Anticancer II Flashcards
Microtubule Structure
:polar structure: plus end (more actively growing end/ dynamic end). Beta subunits form the plus end, alpha subunits form the minus end. The addition and loss of subunits occur only at the ends, never in the middle. hallow tubular structures that are part of the cytoskeletal elements of the cell. Too small to be seen with classic light microscopy. You have to use tricks with phase contrast or differential interference contrast to see the shadows. you can also use labeled antibodies. Or make a fusion to GFP.
The Tubulin Dimer
Guanine nucleotides bind to both alpha and beta tubulin. Alpha and beta proteins are about 90% identical in amino acid structure. When GTP binds in alpha tubulin, it never hydrolyzes unless the protein denatures.
If GTP binds to beta tubulin, upon incorporation of the dimer into a microtubule, you get instantaneoushydrolysis of the GTP to GDP. So in the microtubule, the dimers have GTP in the alpha subunits and GDP in the beta subunits. At the ends of the microtubules, you may have a transient region of GTP region, or you could have the GDP with the inorganic phosphate trapped in the binding place. GTP, microtubule tends to be straight. GDP, individual protofilaments tend to curve.
Microtubules Are Dynamic Polymers
- Dynamic Instability: length change over time of microtubules. Changes in size and length of them over time. Spontaneous process.
- Treadmilling: progressive loss of subunits at the minus end and progressive gain at the plus end. Even though there may be loss at the minus end, there is a net gain in the amount of tubulin added. The gain dominates.
- Both of these parameters take place in living cells. Evidence says that both play roles in the process of separating the sister chromatids that is the target for microtubule targeted drugs.
What’s a key feature of microtubule targeted drugs?
Key feature of microtubule-targeting drugs is that they block cells in mitosis at the transition between metaphase to anaphase.
What do some microtubule targeted drugs promote?
Some drugs promote the rescues to happen more frequently, and thus decrease the instability that would normally happen otherwise.
Most of these drugs bind to the end of the microtubule. By blocking the dynamic instability, you suppress the motion of the kinetochores, which
in turn triggers the spindle-assembly checkpoint (mechanism for detecting errors in mitotic cell). If there are errors, the cell will freeze at metaphase
until the cell corrects the problem. If the cell does in fact carry on, aneuploidy may occur or some form of DNA damage. Apoptosis results.
Why Are Microtubules Good Drug Targets?
- Essential for mitosis.
- Also important for cell migration.
The Spindle Assembly Checkpoint
- Prevents cells from entering anaphase unless all chromosomes are properly attached to the mitotic spindle and kinetochores are experiencing a balance of pushing and pulling forces
- Promotes a mitotic arrest when spindle microtubules are perturbed by low (nM) concentrations of microtubule-targeted drugs
- A prolonged (48-72hr) SAC-dependent mitotic arrest activates an apoptotic pathway
What are most microtubule targeted drugs derived from?
Most Microtubule-Targeted Anti-Cancer Drugs are Derived from Natural Sources
What are the two main classes of microtubule targeted drugs?
Microtubule-Targeted Drugs are Divided into Two Main Classes
• Microtubule depolymerizersMT (vin blasting, colchicine)
• Microtubule polymerizers (taxol)
- At micro molar concentrations, these two effects are observed depending on which drug you’re using. At nano molar concentrations, all of these drugs do the same thing; that is to suppress the dynamic instability of microtubules.
Why is DNA a target for anticancer drugs?
In order to divide you need to replicate your DNA. Why are microtubules a target? You need to be able to separate
your DNA when dividing, and this is where microtubules come in.
