Review of the genotoxicity of styrene in humans

Styrene lung cancer risk assessment: an alternative evaluation of human lung cancer risk assuming mouse lung tumors are potentially human relevant and operating by a threshold-based non-genotoxic mode of action

Rodent inhalation studies indicate styrene is a mouse lung-specific carcinogen. Mode-of-action (MOA) analyses indicate that the lung tumors cannot be excluded as weakly quantitatively relevant to humans due to shared oxidative metabolites detected in rodents and humans. However, styrene also is not genotoxic following in vivo dosing. The objective of this review was to characterize occupational and general population cancer risks by conservatively assuming mouse lung tumors were relevant to humans but operating by a non-genotoxic MOA. Inhalation cancer values reference concentrations for respective occupational and general population exposures (RfCcar-occup and RfCcar-genpop) were derived from initial benchmark dose (BMD) modeling of mouse inhalation tumor dose-response data. An overall lowest BMDL10 of 4.7 ppm was modeled for lung tumors, which was further duration- and dose-adjusted by physiologically based pharmacokinetic (PBPK) modeling to derive RfCcar-occup/genpop values of 6.2 ppm and 0.8 ppm, respectively. With the exception of open-mold fiber reinforced composite workers not using personal protective equipment (PPE), the RfCcar-occup/genpop values are greater than typical occupational and general population human exposures, thus indicating styrene exposures represent a low potential for human lung cancer risk. Consistent with this conclusion, a review of styrene occupational epidemiology did not support a conclusion of an association between styrene exposure and lung cancer occurrence, and further supports a conclusion that the conservatively derived RfCcar-occup is lung cancer protective.

Styrene targets sensory and neural cochlear function through the crossroad between oxidative stress and inflammation

BACKGROUND

Although styrene is an established ototoxic agent at occupational exposure levels, the mechanisms of styrene toxicity in the auditory system are still unclear.

OBJECTIVES

The aim of this study was to identify the consequences of styrene chronic exposure in cochlear structures, looking for the mechanisms of ototoxicity of this organic compound and focusing on cell targets and oxidative stress/inflammatory processes.

METHODS

Male adult Wistar rats were exposed to styrene (400 mg/kg by gavage for 5 days/week, 3 consecutive weeks). Hearing loss was evaluated by measuring auditory brainstem responses (ABR), morphological analysis were performed to evaluate hair cell and spiral ganglion neuron survival, as well as synaptic damage. Analysis of apoptotic (p53) and inflammatory (NF-κB, TNF-α, IL-1β and IL-10) mediators were performed by immunofluorescence analysis and western blot.

RESULTS

Styrene ototoxic effects induced a hearing loss of about 35-40 dB. Immunofluorescence and western blotting analyses demonstrated that styrene administration induced redox imbalance and activated inflammatory processes, targeting sensory hair cell and neural dysfunction by a cross-talk between oxidative and inflammatory mediators.

DISCUSSION

Major findings connect styrene ototoxicity to an interplay between redox imbalance and inflammation, leading to the intriguing assumption of a mixed sensory and neural styrene-induced ototoxicity. Thus, in a clinical perspective, data reported here have important implications for styrene risk assessment in humans.

A critical review and meta-analysis of epidemiology studies of occupationally exposed styrene workers evaluated for chromosomal aberration incidence

Chromosome aberrations in the peripheral blood lymphocytes of styrene exposed workers have been suggested as a potential early marker for cancer risk. We performed a critical review and abstracted data from all studies using current chromosome aberration scoring criteria and providing at least a mean and standard deviation or standard error for the exposed and comparison groups. Using these data, we conducted a meta-analysis of occupational styrene exposed workers and incidence of chromosome aberrations. Our meta-analysis used the standardized mean difference as the summary statistic since all studies assess the same outcome but use different comparison populations. The primary meta-analysis of the 20 comparisons of 505 styrene exposed workers to 532 comparison workers found a meta-mean difference of 0.361 (95 % CI -0.084 to 0.807, random effects model), but there was substantial lack of consistency across studies (I2 of 90.11, p-value <0.001, fixed effect model). Studies with higher styrene exposures had lower mean standard differences compared to studies with lower styrene exposures. While studies of styrene workers overall had a slight increase in chromosomal aberrations relative to comparison groups, the lack of consistency across studies and the absence of an exposure response and other limitations of the reviewed studies including inadequate exposure assessment, small numbers of participants per study, and poorly matched exposed and comparison workers, we find insufficient evidence to support a conclusion that styrene exposure increases chromosome aberration frequencies in styrene workers.

