Leicester Warwick Medical School

1. Leicester Warwick Medical School Cellular Adaptations Dr Gerald Saldanha Department of Pathology Email: gss4@le.ac.uk

2. Introduction • This presentation will …. • Focus on adaptive responses in cell growth & differentiation • Describe cell signalling pathways • Introduce the cell cycle

3. Control of cell growth • Cells in a multicellular organism communicate through chemical signals • Hormones act over a long range • Local mediators are secreted into the local environment • Some cells communicate through direct cell-cell contact

5. Control of cell growth • Cells are stimulated when extra cellular signalling molecules bind to a receptor • Each receptor recognises a specific protein (ligand) • Receptors act as transducers that convert the signal from one physical form to another.

6. Signalling molecules • Most signalling molecules cannot pass through the cell membrane • Their receptors are in the cell membrane • Small hydrophobic signal molecules can diffuse directly into the cell cytoplasm • Their receptors are cytoplasmic or nuclear

7. Signalling molecules • Hormones • Insulin, • Cortisol • etc • Local mediators • Epidermal Growth Factor (EGF), • Platelet Derived Growth Factor (PDGF) • Fibroblast Growth Factor (FGF) • TGF • Cytokines, e.g. Interferons, Tumour necrosis factor (TNF)

8. Receptors • There are three main classes of receptors…. • Ion-channel-linked receptors • G-protein-linked receptors • Enzyme-linked receptors

9. Receptors • Ion channel-linked receptors are important in neural signalling • G-protein and enzyme linked receptors respond by activating cascades of intracellular signals • These signals alter the behaviour of the cell

10. G-protein-linked receptors • G-protein-linked receptors activate a class of GTP-binding proteins (G-proteins) • G proteins are molecular switches • They are turned on for brief periods while bound to GTP • They switch themselves off by hydrolysing GTP to GDP

12. G proteins • Some G proteins directly regulate ion channels • Others activate adenylate cyclase, thus increasing intracellular cyclic AMP • Some activate the enzyme Phospholipase C, thus increasing intracellular inositol triphosphate (IP3) and Diacylglycerol (DAG)

14. Enzyme-linked receptors • Many receptors have intracellular domains with enzyme function • Most are receptor tyrosine-kinases • They phosphorylate tyrosine residues in selected intracellular proteins • These receptors are activated by growth factors, thus being important in cell proliferation

16. Receptor tyrosine kinases • Receptor tyrosine kinase activation results in assembly of an intracellular signalling complex • This complex activates a small GTP-binding protein, Ras • Ras activates a cascade of protein kinases that relay the signal to the nucleus • Mutations that make Ras hyperactive are a common way of inducing increased proliferation in cancer

18. Signalling: cytoplasm to nucleus • Many signalling cascades culminate in activation of nuclear transcription factors • Transcription factors alter gene expression • C-jun and c-fos ( that form an AP1 complex) and c-myc are three important transcription factors

19. Signalling pathway interactions • There are many signalling molecules and receptors • A given cell expresses only a subset of receptors • Different intracellular signalling pathways interact • This enables cells to respond appropriately to complex combinations of signals

22. Cell signalling and proliferation • Animal cells proliferate when stimulated by growth factors • These bind mainly to receptor tyrosine kinases • These signalling pathways override the normal brakes on proliferation • These brakes are part of the cell cycle control system • This ensures that cells divide only under appropriate circumstances

23. The cell cycle • The eukaryotic cell cycle consists of distinct phases • The most dramatic events are nuclear division (mitosis) and cytoplasmic division (cytokinesis) • This is the M phase • The rest of the cell cycle is called interphase which is, deceptively, uneventful • During interphase the cell replicates its DNA, transcribes genes, synthesises proteins and grows in mass

24. Phases of the cell cycle • S phase – DNA replicates • M phase – nucleus divides (mitosis) and cytoplasm divides (cytokinesis) • G1 phase – gap between M and S phase • G2 phase – between S and M phase

25. Cell cycle control • Cell cycle machinery is subordinate to a cell cycle control system • The control system consists mainly of protein complexes • These complexes consist of a cyclin subunit and a Cdk subunit • The cyclin has regulatory function, the Cdk catalytic function

26. Cell cycle control • Cdk expression is constant, but cyclin concentrations rise and fall at specific times in the cell cycle • The Cdks are cyclically activated by cyclin binding and by phosphorylation status • Once activated, Cdks phosphorylate key proteins in the cell

28. Cell cycle control • Different cyclin-Cdk complexes trigger different cell cycle steps • Some drive the cell into M phase, others into S phase • The cell cycle control system has in-built molecular breaks (checkpoints) • The checkpoints ensure that the next step does not begin until the previous one is complete

30. The G1 checkpoint • The G1 checkpoint has been widely studied • The retinoblastoma (Rb) protein plays a key role at this checkpoint • The Rb protein function is determined by its phosphorylation status • S phase cyclin-Cdk complexes phosphorylate Rb

32. The G1 checkpoint • This checkpoint is influenced by the action of cyclin-dependant kinase inhibitors (CKIs, e.g. p21, p16) • E.g. p53 senses DNA damage and induces p21 expression • CKIs inactivate cyclin-Cdk complexes

34. Cellular adaptations of growth and differentiation • Cells must respond to a variety of stimuli that may be hormonal, paracrine or through direct cell contact • These stimuli may arise under physiological or pathological conditions • The way that cells adapt in terms of growth and differentiation depends in part on their ability to divide

35. Cellular proliferative capacity • Tissues can be classified according to the ability of their cells to divide • Some tissues contain a pool of cells that move rapidly from one cell cycle to the next. These are labile cells

36. Cellular proliferative capacity • Some cells dismantle their cell cycle control machinery and exit the cell cycle • These cells are said to be in G0. • Some of these cellscan re-enter the cell cycle when stimulated, e.g. by growth factors. These are stable cells • Others are unable to re-enter the cell cycle. These are permanent cells

37. Growth and differentiation responses • Hyperplasia • Hypertrophy • Atrophy • Metaplasia

38. Hyperplasia • Increase in the number of cells in an organ or tissue, which may then have an increased size

39. Hyperplasia: causes • Hyperplasia can only occur in tissues containing labile or stable cells • Hyperplasia may occur under pathological or physiological conditions

40. Physiological Hyperplasia • Hormonal e.g. endometrium • Compensatory, e.g. partial hepatectomy • TGF alpha, HGF • TGF beta

41. Pathological hyperplasia • Excessive hormone/growth factor stimulation • Often occurs alongside hypertrophy • Associated with increased risk for cancer • E.g. Prostate, endometrium

46. Hypertrophy • An increase in cell size, and resultant increase in organ size

47. Hypertrophy: causes • Occurs in permanent cells • Due to synthesis of more cellular structural components • Physiological or pathological causes

48. Physiological hypertrophy • Increased functional demand, e.g. skeletal muscle • Mechanical • Hormonal, e.g. Uterus in pregnancy • Usually a combination of hypertrophy and hyperplasia

49. Pathological hypertrophy • Increased functional demand e.g. cardiac muscle • Hypertension • valvular heart disease