lecture 11 - spindle poisons Flashcards
Describe the structure of microtubules
long, slender, filamentous tubes comprised of a-tubulin and b-tubulin heterodimers
they have a + end associated with GTP cap
+ end has faster dynamics than the - end, although both can add/lose new heterodimer units
Describe microtubule dynamics
heterodimers can add or leave from both + and - end
catastrophe - frequency of transition from growth to shrinking
rescue - frequency of transition from shrinking to growth
MT can also undergo tread milling - units are added to one end and leave from the other
this allows the MT to ‘move’ while individual heterodimers remain stationary
What is the role of GTP/GDP in MT stability?
Both a and b units have a GTP/GDP binding site at the + end
within heterodimers, the a retains a GTP tightly bound (due to interaction to the B -end) and is non-exchangeable
in contrast, the B +end is accessible and hydrolysis of GTP to GDP occurs at the same time, or shortly after addition to the growing MT
normally the cap has either GTP or GDP/Pi, and is stable
if the cap is lost, the ends fray and catastrophe occurs
Why are microtubules a good cancer target?
during mitosis, the microtubules play a key role in organising chromosomes and cleaving to the daughter cells at the right time
since cancer is characterised by rapid replication, targeting this key step in cell division makes sense
One of the best cancer targets to date
How do anti mitotic drugs (generally) work and what are some examples?
the surface of the globular part of tubular has several binding sites that allow agents to bind
these microtubule targeting agents (MTAs) alter the dynamics, ultimately leading to mitotic arrest and cell death
they are either microtubule stabilising agents (MTAs) - taxane ligands, laulimalide/peluroside ligands - or microtubule destabilising agents (MDAs) - vinca, maytensine, colchicine and pironetin ligands
How do taxane ligands work?
Paclitaxel is commonly used to treat ovarian cancers
The taxane binding pocket is on the inside of the tubular - it acts to strengthen lateral contacts between adjacent protofilaments, leading to MT stabilisation
they bind with high affinity to the site in the b subunit along the inside of the microtubule
just one paclitaxel blocks mitosis by 50% - it is the disruption of dynamics, not the increased polymer mass, that is crucial for interruption of mitosis and ultimately cell death
very effective for ovarian, mammary and lung tumours, but ineffective for many other solid tumours
How do vinca alkaloids work?
They are similar to taxanes - the vinca site is located at the plus end surface of beta-tubulin
the binding alters the surface of the plus end, forming a wedge which interferes with the incorporation of new heterodimers
the plus end remains curved and the microtubule wall cannot form
they are very effective for haematological cancers, but usually ineffective against solid tumours
How does colchicine work?
The binding site is located between the alpha and beta units - it is slow binding and practically irreversible
binding to the heterodimer stabilises it in the survey conformation
during polymerisation, the ends transition from curved at the tip, to straight in the body - the presence of a colchicine ligand prevents this, inhibiting polymerisation
What are possible modes of resistance to these drugs?
Over expression of membrane transport proteins (ABC-transporters), microtubule associated proteins, post-translational modification, or up and/or down regulation of different tubular isotypes
Is it total polymer mass or changes in dynamics that is most lethal for cells?
Could be both
A study where cells became dependent on paclitaxel for slow dynamics showed very fast dynamics when the paclitaxel was removed
thus these cells were both resistant and dependent on the drug
this suggests that it is the changes in dynamics that is the most important role for microtubule interacting drugs