Molecular mechanisms of action of styrene toxicity in blood plasma and liver

Styrene is an aromatic colorless hydrocarbon available in liquid form and highly volatile. In its pure form, it gives a sweet smell. The primary source of exposure in the environment is from plastic materials, rubber industries, packaging materials, insulations, and fiber glass and carpet industry. Natural sources of styrene include: few metabolites in plants which are transferred through food chain. The current study was designed to evaluate styrene toxicity, including: superoxide dismutase (SOD) and protein carbonyl, oxidative stress, glucose-6-phosphatase (G6Pase), glycogen phosphorylase (GP), and phosphoenolpyruvate carboxykinase (PEPCK) activities, adenosine triphosphate (ATP) to adenosine diphosphate (ADP) ratio, and changes in gene expressions such as glutamate dehydrogenase 1 (GLUD1), glucose transporter 2 (GLUT2), and glucokinase (GCK) in the rat liver tissue. For this purpose, styrene was dissolved in corn oil and was administered via gavage, at doses 250, 500, 1000, 1500, 2000, mg/kg/day per mL and control (corn oil) to each rat with one day off in a week, for 42 days. Plasma SOD and protein carbonyl of plasma were significantly up-regulated in 1000, 1500, and 2000 mg/kg/day styrene administrated groups (P < .001). In addition, styrene caused an increase in lipid peroxidation (LPO) and reactive oxygen species (ROS) in the dose-dependent manners in liver tissue (P < .001). Furthermore, the ferrous reducing antioxidant power (FRAP) and total thiol molecules (TTM) in styrene-treated groups were significantly decreased in liver tissue (P < .001) with increasing doses. In treated rats, styrene significantly increased G6Pase activity (P < .001) and down-regulated GP activity (P < .001) as compared to the control group. The PEPCK activity was significantly raised in a dose-dependent manner (P < .001). The ATP/ADP ratio of live cells was significantly raised by increasing the dose (P < .001). There was significantly an up-regulation of GLUD1 and GCK at 2000 mg/kg group (P < .01) and a down-regulation for GLUT2 at the same dose. While in the rest of group, GLUT2 showed up-regulation of relative fold change. By targeting genes such as GLUD1, GLUT2, and GCK, disruption of hepatic gluconeogenesis, glycogenolysis, and insulin secretory functions are obvious. The present study illustrates that induction of oxidative stress followed by changes in G6Pase, GP, and PEPCK activities and the genes responsible for glucose metabolism are the mechanisms of styrene's action in the liver.

Safety evaluation of styrax liquidus from the viewpoint of genotoxicity and mutagenicity

ETHNOPHARMACOLOGICAL RELEVANCE

Styrax liquidus is a resinous exudate (balsam) obtained from the wounded trunk of the Liquidambar orientalis Mill. (Hamamelidaceae). Styrax has been used for treatment of various ailments in Turkish folk medicine such as skin problems, peptic ulcers, nocturnal enuresis, parasitic infections, antiseptic or as expectorant.

AIM OF STUDY

In spite of frequent use of styrax in Turkish folk medicine as well as once as a stabilizer in perfumery industry, negative reports have been noticed by the international authority for restriction its use based on some limited evidences from an in vitro study. The aim of the present study was to evaluate the genotoxic and cytotoxic potential of styrax and its ethanolic extract using in vivo and in vitro assays, as well as an antimutagenic assay and also to determine its phenolic constituents with chromatographic analysis.

MATERIALS AND METHODS

In vitro mutagenicity and antimutagenicity of styrax and its ethanolic extract were evaluated by Ames test performed on Salmonella TA98 and TA100 strains with and without metabolic activation (10- 30,000µg/plate). The genotoxicity was also studied in vivo by chromosomal aberrations assay on bone marrow of Balb C mice with different its concentrations (500-2000mg/kg body weight). Cytotoxicity has been evaluated by the MTT assay using L929 cell line. Its phenolic constituents were determined by HPLC analysis.

