Breakthrough in understanding cell processes could lead to new
22 November 2012
Researchers at Tokyo Institute of Technology and the Weizmann
Institute of Science have identified a means of controlling biological
processes that could help treatments for immune disease, neurological
disorders and cancer.
The cellular response to a number of signals including
inflammatory cytokines, tumour promoters, carcinogens, and
chemotherapeutic agents hinge on the transcription factor NF-κB.
Rivka Dikstein and colleagues at Tokyo Institute of Technology in
Japan and the Weizmann Institute of Science in Israel have
identified how the protein DSIF controls NF-κB activity. The
research may lead to methods for controlling inflammation, and
immune responses, as well as the cell cycle processes that can lead
Activity of NF-κB is usually transient because some of the gene
products function as negative regulators of NF-κB, resulting in a
negative feedback loop. This is desirable because persistent
activation of NF-κB can lead to various pathogenic conditions such
as chronic inflammation, autoimmune diseases, and cancer.
Dikstein and colleagues treated cells to reduce the expression of
DSIF and monitored the effect on protein levels of NF-κB’s target
genes. They found that in the treated ‘DSIF knockdown’ cells the
levels of these proteins failed to recover following a degradation
process. Further investigation revealed that DSIF plays an integral
role in the maturation and transport of messenger RNAs (Figure 2).
Figure 2. The role of DSIF in the control of
inflammatory responses. Upon NF-κB activation, DSIF is recruited to
the A20 and IκBα genes and contributes to the negative feedback
regulation in a unique manner. DSIF facilitates the expression of
these genes by stimulating capping, splicing, and export of their
mRNAs. Otherwise, these steps are rate-limiting, and A20 and IκBα
proteins are not synthesized successfully.
“The unique control of the negative feedback regulator genes by
DSIF may be utilized by cells under circumstances in which prolonged
NF-κB activity is needed,” explain the authors. Their future studies
will focus on DSIF under specific settings with a view to
identifying drug targets for selective manipulation of NF-κB
NF-κB induces gene expression responsible for a number of
biological processes including inflammation and cell survival. Its
deregulation is linked to chronic inflammation and cancer but so far
the mechanisms behind these processes have not been fully
Nuclear transport is integral to NF-κB activity. In unstimulated
cells it is retained inactive in the cytoplasm. Signals that trigger
its activation result in its transport into the nucleus where it
activates responsive genes, such as A20 and IκBα.
Activity of NF-κB is usually transient because products of these
genes function as negative regulators of NF-κB, resulting in a
negative feedback and limiting the duration of NF-κB activation
(Figure 1). In certain diseases NF-κB activation becomes persistent,
perhaps due to an interruption of this feedback loop.
NF-κB activation results in recruitment of the protein DSIF at
the target genes A20 and IκBα. The researchers confirmed the role of
DSIF in the negative feedback regulation of NF-κB activity by
downregulating a protein subunit of DSIF and comparing the treated
cells with controls. In untreated cells the levels of proteins for
the target genes A20 and IκBα are high. Although the levels diminish
significantly in 30 minutes following a process called TNF-α
induction, which is caused by degradation of the proteins, within 2
hours the levels recover. In knockdown cells, however, the levels
remained diminished 2 hours after TNF-α induction.
Further investigations using chromatin immunoprecipitation assays
revealed abnormalities in the synthesis of A20 and IκBα proteins in
the knock down cells. A significant portion of the A20 and IκBα
mRNAs in the knockdown cells were uncapped and unspliced (Fig. 2).
The researchers also studied how export was affected in DSIF
knockdown cells. Export of mature mRNAs from the nucleus to the
cytoplasm is a highly regulated process incorporating quality
assurance checks. The researchers prepared RNA from cytosolic and
nuclear cell fractions for comparison. In knockdown cells following
TNF-α induction, the nuclear fractions had much larger amounts of
A20 and IκBα than the cytosolic fractions, suggesting accumulation
in the nucleus.
The impact observed on the synthesis and export of A20 and IκBα
in DSIF knockdown cells was striking. A20 and IκBα are also
responsible for regulating other cell signalling processes and it is
likely that the effects on associated signalling processes
contribute to the overall impact of DSIF knockdown.
As the authors point out, diminished DSIF activity could
contribute to pathological states in which NF-κB becomes
constitutively active, such as inflammatory and autoimmune diseases,
neurological disorders, and cancer. Further work on DSIF activity
could help develop new approaches to treating these diseases.