Signal Transduction

Fig 12.23  Direct phosphorylation of STAT transcription factors.

STAT1a and STAT1b bind to the tyrosine-phosphorylated receptor and themselves become phosphorylated. They form a dimer (Sis-inducible factor, SIF) which translocates to the nucleus, where it binds to a Sis-inducible element (SIE) within the fos promoter. The domain organization of STAT1 is shown in Figure 12.6.

A switch in receptor signalling: activation of ERK by 7TM receptors

Pathway switching mediated by receptor phosphorylation

G-protein-linked receptors are themselves substrates not only for PKA and PKC, but also for receptor-specific kinases, which preferentially target occupied (and therefore activated) receptors (see Figure 4.14, page 100). These receptor-specific kinases (GRKs, such as the -adrenergic receptor kinase GRK2) are only called into action under conditions of robust

stimulation. On the other hand, phosphorylation by PKA and PKC (triggered by an increase in second messenger production) affects occupied and

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Signalling Pathways Operated by Receptor Protein Tyrosine Kinases

Fig 12.24  Phosphorylation of the -adrenergic receptor by GRK2 activates the Ras–ERK pathway. Phosphorylation of the -adrenergic receptor by GRK2/ ARK1 (see Chapter 4) allows recruitment of the adaptor -arrestin2 and terminates communication with the G protein. It also activates the Ras–ERK pathway by recruitment of Src and MEK1/ERK2. -Arrestin2 also directs the receptor to clathrin-coated pits, there to be removed from the cell surface by endocytosis and degraded in the lysosomal pathway.164

unoccupied receptors alike. The effect of phosphorylation by these kinases is to switch the attention of the receptors to alternative G-proteins. As described in Chapter 4, -adrenergic receptors phosphorylated by PKA now communicate with the Gi proteins instead of Gs, so opening the door to the Ras–ERK pathway188,189 (see Figure 9.5, page 251). The sites phosphorylated

by GRK2, though also present in the C-terminal region, are distinct from those targeted by PKA.

-Receptors phosphorylated by GRK2 bring -arrestin (see page 98) to a segment of the third intracellular loop that would normally interact with G-proteins, so blocking the transmission of signals through Gs and initiating receptor removal (Figure 12.24). Importantly, the bound -arrestin offers an alternative means to activate the Ras–ERK pathway through interaction with the SH3 domain of the Src tyrosine kinase.190 This also initiates a signalling pathway that activates ERK. The steps following the recruitment of Src are not yet clear. It is possible that it initiates a series of events resulting in the phosphorylation of the adaptor Shc-1, which in turn, binds to the Grb2/Sos

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complex. Finally, -arrestins also act as scaffolds for other MAP kinases, binding JNK3, Ask1, and MKK4.164

Other 7TM receptors may employ different pathways to reach Ras. For instance, the lysophosphatidic acid receptor also activates ERK, but here dominant negative Src is without effect. The signal appears to involve

PI 3-kinase, an unidentified tyrosine kinase, a docking protein, and finally Grb2/Sos191.

Pathway switching by transactivation

Another way of switching, that also allows signals emanating from G-protein-linked receptors to activate the Ras–ERK pathway, is by receptor transactivation.192,193 As an example, activation of muscarinic acetylcholine or thrombin receptors releases HB-EGF (an EGF-like factor) by cleavage from its inactive membrane-bound precursor by the metalloproteinase ADAM17. HB-EGF then acts in an autocrine/paracrine manner to stimulate the EGF receptor, resulting in the activation of ERK193,194 (Figure 12.25). In effect, the first ligand induces the release of a second, quite unrelated ligand, which in turn sets in train its own distinct signalling pathways. Receptor transactivation enormously expands the repertoire of signalling systems that a cell can apply

Fig 12.25  Transactivation of receptors.

Activation of the 7TM receptor results in activation of ADAM17, a transmembrane endoproteinase with release of HB-EGF, cleaved from its the membrane-bound precursor. HB-EGF then binds to the EGF receptor and induces yet another set of intracellular signalling pathways that include the Ras–ERK kinase pathway.

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in mounting a response. In this example, the mechanism of transactivation depends, in the first place, upon intracellular signals that activate the transmembrane metalloproteinase which has its catalytic site situated in the extracellular domain.

Pathway switching, transactivation, and metastatic progression of colorectal cancer

Both pathway switching and transactivation make contributions in the malignant behaviour of already transformed epithelial cells of the intestinal tract. The inflammatory conditions caused by tumour cells raise the local production of prostaglandins by leukocytes, in particular PGE2 which acts through G-protein-coupled EP receptors. EP1 receptors activate Gq and raise cytosol Ca2 , EP3 receptors inhibit cAMP production through Gi, and EP2 and EP4 receptors cause transactivation of EGF receptors as described in the previous paragraph.195,196 In addition, EP2 and EP4 bind to -arrestin to activate Src. This then, both through transactivation and pathway switching,

contributes to cell migration in vitro and metastasis in vivo.197 Perhaps aspirin, that old favourite among remedies, might offer some protection against colorectal tumour invasion through its action as an inhibitor of prostaglandin production198.

List of Abbreviations

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Signal Transduction

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