RESULTS

Genotoxicological investigations of styrax or its ethanolic extract showed that none of the tested concentrations induced a significant increase in the revertant number of TA98 and TA100 strains with or without metabolic activation, indicating no mutagenicity to the tested strains. Also results indicated that up to 2000mg/kg body weight, styrax is not genotoxic in mammalian bone marrow chromosome aberration test in vivo. In cytotoxicity study, the IC₅₀ values of styrax and its ethanolic extract were found to be 50.22±1.80 and 59.69±11.77µg/mL, respectively. Among the studied reference standards the major phenolic acids in styrax balsam was found to be p-coumaric acid (2.95mg/g), while in its ethanolic extract not only p-coumaric acid (11.46mg/g), but also gallic acid (1.60mg/g) were found to the main components.

CONCLUSION

The findings of the present study provide scientific basis to the safety of styrax from the viewpoint of genotoxicity risk, and in fact, it was found to be beneficial against genotoxicity.

Styrene enhances the noise induced oxidative stress in the cochlea and affects differently mechanosensory and supporting cells

Experimental and human investigations have raised the level of concern about the potential ototoxicity of organic solvents and their interaction with noise. The main objective of this study was to characterize the effects of the combined noise and styrene exposure on hearing focusing on the mechanism of damage on the sensorineural cells and supporting cells of the organ of Corti and neurons of the ganglion of Corti. The impact of single and combined exposures on hearing was evaluated by auditory functional testing and histological analyses of cochlear specimens. The mechanism of damage was studied by analyzing superoxide anion and lipid peroxidation expression and by computational analyses of immunofluorescence data to evaluate and compare the oxidative stress pattern in outer hair cells versus the supporting epithelial cells of the organ of Corti. The oxidative stress hypothesis was further analyzed by evaluating the protective effect of a Coenzyme Q₁₀ analogue, the water soluble Q_ter, molecule known to have protective antioxidant properties against noise induced hearing loss and by the analysis of the expression of the endogenous defense enzymes. This study provides evidence of a reciprocal noise-styrene synergism based on a redox imbalance mechanism affecting, although with a different intensity of damage, the outer hair cell (OHC) sensory epithelium. Moreover, these two damaging agents address preferentially different cochlear targets: noise mainly the sensory epithelium, styrene the supporting epithelial cells. Namely, the increase pattern of lipid peroxidation in the organ of Corti matched the cell damage distribution, involving predominantly OHC layer in noise exposed cochleae and both OHC and Deiters' cell layers in the styrene or combined exposed cochleae. The antioxidant treatment reduced the lipid peroxidation increase, potentiated the endogenous antioxidant defense system at OHC level in both exposures but it failed to ameliorate the oxidative imbalance and cell death of Deiters' cells in the styrene and combined exposures. Current antioxidant therapeutic approaches to preventing sensory loss focus on hair cells alone. It remains to be seen whether targeting supporting cells, in addition to hair cells, might be an effective approach to protecting exposed subjects.

Significant association between decreased ALDH2 activity and increased sensitivity to genotoxic effects in workers occupationally exposed to styrene

ALDH2 is involved in the metabolism of styrene, a widely used industrial material, but no data are available regarding the influence of this enzyme on the metabolic fate as well as toxic effects of this chemical. In this study, we recruited 329 workers occupationally exposed to styrene and 152 unexposed controls. DNA strand breaks, DNA-base oxidation in leukocytes and urinary 8-hydroxydeoxyguanosine (8-OH-dG) were assayed as biomarkers to measure genotoxic effects. Meanwhile, we examined the genetic polymorphisms, including ALDH2, EXPH1, GSTM1, GSTT1 and CYP2E1, and also analyzed the levels of styrene exposure through detecting urinary styrene metabolites and styrene concentration in air. In terms of DNA damage, the three genotoxic biomarkers were significantly increased in exposed workers as compared with controls. And the styrene-exposed workers with inactive ALDH2 *2 allele were subjected to genotoxicity in a higher degree than those with ALDH2 *1/*1 genotype. Also, lower levels of urinary styrene metabolites (MA + PGA) were observed in styrene-exposed workers carrying ALDH2 *2 allele, suggesting slower metabolism of styrene. The polymorphisms of other enzymes showed less effect. These results suggested that styrene metabolism and styrene-induced genotoxicity could be particularly modified by ALDH2 polymorphisms. The important role of ALDH2 indicated that the accumulation of styrene glycoaldehyde, a possible genotoxic intermediate of styrene, could account for the genotoxicity observed, and should be taken as an increased risk of cancer.

