Cell Signaling

Index to this page

Introduction

Cells must be ready to respond to essential signals in their environment. These are often chemicals in the extracellular fluid (ECF) from: They may also respond to molecules on the surface of adjacent cells (e.g. producing contact inhibition).

Signaling molecules may trigger:

  1. an immediate change in the metabolism of the cell (e.g., increased glycogenolysis when a liver cell detects adrenaline);
  2. an immediate change in the electrical charge across the plasma membrane (e.g., the source of action potentials);
  3. a change in the gene expression — transcription — within the nucleus. (These responses take more time.)
It is the third category that is the topic of this page.

So this page examines some of the major pathways by which the arrival of a chemical signal at the cells turns on a new pattern of gene expression.

Two categories of signaling molecules (steroids and nitric oxide) diffuse into the cell where they bind internal receptors.

The others, e.g., proteins, bind to receptors displayed at the surface of the cell. These are transmembrane proteins whose

Steroid Receptors

Steroids are small hydrophobic molecules that can freely diffuse across the plasma membrane, through the cytosol, and into the nucleus.

Receptors

Steroid receptors are dimers of zinc-finger proteins that reside within the nucleus (except for the glucocorticoid receptor which resides in the cytosol until it binds its ligand).

Until their ligand finds them, some steroid receptors within the nucleus associate with histone deacetylases (HDACs), keeping gene expression repressed in those regions of the chromosome.

Ligands

Some steroids that regulate gene expression:

Mechanism

Link to stereoscopic view of the glucocorticoid receptor bound to the promoter sequence of DNA.

Nitric Oxide (NO) Receptors

Receptors

The signaling functions of NO begin with its binding to protein receptors in the cell. The binding sites can be either:

Mechanisms

In either case, binding triggers an allosteric change in the protein which, in turn, triggers the formation of a "second messenger" within the cell. The most common protein target for NO seems to be guanylyl cyclase, the enzyme that generates the second messenger cyclic GMP (cGMP).
Link to discussion of the various functions that depend on NO signaling.

G-Protein-Coupled Receptors (GPCRs)

Receptors

These are transmembrane proteins that wind 7 times back and forth through the plasma membrane.

Ligands

Some of the many ligands that alter gene expression by binding GPCRs:

Mechanisms

In addition to their roles in affecting gene expression, GPCRs regulate many immediate effects within the cell that do not involve gene expression. Links to some examples.

Frizzled Receptors and Wnt Signaling

Receptors

Frizzled receptors, like GPCRs, are transmembrane proteins that wind 7 times back and forth through the plasma membrane.

Ligands

Their ligands are Wnt proteins. These get their name from two of the first to be discovered, proteins encoded by

The roles of β-catenin

β-catenin molecules connect actin filaments to the cadherins that make up adherens junctions that bind cells together.

Any excess β-catenin is quickly destroyed by a multiprotein degradation complex. (One component is the protein encoded by the APC proto-oncogene.)

The degradation complex

But undegraded β-catenin takes on a second function: it becomes a potent transcription factor.

Mechanism

(Note the similarities to the strategy used by the NF-κB signaling pathway.)

Wnt-controlled gene expression plays many roles in embryonic development as well as regulating activities in the adult body.


Cytokine Receptors

Dozens of cytokine receptors have been discovered. Most of these fall into one or the other of two major families:
  1. Receptor Tyrosine Kinases (RTKs) and
  2. Receptors that trigger a JAK-STAT pathway.

1. Receptor Tyrosine Kinases (RTKs)

Receptors

The receptors are transmembrane proteins that span the plasma membrane just once.

Ligands

Some ligands that trigger RTKs:

Mechanisms

Turning RTKs Off

A cell must also be able to stop responding to a signal. For growth factor receptors, failure to do so could lead to uncontrolled mitosis = cancer. For the RTKs, this is done by quickly engulfing and destroying the ligand-receptor complex by receptor-mediated endocytosis.

Proto-Oncogenes

One might expect that anything which leads to the inappropriate expression of receptors that trigger cell division could lead to cancer (uncontrolled cell division). And, in fact, Mutant versions of some of the "second-order" kinases are also associated with cancer:

2. JAK-STAT Pathways

Receptors

These are also single-pass transmembrane proteins embedded in the plasma membrane.

Ligands

Many ligands trigger JAK-STAT pathways:

Mechanisms

The JAK-STAT pathways are much shorter and simpler than the pathways triggered by RTKs and so the response of cells to these ligands tends to be much more rapid.

Transforming Growth Factor-beta (TGF-β) Receptors

Receptors

Two types of single-pass transmembrane proteins that, when they bind their ligand, become kinases that attach phosphate groups to serine and/or threonine residues of their target proteins.

Ligands

Ligands for these receptors include:

Mechanisms

Tumor-Suppressor Genes

The TGF-β signaling pathway suppresses the cell cycle in several ways. So it is not surprising that defects in the pathway are associated with cancer.

Mutations in the genes encoding are found in many cancers including pancreatic and colon cancer. Thus all these genes qualify as tumor-suppressor genes.

Tumor Necrosis Factor-alpha (TNF-α) Receptors and the NF-κB Pathway

TNF-α is made by macrophages and other cells of the immune system.

Receptors

Trimers of 3 identical cell-surface transmembrane proteins.

Ligands

Mechanisms

In May 2003, the US FDA approved a proteasome inhibitor, called bortezomib (Velcade®) for treatment of multiple myeloma, a cancer of plasma cells. [More]

The monoclonal antibody Infliximab binds to TNF-α, and shows promise against some inflammatory diseases such as rheumatoid arthritis.

In addition to its effects of gene expression, activation of the TNF-α receptor can lead to apoptosis of the cell. [Link]

The T-Cell Receptor for Antigen (TCR)

T cells use a transmembrane dimeric protein as a receptor for a particular combination of antigen fragment nestled in the cleft of a glycoprotein encoded by genes in the major histocompatibility complex.

Receptors

Link to How the T-cell receptor (TCR) is formed from gene fragments.

Ligands

Link to discussions of

Mechanisms

Activation of the TCR (when aided by costimulator molecules also present in the plasma membrane) The immunosuppressant drugs FK506 and cyclosporine inhibit calcineurin thus reducing the threat of transplant rejection by T cells.

Conclusions

Reading this page may help explain why such a large proportion of the genome of animals is devoted to genes involved in cell signaling.
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14 October 2005