C2006/F2402 '07 OUTLINE OF LECTURE #16

(c) 2007 Dr. Deborah Mowshowitz, Columbia University, New York, NY. Last update 03/25/2007 02:06 PM . See corrections/clarifications about action of p53 in I-D-2.

Handouts: Need 16A (Cell cycle revisited); 16B (Hormones)

I. Growth Factors, Growth Control, & Cancer -- Putting it All Together

This material is covered in Becker, chap 24 ( Ch17 p. 562 to end), and Purves Chap 17 pp350-355.

   A. What is wrong with cancer cells? Summary from last time

1. Usually G1-S switch is defective. Cycle is normal length (not shorter), but cell does not pause normally at checkpoint --  proceeds too readily from G1 to S.

2. Problem can be at many different steps -- with production of CDK/cyclin complex, with its activation, &/or with it's targets. (See handout 15B, 16A)

3. Two major types of mutations responsible -- as explained previously, these result in either

a. Stuck accelerator (stimulation uncontrolled)

b. Brake failure (inhibition fails)

     B. Types of mutations that cause cancer

1. Can over-activate an activating (turn on) gene = "stuck accelerator" type of alteration -- activator is always 'on.'

a. Examples: Changes in genes for ras (so it's always in active form), or GF (always produced as autocrine), or GF receptor (always activated even without GF present), etc.

b. Terminology:

(1). Normal activator gene is called a proto-oncogene

(2). Over-active version is called an oncogene. (See Becker table 24-1 (17-2) for a list)

c. Oncogene can "over do it" 2 ways (see Becker fig. 24-15)

(1). Altered gene expression: Gene can be overexpressed  -- problem is with gene expression = level of mRNA production and/or translation = level of protein synthesis. Protein is normal, but you make too much of it (or you make it at the wrong time/place). Mutation is in regulatory sequence controlling gene, not in coding part. (ex: constitutive production of a GF)

(2). Altered gene product: Gene can be altered so protein made is constitutively active -- problem is with regulation of protein (not gene) activity -- protein is altered so it is always active. Mutation is in protein coding part of gene. Normal amount of protein is made, but protein is altered. (ex: production of altered GF receptor that is active without its ligand.)

2. Can inactive a blocking gene = "brake failure" type of alteration -- inhibitor is always 'off.'

a. Terminology; Normal "brake" genes are called tumor suppressors (see Purves 17.17 (18.17) for picture or Becker table 24-2 (17-3) for a list)

b. Examples: Most famous examples of tumor suppressors = rb and p53.

(1). p53 -- protein that causes block in cell cycle in damaged cells. Normal p53 acts as 'brake' on activation of CDK/cyclin complex . (see Becker fig. 19-39 (17-40))

(2b). rb -- one of the targets of start kinase. rb protein must be phosphorylated (and inactivated) for cell to enter S.  Rb 'holds up' the cycle until CDK/cyclin complex is properly activated. (See Becker fig. 19-38 (17-36)). 

c. How mutation causes cancer: When p53 or rb (or other brake protein) is defective, cell keeps on growing when it should not -- in spite of damage (p53) or lack of growth factor signal (rb), etc. Result is often a tumor.

3. Types of mutations that cause cancer are used to unravel normal G1/S signaling pathways & vice versa

Try problems 15-1, 15-10 & 15-11.

    C. How do Cancer Causing Mutations occur?

1. Somatic mutations can cause cancer (see Becker fig. 24-12)

a. What's a somatic mutation? A mistake in DNA replication that occurs in somatic cells, not in germ cells. Is not inherited or passed on. Can be a point mutation (change in a few base pairs) or a rearrangement (deletion, insertion, inversion, translocation, etc. ).

b. Changes in Regulation. Genes can become over-active [or inactive] because of rearrangements or other mutations that alter effects of enhancers, silencers, etc. Example of cancer caused by rearrangements of regulatory sequences: Burkitt's lymphoma. (Proto-oncogene is placed next to a very strong enhancer.) See Becker fig. 24-13.

c. Changes in Protein Structure. Protein can become over-active [or inactive] because of changes in the gene that alter the amino acid sequence of the protein. Example caused by rearrangement: chronic myelogenous leukemia (CML -- too many white blood cells) -- cells make a chimeric TK that is constitutively active. Activity of normal TK is regulated; activity of chimeric TK cannot be turned off. See Becker Fig. 24-14 & 24-15.

