Question

• Know-how p53 is activated during the cell cycle and what two outcomes are possible – know the signaling pathways (you do not have to know how PUMA induces apoptosis, just that it does)

• Know the types of chemical modulators of receptors – agonist/antagonist

• Know the G-protein-linked signaling pathway – how it is activated and what happens downstream – be able to describe the signaling pathways that lead to PKA and PKC activation

• Know the tyrosine-kinase signaling pathway – how it is activated and what happens downstream

• Be able to explain a dominant-negative mutant and constitutive mutant receptor in signaling

• Know the types of cell junctions

• Difference between a benign and malignant tumor

• “Seed & soil” hypothesis of cancer metastasis

• Know & describe the 10 hallmarks of cancer

• Know the four most common causes of cancer

• Know the difference between an oncogene and a tumor suppressor

• Understand the immune surveillance theory

• Know the phases of the cell cycle and what occurs at each stage

• Know where the checkpoints (restriction points) occur in the cell cycle and what influences each checkpoint

• Understand the role of cyclins and CDKs – which one is regulatory and which one has the enzyme activity

Answer 1

• p53 is known as the guardian of genome or cellular gatekeeper. In normal cell, p53 protein is degraded by an E3 ubiquitin ligase called MDM2. When DNA is damaged, ATM (a protein kinase) phosphorylates p53 and activate it. Activated p53 prevent binding of MDM2 resulting p53 is established, p53 act as transcriptional factor for several genes such as p21CIPs: arrest cell cycle; p21CIPs is a Cdk inhibitory protein that binds and inhibit the activity of G1 Cdk cyclin complex as a result cell cycle arrest in G1 phase, DNA repair genes: product of these genes involved in repairing of damaged DNA and pro-apoptotic genes: induce apoptosis,example Bax, Bak, Buk, Noxa, Puma.
• Agonist is a chemical modulator of receptor which binds to the receptor and activates it so that biological response is produced whereas antagonist is the one which blocks the biological response by binding to the receptor. For example calcium act as agonist for inositol phospholipid pathway. Inositol triphosphate (IP3) is a hyrophilic secondary messenger that diffuse into cytosol and binds with ER membrane bound Ca2+ channel thus calcium efflux from ER lumen. High cytosolic Ca2+ binds and translocate a protein called protein kinase C (PkC) from cytoplasm to membrane where it is activated by diacyl glycerol along with phosphatidyl serine. Active PkC phosphorylates various target proteins such as transcriptional factors, cytoskeletal proteins and several enzymes that change cell physiology. Antagonist works opposite to agonist and also called blockers hence in the above pathway calcium blocker will work as antagonist and stop the signaling.
• GPCR stands for G protein coupled receptor. GPCR signaling system consists of three essential components: A transmembrane protein: consists of single polypeptide and spans the membrane 7 times and hence called serpentile receptor or 7 transmembrane, recognize external signal and activates G protein via its GEF (Guanine nucleotide exchange factor) activity. A trimeric G protein: There are three types of trimeric G protein: GS (activate adenylate cyclase enzyme which catalyze production of a potent secondary messenger called cAMP), GI (activates cyclic nucleotide phosphodiesterase enzyme which converts cyclic AMP to 5'AMP) and Gq (activates phospholipase C enzyme which acts on a membrane lipid called PIP2 and cleaves it in two secondary messengers called IP3 and DAG). An effector enzyme: Activated effector enzyme can produce or destroy secondary messenger that carry message from cell surface to cell interior.

Activated 7 transmembrane acts as GEF and replace GDP from GTP of trimeric G protein (GS). Activated GS protein dissociate in two components GSGTP and G. GSGTP moves along with membrane and binds with effector enzyme called adenylate cyclase. Activated adenylate cyclase catalyze synthesis of a potent secondary messenger called cAMP. High level of cAMP activates a protein called protein kinase A (PKA) which phosphorylates phosphorylase kinase and activates it, this further phosphorylates glycogen phosphorylase and makes it active and glycogen phosphorylase catalyse breakdown of glycogen.

Activated Gq protein binds and activates membrane bound effector enzyme phospholipase C (PLC). Activated PLC act on membrane lipid called PIP2 and cleaves it in two potent secondary messengers called diacyl glycerol (DAG) and Inositol triphosphate (IP3). IP3 diffuse into cytosol and binds with ER membrane bound Ca2+ channel thus calcium efflux from ER lumen. High cytosolic Ca2+ binds and translocate a protein called protein kinase C (PKC) from cytoplasm to membrane where it is activated by DAG along with phosphatidyl serine. Active PKC act as serine threonine kinase and phosphorylates various target proteins such as transcriptional factors, cytoskeletal proteins and several enzymes that change cell physiology.

• Receptor Tyrosine Kinase (RTK) is the second largest family of receptors that relay signal from cell surface to the interior. Receptor of RTK family consists of single polypeptide that spans the membrane one time and organised in three structural domains: 1) Extracytoplasmic domain act as binding site for primary messenger molecule, 2) Transmembrane domain holds receptor within membrane and 3) Cytosolic domain has intrinsic tyrosine kinase activity. Normally RTK exist in monomeric form in absence of hormone but they start to dimerize when binds with extracellular hormone. Receptor dimerization activates intrinsic tyrosine kinase activity by which they phosphorylate its own tyrosine residue on cytosolic phase. Each phosphorylated residue of RTK act as docking site for different intracellular signaling proteins, which then help to relay the signal to cells interior. Protein-protein interaction leads to assembly of signaling complex that transmit signals from cell surface to the interior.