11/2- Control of Cell Proliferation: Relationship with Aging Flashcards
What are the factors that contribute to aging?
Extrinsic/Stochastic/Environmental
- Free radical damage
- Wear and tear theories
- Error catastrophe theory
- AGE/glycosylation
Intrinsic/Programmed/Genetic
- Centralized clock theory
- Glucocorticoid theory
- Immunologic theory of aging
- Apoptosis If we do age because of a genetic program, then we should be able to induce that program in culture and examine its effects
What are theories of aging?
- Homeostatic failure: either in regulatory control systems (like the immune or endocrine systems) or generalized cellular failure (like free radial or AGE-mediate damage)
- Intrinsic: genetic program similar to Development-Antagonist Pleiotropy or telomere shortening
If we age because of a genetic program, then we should be able to induce that program in culture and examine its effects in vitro
What are some reactive molecules (internal) contributing to aging?
Glucose
- Non-enzymatic glycosylation
- AGE (advanced glycosylation end products)
Oxygen/ROS (includes free radicals)
- Peroxide
- Hydroxyl
- Peroxynitrate
Oxidation and glycation can produce damage and modify function of cellular components
What is the wear and tear theory?
- If it were true then low use would be associated with survival
- Failure (or death) would occur at a constant rate, not increasing with age.
- Even today… we see some incredibly old cars still running (but people are not cars)
Extrinsic damage can lead to DNA damage- how is the cell affected?
- Extrinsic damage can occur to cellular and/or mitochondrial DNA
- Repair enzymes work to correct the damage, but mitochondria have less active DNA repair
Single cells with _____ may ____ (increase/decrease) with age
Single cells with MitoDNA deletions may increase with age
- Mitochondria generate the most free radicals
- Single cardiac myocytes had mito DNA expanded
- Frequency of cells with mitoDNA deletion increases with age (closed circles)
What is antagonistic pleiotropy?
- Genes that are beneficial in early life up to sexual maturity carry harmful effects later in the lifespan display antagonistic pleiotropy
- A gene that facilitates calcium deposition will lead to more rapid development of strong bones (benefit) and predispose to arterial calcification later in life (harmful)
- There is NO GENETIC PRESSURE on genes that manifest their harmful effects after reproductive age (even if the gene’s only effects are negative)
What are examples of dysregulation of cell proliferation in aging?
Inappropriate cell division
- Atherosclerosis (smooth muscle cells)
- Prostatic hypertrophy
- Cancers
Inadquate cell division
- Decreased T cell response to immunologic stimuli
- Atherosclerosis (endothelial cells)
- Slow wound healing
- Balding
What is cellular senescence?
- All normal diploid cells from humans and other species will NOT divide indefinitely.
- After a reproducible number of divisions, they will cease dividing, unresponsive to any “stimulus” to divide. They will remain, senescent, quite alive for prolonged periods of time without loss of other function
Exceptions:
- Some stem cells will divide indefinitely
- Cells isolated from malignant tumors will not senesce, but will divide ad infinatum
- Normal cells have limited proliferative potential
How is senescence different from quiescence?
Neither are actively dividing, but if growth factors are added back, quiescent cells will divide (senescent will not)
What is the typical pattern of cell growth in continued culture?
- After adjustment to conditions (Phase I) cell growth usually follows a linear growth for 20-50 population doublings (phase II)
- As cells approach their maximum, their growth rate slows down
There is good correlation between survival and ____
- What is a potential confounder
There is good correlation between survival and fibroblast replicative potential
- Potential confounder is size of organism
What cell types are immortal?
(Immortal = unlimited divisions in culture)
- Cancer cells
- Germ line cells
- Certain “stem cells”, likely, but it may depend on their “sternness”
Senescent cells produce proteins in their cytoplasm that do what?
suppress replication
Found because:
- Put young nucleus in young cytoplast: rapid mitosis
- Put young nucleus in old cytoplast: slow mitosis
- Put old nucleus in old cytoplast: slow mitosis
- Put old nucleus in young cytoplast: slow mitosis
So, slow division is dominant (factors in both cytoplasm and nucleus lead to this)
What was found following fusion studies regarding the pathways leading to senescence/immortality?
There are only 4 complementation groups suggesting that there are only 4 pathways that determine senescence from immortalization;
all cancers tested to date fit in the 4 groups:
- Group A: chr 16
- Group B: chr 4, MORF4
- Group C: chr 1
- Group D: chr 7
What is the typical pattern of cell growth in continued culture?
