Biological effects of inhaled hydraulic fracturing sand dust. II. Particle characterization and pulmonary effects 30 d following intratracheal instillation

High-fat Western diet alters crystalline silica-induced airway epithelium ion transport but not airway smooth muscle reactivity

OBJECTIVES

Silicosis is an irreversible occupational lung disease resulting from crystalline silica inhalation. Previously, we discovered that Western diet (HFWD)-consumption increases susceptibility to silica-induced pulmonary inflammation and fibrosis. This study investigated the potential of HFWD to alter silica-induced effects on airway epithelial ion transport and smooth muscle reactivity.

METHODS

Six-week-old male F344 rats were fed a HFWD or standard rat chow (STD) and exposed to silica (Min-U-Sil 5®, 15 mg/m3, 6 h/day, 5 days/week, for 39 d) or filtered air. Experimental endpoints were measured at 0, 4, and 8 weeks post-exposure. Transepithelial potential difference (Vt), short-circuit current (ISC) and transepithelial resistance (Rt) were measured in tracheal segments and ion transport inhibitors [amiloride, Na+ channel blocker; NPPB; Cl- channel blocker; ouabain, Na+, K+-pump blocker] identified changes in ion transport pathways. Changes in airway smooth muscle reactivity to methacholine (MCh) were investigated in the isolated perfused trachea preparation.

RESULTS

Silica reduced basal ISC at 4 weeks and HFWD reduced the ISC response to amiloride at 0 week compared to air control. HFWD + silica exposure induced changes in ion transport 0 and 4 weeks after treatment compared to silica or HFWD treatments alone. No effects on airway smooth muscle reactivity to MCh were observed.

Airway contraction and cytokine release in isolated rat lungs induced by wear particles from the road and tire interface and road vehicle brakes

The dominant road traffic particle sources are wear particles from the road and tire interface, and from vehicle brake pads. The aim of this work was to investigate the effect of road and brake wear particles on pulmonary function and biomarkers in isolated perfused rat lungs. Particles were sampled from the studded tire wear of three road pavements containing different rock materials in a road simulator; and from the wear of two brake pad materials using a pin-on-disk machine. Isolated rat lungs inhaled the coarse and fine fractions of the sampled particles resulting in an estimated total particle lung dose of 50 μg. The tidal volume (TV) was measured during the particle exposure and the following 50 min. Perfusate and BALF were analyzed for the cytokines TNF, CXCL1 and CCL3. The TV of lungs exposed to rock materials was significantly reduced after 25 min of exposure compared to the controls, for quartzite already after 4 min. The particles of the heavy-duty brake pads had no effect on the TV. Brake particles resulted in a significant elevation of CXCL1 in the perfusate. Brake particles showed significant elevations of all three measured cytokines, and quartzite showed a significant elevation of TNF in BALF. The study shows that the toxic effect on lungs exposed to airborne particles can be investigated using measurements of tidal volume. Furthermore, the study shows that the choice of rock material in road pavements has the potential to affect the toxicity of road wear PM10.

High-fat western diet-consumption alters crystalline silica-induced serum adipokines, inflammatory cytokines and arterial blood flow in the F344 rat

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Silica reduced serum leptin and adiponectin, no effects on body or fat pad weight.

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HFWD-consumption altered pro-inflammatory cytokines in silica-exposed animals.

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Silica altered pulse frequency; HFWD increased mean blood flow; effects additive.

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HFWD affected silica-induced metabolic effects.

Adipose tissue (AT) plays a central role in the maintenance of whole-body energy homeostasis through release of adipokines. High-fat Western diet (HFWD)-consumption contributes to obesity, disruption of adipocyte metabolism, chronic systemic inflammation, and metabolic dysfunction (MetDys). MetDys is associated with impaired lung function, pulmonary hypertension, and asthma. Thirty-five percent of adults in the U.S. have MetDys, yet the impact of MetDys on susceptibility to occupational hazards is unknown. The aim of this study was to determine the potential of HFWD-consumption to alter inhaled crystalline silica dust-induced metabolic responses. Six-wk old male F344 rats were fed a HFWD (45 kcal % fat, sucrose 22.2 % by weight) or standard rat chow (STD, controls), and exposed to silica-inhalation (6 h/d, 5 d/wk, 39 d; Min-U-Sil 5®, 15 mg/m³) or filtered air. Indices of MetDys and systemic inflammation were measured at 0, 4, and 8 wk following cessation of silica exposure. At 8 wk post-exposure, silica reduced serum leptin and adiponectin levels, and increased arterial pulse frequency. HFWD-consumption induced weight gain, altered adipokines, liver, kidney, and pancreatic function, and increased tail artery blood flow. At 8 wk in HFWD + SIL-treated animals, the levels of serum pro-inflammatory cytokines (IFN-γ, CXCL-1, TNF-α, IL-1β, IL-4, IL-5, IL-6, IL-10 and IL-13) were increased compared to STD + SIL but were less than HFWD + AIR-induced levels. In conclusion, consumption of a HFWD altered silica-induced metabolic responses and silica exposure disrupted AT endocrine function. These findings demonstrate previously unknown interactions between HFWD-consumption and occupational silica exposure.

