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Transcript
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 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 • Phospholipase C IP2 IP3 • Calcium Protein kinase C Calmodulin Ca2+ 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 Insulin Insulin signaling in muscle P P P P SHC P IRS P 1,2,3 P P Endosome GLUT-4 vesicles 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 • 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. The base sequences of the estrogen and glucocorticoid response elements are shown below DNA Histones 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 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 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