resistance transporters in biology and medicine
work centers on finding new approaches to combat multidrug resistances
in cancer chemotherapy treatments
Recent press links:
models of drug resistance proteins
Video popularizations of our work:
What we do
and why we do it:
occur when cells in our bodies lose control of the reproduction of
new offspring cells. This results in the uncontrolled proliferation
of cells which can manifest as a solid tumor or as a liquid cancer
such as in leukemias.
Genetic changes in cancer cells that generally make the cancer more
aggressive and harder to treat are associated with the growth and
aging of the cancer itself. This is the reason that early detection
of cancers is so important: It is often true that younger cancers are
much less aggressive than older cancers.
the changes that cancers undergo during the aging or maturation
process is to adapt to the cancer chemotherapeutics that are
administered to kill them. These “resistances” can occur in many
different ways, from very specific inactivations of individual drugs
to more general methods that simply limit the exposure of the cancer
cell to the drugs. These latter general methods are very dangerous,
since whole categories of chemotherapeutics can be made ineffective
by one or two changes in the cancer cell. The most dangerous of these
adaptations is the “over-expression” of members of a class of
protein called “efflux transporters”.
means that the cells produce more than the normal functional amount
of a protein as compared to when it is found in a normal cell.
“Efflux transporters” are machines made of protein that are found
within our cells that take chemicals from the inside of a cell and
push them to the outside. The efflux process is generally reserved
for chemicals that can harm the cell. It turns out that many cells,
cancers included, treat medicines as toxins and actively pump the
medicines out of the cell, where they then lose any positive
efflux pumps have evolved the ability to push many different
substances across the cell membrane and these are especially
dangerous when over expressed in cancer, since they then confer
resistances to a wide variety of drugs. Of the nearly 100 cancer
chemotherapeutics that have been approved by the FDA, nearly all of
them can be removed from a cancer cell that overexpresses as few as
one of these more generic efflux pumps. These cancers are called
multidrug resistant (MDR).
resistant cancers develop with relatively high frequency especially
in recurring cancers. They are a very significant health problem as
they render therapeutic approaches ineffective. One main cause for
MDR, both in cancer and in the chronic treatment of infectious
diseases like AIDS, is the overexpression of multidrug resistance
membrane transporters like breast cancer resistance protein (BCRP),
P-glycoprotein (P-gp), and the multidrug resistance associated
Multidrug resistances develop with high frequency (up to 40%) after
and are associated with very poor patient prognoses.
lab is dedicated to finding inhibitors for these multidrug resistance
pumps and in understanding how these transporters work on the molecular
The human P-glycoprotein (P-gp) is a
member of a large family of evolutionarily conserved membrane
transporters called the ABC-transporters.
These proteins are used to move many
different types of substances across cell membranes.
Human P-gp has caused many problems in
the medical treatment of cancers and viral infections like HIV-AIDS
because this transporter is able to "pump" chemotherapeutic drugs out
of the cell.
P-gp is also able to bind many
different drugs, which compounds the problem. Cells expressing P-gp or
over-expressing P-gp become resistant to the effects of many different
drugs because of its ability to "pump them overboard".
See below for an introduction to our
current studies on this fascinating enzyme.
Important remaining questions:
What is its structure and how does
In order to understand how the
transporter works, we need to know what it looks like.
Before we can attempt to fix the
medical problems associated with P-gp, we need to know how it works.
How can we inhibit this Multidrug
If we can find a compound that
inhibits the pump, we may be able to use it as part of the treatment
with the chemotherapeutics.
Knocking out the pump may then
destroy the drug resistance so the chemotherapy works again.
Recent progress and publications:
Identification of new inhibitors of P-glycoprotein
using computer models
In 2014 we found four
inhibitors of P-glycoprotein in ultra-high-throughput computational
screening experiments using the SMU High Performance Computing system.
These inhibitors were targeted at P-glycoproteins power source to
prevent them also from being pumped out of the cell. The four
inhibitors were biochemically and biophysically characterized.
Get the paper here.
Molecular pharmacology 86 (6), 716-726 2015
Our new inhibitors of P-glycoprotein
reverse multidrug resistances in
In this 2015 paper, we show that three of the four
inhibitors we found in 2014 actually reverse multidrug resistances in
prostate cancer cells. Not only that, but they don't seem to have
significant toxicities on their own without the co -administered
Get the paper here.