Biological effects of inhaled hydraulic fracturing sand dust. IX. Summary and significance

An investigation into the potential toxicological effects of fracking sand dust (FSD), collected from unconventional gas drilling sites, has been undertaken, along with characterization of their chemical and biophysical properties. Using intratracheal instillation of nine FSDs in rats and a whole body 4-d inhalation model for one of the FSDs, i.e., FSD 8, and related in vivo and in vitro experiments, the effects of nine FSDs on the respiratory, cardiovascular and immune systems, brain and kidney were reported in the preceding eight tandem papers. Here, a summary is given of the key observations made in the organ systems reported in the individual studies. The major finding that inhaled FSD 8 elicits responses in extra-pulmonary organ systems is unexpected, as is the observation that the pulmonary effects of inhaled FSD 8 are attenuated relative to forms of crystalline silica more frequently used in animal studies, i.e., MIN-U-SIL® 5. An attempt is made to understand the basis for the extra-pulmonary toxicity and comparatively attenuated pulmonary toxicity of FSD 8.

Biological effects of inhaled hydraulic fracturing sand dust. IV. Pulmonary effects

Hydraulic fracturing creates fissures in subterranean rock to increase the flow and retrieval of natural gas. Sand ("proppant") in fracking fluid injected into the well bore maintains fissure patency. Fracking sand dust (FSD) is generated during manipulation of sand to prepare the fracking fluid. Containing respirable crystalline silica, FSD could pose hazards similar to those found in work sites where silica inhalation induces lung disease such as silicosis. This study was performed to evaluate the possible toxic effects following inhalation of a FSD (FSD 8) in the lung and airways. Rats were exposed (6 h/d × 4 d) to 10 or 30 mg/m3 of a FSD collected at a gas well, and measurements were performed 1, 7, 27 and, in one series of experiments, 90 d post-exposure. The following ventilatory and non-ventilatory parameters were measured in vivo and/or in vitro: 1) lung mechanics (respiratory system resistance and elastance, tissue damping, tissue elastance, Newtonian resistance and hysteresivity); 2) airway reactivity to inhaled methacholine (MCh); airway epithelium integrity (isolated, perfused trachea); airway efferent motor nerve activity (electric field stimulation in vitro); airway smooth muscle contractility; ion transport in intact and cultured epithelium; airway effector and sensory nerves; tracheal particle deposition; and neurogenic inflammation/vascular permeability. FSD 8 was without large effect on most parameters, and was not pro-inflammatory, as judged histologically and in cultured epithelial cells, but increased reactivity to inhaled MCh at some post-exposure time points and affected Na+ transport in airway epithelial cells.

Biological effects of inhaled hydraulic fracturing sand dust. I. Scope of the investigation

Hydraulic fracturing ("fracking") is a process in which subterranean natural gas-laden rock is fractured under pressure to enhance retrieval of gas. Sand (a "proppant") is present in the fracking fluid pumped down the well bore to stabilize the fissures and facilitate gas flow. The manipulation of sand at the well site creates respirable dust (fracking sand dust, FSD) to which workers are exposed. Because workplace exposures to FSD have exceeded exposure limits set by OSHA, a physico-chemical characterization of FSD along with comprehensive investigations of the potential early adverse effects of FSDs on organ function and biomarkers has been conducted using a rat model and related in vivo and in vitro experiments involving the respiratory, cardiovascular, immune systems, kidney and brain. An undercurrent theme of the overall hazard identification study was, to what degree do the health effects of inhaled FSD resemble those previously observed after crystalline silica dust inhalation? In short-term studies, FSD was found to be less bioactive than MIN-U-SIL® 5 in the lungs. A second theme was, are the biological effects of FSD restricted to the lungs? Bioactivity of FSD was observed in all examined organ systems. Our findings indicate that, in many respects, the physical and chemical properties, and the short-term biological effects, of the FSDs share many similarities as a group but have little in common with crystalline silica dust.

