6 Cell Birth & Death

• altered proliferative states of cells that are reversible: proliferation stops when stimulus that provoked it is removed

• listed in order of how serious dysregulation is

• Regeneration
• Hyperplasia
• Metaplasia
• Dysplasia

• altered growth states in tissues can reverse back to normal or at least stop progressing if the stimulus that provoked the proliferation is removed

irreversible proliferation of cells

• proliferation continues EVEN IN THE ABSENCE of an external stimulus

proliferation continues even in the absence of an external stimulus (NOT CANCER)

• NON-reversible proliferation state

• when there is a loss of proliferation control, but NO loss of position control

• means that a tumor will result but can't spread

• • most still retain enough of the characteristics of the original cell to detect what type of cell it is (whereas for many cancers the only way to determine the original cell type is to look at filaments & such)
• • eg. uterine fibroids

loss of BOTH proliferative & positional controls

• eg. metastatic tumors, "cancer"

• malignant neoplasia or malignant tumor
• • cell has to lose both proliferation controls & positional controls to be cancerous
• • all cancers are neoplasia, but not all neoplasia is cancer

a one-for-one replacement of cells

• • type of reversible proliferation state
• • eg. epithelia will hopefully regenerate after an angioplasty

• an increase in number of fully differentiated & functional cells in a tissue
• • corresponds to more cells than normal in response to some kind of stimulus (eg. injury)
• • disease examples: Restenosis [following vascular surgery], Grave's disease

• if endothelial cells don’t regenerate quickly enough after a balloon angioplasty, then smooth muscle cells can become hyperplastic

• this is problematic because excessive smooth muscle cells causes re-blockage (restenosis) of the blood vessel that was JUST opened

an increase in cell size; also used to describe tissues & organs that have enlarged (NOT an altered proliferative state - don’t confuse with Hyperplasia)

• • a form of hyperthyroidism in which there is a pathological increase in fully differentiated thyroid cells
• • ↑ # of cells → ↑ thyroid hormone production → ↑ metabolic rate, weight loss, bulging eyes, & strabismus (eyes don't look in the same direction at the same time)
• • HYPERPLASIA
• • most common form of hyperthyroidism

• • bone marrow is signaled to make more RBCs
• • normally hematopoietic cells are interspersed with fat droplets
• • serious blood loss/high altitude → lipid droplets disappear in favor of hyperplastic hematopoetic cells
• • eg. of Hyperplasia being useful
• 

replacement (adaptive substitution) of one cell type with another cell type; ALWAYS pathogenic (net harm to individual)

• 1. Lung: in smokers heat & smoke cause normal cells to be replaced by other, more protective cells
• 2. Endocervix: Chronic Pelvic Inflammatory Disease
• • normal columnar is replaced by a stratified squamous epithelium; more protective squamous epithelium lacks cilia & can't move mucus along well creating a rich environment for bacterial/viral replication

• changes in mitotic rate of cells, loss of positional control, & loss in the uniformity of cell shape (pleiotropy)
• • often a precursor to cancer
• • seen in the Exocervix where it is often a precursor to CERVICAL CANCER (reason for pap smears)
• • also seen in Skin Biopsies: dysplastic nevi (mole) can go on to become cancerous (melanoma)

• • BENIGN NEOPLASIA (tumor) of smooth muscle cells that grow in the uterus & cause severe pain, pressure, heavy bleeding, & fertility problems
• • positional control is retained
• • most common tumor of women

• its proliferation rate
• • partly because information in the extracellular matrix helps regulate cell proliferation
• • eg. epithelial cells in intestinal crypt
• 

• G1
• S
• G2
• M
• 

chromosome segregation (mitosis) & separation of daughter cells (cytokinesis)

• mitosis checkpoint: are all chromosomes attached to the spindle

prepares cell for replicating DNA; cell doubles its contents (eg. protein) in preparation for an eventual cell division

checkpoint between G1 & S where the cell has the option to exit the cell cycle & enter a quiescent state called G0

• • cells go into G0 based on their extracellular environment, nutritional state, growth factors, amount of DNA damage & positional information
• • R point is defective in almost all cancer cells

CANCER cells; they plough through & if they start making DNA but don't have enough material they'll happily die

• cell cycle phase during which DNA replication occurs
• • there’s a checkpoint at the end of S during which the cell makes sure all DNA has been replicated (makes sure no DNA was left out)

• prepares cell for segregation/division of genome & cytoplasm
• • G2 checkpoint corresponds to making sure all DNA has been replicated ONLY once & that none of the DNA is damaged

