DƯỢC LÍ Goodman & Gilman's The Pharmacological Basis of Therapeutics 12th, 2010

Activation of members of the Ras family leads in turn to activation
of a protein kinase cascade termed the mitogen-activated
protein kinase (MAP kinase or MAPK) pathway. Activation of the
MAPK pathway is one of the major routes used by growth factor
receptors to signal to the nucleus and stimulate cell growth. The first
enzyme in the pathway is Rap which is a MAP kinase kinase kinase
(MKKK). Rap phosphorylates and activates a MAP kinase kinase
(MKK) termed MEK. MEK phosphorylates a MAP klnase termed
ERK. ERK is an interesting member of the kinase family; its activation
is achieved by phosphorylation of closely spaced tyrosine and
threonine residues in the kinase activation loop. ERK phosphorylates
a number of transcription factors in the nucleus, including
Elk-1 and CREB, to regulate gene transcription and cause cell proliferation
(Manning and Davis, 2003). Drugs that act at receptors in
this diverse family include insulin for the treatment of diabetes mellitus
and imatinib, a small molecule protein kinase inhibitor designed
to inhibit both receptor and non-receptor tyrosine kinases. Imatinib
is used to treat chronic myelogenous leukemia and several solid
tumors with dysregulated tyrosine kinases.
JAK-STAT Receptor Pathway. Cells express a family of
receptors for cytokines such as γ-interferon and hormones
like growth hormone and prolactin, which signal
to the nucleus by a more direct manner than the receptor
tyrosine kinases. These receptors have no intrinsic
enzymatic activity, rather the intracellular domain binds
a separate, intracellular tryosine kinase termed a Janus
kinase (JAK). Upon the dimerization induced by ligand
binding, JAKs phosphorylate other proteins termed signal
transducers and activators of transcription (STATs),
which translocate to the nucleus and regulate transcription
(Figure 3–10B). The entire pathway is termed the
JAK-STAT pathway (Gough et al., 2008; Wang et al.,
2009). There are four JAKs and six STATs in mammals
which, depending on the cell type and signal, combine
differently to activate gene transcription. For example,
prolactin appears to use JAK1, JAK2, and STAT5 to
stimulate milk production.
Receptor Serine-Threonine Kinases. Protein ligands
such as TGF-β activate a family of receptors that are
analogous to the receptor tyrosine kinases except that
they have a serine/threonine kinase domain in the cytoplasmic
region of the protein. There are two isoforms of
the monomeric receptor protein, type I (7 forms) and
type II (5 forms). In the basal state, these proteins exist
as monomers; upon binding an agonist ligand, they
dimerize, leading to phosphorylation of the kinase
domain of the type I monomer, which activates the
receptor. The activated receptor then phosphorylates a
gene regulatory protein termed a Smad. There are multiple
Smads in cells; once phosphorylated by the activated
receptor on a serine residue, Smad dissociates
from the receptor, migrates to the nucleus, associates
with transcription factors and regulates genes leading to
morphogenesis and transformation. There are also
inhibitory Smads (the Smad6 or Smad7 isoforms) that
compete with the phosphorylated Smads to terminate
signaling.
Toll-Like Receptors. Signaling related to the innate
immune system is carried out by a family of over ten
single membrane-spanning receptors termed Toll-like
receptors (TLR), which are highly expressed in
hematopoeitic cells. In a single polypeptide chain, these
receptors contain a large extracellular ligand-binding
domain, a short membrane-spanning domain, and a
cytoplasmic region termed the TIR domain that lacks
intrinsic enzymatic activity. Ligands for TLR are comprised
of a multitude of pathogen products including
lipids, peptidoglycans, lipopeptides, and viruses.
Activation of these receptors produces an inflammatory
response to the pathogenic microorganisms. As with all
single membrane-spanning receptors, the first step in
activation of TLR by ligands is dimerization, which in
turn causes signaling proteins to bind to the receptor to
form a signaling complex.
Ligand-induced dimerization recruits a series of adaptor proteins
including Mal and the myeloid differentiation protein 88
(MyD88) to the intracellular TIR domain, which in turn recruit the
interleukin-associated kinases termed IRAKs. The IRAKs autophosphorylate
in the complex and subsequently form a more stable complex
with MyD88. The phosphorylation event also recruits TRAF6
to the complex, which facilitates interaction with a ubiquitin ligase
that attaches a polyubiquitin molecule to TRAF6. This complex can
now interact with the protein kinase TAK1 and the adaptor TAB1.
TAK1 is a member of the MAP kinase family, which activates the
NF-κB kinases; phosphorylation of the NF-κB transcription factors
causes their translocation to the nucleus and transcriptional activation
of a variety of inflammatory genes (Gay and Gangloff, 2007).
TNF-α Receptors. The mechanism of action of tumor
necrosis factor α (TNF-α) signaling to the NF-κB transcription
factors is very similar to that used by Toll-like
receptors in that the intracellular domain of the receptor
has no enzymatic activity. The TNF-α receptor is another
single membrane-spanning receptor with an extracellular
ligand-binding domain, a transmembrane domain,
and a cytoplasmic domain termed the death domain.
TNF-α binds a complex composed of TNF-receptor1 and
TNF-receptor2. Upon trimerization, the death domains bind the
adaptor protein TRADD, which recruits the receptor interacting protein
1 (RIP1) to form a receptor-adaptor complex at the membrane.
RIP1 is poly-ubiquinated, resulting in recruitment of the TAK1
kinase and the IκB kinase (IKK) complex to the ubiquinated molecules
(Skaug et al., 2009). The activation loop of IKK is phosphorylated
in the complex eventually resulting in IκBα being released
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CHAPTER 3
PHARMACODYNAMICS: MOLECULAR MECHANISMS OF DRUG ACTION