2. Viruses can cause cancer --  viral DNA integrates into normal cell DNA and brings in new genes (or messes up old ones of host).

a. Virus can carry in a new gene/protein -- an oncogene or a gene that codes for the inhibitor of a tumor suppressor. See Becker fig. 24-18. Ex: HPV (which can cause cervical cancer) makes two proteins that inhibit the tumor suppressors p53 and rb.

b. Virus DNA can destroy a host gene -- virus can integrate into DNA in middle of a normal gene and inactivate that gene -- can knock out a tumor suppressor.

c. Virus DNA can carry in a new regulatory sequence -- virus can integrate into DNA near a normal host gene and provide an enhancer or silencer that changes expression of that host gene -- can turn on a proto-oncogene.

d. Most cancers are not caused by viruses. A few types of cancer are associated with viruses (see Purves Table 17.1 (18.2)) but even in these cases the viruses alone are usually not sufficient to cause the cancer. An example: Cervical cancer is largely viral in origin, in that HPV infection is usually involved. However HPV infection is not sufficient to cause it. A vaccine has been developed recently to prevent HPV infection, and there is considerable controversy about the use of the vaccine. For more background information, go to http://www.cdc.gov/std/hpv/ . For news on the subject, try Medical News Today.

Note: there is some recent evidence that some cases of prostate cancer are associated with a viral infection. See also medpage for a reference to the original report.

3. You can inherit a predisposition to cancer, not the disease itself. How can you inherit a "predisposition" = high chance of getting cancer?

a. Genes that affect DNA replication &/or repair.  If you inherit versions of genes giving low repair or high mutability, tend to get cancer -- sooner or later, one of the random mutations that occurs is likely to mess up a proto-oncogene or tumor suppressor gene as above. Example: xeroderma pigmentosum, which carries a high risk of skin cancer because of defects in the genes for DNA repair. See Becker, box 24A & fig. 24A-1.

b. Tumor suppressor mutations. If you inherit one defective copy of a tumor suppressor gene, nothing happens unless the second copy of the gene gets messed up in a cell. If both copies of a tumor suppressor gene in one cell get inactivated or lost, cancer can result.  (This is the "two-hit" hypothesis for how mutated tumor suppressors cause cancer. See Purves 17.16 (18.16) or Becker fig. 24-17. Example: retinoblastoma, which causes a high risk of eye and ovarian tumors, is caused by defects in rb.

4. Most cancer is sporadic, due to somatic mutation. Not inherited.

5. Cancer develops in stages

a. Most cancers have more than one mutation

b. Selection -- selection for increasing loss of growth control occurs as disease progresses -- cells that grow more aggressively (due to additional mutations) outgrow the others.

c. Progression: Normal cell benign tumor malignant (invasive) metastasis (spreads) See Purves 17.18 (18.18) or Becker fig. 24-10.

d. What sort of mutation causes cancer? Is cancer caused by a 'lack of function' mutation or 'gain of function' mutation? Usually both.

Try problems 15-3, 15-4, & 15-7.

     D. How do Rb & p53 fit into normal cycle? How do mutant tumor suppressor genes/proteins cause cancer.

1. Rb & E2F -- see handout 16A & Becker fig. 19-38 (17-36)

a. Role of E2F: E2F = TF needed to make proteins to enter S.

b. Role of rb protein: inhibitor of E2F. Rb holds E2F in check until CDK/cyclin is properly activated by ras et al. 

c. Role of start kinase: Active cyclin/CDK complex (= start kinase) phosphorylates rb protein, inactivating it and releasing active E2F.

d. How inactive rb causes loss of growth control: If both copies of RB gene in a cell are knocked out, then cell makes no rb protein, and uncontrolled growth develops (because E2F cannot be inhibited.)

e. How role of rb found:   This discovered because individuals who inherited one defective copy of RB developed tumors. Cells in tumor had both copies of RB knocked out (tumor cells =  rb -/- = homozygous defective). Cells in other tissues were -/+ (heterozygous). 

f. Led to discovery of importance of E2F

2. p53 -- most commonly mutated gene in human cancers. 

a. What normal p53 is/does: 

(1). p53 is a protein that is unstable; it only builds up if there is extensive DNA damage, for example, from irradiation or chemicals. (In the absence of damage,  p53 is degraded by the proteasome.)

(2). When p53 builds up, it causes either

(a). A temporary block in the cell cycle (until damage is repaired), or

(b). Apoptosis (programmed cell death) in irreversibly damaged cells. Cell "commits suicide" and dies without damaging neighbors. See Becker, figs. 14-25 & 14-26.

(3). Normal p53 acts as "brake" on activation of CDK/cyclin start complex. (see handouts or Becker fig. 19-39 (17-40))

(4). Not all DNA changes are detected by p53. Changes in base sequence that do not disrupt the 3D structure of DNA or interfere with DNA replication do NOT generate a 'damage signal' and do not affect p53 stability. Therefore p53 does NOT prevent all mutations, and if a mutation occurs (due to a mistake in DNA replication, etc.) p53 does not interfere with replication of the mutated cells (because their DNA is normal in overall structure even if it is changed in sequence).

b. Results of p53 failure: When p53 (or other brake protein) is defective, and there is extensive DNA damage, there is no block to DNA replication (or cell cycle), and damaged cell keeps on growing   cells with mutations tumors (sometimes).

