Multidrug Resistance Protein Pumps: Nature's Common Currency by Alison Davis
Tiny pumps protect many animals from toxins -
sometimes a little too well.
The pudgy "fat innkeeper" worm Urechis caupo, which looks more like a frankfurter than a worm, tunnels through the salty mudflats of Elkhorn Slough, California. It collects food by spinning a sinewy, mucous net. But the muddy cocktail trapped within the net contains more than lunch: within lies a virtual potpourri of chemical toxins, some natural and some man-made, that aren't meant to be eaten. Yet as soon as the poisons find their way into the marine worm's belly, they are spit back out into the mire.
At a nearby hospital, a breast cancer patient is losing her battle against cancer because the tenaciously dividing cancer cells in her body are using the same molecular gadgetry to jettison chemotherapy drugs back into her bloodstream, preventing them from attacking her tumor.
The fates of the worm and the woman are linked by a sub-microscopic pumping mechanism. A protein called a "multidrug resistance," or MDR pump, is nature's common currency here. Embedded within a cell's membrane, this protein protects a cell by ejecting a variety of molecules, in many cases, toxins, on contact. The cell might be a bacterium, in which case the "toxins" are antibiotics. With cancer cells, the "poisons" are chemotherapy drugs. MDR pumps are being discovered in almost every organism searched, including Urechis caupo that calls polluted, toxin-laden mud home, says Stanford University embryologist David Epel. Detecting traces of MDR in such a wide array of organisms is leading scientists like Epel to believe that this line of defense has persisted for a long, long time, in biology, usually the signature of a very important molecule.
The MDR protein got its name from the place it was originally discovered roughly a decade ago: in the membranes of cancer cells. A huge, snake-like protein, the MDR pump threads through a membrane like a needle through a piece of cloth, back and forth across the membrane a dozen times. The pump uses packets of energy made by the cell, called ATP, to force out toxins.
Biologist Epel agrees that MDR pumps in worms, mussels and humans might all have natural substrates. While the primordial role for these proteins may have been to protect animals from natural toxins, he suggests, MDR pumps might also transport "normal" substances, such as steroids and some fats, out of cells. For instance, yeast cells use MDR-like proteins to pump out hormones involved in mating, he says. Yet, in mammals, MDR pumps are positioned so as to encounter potentially harmful substances face-to-face, in important places like the intestine, the placenta, and the blood-brain and blood-testes barriers, making them an excellent first line of defense, says Epel.
In the toxic battleground of the ocean, creatures that can cope with what their opponent throws at them are more likely to survive. "It's a space war, and the weapons are chemicals," says Epel, who has also found the pumps studding the flapping gills of mussels. "Organisms use them to compete for the best real estate." The MDR protein, he says, is a method animals have evolved to escort toxins out of a cell. He and other researchers are finding that creatures of all types rely on the pump as a first line of defense ,and it's a particularly advantageous one at that. "MDR doesn't even let the things get in the cell," he says.
The presence of MDR pumps in mussels might be an indicator that certain natural and man-made chemicals are present in the sea water,Epel says. And, in fact there may be more than previously suspected,because of the way toxins have been traditionally measured. For more than a decade, a biomonitoring effort called the California State Mussel Watch Program has been gauging water pollution concentrations by measuring levels of contaminants in mussel tissues. Mussels were thought to be ideal for the job, since they eat by filtering water through their gills and thus have a high exposure to whatever is in the water, says Nancy Eufemia, a graduate student in Epel's lab. Also, they are basically stuck in place, she says, so they can't swim away from something noxious like a fish can.
But, as it turns out, the mussels' MDR pumps allow the animals to rid themselves of much of the toxic material they ingest, thus giving researchers a misleadingly low reading of pollutant levels. Epel andhis group tested eight compounds on the Program's list of common pollutants ,half of them were spit right out by the MDR pump, suggesting that water levels of some chemicals could be underestimated, Epel says.
MDR pumps don't always work to benefit the organism. They can bestow deadly power ,in the form of resistance to antibiotics, to harmful bacteria. While bacterial MDR genes are not identical to those from mollusks and people, the three-dimensional shape of all pumps is much the same, and all MDR pumps perform a similar task: preventing potentially harmful molecules from setting up shop inside their own quarters, the cell's interior.
