EVALUATING SIGNAL TRANSDUCTION NETWORKS AND THEIR CELLULAR
EFFECTS
Signal Transduction in the Post-genomic Era
With the complete genomic sequences from a variety of
eukaryotic and prokaryotic organisms now available, there is
increased confidence that an understanding of complex
biological phenomena and systems is now within our reach.
However, cells are dynamic entities constantly bombarded by
many external stimuli. Examples of these stimuli include
soluble growth factors, hormones and cytokines that initiate
and regulate intracellular events. In addition, other
mechanical forces and cell-cell/cell-extracellular matrix
adhesion also regulate a myriad of intracellular events. Thus,
genome information alone is not sufficient to understand
cellular function. Rather, the complex intracellular processes
of gene expression, cell survival, growth, differentiation or
death will require subsets of gene products. Importantly, these
gene products can be up-regulated or down-regulated in total
expression levels, expressed in a tissue-specific manner, or
subject to post-translational modifications to enhance or
inhibit function.
When the genome of the simple multi-cellular nematode worm
Caenorhabditis elegans was first fully examined in 1998, signal
transduction components and transcriptional regulators were
amongst the largest families of proteins. Specifically, the
most common protein domain classified by the Pfam protein
family database was the seven transmembrane chemoreceptors (650
matches) followed by the protein kinase domain (410 matches).
Amongst the 20 most common protein domains were also the seven
transmembrane receptors of the rhodopsin family (140 matches)
and the protein tyrosine phosphatases (90 matches). Within the
Human Genome we see that 90 unique tyrosine kinase genes found
on 19 of the 24 human chromosomes and that there are additional
400 or so serine/threonine protein kinases. Given the
established importance of protein phosphorylation, such a
catalogue of protein kinases has been greeted by the scientific
community with much enthusiasm.
The major aim of our research in the Cell Signalling
Laboratory is to understand how extracellular stimuli can be
translated into final cellular outcomes. This transfer is
accomplished by "relays" or "networks" of protein kinases. In
recent years we have focused on understanding the regulation of
mitogen-activated protein kinase (MAPK) cascades. More
recently, our novel observations of STAT3 activation in the
failing human heart have prompted us to also evaluate the
JAK/STAT signalling cascades. In addition, we are broadening
our interests in protein kinase signalling mechanisms and have
now begun studies on a critical regulator of the cell cycle
called Aurora-A.
MAPK cascades — Understanding and Interrogation?
Fifteen years ago, an insulin-activated protein kinase was
first described. At that time, the signal transduction
mechanisms employed by hormone receptors were poorly defined.
It took an additional two years before purification and cloning
revealed the similarity to protein kinases in the yeast
pheromone response. This protein kinase, now known as ERK,
became the founding member of the superfamily of
mitogen-activated protein kinases or MAPKs.
ERK MAPKs are rapidly activated after exposure of mammalian
cells to growth-promoting stimuli such as growth factors,
hormones and cytokines. It is now well-established that the ERK
MAPKs are essential for the growth and proliferative responses
of cells. In contrast, other MAPKs that are generally termed
the "stress-activated" MAPKs may be activated after exposure of
cells to cellular stresses and inflammatory cytokines. The
cellular stress signals are a diverse group of stimuli and
include hyperosmotic shock, UV irradiation, metabolic poisoning
and inhibition, lack of oxygen, and changes in pH or
temperature. The stress-activated MAPKs include the c-Jun
N-terminal protein kinases (JNKs or also called SAPKs), p38
MAPK, and SAPK-3. Because these kinases are activated following
cellular insults, it is thought that they may play a role in
initiating or regulating cell death. A general scheme of some
of the major MAPK pathways is shown in Figure 1.
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