Polychlorinated Biphenyls (PCBS)-induced oxidative stress and inflammation in human thyrocytes: involvement of AhR and NRF-2/HO-1 pathway.

PURPOSE: In this in vitro study, we investigated the effects of polychlorinated biphenyls (PCBs) on human thyrocytes, with a focus on the involvement of AhR, a key player in xenobiotic response, and the anti-oxidant Nrf-2/HO-1 pathway. METHODS: Primary cultured thyrocytes were exposed to the dioxin-like congeners PCB118 and PCB126 at 2.5 and 5 µM concentrations. mRNA expression was assessed by real-time PCR, and protein expression by Western Blot and ELISA, while protein quantification was assessed by densitometric analysis. RESULTS: In cultured thyrocytes, PCB118 and PCB126 induced a significant (P < 0.01) increase of mRNA and protein levels of the pro-inflammatory cytokines IL-1beta and IL-6, while reducing those of thyroglobulin (TG) and NIS (p < 0.05), indicating down-regulation of these thyroid-specific genes in PCB-induced inflammation. ROS production also increased (p < 0.001). mRNA levels of AhR and the downstream molecules cytochrome P4501A, Nrf-2/HO-1 increased (p < 0.001), as well as related protein levels (p < 0.01), suggesting the activation of AhR and Nrf-2 pathways in response to PCBs exposure. AhR silencing decreased AhR-related gene expression and restored NIS and TG expression, while reducing inflammatory cytokines and oxidative stress markers (p < 0.05). CONCLUSIONS: Dioxin-like PCBs (PCB118 and PCB126) may promote inflammation and oxidative stress in thyrocytes, impairing the expression of genes that are key players of thyroid function. These effects can be partially attributed to the activation of the AhR and Nrf-2 pathways. These data may contribute to explain the mechanisms underlying thyroid toxicity of PCBs, highlighting the potential role of these pollutants as a trigger of autoimmune thyroid inflammation and damage.

Antioxidant Defense Capacity Is Reduced in Thyroid Stem/Precursor Cells Compared to Differentiated Thyrocytes.

There is much evidence linking oxidative stress to thyroid cancer, and stem cells are thought to play a key role in the tumor-initiating mechanism. Their vulnerability to oxidative stress is unexplored. This study aimed to comparatively evaluate the antioxidant capacity of stem/precursor thyroid cells and mature thyrocytes. Human stem/precursor cells and mature thyrocytes were exposed to increasing concentrations of menadione, an oxidative-stress-producing agent, and reactive oxygen species (ROS) production and cell viability were measured. The expression of antioxidant and detoxification genes was measured via qPCR as well as the total antioxidant capacity and the content of glutathione. Menadione elevated ROS generation in stem/precursor thyroid cells more than in mature thyrocytes. The ROS increase was inversely correlated (p = 0.005) with cell viability, an effect that was partially prevented by the antioxidant curcumin. Most thyroid antioxidant defense genes, notably those encoding for the glutathione-generating system and phase I detoxification enzymes, were significantly less expressed in stem/precursor thyroid cells. As a result, the glutathione level and the total antioxidant capacity in stem/precursor thyroid cells were significantly decreased. This reduced antioxidant defense may have clinical implications, making stem/precursor thyroid cells critical targets for environmental conditions that are not detrimental for differentiated thyrocytes.

Heavy Metals in the Environment and Thyroid Cancer.

In recent decades, the incidence of thyroid cancer has increased more than most other cancers, paralleling the generalized worldwide increase in metal pollution. This review provides an overview of the evidence supporting a possible causative link between the increase in heavy metals in the environment and thyroid cancer. The major novelty is that human thyroid stem/progenitor cells (thyrospheres) chronically exposed to different metals at slightly increased environmentally relevant concentrations show a biphasic increase in proliferation typical of hormesis. The molecular mechanisms include, for all metals investigated, the activation of the extracellular signal-regulated kinase (ERK1/2) pathway. A metal mixture, at the same concentration of individual metals, was more effective. Under the same conditions, mature thyrocytes were unaffected. Preliminary data with tungsten indicate that, after chronic exposure, additional abnormalities may occur and persist in thyrocytes derived from exposed thyrospheres, leading to a progeny population of transformation-prone thyroid cells. In a rat model predisposed to develop thyroid cancer, long-term exposure to low levels of metals accelerated and worsened histological signs of malignancy in the thyroid. These studies provide new insight on metal toxicity and carcinogenicity occurring in thyroid cells at a low stage of differentiation when chronically exposed to metal concentrations that are slightly increased, albeit still in the "normal" range.

Thyroid Stem Cells But Not Differentiated Thyrocytes Are Sensitive to Slightly Increased Concentrations of Heavy Metals.

Thyroid cancer incidence is markedly increased in volcanic areas where residents are biocontaminated by chronic lifelong exposure to slightly increased metals in the environment. Metals can influence the biology of living cells by a variety of mechanisms, depending not only on the dose and length of exposure but also on the type and stage of differentiation of target cells. We explored the effect of five heavy metals (Cu, Hg, Pd, W and Zn) at nanomolar concentrations (the biocontamination level in residents of the volcanic area in Sicily where thyroid cancer is increased) on stimulating the proliferation of undifferentiated (thyrospheres) and differentiated human thyroid cells. Thyrosphere proliferation was significantly increased after exposure to each individual metal and a greater stimulating effect was observed when a mixture of the examined metals was used. No effect was seen in differentiated thyrocytes. For all metals, the dose-response curve followed a biphasic pattern that is typical of hormesis. Thyrosphere growth concerned the size rather than number, except with the metal mixture. An altered morphology was also observed in metal-treated thyrospheres. Metal-induced proliferation was due to activation of the ERK1/2 pathway, as confirmed by growth inhibition when ERK1/2 signaling was blocked. These studies show that stem/precursor thyroid cells are sensitive to small increases in environmental metal concentrations that are harmless for differentiated thyrocytes.