Endo. 4 Detecting and signalling Cell surface receptors: G protein linked and tyrosine kinase receptors: second messengers, phosphorylating kinases, activation of transcription Intracellular steroid receptors; transcription factors Receptor regulation Hormone receptors and disease

Hormone receptors Peptide and protein hormones Steroid hormones Water soluble cell surface receptors Activate second messengers and/or enzymes Cytoplasmic and nuclear effects Steroid hormones Lipophilic intracellular receptors in cytoplasm or nucleus. Cell surface receptors? Receptors are transcription factors

General types of cell surface receptors

Protein and peptide hormone receptors G-protein linked Gs, Gi, Gq, G12 Open ion channels Activate enzymes second messengers activation of tyrosine kinases activation of serine/ threonine kinases Enzyme linked or associated Tyrosine kinase on receptor or on molecule associated with receptor activation of serine/ threonine kinases

G-protein linked receptors Receptors with or associated with tyrosine kinase domains

G-protein linked receptors I and E loops 1-3

Activity of trimeric G proteins after hormone binding to a G-protein linked receptor

Second messengers in G-protein linked receptors Adenyl cyclase cAMP Protein kinase A DAG Protein kinase C Phospholipase C IP2 IP3 Ca2+ Calcium Calmodulin Ca-CAM protein kinase

Signalling at the TSH receptor

cAMP signalling system

The phosphinositide signalling system

Signalling at the vasopressin receptor

A CASCADE OF KINASES G-protein receptors cAMP. Phospholipase C, Ca2+/calmodulin Protein kinase A, C, or CaCM cytoplasm Serine/threonine kinases MEK MAP pathway nucleus

Tyrosine kinases in intracellular signalling

G-protein linked receptors Receptors with or associated with tyrosine kinase domains

Signalling at a receptor with tyrosine kinase activity

Increased glycogen synthase Insulin Insulin signaling in muscle P P SHC P P P IRS 1,2,3 P P PI3K GRB/SHP P GLUT-4 vesicles Endosome PDK1 SOS RAS PKB RAF, MEK, MAPK Increased glycogen synthase FOS, ELK--> gene transcription

JAK-STAT signalling pathway - receptor with tyrosine kinase associated molecule

Signalling pathways at G-protein linked and tyrosine kinase receptors

STEROID RECEPTORS Intracellular Membrane receptors Type I - in cytoplasm combined with hsp e.g sex steroids, glucocorticoids. Form homodimers Type II - in nucleus, often bound to DNA e.g. thyroid hormones,vitamin D Form homo- and hetero-dimers Membrane receptors

STEROID HORMONE RECEPTORS Steroid hormone receptors are a family of transcrtiption factors Different functional regions of the receptor are defined as domains - A-F The C domain is the DNA binding region and is highly conserved Both the A/B domains and E/F domains have transcriptional activity Different steroid receptors are continually being discovered

Hormone crosses cell membrane Heat shock protein dissociates from receptor Hormone binds to receptor and dimerization occurs Dimerized receptors translocate to the nucleus Binds to the hormone response element on the DNA Along with other transcription factors transcription is initiated

Steroid hormone binding to its receptor hsp

The C domain The DNA binding region is made up of 2 zinc fingers Each finger contains 3 helical regions, I, II and III The first helix contains the P box which recognizes specific base sequences on the DNA - the HRE The amino acids indicated in brown are those concerned with dimerization of two receptors

Cartoon showing two dimerized receptors linking into the DNA helix Cartoon showing two dimerized receptors linking into the DNA helix. The base sequences of the estrogen and glucocorticoid response elements are shown below

Activation of transcription by steroid hormone receptors Binding of steroid hormone receptors (transcription factors) to the hormone response element on the DNA induces chromatin remodelling There is binding of additional transcription factors and acetylation of histones RNA polymerase is activated and transcription is stimulated When histones are deacetylated transcription is repressed DNA Histones

Homologous and heterologous regulation RECEPTOR REGULATION Affinity of receptors - postitive and negative co-operativity Number of receptors - up and down regulation Desensitization of receptors - receptor phosphorylation, uncoupling Homologous and heterologous regulation

Receptor regulation Phosphorylation of receptor Binding of arrestin complete desensitization Phosphatases remove phosphate and may reactivate receptor Receptor regulation

Hormone receptors and disease Genetic mutations of receptors - vitamin D resistant rickets. Auto-antibodies against membrane receptors - Graves’ disease. Inability to couple receptors with signal transduction pathways - pseudohypoparathyroidism Receptor regulation in excess deficiency - obesity and insulin resistance Inappropriate receptor interaction (specificity) - LH/TSH, prolactin/GH

Grave’s disease