Construction and characterization of a forward subtracted library of blue mussels Mytilus edulis for the identification of gene transcription signatures and biomarkers of styrene exposure

Transcriptional profiling can elucidate adaptive/toxicity pathways participating in achieving homeostasis or leading to pathogenesis in marine biota exposed to chemical substances. With the aim of analyzing transcriptional responses in the mussel Mytilus edulis exposed to the corrosive and putatively carcinogenic hydrocarbon styrene (3-5 ppm, 3days), a forward subtracted (SSH) cDNA library was produced. Female mussels were selected and digestive gland mRNA was isolated. A library with 1440 clones was produced and a total of 287 clones were sequenced, 53% being identified through BlastN analysis against Mytibase and DeepSeaVent databases. Those genes included GO terms such as 'response to drugs', 'immune defense' and 'cell proliferation'. Furthermore, sequences related to chitin and beta-1-3-glucan metabolism were also up-regulated by styrene. Many of the obtained sequences could not be annotated constituting new mussel sequences. In conclusion, this SSH study reveals novel sequences useful to generate molecular biomarkers of styrene exposure in mussels.

Genotoxicity of styrene-7,8-oxide and styrene in Fisher 344 rats: a 4-week inhalation study

The cytogenetic alterations in leukocytes and the increased risk for leukemia, lymphoma, or all lymphohematopoietic cancer observed in workers occupationally exposed to styrene have been associated with its hepatic metabolisation into styrene-7,8-oxide, an epoxide which can induce DNA damages. However, it has been observed that styrene-7,8-oxide was also found in the atmosphere of reinforced plastic industries where large amounts of styrene are used. Since the main route of exposure to these compounds is inhalation, in order to gain new insights regarding their systemic genotoxicity, Fisher 344 male rats were exposed in full-body inhalation chambers, 6 h/day, 5 days/week for 4 weeks to styrene-7,8-oxide (25, 50, and 75 ppm) or styrene (75, 300, and 1000 ppm). Then, the induction of micronuclei in circulating reticulocytes and DNA strand breaks in leukocytes using the comet assay was studied at the end of the 3rd and 20th days of exposure. Our results showed that neither styrene nor styrene-7,8-oxide induced a significant increase of the micronucleus frequency in reticulocytes or DNA strand breaks in white blood cells. However, in the presence of the formamidopyridine DNA glycosylase, an enzyme able to recognize and excise DNA at the level of some oxidized DNA bases, a significant increase of DNA damages was observed at the end of the 3rd day of treatment in leukocytes from rats exposed to styrene but not to styrene-7,8-oxide. This experimental design helped to gather new information regarding the systemic genotoxicity of these two chemicals and may be valuable for the risk assessment associated with an occupational exposure to these molecules.

Comparative analysis of binding affinities between styrene and mammalian CYP2E1 by bioinformatics approaches

Cytochrome P450 2E1 (CYP2E1) is a cytochrome P450 enzyme involved in styrene metabolism. This study compared the binding affinities between styrene and 11 mammalian CYP2E1 systems using bioinformatics methods. Firstly, amino acid sequences of CYP2E1s were obtained from the Swiss-Prot database. Then, taking the crystal structure of human CYP2E1 as a template, 3D models of the CYP2E1s of other mammals were constructed using the SWISS-MODEL program. Finally, the generated homology models were applied to calculate their docking capacities against styrene and polystyrene using the Surflex-Dock program, which could automatically dock ligands into a receptor's ligand binding site using a protomol based approach and assess the affinity by an empirically derived scoring function. Docking experiments showed that the studied mammalian CYP2E1s had high binding affinities with styrene. For polystyrene, the dimmer of styrene has high binding affinities with CYP2E1s, however, trimer and other high polymers were found hard to be docked into the CYP2E1s. The results of this study indicated that bioinformatics approaches might be useful tools to predict styrene and polystyrene affinities with mammalian CYP2E1s.

Epidemiologic studies of styrene and cancer: a review of the literature

OBJECTIVE

To review the epidemiologic literature on styrene and cancer.