In silico identified P-gp inhibitors potentiate the
cytotoxic effects of paclitaxel in the multidrug resistance (MDR) human
cell line DU145TXR. Cells were incubated with the indicated
concentrations of the chemotherapeutic paclitaxel. The upper figure
shows the sensitive prostate cells (open circles) and the multidrug
resistant prostate cancer cells (closed circles). The bottom figure
shows the resistant cells in the presence of our inhibitors. One can see resensitization of the
multidrug resistant cancer cells to the chemotherapeutic paclitaxel.
Notice how the cells with P-gp inhibitors (red, pink
and blue symbols) die when exposed to paclitaxel!
What is the
structure of P-glycoprotein and how does it pump
drugs through a membrane?
In work reported in 2012 and 2015, we have
been able to model the structure of P-glycoprotein as well as show a
plausible dynamic mechanism of how it works. In the 2015 paper we were
able to show how P-glycoprotein moves drugs through the membrane.
Modeling P-glycoprotein - We
were able to build models of the human P-glycoprotein using
evolutionary relationships with related proteins of know structure.
These homolgy models have been used in drug discovery projects (see
above) as well as in mechanistic studies.
Targeted molecular dynamics - This
is a computational technique that can be used to get a first look at
how a transporter like P-glycoprotein might function. The method using
known structural changes in the protein. We arranged four such
"conformations" into a catalytic cycle and then used molecular dynamics
methods to "push" P-gp from one structure to the next.
The four "targets" we used to simulate P-gp function:
These structures were based on seminal work from several other
- Dawson, R. J. P. and K. P.
Locher (2006). "Structure of a bacterial multidrug ABC transporter."
Nature 443(7108): 180-185.
- Dawson, R. J. P. and K. P.
Locher (2007). "Structure of the multidrug ABC transporter Sav1866 from
Staphylococcus aureus in complex with AMP-PNP." FEBS Lett 581(5):
- Jin, M. S., M. L. Oldham, Q.
Zhang and J. Chen (2012). "Crystal structure of the multidrug
transporter P-glycoprotein from Caenorhabditis elegans." Nature
- Li, J., K. F. Jaimes and S. G.
Aller (2014). "Refined structures of mouse P-glycoprotein." Protein Sci
- Ward, A., C. L. Reyes, J. Yu, C.
B. Roth and G. Chang (2007). "Flexibility in the ABC transporter MsbA:
Alternating access with a twist." Proc Natl Acad Sci U S A 104(48):
- Ward, A. B., P. Szewczyk, V.
Grimard, C. W. Lee, L. Martinez, R. Doshi, A. Caya, M. Villaluz, E.
Pardon, C. Cregger, D. J. Swartz, P. G. Falson, I. L. Urbatsch, C.
Govaerts, J. Steyaert and G. Chang (2013). "Structures of
P-glycoprotein reveal its conformational flexibility and an epitope on
the nucleotide-binding domain." Proc Natl Acad Sci U S A 110(33):
the molecular dynamics system for high performace computing use - First
we build the protein model, then add it to a phospholipid bilayer
system that simulates the cell membrane, and then we add water to
hydrate it. After that, we add thermal energy and heat the system to
molecular dynamics - Some
examples: Note that the water and lipids have been taken out so better
views of the drugs are available. All atoms were present in the
simulations presented below. The cell membrane is represented with the
gold spheres (top layer inside the cell / bottom layer outside the
In these videos notice how the drug moves out of the cell
(towards the bottom of the screen).
With an antihypertensive called Verapamil
bound (red spheres).
Close up of verapamil
With Daunorubicin bound (a cancer chemotherapeutic
sometimes called Adriamycin)
Close up of
Daunorubicin being pumped by P-gp
Using these techniques and structures, we have been able to track the
drug movement through P-glycoprotein and watch it push drugs out of the
We use these models for the computational searches for inhibitors
of P-glycoprotein that reverse multidrug resistances in cancer cells
Current research directions:
are optimizing the P-glycoprotein inhibitors we have found,
are searching for new inhibitors of P-glycoprotein and other MDR tranporters, and are
continuing our mechanistic studies of these intriguing ABC transporter
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