Biological effects of inhaled hydraulic fracturing sand dust. III. Cytotoxicity and pro-inflammatory responses in cultured murine macrophage cells

Cultured murine macrophages (RAW 264.7) were used to investigate the effects of fracking sand dust (FSD) for its pro-inflammatory activity, in order to gain insight into the potential toxicity to workers associated with inhalation of FSD during hydraulic fracturing. While the role of respirable crystalline silica in the development of silicosis is well documented, nothing is known about the toxicity of inhaled FSD. The FSD (FSD 8) used in these studies was from an unconventional gas well drilling site. FSD 8was prepared as a 10 mg/ml stock solution in sterile PBS, vortexed for 15 s, and allowed to sit at room temperature for 30 min before applying the suspension to RAW 264.7cells. Compared to PBS controls, cellular viability was significantly decreased after a 24 h exposure to FSD. Intracellular reactive oxygen species (ROS) production and the production of IL-6, TNFα, and endothelin-1 (ET-1) were up-regulated as a result of the exposure, whereas the hydroxyl radical (.OH) was only detected in an acellular system. Immunofluorescent staining of cells against TNFα revealed that FSD 8 caused cellular blebbing, and engulfment of FSD 8 by macrophages was observed with enhanced dark-field microscopy. The observed changes in cellular viability, cellular morphology, free radical generation and cytokine production all confirm that FSD 8 is cytotoxic to RAW 264.7 cells and warrants future studies into the specific pathways and mechanisms by which these toxicities occur.

Biological effects of inhaled hydraulic fracturing sand dust. VIII. Immunotoxicity

Hydraulic fracturing ("fracking") is a process used to enhance retrieval of gas from subterranean natural gas-laden rock by fracturing it under pressure. Sand used to stabilize fissures and facilitate gas flow creates a potential occupational hazard from respirable fracking sand dust (FSD). As studies of the immunotoxicity of FSD are lacking, the effects of whole-body inhalation (6 h/d for 4 d) of a FSD, i.e., FSD 8, was investigated at 1, 7, and 27 d post-exposure in rats. Exposure to 10 mg/m3 FSD 8 resulted in decreased lung-associated lymph node (LLN) cellularity, total B-cells, CD4+ T-cells, CD8+ T-cells and total natural killer (NK) cells at 7-d post exposure. The frequency of CD4+ T-cells decreased while the frequency of B-cells increased (7 and 27 d) in the LLN. In contrast, increases in LLN cellularity and increases in total CD4+ and CD8+ T-cells were observed in rats following 30 mg/m3 FSD 8 at 1 d post-exposure. Increases in the frequency and number of CD4+ T-cells and NK cells were observed in bronchial alveolar lavage fluid at 7-d post-exposure (10 mg/m3) along with an increase in total CD4+ T-cells, CD11b + cells, and NK cells at 1-day post-exposure (30 mg/m3). Increases in the numbers of B-cells and CD8+ T-cells were observed in the spleen at 1-day post 30 mg/m3 FSD 8 exposure. In addition, NK cell activity was suppressed at 1 d (30 mg/m3) and 27 d post-exposure (10 mg/m3). No change in the IgM response to sheep red blood cells was observed. The findings indicate that FSD 8 caused alterations in cellularity, phenotypic subsets, and impairment of immune function.

