Molecular Biology and Genetics

Chapter. 6 Molecular Biology and Genetics



I. Cell Cycle


A. Normal Cell Cycle



Figure 6-2



  • Phases of cell cycle : G1, S, G2, M
  • Duration of cell cycle (e.g., the generation time) : about 24hrs.
  • Three subpopulations of cells



    1. Terminally differentiated : RBC, Striated muscle cells, uterine smooth
      muscle cells
    2. Quiescent (G0) : fibroblasts
    3. Dividing : GI tract, skin, cervix


  1. G1

    ·Synthesis of enzymes & regulatory proteins necessary for DNA synthesis

    : Duration (8 ~ 100 hrs)



  2. S

    ·Nuclear DNA content of the cell is copied.



  3. G2

    ·RNA & Protein synthesis

    Repair of errors of DNA replication



  4. M

    ·Nuclear division occurs.

    Following Mitosis, contain diploid (2n) DNA (44 somatic chromosomes, XX

    or XY)

    cf) exception : hepatocytes(4n), syncytiotrophoblasts


B. Genetic Control of the Cell Cycle



  • To successfully complete the cell cycle, a number of cell-division-cycle(cdc)
    genes are activated.

  • Two checkpoints



    1. G1/S boundary : cell commits to proliferation
    2. G2/M boundary : repair of any DNA damage must be completed




1. Cell Division Cycle Genes

  • Factors that regulate the cell cycle checkpoints : proteins encoded by the
    cdc2 family of genes, cyclin proteins



  • Cyclins



    1. Regulate the checkpoint at the G1/S boundary.
    2. Inhibit progression through the cell cycle in the presence of DNA damage

      cf) p53 tumor suppressor gene : participate in delay of the cell

    3. Mitosis is initiated by activation of the cdc2 gene at the G2/M checkpoint.

    4. MPF(mitosis promoting factor : p34 cdc2 protein + cyclins -> complex

      heterodimer) : catalyzes protein phosphorylation & drives the cell into mitosis


2. Programmed Cell Death (Apoptosis)



  • The regulation & maintenance of normal tissue mass requires a balance
    between cell proliferation & programmed cell death, or apoptosis.


  • Example ;



    • a. Deletion of the interdigital webs
    • b. Palatal fusion
    • c. Development of the intestinal mucosa
    • d. During the menstrual cycle : reduction in the number of endometrial cells.
    • e. Follicular atresia



  • Characteristics



    1. Histological : cellular condensation & fragmentation of the nucleus
    2. Biochemical : increase in transglutaminase expression & fluxes in
      intracellular calcium concentration
    3. Molecular : complex interactions between the bcl-2, c-myc, p53, ced-9



II. Modulation of Cell Growth and Function




  • Three groups of substances that signal extracellular changes



    1. Steroid hormones
    2. Growth factors
    3. Cytokines



A. Oncogenes & Tumor Suppressor Genes




  • Among the genes that participate in cell growth and function,
    proto-oncogenes & tumor suppressor genes are particularly important



  • Proto-oncogenes



  • Encode : growth factors, membrane & cytoplasmic receptors, proteins that
    play key roles in the intracellular signal transduction cascade, nuclear DNA
    binding proteins



  • Positive effects upon cellular proliferation



  • Tumor suppressor genes : inhibitory regulatory effects on cellular
    proliferation


- Table 6-1


- Table 6-2



B. Steroid Hormones



ex) estrogen diffuses through the cell membrane

    -> binds to estrogen receptors that are located in the nucleus

    -> the receptor-steroid complex binds to the DNA (EREs)

    -> gene expression, protein synthesis



C. Growth Factors



  • High-affinity to cell membrane receptor



  • Growth factors exert positive or negative effects upon the cell cycle by
    influencing gene expression related to events that occur at the G1/S cell
    cycle boundary



  • Because of their short half-life in the extracellular space, growth factors
    generally act over limited distances through autocrine or paracrine
    mechanisms.


Table 6-3



  • The regulation of ovarian function occurs through autocrine, paracrine, and
    endocrine mechanisms.


  • The growth & differentiation of ovarian cells : influenced by the IGFs
    (insulin-like growth factors)



    * IGF-1 ---> granulosa cell : increase in cAMP, progesterone, oxytocin,
    proteoglycans, inhibin


    * IGF-1 ---> theca cell : increase in androgen production
    theca cell -> TNF-a, EGF produce


    * EGF : acts on granulosa cells to stimulate mitogenesis


    * TGF-b : activates intracytoplasmic serine-threonine kinase, inhibits cells in
    the late G1 phase of the cell cycle


    * TGF-a : promotes entry of G0 cells into the G1 phase of the cell cycle



Intracellular Signal Transduction



  • Gowth factors trigger intracellular biochemical signals by binding to cell
    membrane receptors (protein kinases - convert an extracellular signal into an
    intracellular signal)



  • Figure 6-4



  • Many of the proteins that participate in the intracellular signal transduction
    system are encoded by proto-oncogenes.



  • Figure 6-5



  • Protein kinase C (PKC) : exhibits serine-threonine kinase activity, plays a
    central role in phosphorylation & cell metabolism and division.


  • G-proteins : guanyl nucleotide-binding proteins.