Microbiologist Kim Lewis of the University of Maryland at Baltimore, studies MDR pumps in so-called "gram-negative" bacteria, a group of microbes that includes the recently notorious E. Coli. One member of this microbial family, S. aureus, can skillfully evade antibiotic-containing hospital handsoaps by immediately rejecting their antiseptic weaponry. This renders the drugs useless, and worse, can lead to menacing and sometimes deadly infections. "It's a pretty dangerous pest," says Lewis.
Many MDR-like pumps also confer resistance against first-line drugs used to treat pathogenic yeasts and parasites, says Lewis. Resistance to drugs called antifungals that are used to treat yeast infections is increasing rapidly, in particular affecting immune-crippled AIDS patients, who can't fight back, he says. And the latest victim that may succumb to MDR is saquinavir, one of a class of anti-AIDS drugs called protease inhibitors, a recent study suggests. Some bacteria, such as P. aeruginosa, which causes an uncontrolled, life-threatening infection called sepsis, have a virtual armamentarium of different MDR pumps. "They extrude practically every [drug] we know of," says Lewis.
In cancer cells, the MDR pump promotes drug resistance as it ejects cell-killing chemotherapy molecules, a victory for the cell, but of course not for the patient, who must then rely on some other form of treatment to keep the cancer at bay.
Many important chemotherapy drugs are denied entry into MDR-laden cancer cells, says Stanford University oncologist Brandy Sikic, who for years has studied multidrug resistance in cancer. "The pump is active in a broad spectrum of human cancers, including lymphoma, leukemia, and breast, lung and ovarian cancers," says Sikic. And in most cases, he says, how much MDR is present in tumor cells can predict how well the patient fares treatment. The more MDR pumps cancer cells have, the less effective chemotherapy is likely to be.
Scientists don't yet fully understand why some cancer cells are packed with MDR and some appear to have little or none at all, but they do know that chemotherapy drugs themselves can sometimes prod MDR production within a select group of cancer cells in the body. When that happens, those cells have a distinct growth advantage and quickly take over, rendering the drug useless for its original purpose: attacking the tumor.
Cancer researchers are now trying to outwit Mother Nature with her own weaponry: MDR. Some cells in the body, including those in the bone marrow called stem cells, have little or no MDR in their membrane. For that reason, they are very sensitive to many cancer-killing drugs: indeed, one of the major drawbacks to chemotherapy is bone marrow toxicity, a serious complication that can leave the patient defenseless against deadly infections. Now, scientists are devising ways to equip the helpless stem cells with MDR armor.
By using gene therapy, in which an MDR gene is "pasted" into the DNA of dividing stem cells in the bone marrow, researchers hope to give stem cells a chance to stand up to the doses of chemotherapy required to kill tumors elsewhere in the body. Though that's easier said than done, says oncologist Sikic, data from preliminary studies suggests that the strategy has promise. Also in progress are clinical trials to determine whether blocking the MDR pump in cancer cells will allow easy access to much-needed chemotherapy drugs that might otherwise be denied entry, says Sikic, who is currently testing such a drug called Valspadar in leukemia and lymphoma patients.
Bacteria probably aren't alone in using MDR pumps, it is likely that there are probably many as yet unidentified such proteins lurking in membranes of all kinds of cells, says Sikic. The latest estimates say 2,000 of the approximately 100,000 human genes are included in the family of proteins to which the MDR pump also belongs, says Sikic. Only a fraction of those genes and the proteins they encode have been studied, he says.
While scientists expect to discover many more MDR-like proteins, they all are puzzled by the broad spectrum of chemicals, called substrates, that the pumps repel. The list of such substrates includes a surprisingly wide variety of compounds, from naturally occurring substances such as alkaloids in plants and antibiotics in dirt, to man-made chemicals such as dyes and some pesticides. Scientists have found only one common thread: all the substances repelled by MDR pumps are relatively hydrophobic. (They don't dissolve easily in water.)
Scientists are consequently coming to believe that there are likely a variety of pumps, each dealing with a group of its own, similar substrates. But researchers such as Lewis also suspect that an as yet unidentified natural chemical substrate has kept the pumps around throughout evolution.
Meanwhile, researchers around the globe continue to search by land and by sea for the MDR signature: indeed, bottom-dwelling flounder are among the most recent club members. As scientists better understand how and why cells universally use MDR pumps, they may well be able to beat nature at its own game.