• • monitor the health of the nuclear genome (eg. DNA damage, completeness of DNA replication, alignment of chromosomes)
• • monitor availability of key nutrients & cytokines in the environment

Cyclin B

• factor made up of Cyclin B & Cdk1 that regulates mitosis
• • was discovered when mature (frog) oocyte cytoplasm was inserted into immature oocyte causing it to go through meiosis
• • amount of protein builds consistently through Interphase & after reaching a critical threshold → induces meio/mitosis
• 

1. lamins (→ nuclear membrane disassembly)

2. histones ( → chromosome condensation)

~ nucleolin is another protein (in the nucleolus) that gets phosphorylated by CyclinB-Cdk1

• • mitotic Cyclin binds to mitotic Cdk
• • are still inactive when complexed together
• • inhibitory & activating kinases phosphorylate the Cdk
• • the complex will be inactive as long as there’s an inhibitory phosphate bound to the Cdk
• • activating phosphatases can activate the complex by removing the inhibitory phosphate
• • complex can only be active if only an activating phosphate is bound
• 

• normally inactive (underphosphorylated) & complexed w/ TFs needed for cell proliferation
• • this prevents them from binding to DNA
• • during correct passage through the R checkpoint, Cyclin D/Cdk4-6 & Cyclin E/Cdk2 complexes phosphorylate Rb, changing its conformation & releasing the TFs that cause the cell to go through the cell cycle

• • active: underphosphorylated, complexed with Tx factors
• • inactive: phosphorylated, NOT complexed with Tx factors
• • in tumor cells, the Rb protein is missing or defective

• 1. external signals (growth factors) stimulate synthesis of Cyclins D & E
• • Cyclin D partners w/ Cdk4 or 6
• • Cyclin E partners w/ Cdk2
• 2. when enough Cyclin D/Cdk4-6 & Cyclin E/Cdk2 are activated they phosphorylate Rb protein
• 3. the TFs released from Rb bind to DNA & activate transcription that encodes proteins which push cells through the R-point into S-phase
• • once DNA synthesis starts a protease is activated & destroys Cyclins D & E inactivating kinase complexes

Cyclins D & E drive the cell through the R-point by inactivating Rb protein via phosphorylation

• 10¹⁶ times every gene in a cell is replicated (aka 10¹⁶ gene duplications)
• • human mutation rate: 10^-6?
• • meaning there are 10¹⁰th mutations in every gene in our lifetime

• the tissue environment that a cell finds itself in is critical to it’s development as a cancer cell (or not developing into a cancerous cell)
• • cancer is a complex pathogenesis that isn’t just caused by mutations or signaling pathway defects, but also caused by interactions with the environment
• • eg. ErbB (tyrosine kinase) is the receptor for EGF (a proliferative factor for epithelial cells)
• - mutated ErbB sits on a cell membrane & doesn’t need to see EGF (external signal/stimulus) to be active → is constitutively firing & causes cell proliferation

• • to balance the size of tissues
• • to maintain tissue homeostasis the number of cells dying should equal the number of new cells born

cell proliferation

• • seen in thymus, spleen, small intestine, epidermis, ovarian follicles
• • wherever there are high proliferation rates there is a high rate of apoptosis

• • accidental/unplanned cell death
• • triggered by sustained ischemia or physical or chemical trauma (environmental changes)
• • cells & organelles swell → organelle damage
• • chromatin is randomly degraded
• • cells LYSE
• • INFLAMMATION results

• • genetically programed cell death
• • triggered by specific signals that activate specific genes
• • cells shrink & organelles remain intact
• • chromatin is degraded systematically
• • membrane blebs (chunks of cell) can be phagocytosed by neighboring cells
• • PHAGOCYTOSIS results (there is no inflammatory response; happens in an orderly fashion)

• Necrosis: just a smear from - → +, no distinct banding because chromatin is degraded randomly

• Apoptosis: distinct banding because cell contents (including chromatin) are kept intact

• digit formation
• • at a certain time in development, cells that make up webbing between digits undergo apoptosis so distinct digits are created
• 
• • syndactyly is when the process isn’t done correctly

• • autosomal dominant disease caused by defect in the PKD1 (or 2) gene that results in uncontrolled APOPTOSIS of kidney cells
• • kidney tissue becomes full of cysts because cells that’ve died from apoptosis leave spaces → become filled with fluid → creating cysts
• • clinical correlate: APOPTOSIS
• 

• apoptitic endonuclease cleaves DNA in linker region between nucleosomes - why you get multiples of 180 bp banding
• 