Biological effects of inhaled hydraulic fracturing sand dust. V. Pulmonary inflammatory, cytotoxic and oxidant effects

The pulmonary inflammatory response to inhalation exposure to a fracking sand dust (FSD 8) was investigated in a rat model. Adult male Sprague-Dawley rats were exposed by whole-body inhalation to air or an aerosol of a FSD, i.e., FSD 8, at concentrations of 10 or 30 mg/m3, 6 h/d for 4 d. The control and FSD 8-exposed rats were euthanized at post-exposure time intervals of 1, 7 or 27 d and pulmonary inflammatory, cytotoxic and oxidant responses were determined. Deposition of FSD 8 particles was detected in the lungs of all the FSD 8-exposed rats. Analysis of bronchoalveolar lavage parameters of toxicity, oxidant generation, and inflammation did not reveal any significant persistent pulmonary toxicity in the FSD 8-exposed rats. Similarly, the lung histology of the FSD 8-exposed rats showed only minimal changes in influx of macrophages following the exposure. Determination of global gene expression profiles detected statistically significant differential expressions of only six and five genes in the 10 mg/m3, 1-d post-exposure, and the 30 mg/m3, 7-d post-exposure FSD 8 groups, respectively. Taken together, data obtained from the present study demonstrated that FSD 8 inhalation exposure resulted in no statistically significant toxicity or gene expression changes in the lungs of the rats. In the absence of any information about its potential toxicity, a comprehensive rat animal model study (see Fedan, J.S., Toxicol Appl Pharmacol. 000, 000-000, 2020) has been designed to investigate the bioactivities of several FSDs in comparison to MIN-U-SIL® 5, a respirable α-quartz reference dust used in previous animal models of silicosis, in several organ systems.

Biological effects of inhaled hydraulic fracturing sand dust. VI. Cardiovascular effects

Hydraulic fracturing is used to access oil and natural gas reserves. This process involves the high-pressure injection of fluid to fracture shale. Fracking fluid contains approximately 95% water, chemicals and 4.5% fracking sand. Workers may be exposed to fracking sand dust (FSD) during the manipulation of the sand, and negative health consequences could occur if FSD is inhaled. In the absence of any information about its potential toxicity, a comprehensive rat animal model study (see Fedan et al., 2020) was designed to investigate the bioactivities of several FSDs in comparison to MIN-U-SIL® 5, a respirable α-quartz reference dust used in previous animal models of silicosis, in several organ systems. The goal of this study was to assess the effects of inhalation of one FSD, i.e., FSD 8, on factors and tissues that affect cardiovascular function. Male rats were exposed to 10 or 30 mg/m³ FSD (6 h/d for 4 d) by whole body inhalation, with measurements made 1, 7 or 27 d post-exposure. One day following exposure to 10 mg/m³ FSD the sensitivity to phenylephrine-induced vasoconstriction in tail arteries in vitro was increased. FSD exposure at both doses resulted in decreases in heart rate (HR), HR variability, and blood pressure in vivo. FSD induced changes in hydrogen peroxide concentrations and transcript levels for pro-inflammatory factors in heart tissues. In kidney, expression of proteins indicative of injury and remodeling was reduced after FSD exposure. When analyzed using regression analysis, changes in proteins involved in repair and remodeling were correlated. Thus, it appears that inhalation of FSD does have some prolonged effects on cardiovascular, and, possibly, renal function. The findings also provide information regarding potential mechanisms that may lead to these changes, and biomarkers that could be examined to monitor physiological changes that could be indicative of impending cardiovascular dysfunction.

Biological effects of inhaled hydraulic fracturing sand dust VII. Neuroinflammation and altered synaptic protein expression

Hydraulic fracturing (fracking) is a process used to recover oil and gas from shale rock formation during unconventional drilling. Pressurized liquids containing water and sand (proppant) are used to fracture the oil- and natural gas-laden rock. The transportation and handling of proppant at well sites generate dust aerosols; thus, there is concern of worker exposure to such fracking sand dusts (FSD) by inhalation. FSD are generally composed of respirable crystalline silica and other minerals native to the geological source of the proppant material. Field investigations by NIOSH suggest that the levels of respirable crystalline silica at well sites can exceed the permissible exposure limits. Thus, from an occupational safety perspective, it is important to evaluate the potential toxicological effects of FSD, including any neurological risks. Here, we report that acute inhalation exposure of rats to one FSD, i.e., FSD 8, elicited neuroinflammation, altered the expression of blood brain barrier-related markers, and caused glial changes in the olfactory bulb, hippocampus and cerebellum. An intriguing observation was the persistent reduction of synaptophysin 1 and synaptotagmin 1 proteins in the cerebellum, indicative of synaptic disruption and/or injury. While our initial hazard identification studies suggest a likely neural risk, more research is necessary to determine if such molecular aberrations will progressively culminate in neuropathology/neurodegeneration leading to behavioral and/or functional deficits.