METHODS

We reviewed studies of workers exposed to styrene in manufacturing and polymerization, in the reinforced plastics industry, and in styrene-butadiene rubber production. We also reviewed studies of workers monitored for styrene exposure, studies of environmental exposure, community-based case-control studies of lymphoma and leukemia, and studies of DNA adducts. Studies of workers in the reinforced plastics industry were considered more informative because of higher worker exposure and less confounding by other carcinogens.

RESULTS

We found no consistent increased risk of any cancer among workers exposed to styrene. A study of reinforced plastic workers reported an association between average estimated styrene exposure and non-Hodgkin lymphoma (NHL, P = 0.05) but no trend with increasing duration of exposure. Other studies of styrene exposure and NHL found no increased risk. In two US studies of reinforced plastic workers, esophageal cancer mortality was increased, but these findings were generated in a background of multiple comparisons. Results for other cancers were unremarkable.

CONCLUSIONS

The available epidemiologic evidence does not support a causal relationship between styrene exposure and any type of human cancer.

Investigation on the mechanisms of genotoxicity of butadiene, styrene and their combination in human lymphocytes using the Comet assay

The toxicity of butadiene and styrene is exerted by their metabolites. Such metabolites have been extensively scrutinized at the in vitro level demonstrating evident genotoxic properties. In monitoring, a diverse range of outcomes has been produced. Additionally, epidemiological studies in rubber workers face difficulties of data interpretation due to the changeability and multiple exposures of the workers as well as to confounding factors inherent to the cohorts. Nevertheless, toxicity has been associated with a significant trend of increasing the risk of leukaemia in employees at the styrene-butadiene rubber industry. Thus, further effort must be made to distinguish the exposures to each chemical over time and to characterize their interrelationships. The present investigation focuses on the effects and mechanisms of damage of the mixture styrene-butadiene by examining its metabolites: styrene oxide (SO), butadiene monoepoxide (BME) and butadiene diepoxide (BDE) respectively. The in vitro Comet assay on frozen lymphocytes has been employed to ascertain the DNA damage patterns for the styrene-butadiene metabolites combined and on their own. Different patterns were observed for the mixture and each of its components. This study has also led to determining the mechanism of damage of the mixture and the compounds. With regard to the presence of reactive oxygen species (ROS), co-treatment with catalase does not modulate the genotoxicity of the mixture but it does modulate its components. The outcomes also indicate that the mixture induces cross-links and this is due to the influence of BDE in the mixture, being more evident as the concentration of BDE increases. An investigation on the sensitivity of lymphocytes from occupationally un/exposed subjects to in vitro exposure of the mixture and its components revealed that occupationally exposed subjects had a substantially higher background of DNA damage and a lower sensitivity to the metabolites of styrene, 1,3-butadiene and its mixture.

Styrene Metabolism, Genotoxicity, and Potential Carcinogenicity

This report reviews styrene biotransformation, including minor metabolic routes, and relates metabolism to the genotoxic effects and possible styrene-related carcinogenicity. Styrene is shown to require metabolic activation in order to become notably genotoxic and styrene 7,8-oxide is shown to contribute quantitatively by far the most (in humans more than 95%) to the genotoxicity of styrene, while minor ring oxidation products are also shown to contribute to local toxicities, especially in the respiratory system. Individual susceptibility depending on metabolism polymorphisms and individual DNA repair capacity as well as the dependence of the nonlinearity of the dose-response relationships in the species in question and the consequences for risk evaluation are analyzd.

Association between genetic polymorphisms in styrene-metabolizing enzymes and biomarkers in styrene-exposed workers

Single-nucleotide polymorphisms (SNP) in genes of styrene-metabolizing enzymes could modulate biomarker concentrations in blood or urine after exposure to styrene. Ten SNP were analyzed to study their influence on styrene-specific biomarkers in 89 workers of a fiber-reinforced plastic boat building factory. The internal styrene body burden was analyzed in post-shift blood and urine samples. External styrene exposure was measured by passive samplers. Spearman rank correlations between styrene exposure and biomarkers were calculated and distributions of biomarkers were checked for lognormality. Mixed linear models were applied to analyze the influence of genotypes and styrene exposure, on styrene in blood (Monday and Thursday post-shift) and on phenyglyoxylic acid (PGA; adjusted for day of measurement, Monday to Thursday) due to a lognormal distribution, smoking (current, not current), and use of respirators. Stratified analyzes for workers without and with different types of respirators were also performed. The models of both the subgroups revealed a significant influence dependent on the respirator type that workers used for inhalation protection. An influence of the external styrene concentration on the urinary PGA concentration was not observed. After implementation of the SNP into the model significant lower adjusted means of urinary PGA concentrations were found for GSTP1 105IleVal and CYP2E1 -71TT. For styrene levels in blood no significant effect was observed. A significant influence on styrene levels in blood was correlated with external styrene concentration only in workers without use of respirators. The effects of two SNP on urinary PGA decrease indicated a limited modulating SNP effect. The most effective prevention for styrene exposure was obtained with the wearing of respirators.

