Hormonal Biochemistry

1. Babcock University
Ben Carson (SNR.) School of Medicine
Department of Biochemistry
Signaling Pathways From The Membrane To
Nucleus
November, 2014

2. CONTRIBUTORS
Kolawole, Kayode
Wokomaty, Jane
Oyibo, Jane
Oke, Ibukunoluwa

3. INTRODUCTION
 There are three categories of signaling molecules;
steroid, nitric oxide and others e.g. Proteins.
 Steroids and nitric oxide diffuse into the cell where
they bind internal receptors while the others bind to
receptors displayed at the surface of the cell.
 These (others) are transmembrane proteins whose
extracellular portion has the binding site for the
signaling molecule; intracellular portion activates
proteins in the cytosol that in different ways
eventually regulate gene transcription in the nucleus.

5. A. G-PROTEIN-COUPLED RECEPTOR (GPCRS)
 The ligand binds to a site on the extracellular portion
of the receptor
 This binding
 activates a G protein associated with cytoplasmic C-terminal
 Initiates the production of a “second messenger” e.g. cAMP,
and Inositol 1,4,5-triphosphate (IP3)
 The second messenger, in turn, initiates a series of
intracellular events such as
 Phosphorylation and activation of enzymes
 Release of Ca2+ into the cytosol from stores within the ER

6.  In the case of cAMP, these enzymatic changes
activates the transcription factor CREB (cAMP
response element binding protein)
 Bound to its response element
5ˈ TGACGTCA 3ˈ
in the promoter of the genes that are able to respond
to the ligand
 Activated CREB turns on gene transcription

7. B. THE HEDGEHOG SIGNALING PATHWAY
 In mammals, when there is no hedgehog protein
present, the patched (Ptc) receptors bind a second
transmembrane protein called smoothened (Smo)
 When Hedgehog (Hh) protein binds to patched, the
Smo protein seperates from Ptc
 enabling Smo to activate a zinc-finger transcription
factor designated Glucagon-Like Immunoreactive
factor (GLI)
 GLI migrates into the nucleus when it activates a
variety of target genes.

8. C. THE NOTCH SIGNALING PATHWAY
 Ligands and Receptors are transmembrane proteins
embedded in the plasma membrane of cells.
 Ligands include the serrate and delta protein
families
 When a cell bearing the ligand comes in contact
with a cell displaying the notch receptor, the
external portion of the notch is cleaved away from
the cell surface (and engulfed by the ligand-bearing
cell by endocytosis).

9.  The internal portion of the notch receptor is cut
away from the interior of the plasma membrane and
travels into the nucleus where it activates
transcription factors that turn the appropriate genes
on (off)
 It would appear that proper development of virtually
all organs depends on notch signaling.

10. D. CYTOKINE RECEPTORS
 Receptor Tyrosine Kinases (RTKs)
 Binding of the ligand to two adjacent receptors forms an
active homodimer
 Activated dimer is a tyrosine kinase; an enzyme that adds
phosphate groups to tyrosine residues and other proteins
converting them into an active state
This leads to a cascade of expanding phosphorylations
in the cytosol
 Some cytosolic tyrosine kinases act directly on gene
transcription by entering nucleus and transfering their
phosphate to transcription factors thus activating them
 Others act indirectly by producing second messengers

11.  RAF
 This kinase participates in a signaling pathway that links
RTKs to gene activation
 Binding of a ligand to the RTK activates an intracellular
molecule called RAS, which then activates RAF.
 In mammals, this pathway promotes mitosis
 Excess activity of the RAS gene or mutations in RAS
and/or RAF are associated with many types of cancer.

12.  JAK-STAT Pathway
 Binding of the ligand (Leptin, Interferons, prolactin etc)
activates the JAK molecules which
 Phosphorylates certain tyrosine (Tyr) residues on each
other as well as on one or another of several STAT
(Signal Transducer and Activator of Transcription)
proteins.
 These, in turn, form dimers which enter the nucleus and
bind to specific DNA sequences in the promoters of
genes that begin transcription

13. E. TRANSFORMING GROWTH FACTOR-BETA
(TGF-Β) RECEPTORS
 The ligand binds to the extracellular portion of the
receptors,
 which then phosphorylates one or
more SMAD proteins in the cytosol.
 The SMAD proteins move into the nucleus where
they form heterodimers with another SMAD protein
designated SMAD4.
 These dimers bind to a DNA sequence (CAGAC) in
the promoters of target genes and with the aid of
other transcription factors enhance, or repress, as the
case may be, gene transcription.

14. F. TUMOR NECROSIS FACTOR-ALPHA (TNF-
Α) RECEPTORS AND THE NF-ΚB PATHWAY
 NF-κB resides in the cytosol bound to
an inhibitor called IκB.
 Binding of ligand to the receptor triggers
Phosphorylation of IκB
 IκB then becomes ubiquinated and destroyed
by proteasomes.
 This liberates NF-κB so that it is now free to move
into the nucleus where
 it acts as a transcription factor binding to the
promoters and/or enhancers of more than 60 genes

15. + NF-κB got its name from its discovery as a
transcription factor bound to the enhancer of
the kappa light chain antibody gene.
However, it also turns on the genes
encoding IL-1 and other cytokines that
promote inflammation.
NF-κB also turns on genes needed for cell
proliferation, cell adhesion, and angiogenesis.