• 1. Induction
• 2. Modulation
• 3. Execution
• (I'm Meredith Elman)

• can be physiologic, damage-related, or therapy-associated
• • 2 major pathways: intrinsic & extrinsic

applies mostly to the Intrinsic pathway, what tells a cell to speed up or slow down apoptosis (pro- or anti-apoptotic)

• • carried out by the Bcl proteins
• • balance of pro or anti-apoptotic Bcl proteins determines whether a cell is going to live or die
• • tumor cells either grossly over-express anti-apoptotic Bcl (Bcl-2) or under express pro-apoptotic Bcl (BAD)

• caspases (what are directly responsible for blebbing) followed by endonucleases
• • caspases degrade cell structures (eg. proteins)
• • endonucleases chop up DNA into nuclesome sized fragments

• intrinsic: growth factor withdrawal, survival factor withdrawal, glucocorticoids

• extrinsic: TNF-α, FasL

• viral infection, heat shock, toxins, tumor suppressors, oxidants/free radicals
• • all damage related activators of apoptosis do so via the intrinsic pathway

• UV/gamma radiation, chemotherapeutic drugs
• • all therapy associated activators of apoptosis do so via the intrinsic pathway

• • upon mild ischemia, removal of nutrients, there is a withdrawal of growth factors (& the like)
• • when apoptosis signal is received, pro-apoptotic Bcl proteins (eg. BAD) are DEphosphorylated
• • they bind to others on the outer mitochondrial membrane
• • this results in loss of mitochondria membrane channel control & cytochrome C is released
• • helps cleave caspase 9 → begins caspase amplification
• • if you want cell to LIVE, mitochondria membrane channels must stay closed

cleaves cytoskeletal fibers & activates specific endonuclease that makes the ladder; common enzyme in apoptosis cascade

• TNF-α (tumor necrosis factor: used to be known as cachexin because it was found in end stage cancer patients that had massive death of tissue)

• FasL (Fas Ligand)

• • via receptor mediated apoptosis
• • 2 death receptors may be expressed on a cell's membrane: TNF-α & FasL (fas ligand system, or FasL receptor)
• - almost all cells have TNF-α receptors in their membrane
• • a cell presenting a ligand will live
• • cells with receptors will die
• • once bound → receptor's death domain is activated & initiates caspase cascade

• • via the EXTRINSIC pathway
• • all T-cells have FasL RECEPTORS on their surface
• • blood vessels lining immune privileged sites constitutively express FasL on their endothelial cells
• • when T-cells enter blood vessels lining privileged sites, FasL receptors on T-cells interact with FasL on endothelial cells
• • this causes T cells to undergo rapid apoptosis

autoimmune disease in which antibodies against the thyroid epithelial cells attack & induce apoptosis → leads to compromised thyroid function

• • inhibitors of apoptosis (IAPs) exist in humans; neuronal IAP can protect stroke victims from excessive loss of neurons if given in time
• • investigators induce stroke in rats: control rats (no apoptosis inhibitor given after stroke) are left with fewer brain cells than experimental rats given the apoptosis inhibitor drugs
• • clinical correlate: anti-apoptosis drugs
• 

both pro- & anti-apoptotic family of proteins

• • individual produces LOTS of pro-life Bcl proteins (eg. Bcl-2, Bcl-X1) but low levels apoptosis-inducing Bcl proteins (eg. BAD)
• • results in excessive overgrowth of cells
• • clinical correlate: apoptosis modulation, Bcl proteins

• • intrinsic: Bcl protein DEphosphorylation, complexing, & openings of mitrochondrial membrane channels to release cytochrome C
• • extrinsic: binding of a TNF-α or FasL ligand to their respective receptors

• 1. intrinsic pathway is activated by: growth factor/survival factor withdrawal, glucocorticoids, viruses, heat-shock, toxins, tumor suppressors, oxidants/free radicals, DNA damage-causing events (UV/gamma irradiation), chemo drugs & ischemia
• • extrinsic pathway is activated by TNF-α or FasL binding to their receptors on the cell surface
• 2. the intrinsic (NOT extrinsic) pathway can be modulated by Bcl proteins
• 3. Similarity: both pathways result in massive amounts of caspase-3 activity → the chief executioner of cells undergoing apoptosis

• • Syndactyly + polydactyly
• • cancers expansion
• • cachexia (“wasting”) seen in some late-stage cancer patients
• • Polycystic Kidney Disease
• • Hashimoto’s (autoimmune form of hypothyroidism)