14th April 15:13
Benzene's Adverse Effects (stress tuberous sclerosis leukemia)
Benzene's Adverse Effects
Microarrays Reveal Breadth of Toxicity
Benzene is both widely used and widely studied. Yet, although the
chemical is strongly associated with leukemia in humans, questions
remain regarding its mechanism of action. Hoping to better understand
the genetic mechanisms behind benzene's hematotoxicity and
leukemogenicity, a group of researchers from Japan and Korea used cDNA
microarrays to ****yze mouse bone marrow tissue both during and after a
two-week exposure to the compound by inhalation [EHP 111:1411-1420]. The
researchers found, among other discoveries, that benzene may perturb
cell cycling that is mediated by the gene for the protein p53,
triggering a host of fatal problems at the cellular level and thus
causing blood cell malignancies epigenetically (that is, without
encoding the information in the genetic code).
A bad actor in blood. New research shows that benzene's toxic
effects--including leukemogenicity and hematotoxicity--are wrought
through many pathways. Thus, multiple genes may be implicated.
image credit: Photodisc, Christopher G. Reuther/EHP
Benzene is used in fuels, as an industrial solvent, and in other
manufacturing applications, and is also found in cigarette smoke. Human
populations generally are exposed through polluted ambient air or
contaminated water. Benzene is known to cause hematotoxicity and blood
tumors in humans and mice. Studies so far have focused on benzene's
carcinogenic and genotoxic metabolites, which cause various types of
tumors in a number of mouse organ systems. Hepatic enzymes convert
inhaled benzene into genotoxic metabolites. Then, to add insult to
injury, a number of these benzene metabolites (primarily phenol,
hydroquinone, catechol, and trans-trans muconic acid) actually intensify
the chemical's toxic effect on an organ.
Past studies have suggested that benzene's toxic effects on bone marrow
tissue--its major target organ--may be enacted through multiple
pathways, including growth factor regulation, oxidative stress
reduction, DNA damage repair, cell cycle regulation, and apoptosis.
Also, genetic variations may upset the cellular-environmental
homeostasis that protects bone marrow cells from toxic effects such as
those caused by benzene, resulting in altered gene expression.
Therefore, the authors suggest, studying just a few specific genes may
not be enough to thoroughly explain the complex molecular mechanisms of
benzene-induced hematotoxicity and leukemogenicity.
With this in mind, the research team conducted broad cDNA microarray
****yses using multiple gene expression profiling technologies. The team
****yzed mouse bone marrow tissue during and after exposure to 300 parts
per million benzene over a 2-week period for 6 hours a day, 5 days a
week. Two types of C57BL/6 mice were used--standard wild-type mice
possessing the gene for p53 and p53-knockout mice. The mice were
randomly grouped into control and benzene-exposed groups.
Twice during the exposure period and then 3 days after the full 2-week
exposure, the researchers collected bone marrow from both femurs of each
mouse in each group. RNA was extracted from this tissue and used to
synthesize cDNA, which was then hybridized onto a microarray chip. The
resulting array of gene fragments was scanned as a digital image and
****yzed using software that searched for clustering genes specifically
expressed and/or suppressed in each group.
The researchers found that benzene caused DNA damage in cells during all
phases of the cell cycle. In the benzene-exposed wild-type mice, DNA
repair genes were activated, but they were suppressed in the
p53-knockout mice. Mice in the latter group were therefore susceptible
to benzene's direct genotoxic leukemogenicity, whereas those in the
former still experienced epigenetic leukemogenicity via cell-cycle
perturbations despite DNA repair.
Besides the p53-mediated pathway, the investigators identified other
specific genes that may be involved in G1 cell cycle arrest and
apoptosis following benzene exposure, and confirmed that certain repair
genes--including the tuberous sclerosis gene and the metallothionein 1
gene--are also triggered by such exposure. They also found that, during
benzene exposure, the production of blood cells was arrested due to
alterations in the expression of cell cycle checkpoint genes in the
wild-type mice. However, production continued in the p53-knockout mice,
an important difference that the researchers say could point to
mechanisms of benzene's hematotoxicity.
The researchers' cDNA microarray ****yses supported the theory that the
gene for p53 mediates the effect of benzene on bone marrow tissue by
regulating specific genes instrumental in cell cycle arrest, apoptosis,
and DNA repair. Because careful simultaneous screening of different
expression patterns of many interrelated genes between the two groups is
necessary, the researchers write, toxicogenomics should prove extremely
useful for future investigations into the toxicity and leukemogenicity
mechanisms of benzene.