavy metals are constituents of the natural environment and are of great importance to both natural and artificial processes. But in recent times the indiscriminate use of heavy metals especially for human purposes has caused an imbalance in natural geochemical cycles. This imbalance has caused contamination of heavy metals into natural resources and such as soil and a marine ecosystem. Long exposure and higher accumulation of given heavy metals are known to impose detrimental and even lethal effects on humans. Conventional remediation techniques of heavy metals provide good results but have negative side effects on surrounding environment. The role played by microbes in bioremediation of heavy metals is well reported in the literature and understanding the role of molecules in the process of metal accumulation its reduction and transformation into less hazardous state, has myriads of biotechnological implications for bioremediation of metal-contaminated sites. The current review presents the implications of heavy metals on human health and marine ecosystems, conventional methods of heavy metal removal and their side effects on the environment. Bioremediation approaches have been discussed as well in this review, proving to be a more sustainable and eco-friendly approach towards remediation of heavy metals.

Elucidating the link between thyroid cancer and mercury exposure: a review and meta-analysis

Mercury (Hg) is a widely distributed and bioavailable metal of public health concern, with many known human toxicities, but data regarding mercury's influence on thyroid cancer (TC) is scarce. Mercury is known to impact several molecular pathways implicated in carcinogenesis, and its proclivity for bioaccumulation in the thyroid suggests a potential modulatory effect. We conducted a literature/systematic review of studies between 1995-2022 intending to define better and establish relationships between these two entities, congregate the evidence for mercury's potential role in thyroid carcinogenesis, and identify populations of interest for further study. Insufficient evidence precludes definitive conclusions on dietary mercury as a TC risk factor; however, several common mechanisms affected by mercury are crucial for TC development, including biochemical, endocrine, and reactive oxygen species effects. Quantitative analysis revealed associations between TC risk and mercury exposure. In three mercury studies, average urine levels were higher in TC patients, with a mean difference of 1.86 µg/g creatinine (95% CI = 0.32-3.41). In two studies investigating exposure to elevated mercury levels, the exposed group exhibited a higher risk of developing TC, with a relative risk of 1.90 (95% CI = 1.76-2.06). In three thyroid tissue studies, mercury levels (ppm) were higher in TC patients, averaging 0.14 (0.06-0.22) in cancerous cases (N = 178) and 0.08 (0.04-0.11) in normal thyroids (N = 257). Our findings suggest an association between mercury exposure and TC risk, implying a possible predisposing factor. Further research is necessary to reveal the clinical relevance of dietary and environmental mercury exposures in TC pathogenesis.

Alterations of Bax/Bcl-2 ratio contribute to NaAsO2 induced thyrotoxicity in human thyroid follicular epithelial cells and SD rats

The environmental toxicant arsenic causes various human diseases and threatens millions of people worldwide. Recently, a limited number of studies have revealed that exposure to arsenic is associated with thyroid dysfunction, indicating its toxicological impact on the thyroid gland, however, its precise forms of damage and underlying mechanisms remain largely unknown. Here, we sought to observe the thyrotoxicity of sodium arsenite (NaAsO2) on human thyroid follicular epithelial cells (Nthy-ori 3-1) and SD rats, and explore the role of Bax/Bcl-2 ratio in the above process. Our results displayed that NaAsO2 exerted a dose-dependent inhibitory effect on the viability of Nthy-ori 3-1 cells. Alongside the increase doses of NaAsO2 exposure, morphological changes and elevated LDH levels were observed. Furthermore, apoptosis rates increased in a dose- and time-dependent manner, accompanied by a decrease in Bcl-2 and an opposite change in Bax expression. SD rats were treated with 0, 2.5, 5, and 10 mg/kg NaAsO2 for 36 weeks. Our findings revealed that NaAsO2 exposure resulted in arsenic accumulation in thyroid tissue, elevated ratio of Bax/Bcl-2, and histopathological changes of thyroid in rats, which accompanied by the decreased serum T3 and T4 levels and the increased serum TSH level. Furthermore, T3 and T4 levels were negatively correlated with Bax expression, whereas positively correlated with Bcl-2 expression. Collectively, our results suggest that NaAsO2 exposure induces cytotoxicity in Nthy-ori 3-1 cells, causes structural damages and dysfunction of thyroid in SD rats, in which the imbalance of Bax/Bcl-2 ratio may play a significant role.

Environmental arsenic exposure and its toxicological effect on thyroid function: a systematic review

OBJECTIVES

This study was performed to review epidemiological evidence related to Arsenic (As) effects on the thyroid function by focusing on the serum thyroid hormone concentration.

CONTENT

As, one of the main pollutants, has been recognized as an endocrine-disrupting agent that may affect the function of thyroid as shown by experimental studies.

SUMMARY

This systematic study indicates the association between As exposure and thyroid dysfunction. The studies have shown an association between serum and urine concentration of arsenic and thyroid dysfunction. Most of them reported the association between increase in the serum or urine As levels and decrease in the triiodothyronine (T3) and thyroxine (T4), and also elevation in the thyrotropic hormone (TSH) levels.

OUTLOOK

Our findings related to the effects of As on the function of thyroid in humans are still limited and future studies should be done to address this question.

Increased Thyroid Cancer Incidence in Volcanic Areas: A Role of Increased Heavy Metals in the Environment?

Thyroid cancer incidence is significantly increased in volcanic areas, where relevant non-anthropogenic pollution with heavy metals is present in the environment. This review will discuss whether chronic lifelong exposure to slightly increased levels of metals can contribute to the increase in thyroid cancer in the residents of a volcanic area. The influence of metals on living cells depends on the physicochemical properties of the metals and their interaction with the target cell metallostasis network, which includes transporters, intracellular binding proteins, and metal-responsive elements. Very little is known about the carcinogenic potential of slightly increased metal levels on the thyroid, which might be more sensitive to mutagenic damage because of its unique biology related to iodine, which is a very reactive and strongly oxidizing agent. Different mechanisms could explain the specific carcinogenic effect of borderline/high environmental levels of metals on the thyroid, including (a) hormesis, the nonlinear response to chemicals causing important biological effects at low concentrations; (b) metal accumulation in the thyroid relative to other tissues; and (c) the specific effects of a mixture of different metals. Recent evidence related to all of these mechanisms is now available, and the data are compatible with a cause–effect relationship between increased metal levels in the environment and an increase in thyroid cancer incidence.

Low Doses of Methylmercury Induce the Proliferation of Thyroid Cells In Vitro Through Modulation of ERK Pathway

Exposure to environmental endocrine disruptors has been associated with an increased frequency of thyroid pathology. In this study, we evaluated the effects of various concentrations of methylmercury (MeHg) on immortalized, non-tumorigenic thyroid cells (Nthy-ori-3-1). Exposure to MeHg at 2.5 and 5 µM for 24 h caused a reduction in cell viability with a decrease of the cell population in sub-G0 phase, as detected by MTT and flow cytometry. Conversely, MeHg at the lower concentration of 0.1 µM increased the cell viability with a rise of G2/M phase. An immunoblot analysis showed higher expression levels of phospho-ERK and not of phospho-Akt. Further enhancement of the cell growth rate was observed after a prolonged exposure of the cells up to 18 days to MeHg 0.1 µM. The present findings demonstrate the toxicity of high concentrations of MeHg on thyroid cells, while showing that treatment with lower doses of Hg, as may occur after prolonged exposure to this environmental contaminant, exerts a promoting effect on thyroid cell proliferation, by acting on the ERK-mediated pro-oncogenic signal transduction pathway.