    (1) Heterotrimeric, large G-proteins : link receptor activation with effector
    proteins such as adenyl cyclase (--> activates the cAMP-dependent,
    kinase-signaling cascade.)


    (2) Monomeric, small G-proteins : encoded by the ras proto-oncogene family,
    are designated p21



    • ras protein exhibits GTP binding/GTPase activity.
      GTP -> GDP : terminates p21 ras activity.


    • p21 ras protein influences the production of deoxyguanosine(dG) & inositol
      phosphate(IP)3, arachidonic acid production, and inositol phosphate
      turnover.



1. Gene Expression



  • Transmission of external signals to the nucleus by way of the intracellular
    signal transduction cascade culminates in the transcription and translation of
    specific genes that ultimately affect the structure, function, and proliferation
    of the cell.


2. Genetic abnormalities



  • Amplification : an increase in the copy number of a gene
  • Point mutations
  • Deletion & Rearrangements




III. Immunology



A. Immunologic Mechanisms


    * Adaptive immune response


    1) Humoral immune responses : production of antibodies that are
    antigen-reactive, soluble, bifunctional molecules composed of specific
    antigen-binding sites that react with foreign antigens


    2) Cellular immune responses : antigen-specific immune responses mediated
    directly by activated immune cells



1. B Cells, Humoral Immunity, and Monoclonal Antibodies

; bone marrow stem cell -> Pre-B cell -> B cell -> plasma cell : produce
antibodies.



  • Monoclonal antibodies that react with tumor-associated antigens.


  • Immunotoxin-conjugated monoclonal antibodies directed to human ovarian
    adenocarcinoma antigens can induce tumor cell killing and can prolong
    survival in mice implanted with a human ovarian cancer cell line.


    cf) Obstacles : tumor cell antigenic heterogeneity, modulation of
    tumor-associated antigens, cross-reactivity of normal host &
    tumor-associated antigens,



2. T Lymphocytes and Cellular Immunity




    1) T lymphocytes : acting as helper cells in both humoral & cellular
    responses acting as effector cells in cellular responses



    2) T cell surface phenotype : CD3 molecular complex, T-cell antigen receptor



    3) T cells recognize antigen through the cell-surface T-cell antigen receptor.



    4) T cells can respond to antigens only when these antigens are presented in
    association with MHC molecules on antigen-presenting cells.



    5) B cells : bind antigen directly, without processing & presentation by
    antigen-presenting cells



    6) Two major subsets of mature T cells



  • T helper/inducer cells : express the CD4 cell surface marker
  • T suppressor/cytotoxic cells : express the CD8 marker



3. Monocytes and Macrophages



    1) Macrophages (and B cells) express MHC class II molecules and are
    effective antigen-presenting cells for CD4 T cells.


    2) T cells do not respond to foreign antigens unless those antigens are
    processed and presented by antigen-presenting cells.



4. Natural Killer cells.



    a. Nonspecific killing of tumor cells and virus-infected cells.



    b. NK activity represents an innate form of immunity that does not require
    an adaptive, memory response for optimal biological function.
    But the anti-tumor activity can be increased by exposure to several
    agents(; IL-2)



5. Biological Response Modifiers


6. Cytokines, Lymphokines, and Immune Mediators



·Table 6-4



# Interleukins



1. IL-1



  • Involved in fever & inflammatory responses



  • Source : macrophages, phagocytic cells of the liver & spleen, some B cells,
    epithelial cells, certain brain cells, the cells lining the synovial spaces.



  • Initiation of early events in immune responses



  • Act as a B-cell activation-inducing factor


2. IL-2 : T-cell growth factor



  • Proliferation-inducing effects


  • Source : activated T cells



3. IL-3 : increase the early differentiation of hematopoietic cells



4. IL-4, IL-5, IL-6 : B-cell stimulating factor



* IL-6



    a. Induction of cytotoxic T-lymphocyte differentiation

    b. Induction of acute phase reactant production by hepatocytes

    c. Activity as a colony-stimulating factor for hematopoietic stem cell



5. IL-8, IL-10 : cytokine synthesis inhibitory factor



# Interferons



* Three types : IFN-a, IFN-b, IFN-r --> interfere with viral production in
infected cells, direct antitumor effects



7. Adoptive Immunotherapy



  • Exposure of peripheral blood monoclonal cells to cytokines(;IL-2) in vitro
    leads to the generation of cytotoxic effect cells called " LAK cells "


  • Adoptive immunotherapy with IL-2 : produce regression of tumors
    (melanoma, renal cell carcinoma)



IV. Factors that Trigger Neoplasia



A. Advanced Age



  • Single most important risk factor
  • Cancer Dx : 50 % of the population by 75 years of age
  • Accumulation of critical genetic mutations over time
  • Exposure to exogenous mutagens, altered host immune function



B. Environmental Factors



ex) activated hydrocarbons ---> produce G-T transversion




  • Smoking : cigarette smoking and cervical ca. is associated.
  • Radiation



C. Immune Function



ex) immunosuppressed renal transplant pt. ---> 40-fold increased risk of
cervical cancer


HIV-infected Pt. with depressed CD4 cell count --> cervical. dysplasia,
invasive disease



D. Diet


ex) dietary fat ---> risk of colon & breast cancer
deficiency of folic acid & vitamin A & C ---> cervical dysplasia &
cervical cancer