Relationship between the capacity to repair 8-oxoguanine, biomarkers of genotoxicity and individual susceptibility in styrene-exposed workers

Genotoxic effects related to exposure to styrene have been a matter of investigation for many years by employing markers of exposure, effect and susceptibility. The role of individual DNA-repair capacity in response to exposure to styrene may explain the controversial results so far obtained, but it is still scarcely explored. In the present study, we measured capacity to repair oxidative DNA damage in cell extracts obtained from 24 lamination workers occupationally exposed to styrene and 15 unexposed controls. The capacity to repair oxidative DNA damage was determined by use of a modified comet assay, as follows: HeLa cells, pre-treated with photosensitizer and irradiated with a halogen lamp in order to induce 7,8-dihydroxy-8-oxoguanine, were incubated with cell extracts from mononuclear leukocytes of each subject. The level of strand breaks reflects the removal of 7,8-dihydroxy-8-oxoguanine from substrate DNA by the enzymatic extract. In styrene-exposed subjects a moderate, non-significant increase in oxidative DNA repair was observed. Stratification for sex and smoking habit showed that unexposed males (P=0.010) and unexposed smokers (P=0.037) exhibited higher DNA-repair rates. The repair capacity did not correlate with parameters of styrene exposure and biomarkers of genotoxic effects (DNA strand breaks, N1-styrene-adenine DNA adducts, chromosomal aberrations and mutant frequencies at the HPRT locus). Significantly higher levels of DNA-repair capacity were observed in carriers of GSTM1-plus, compared to those with a deletion in GSTM1. The DNA-repair capacity was significantly lower in individuals with variant Gln/Gln genotype in XRCC1 Arg399Gln than in those with heterozygous Arg/Gln and wild-type Arg/Arg genotypes. Significantly lower repair capacity was also found in individuals with the wild-type Lys/Lys genotype in XPC Lys939Gln as compared with those homozygous for the Gln/Gln variant genotype.

Cloning and functional characterization of the styE gene, involved in styrene transport in Pseudomonas putida CA-3

A 1.5-kb region immediately downstream of the styABCD operon involved in styrene degradation in Pseudomonas putida CA-3 has been cloned. Sequence analysis revealed a 1,296-bp open reading frame, designated styE, and BLAST P database comparisons of the deduced StyE amino acid sequence revealed 33 to 98% identity with several membrane-associated ATPase-dependent kinase proteins involved in the active transport of aromatic hydrocarbons across bacterial membranes and also with FadL, an outer membrane protein necessary for the uptake of long-chain fatty acids in Escherichia coli. Transcription of styE is styrene dependent, and the gene is cotranscribed with the styABCD structural genes. StyE appears to be membrane associated, with a corresponding 45.9-kDa band being identified following sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of membrane preparations from styrene-grown cells. P. putida CA-3 cells in which the styE gene had been interrupted were no longer capable of growth on styrene. In contrast, overexpression of styE in P. putida CA-3 resulted in a 4.2-fold increase in styrene monooxygenase activity compared with wild-type cells grown on styrene, with a concomitant 8-fold increase in styA mRNA transcript levels. Experiments with the classic, ATPase inhibitor vanadate revealed that growth of wild-type cells on styrene was inhibited at a concentration of 1 mM, while 1.75 mM was required to achieve a similar effect in the StyE overexpression strain. Growth of either strain on citrate was not inhibited in the presence of up to 7 mM vanadate. These findings suggest a role for StyE in the active transport of styrene in Pseudomonas putida CA-3 and identify styrene transport as a potentially limiting factor with respect to mRNA transcript levels and associated enzymatic activity of the styrene degradative pathway.