3. Rb vs p53: p53 regulates activation of CDK/cyclin complex. Rb is phosphorylated by active complex. So rb acts "downstream" of p53 = after p53, at a later step in the pathway. If rb is out of commission, it doesn't matter what p53 does -- p53 can't block the cycle.

Note: the real situation is quite complex and there is some evidence that ras may also directly effect rb without cyclin. This complexity and/or uncertainty is reflected in your texts -- the different diagrams in the texts differ in where they put cyclin relative to rb/E2F. Hopefully, this matter will be cleared up shortly and the information we get will be useful in preventing growth of cancerous cells.

Try Problems 15-2, 15-5, & 15-6.

   D . What can we do to treat/cure cancer? What use is all this?

1. Classic methods. Usual methods depend on fact that cancer cells are actively dividing and most cells of adult are not. So cure/treatment is to try to remove as many cancerous cells as possible (by surgery) and then destroy any remaining cancer cells with drugs (chemotherapy) and/or radiation. This destroys any normal dividing cells as well, so it has serious side effects. (It also loses effectiveness with time because it selects for growth of drug/radiation resistant cells.) See Purves 17.19 (18.19).

2. New methods (mostly still under development). Target specific protein(s) in cancer cells that allow unregulated growth or metastasis. See articles & Becker Ch 24. Examples:

a. Small molecules that block enzymes/receptors:

Gleevec (A review of Gleevec) -- inhibits a constitutive kinase; binds to and blocks the ATP binding site.

Tamoxifen -- binds to and inhibits an estrogen receptor.

b. Monoclonal antibodies (See Becker box 24B)

Herceptin, Iressa & Erbitux are monoclonal antibodies to GF receptors on tumor cells; Selling Erbitux stock is what landed Martha Stewart in jail. (A sobering note: current prices for these drugs run from $3000 to over $9000 per month.)

Avastin is a monoclonal antibody to a VEGF -- a growth factor that promotes vascularization (growth of blood vessels to support the tumor). See Becker fig. 24-22 for effects of blocking vascularization.

By this point, you should be able to do all the problems in Problem Set 15. You do not need to memorize the names of all the proteins and genes involved in the cell cycle. You need to be able to do the problems if you have clean copies (no annotations) of the class handouts in front of you.

II. Introduction to Hormones (Primarily Endocrines)

    A. How to describe or classify hormones?

1. Many Possible Classification Schemes -- Hormones can be classified by effect, chemical nature, source (which gland?), target cells, etc. etc. See Topic III for a extensive list.    

2. Today: We will look at (1) processes controlled by hormones,  and then (2) the major hormone producing glands. Next time we'll look at details for specific hormones.

    B.  Summary of typical hormone roles and examples.  See Becker Table 14-3 (10-3) or Purves table 42.1 (41.1) for a list of hormones by type of function (Becker) or by source (Purves).

1. Stress response -- cortisol, epinephrine. Regulate heart rate, blood pressure, inflammation, etc.

2. Maintainance of Homeostasis -- insulin, glucagon. Regulate blood glucose/energy supplies and concentrations of substances in general. Maintain more or less constant conditions = homeostasis. (To be discussed next time.)

3. Regulation of episodic or cyclic events -- estrogen, insulin, oxytocin -- regulate lactation, pregnancy, effects of eating, etc.

4. Growth/overall regulation -- growth factors, tropic hormones  -- regulate production of other hormones. (Note: not all GF's are endocrines.)

    C. Overview of Major Glands & Hormones -- see handout 16B for overview. For a complete list see Purves Table 42.1 (41.1)

1.  Hypothalamus (HT) / Pituitary Axis -- Two Parts

a. HT/Ant. Pit -- 3 stages (more details next time)

(1). HT hormones (releasing factors) that signal the AP

(2). AP (anterior pituitary) → tropic hormones (ACTH, LH, etc.) that signal to glands (endocrine tissue)

(3). Glands lipid soluble hormones (steroids & TH) which control their target organs. Overall:

 HT releasing hormone AP   tropic hormone   TARGET GLAND hormone TARGET TISSUE action.

(4) AP also  "other hormones" (GH, Prolactin, etc.) that signal to nonendocrine tissues

b. HT/Post Pit. (Purves 42.5 (41.5).

                    (1): One Stage: Direct connection --  HT secretes hormones (neuroendocrines) from nerve endings in Post Pit.

(2). Hormones =  ADH (vasopressin) and oxytocin. Peptides are very similar in structure (homologous = share common evolutionary origin)  but bind to different (G protein linked) receptors dif. effects.

