Leicester Warwick Medical School

Cellular Adaptations

Dr Gerald SaldanhaDepartment of Pathology

Email: gss4@le.ac.uk

Introduction

• This presentation will ….– Focus on adaptive responses in cell growth

& differentiation– Describe cell signalling pathways– Introduce the cell cycle

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

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.

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

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)

Receptors

• There are three main classes of receptors….

• Ion-channel-linked receptors

• G-protein-linked receptors

• Enzyme-linked receptors

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

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 cells can 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

Growth and differentiation responses

• Hyperplasia

• Hypertrophy

• Atrophy

• Metaplasia

Hyperplasia

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

Hyperplasia: causes

• Hyperplasia can only occur in tissues containing labile or stable cells

• Hyperplasia may occur under pathological or physiological conditions

Physiological Hyperplasia

• Hormonal e.g. endometrium

• Compensatory, e.g. partial hepatectomy– TGF alpha, HGF– TGF beta

Pathological hyperplasia

• Excessive hormone/growth factor stimulation

• Often occurs alongside hypertrophy

• Associated with increased risk for cancer

• E.g. Prostate, endometrium

Hypertrophy

• An increase in cell size, and resultant increase in organ size

Hypertrophy: causes

• Occurs in permanent cells

• Due to synthesis of more cellular structural components

• Physiological or pathological causes

Physiological hypertrophy

• Increased functional demand, e.g. skeletal muscle– Mechanical

• Hormonal, e.g. Uterus in pregnancy– Usually a combination of hypertrophy and

hyperplasia

Pathological hypertrophy

• Increased functional demand e.g. cardiac muscle– Hypertension – valvular heart disease

Atrophy

• Shrinkage in cell size by loss of cell substance

– Term is often used loosely to describe reduced organ size that may be related to cell loss rather than shrinkage

Atrophy: causes

• Reduced workload

• Loss of innervation

• Reduced blood supply

• Inadequate nutrition

• Loss of endocrine stimulation

• Ageing

Metaplasia

• Reversible change of one adult cell type to another adult cell type

Metaplasia: causes

• An adaptive response to various stimuli• New cell type is better adapted to

exposure to the stimulus• The stimulus that induced metaplasia

may, later, induce cancer, e.g. squamous cell carcinoma of the bronchus

• Metaplasia in mesenchymal tissues is often less clearly adaptive

Hypoplasia

• Incomplete development of an organ with reduced cell numbers

                                                            

Summary

• Cells communicate through signalling pathways

• Signalling pathways influence the cell cycle control system

• This determines a cells ability to divide• A cells replicative capacity influences its

adaptive responses to changes in the tissue environment