By David N. Leff
Four years ago, when molecular geneticists at Rockefeller University, in New York, discovered a hormone that acts in the brain to curb appetite, they were happy to find that the Greeks had a word for it — leptos. It means fine, thin, slender, so the scientists named their anti-obesity molecule leptin. (See BioWorld Today, Dec. 1, 1994, p. 1.)
Unsurprisingly, leptin has since been the subject, and object, of wide research, clinical trials, patents, partnerships and popular hopes that its promise of effortless weight reduction may come true.
Leptin has since shown that, besides putting the kibosh on overeating, it plays a physiological hand in blood formation, insulin-cell function, ovarian function, fat metabolism and body-heat generation. But its mechanism of action remained largely unknown.
"One thing that was missing," observed molecular biologist Jaime Flores-Riveros, "was whether the leptin receptor was expressed in the relevant tissues." Then, last year, serendipity took a hand.
"We were doing control experiments," Flores-Riveros recalled, "using endothelial cells that we never suspected would be expressing the leptin receptor. We found that these cells were positive with the receptor-specific antibodies we were using. Once we had this finding in front of us, we began speculating what the biological action of recombinant leptin might be in that cell type.
"We were simply trying to transfect the leptin receptor in those cell lines," he said. "We were interested in the signaling mechanism from the receptor to its effector systems. That's when we made a little bit of an accidental discovery."
Flores-Riveros and his colleagues document that discovery in Science, dated Sept. 11, 1998, in a paper titled "Biological action of leptin as an angiogenic factor."
"We were certainly puzzled by our initial observation that we found leptin receptors present in endothelial cells lining blood vessels, a location never suspected to be a site of leptin action," said Flores-Riveros, senior author of the Science paper and director of molecular biology and obesity research at the privately held Institutes For Pharmaceutical Discovery, in Branford, Conn.
His wife, vascular biologist M. Rosio Sierra-Honigmann, the paper's first author, is a research scientist in the pathology department of Yale University, in New Haven, Conn. It was she who actually made the discovery that leptin is angiogenic.
"Now we have strong evidence," Sierra-Honigmann observed, "for the existence of functional leptin receptors outside the brain. This could give us new insights into how leptin controls body fat besides blocking hunger."
She added, "Because leptin is produced mainly in fat cells, it is reasonable to speculate that angiogenesis — formation of new blood vessels — may actively occur around those adipose cells."
Angiogenesis normally occurs by equipping infants and children with their circulatory systems. It also acts in wound healing, menstruation and in the breast tissue of nursing mothers. Abnormally, it aids cancer progression and metastasis by supplying growing tumors with blood-borne nutrients.
Fat Cells, Like Tumors, Need Blood Vessels
Adipocytes, which store and release the body's energy in the form of fat globules, also require a blood supply. "Leptin can increase energy expenditure," Flores-Riveros pointed out. "It not only blocks hunger but increases energy expenditure by oxidizing and catabolizing lipids. As it forms more blood vessels around adipocytes that are in the process of releasing that stored energy," he went on, "we think leptin fulfills a fascinating regulatory mechanism."
To demonstrate that leptin actually grows blood vessels, he and his co-authors — after extensive in vitro experiments — turned to two strains of laboratory rat. One was normal, the other grossly obese, because it lacked the leptin receptor gene. Co-author Peter Polverini, at the University of Michigan, in Ann Arbor, surgically implanted leptin-loaded polymer pellets into the corneas of both rat models, normals and knockouts.
The former cohort responded to the ocular leptin by growing a dense network of blood vessels across their eyeballs within a week. The receptor-lacking animals showed no such angiogenesis.
"This may be a crucial adaptive response," Flores-Riveros suggested, "that provides more efficient release of metabolic energy, and thereby, weight loss."
As for the putative molecular mechanism involved, he observed, "I'm not necessarily thinking in terms of new therapies. We just wanted to portray this as a new paradigm of leptin action that could lead to a better understanding of how the vasculature in adipose tissue plays a role in maintaining a particular fat mass.
"We're not prepared, obviously," he went on, "to offer a detailed molecular explanation as to how this happens. But it makes sense to think of adipose tissue as composed of various cell types, of which fat cells and endothelial cells would be the most abundant."
Leptin Acts Not By Brain Alone
Flores-Riveros said having continuous formation of blood vessels "in proportion to the number of adipocytes, or their sites, I think, offers a new way to look at the regulation of adipose tissue by leptin, a molecule that so far has been extensively viewed as being a brain-centered regulator of energy intake."
Sierra-Honigmann added, "The growth of new fat-cell blood vessels might also serve as a 'cooling system' that dissipates heat generated as stored fat breaks down — especially in 'brown fat' of hibernating animals, and fat stored in human newborns and cold-acclimatized individuals."
Flores-Riveros called the research "the first unequivocal evidence for cross-talk between fat cells and endothelial cells. The discovery is likely to be helpful in the development of new pharmaceuticals to treat obesity.
"Our major focus right now [at the Institutes for Pharmaceutical Discovery, which is not affiliated with Yale] is on discovery research," Flores-Riveros said. "We are not necessarily involved in developing or marketing any products. Rather, we work in association with pharmaeutical partners, of whom we have several, to develop a particular product or target of disease. That's really the angle that we come from." *