(a). ADH. Affects (primarily) water retention; has 2 names because discovered twice from different effects. Details of action to be described when we get to kidney. (Works through IP3 or cAMP.)

(b). Oxytocin. Affects milk ejection, uterine contractions -- works (at least in part) through IP3 to affect Ca++ and therefore contraction

(3). Details of Structure/hormone release -- Two parts of pituitary (anterior pituitary and posterior pituitary) develop and function separately; connected differently to HT. 

(a). Ant. Pit. (epithelial) -- connected by portal vessel to HT.
See handout 16B and/or Purves  42.5 & 42.7 (41.5 & 41.7).

  • Normally, blood flows from artery → capillary bed in some tissue → vein. Does not go from one capillary bed to another. Blood flows through separate capillary beds in parallel, not in series.

  • Portal vessel connects two capillary beds (in series).

(b). Post. Pit. (nervous)  -- Cells of HT have bodies in HT and axons/terminals in posterior pituitary. 

(i). Release hormones (neuroendocrines) from endings (terminals) in post. pit   blood supply.

(ii). Hormones are peptides. Made in cell body, packaged in vesicles, vesicles travel down MT's to end of neurons, hormones released by exocytosis.

2.  Adrenal Medulla & Cortex See Purves 42.10 (41.11).

a. Cortex (epithelial)

(1). stimulated by ACTH (tropic hormone from ant. pit.)

(2). produces three major types of steroids  = corticosteroids. For structures see Purves 42.11 (41.12) .

(a). Glucocorticoids. Ex: cortisol -- involved in long term stress response (after epinephrine wears off --more details after nerves)

(b). Mineralocorticoids. Ex: aldosterone -- regulates salt balance (to be discussed when do kidney)

(c). Sex Steroids -- cortex produces low levels of sex hormones (both androgens and estrogens) in both sexes. That's how females get 'male' hormones and vice versa.

b. Medulla (nervous)

(1). Stimulated by nerves

(2). Is neural tissue

(3). Secretes compounds that can act as transmitters (when signal cell to cell) but act as hormones (neuroendocrines) here -- are released into the blood.  Note same compound can act as a transmitter or a neuroendocrine. (Roles as transmitters to be discussed later.)

(4). Major hormone = epinephrine (adrenaline); also secretes some norepinephrine (noradrenaline).

(5). Receptors. Receptors for these hormones/transmitters are same adrenergic receptors (α & β) discussed previously.

3. Pancreas -- secretes glucagon and insulin -- Control blood sugar balance. Details next time

4. There are other glands/hormones -- the list so far is not exhaustive but covers most of the major players. See texts for complete lists.

Wait on problem set 7 until we discuss more on hormones. It is worthwhile to memorize most of handout 16B in order to keep all the hormones and glands straight.

III. How to Keep Track of Hormones --  How to Classify Hormones & Growth Factors (or Signal Molecules in General). The following is meant as a check list to help you keep track of the various signal molecules. It is for reference & study purposes; it will not be discussed in class.
    Some of these questions/categories overlap, and you can't answer all the questions for all the hormones, growth factors, etc., but the list helps to organize the information you do have.

1. Type of Action -- Is it paracrine, endocrine (hormone), growth factor, neurotransmitter, etc.? (See handout 11B or Purves Table 42.1 (41.1 )

2. Chemical nature -- Is it a peptide, amino acid or derivative, fatty acid or derivative, or steroid? See Becker table 14-4.

3. Where is it made? In what gland or tissue? (HT? pancreas?)

4. Target Cells -- where does it act? (Muscle and liver or just liver?)

5. Mechanism of signal transduction

A. Location/type of receptor on target cells -- Is receptor on surface or intracellular? TK or G protein linked?

B. Type of signal transduction -- Is there a second messenger? Which one? If none, what links receptor to intracellular events?

C. Intracellular mode of action --  what mechanism is used to get the end result? Is there a change in enzyme activity? change in transcription? both? change in state of ion channel?

6. What actually gets done? What happens?

A. Biochemically speaking: Which target enzymes, proteins or genes are affected (glycogen phosphorylase activated? Cyclin gene transcribed?)

B. Physiological End Result: Another hormone secreted? Glycogen broken down, & Glucose in blood up? Note the "result" may have several steps, and more than one can sometimes be considered "the end."

C. What's the (teleological) point? What overall function is served by the signal molecule's action?

1. One list of possibilities: Homeostasis, response to stress, growth, maintenance of some cycle;

2. An alternative version of the list: Regulation of rates of processes, growth & specialization, Conc. of substances, and response to stress.

3. The 2 lists are really the same = homeostasis (control of rates & concentrations), response to stress, & regulation of  growth (unidirectional and cyclic).

Next time: HT/Anterior Pituitary Axis -- Set up & Regulation of overall circuit (HT Ant. Pit. target)