In the study, published in the June issue of Nature Medicine, it took only weeks of treatment by an experimental drug to restore the normal weight of mice that had doubled their size by eating a high-fat "cafeteria" diet.
Although the work has only been conducted in animals, the authors say it may one day lead to the development of targeted therapies to treat human obesity, which is a risk factor for numerous conditions including cancer, diabetes and cardiovascular diseases.
"When you inject our drug into mice, it homes in on and promotes the death of blood vessels associated with white fat tissue, which is then reabsorbed and metabolized," says Wadih Arap, M.D., Ph.D., a professor of medicine and cancer biology at M. D. Anderson.
"If even a fraction of what we found in mice relates to human biology, then we are cautiously optimistic that there may be a new way to think about reversing obesity," says Renata Pasqualini, Ph.D., also a professor at M. D. Anderson. Arap and Pasqualini are co-lead investigators on the study.
"Most current obesity treatments involve efforts to prevent new fat accumulation," notes the study's first author, Mikhail Kolonin, Ph.D., an instructor who carried out the project in the Arap/Pasqualini laboratory. "This makes our approach unique and exciting, because it shows in animal models that we can remove already-formed fat by a non-surgical method, a molecular liposuction."
Moreover, the study is another proof of the concept of vascular "zip codes" which was first pioneered by Arap and Pasqualini. Blood vessels are not simply a uniform and ubiquitous "pipeline" that services all parts of the body, but are different depending on the kind of organ or tissue they are supporting, the researchers say. Their idea is that if unique molecules can be located that define each different kind of blood vessel, then drugs can be targeted to those "addresses" to treat associated diseases.
Blood vessels that service adipose tissue that contains white fat cells is an example of this distinctive type of vasculature, they say. "White adipose tissue is unique because it resembles a tumor in that it can rapidly expand," says Kolonin. "For such rapid expansion, there must be a very active production of blood vessels to deliver oxygen, and in fact, every single fat cell is encased by capillaries."
Because of such a potential for biological activity, a vast network of blood vessels is needed to deliver oxygen to these cells - many more blood vessels than are needed to support other normal tissues, the researchers say. In fact, one pound of fat contains a mile of blood vessels, according to an estimate quoted by Harvard Medical School Professor Judah Folkman, M.D., the first researcher to promote the concept that adipose tissue mass may be controlled by angiogenesis (the process of new blood vessel formation from established ones).
Given their belief that blood vessels that feed adipose tissue are different from all other kinds of blood vessels, the M.D. Anderson researchers set out to find a protein marker or "zip code address" that would identify those particular vessels.
They did this by using an in vivo "phage display library" screening technique that the group developed. This method uses billions of viral particles that each displays a different peptide on its outer coat. In this study, researchers injected these genetically modified viruses into obese mice, hoping several peptides would bind onto protein receptors found only on the inside lining of adipose tissue blood vessels. The process was laborious, but after years of searching, they located a receptor that appears to be over-expressed on blood vessels serving white fat tissue, the kind of adipose tissue that is problematic in obesity.
They then identified the receptor as prohibitin, a protein which is known to regulate cell survival and growth. "The expression of prohibitin in organelles inside the cells such as the mitochondria, and the nucleus, has been established, but its presence and function on the cell surfaces of blood vessels associated with white fat tissue has not been explored," says Arap. "We don't know what prohibitin is doing there, but we know that this protein is not normally found in blood vessels of several other tissues and also of tumors derived from fat tissue."
Once that they had a "docking receptor" specific to white fat tissue, the researchers designed a drug. They created a synthetic ligand - a sequence of amino acids that would fit neatly into the prohibitin receptor, like a key into a lock - and fused to it a corkscrew-shaped "drug" that they knew could induce a cell to self-destruct.
"We have previously shown that this drug can be targeted to blood vessel cells of tumors of the prostate in mice, resulting in reabsorbtion of these tissues due to oxygen deficiency," says Pasqualini.
"The ligand - the guidance system - brings the therapeutic drug to the prohibitin receptor, where it locks on, and then is internalized into the blood vessel cell," she says. "It selectively destroys these blood vessels."
In a series of experiments, the investigators found that one month of treatment in severely obese mice was enough to restore the animal's normal body weight. None of the mice used in the therapy experiment were genetically altered or prone to obesity prior to treatment; they gained weight because they ate a high-fat diet.
The authors then conducted other experiments to make sure that the animals were not loosing weight because of nutrient malabsorption or appetite suppression. They note that there were no toxic side effects to the treatment and that the structure of the human prohibitin receptor is similar to the molecule found in mice.
But the authors caution that further studies are needed, especially to ensure that a drug that targets prohibitin receptors will not damage other vital tissues or vascular systems in the context of human disease and there is much more to be learned. Other collaborating investigators include Pradip Saha, Ph.D., an instructor in the laboratory of Lawrence Chan, M.D., a professor of medicine and molecular and cellular biology at Baylor College of Medicine. "Rapid loss of body fat in animals is often associated with undesirable side effects such as fat accumulation in the liver and in blood," says Chan. "So we were pleasantly surprised that the treatment totally reversed the obesity without producing any of the usual complications."
The study was funded by the National Institutes of Health, as well as numerous foundation grants. Pasqualini and Arap say that they are planning a trial in obese non-human primates as a starting point for further validation and a stepping stone to trials in patients.