CANCER AND CYTOKINE RECEPTORS
The large and diverse family of cytokine signalling proteins includes lymphokines, interleukins and chemokines and the members play major roles in the immune system and hematopoietic cell development. The major classes of cytokine receptors also signal via tyrosine kinase activation, but for these, the enzyme is a Janus kinase (JAK) – a distinct protein that associates non-covalently with the receptor (Fig. 1).
1. Cytokine signalling.
Receptor dimerisation activates JAKs to trans-phosphorylate each other and to phosphorylate the C-termini of the receptors, to which STAT (signal transduction and activator of transcription) proteins bind before becoming phosphorylated. STATs then dimerise and translocate to the nucleus to direct gene transcription. STATs may form stable homodimers or heterodimers that are active as transcription factors.
In addition to cytokines, growth factors (e.g. EGF) can also activate the JAK/STAT signalling pathway through the recruitment of the SRC tyrosine kinase to the EGFR. Activated SRC can then phosphorylate both JAKs and STATs. Conversely, a number of cytokines (e.g. IL3 and granulocyte-macrophage colony stimulating factor) can activate both JAK/STAT and MAPK pathways and the phospho-tyrosyl moieties of some activated cytokine receptors can provide binding sites for the regulatory subunit of phosphatidylinositol 3-kinases (PI3Ks: produce PIP, PIP2 and PIP3 from PI), as do EGFR family members.
G-protein-coupled receptors (GPCRs), (also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors and G-protein-linked receptors (GPLR)), comprise a superfamily of over 1,000 genes that contribute to the control of most aspects of cellular behaviour. On the basis of shared sequence motifs and functional similarity they are grouped into four classes:
Class A (or 1) (Rhodopsin-like)
Class B (or 2) (Secretin receptor family)
Class C (or 3) (Metabotropic glutamate/pheromone) Class D (or 4) (Fungal mating pheromone receptors) Class E (or 5) (Cyclic AMP receptors)
Class F (or 6) (Frizzled/Smoothened)
GPCRs activate intracellular signal pathways via trimeric G proteins (Fig. 2).
2. Hormonal activation of G-protein signalling via a G-protein-coupled receptor (GPCR).
Ligand binding induces a conformational change in the receptor that promotes guanine nucleotide exchange from an interacting G protein (when the α subunit dissociates from the complex of β and γ subunits). This results in the activation of one of four classes of Gα subunits (Gαs (stimulatory), Gαi (inhibitory), Gαq/11 or Gα12/13).
The two major signal pathways that are activated as a result are cyclic adenosine monophosphate (cAMP) and phosphatidylinositol (PI). Gs proteins activate adenylyl cyclase, which catalyses the production of cAMP from ATP, thereby activating protein kinase A (PKA), a major regulator of cell metabolism. Gαi inhibits PKA.
GPCRs activate PI signalling when the receptor binds to a Gq subunit: this activates phospholipase Cβ causing the hydrolysis of phosphatidylinositol 4, 5-bisphosphate (PIP2) into two-second messengers, inositol 1, 4, 5-trisphosphate (IP3) and diacylglycerol (DAG). The release of calcium from the endoplasmic reticulum raises the cytosolic concentration of calcium, which, together with DAG, activates the serine/threonine kinase protein kinase C (PKC).
Ligand-gated ion channel receptors (LGICs) are trans-membrane pores, typically highly selective (e.g. for Na+, K+, Ca2+ or Cl-), that are opened or closed in response to the binding of a ligand (usually a neurotransmitter). These ion channels are either an integral part of the receptor molecule (ligand-gated ion channels) or are linked to the receptor through a G-protein-mediated mechanism (ion channel-linked receptors). Ligand binding induces a conformational change in the channel-forming protein that increases ion flux across the membrane.
There are three LGIC superfamilies: Cys-loop receptors (e.g. GABAA receptor, nicotinic acetylcholine receptor), ionotropic glutamate receptors (e.g. NMDA receptor) and ATP-gated channels (e.g. P2X) and each of these channels can be switched on and off rapidly. There is no evidence that abnormal LGICs contribute to cancers although there is evidence that altered expression of some voltage-gated ion channels is associated with the invasive capacity of some tumour cells.
These two classes of receptors thus play a less prominent role in cancer than the RTKs. Nevertheless, many forms of GPCR, some of their ligands and the coupling G proteins are aberrantly expressed in a range of cancers. Their activation may be autocrine (affecting the cells in which it is produced) or paracrine and promoted by tumour cells themselves (e.g. transforming growth factor α, insulin-like growth factor) or by stromal cells in the vicinity of a tumour. Some tumour cells use detection of the level of the ligand stromal-derived-factor-1 (SDF1) by CXCR4 (a GPCR sometimes called fusion) as a guide to secondary sites during metastasis. Endothelial cells that form the inner lining of blood vessels are characterised by strong expression of S1PR1, which when activated by its ligand, sphingosine 1-phosphate, switches on cAMP, RHO and RAC GTPases and phosphatidylinositol signalling pathways, acting as an important mediator of angiogenesis. An emerging complexity is cross-talk between GPCR pathways and the RTK network. Thus, for example, S1P receptors are trans-activated by RTKs and the prostaglandin receptor EP2 regulates the activity of the EGFR.