Cells made senescent by freezing still remember how many divisions they have
T/F: Senescent cells are present int issue from older people
True
- Senescent human fibroblasts seen as blue (senescence-associated beta-galactosidase turning X-gal blue at pH = 6)
T/F: senescent cells are innocent bystanders
False!
In what ways are senescent cells not benign/innocent bystanders?
- Adopt the Senescence Associated Secretory Phenotype (SASP) driven by NF-KB
- Not depend on cell type (fibroblast, adipocyte)
- Release GFs, proteases, pro-inflammatory cytokines (IL-6, IL-1a, IL-8, PAI-1)
- In time, SASP becomes persistent source of low levels of pro-inflammatory signals
- Process of “cleaning up” senescent cells is not well characterized, senolytic drugs under study
- Not only fibroblasts, but fat cells as well. They contribute to insulin resistance and glucose intolerance
What are senolytic drugs?
Drugs to kill the senescent cells
- In vitro testing found pairs of compounds that killed senescent cells in culture
- Old mice given comparable concentrations of one pair quercetin and casatinib resulted in reduced senescent cells by beta-galactose staining
- Improved glucose tolerance
- Decreased inflammation
How do telomeres contribute to cell aging?
- What is the role of telomerase?
Telomeres are the clock/counting mechanism in human cells
- In human AGGGTT and mouse repeats at ends of chromosomes
- Typical newborn human has 2000 repeats at each end; old people have less
- Requires enzyme telomerase to keep up number, otherwise few repeats are lost with each division because DNA polymerase cannot get to end of chromosome
- Tumor cells (90%) and germ line have telomerase; not those that senesce
- 70% of immortalized cells have telomerase
- If you knock out telomerase in tetrahymena, it senesces
- Introducing teloemrase will delay senescence
How can genetic manipulation increase lifespan?
- Stress free environment (dwarf mice have lived longer)
- A number of manipulations have increased survival of C. Elegans worm
- Dauer rod genes (prolong development and hibernation)
- Insulin/IGF-1 like signaling pathway mutations have increased survival and slowed aging
- Drosphilia mutations also have long survival and some are associated with paraquat and oxidative stress resistance
How do epigenetics affect aging?
- If genetic were the sole determinant of aging, than one would expect monozygotic twins to have very similar survival
- BUT differences between twins seem to increase with increasing age
- DNA methylation and histone acetylation are modifications that alter gene expression
- Histone acetylation also varies with age among twin pairs
What is Hutchinson Gilford Syndrome (broadly)?
- Osler’s words on progeria, rarely seen after adolescence:
- Child does not remain infantile but skips adolescence, maturity, and manhood, and passes at once to senility, looking at 11 or 12 yrs like a miniature Tithomas marred and wasted, wrinkled and stunted, a little old man among his toys
What is Hutchinson Gilford syndrome:
- Prevalence
- Inheritance patter
- Gene mutation
- Median age of death
- Cause of death
- Features
- Very rare; 1/8 million
- Autosomal dominant
- Mutation in lamin-A gene (G608G)
- Altered hyaluronic acid metabolism, nuclear structure
- Median age at death = 12 yrs
- Most die from MI or heart failure
Features:
- Accelerated atherosclerosis is indistinguishable from that occurring in older adults
- Diminished subQ fat, scalp hair
- Normal intelligence and no CNS pathology
What is the lamin-A gene responsible for?
It’s a critical component of scaffold of the nucleus that gives nuclear membrane its shape
- Mutation activates a cryptic splice donor site in exon 11 that produces progerin
- Progerin is Lamin A with a 50 AA internal deletion and is missing the final cleavage site
- Progerin is permanently modified by a lipid farnesyl group
- Nuclear envelope has structure maintained by Lamin; nucleus with mutation is very lumpy, bumpy
- Progerin may critically interfere with movement across the nuclear membrane
T/F: Progeria is accelerated aging
False!
- It has aspects of aging, but does not have others
- For example, there are no cognitive changes
- Prominent atherosclerosis
- It is a segmental progeroid syndrome, not aging
Is the lamin distribution modified by aging in nuclei from old cells?
Maybe; it appears somewhat like Progeria changes
What are the long range goals of studies focusing on cellular aging?
- Specific gene therapy may improve quality of life for older people
- Therapies could increase or decrease replicative potential
- Increasing T cell replications may bolster immune function
- Increasing cell replications may improve cell based gene therapy
- Decreasing prostate cell replications may improve cell based gene therapy
- Reinstitution of dominant phenotype (sensecence) might remove aggressiveness from malignancies
Summary
- Normal growth control is a dominant process
- Cells become immortal because of recessive gene defects
- Cell proliferative control and cellular senescence are dictated by active, genetic events, not random accumulation of damage
