This is an update to our recent discussion on targeting the physiology of brown fat [or brown adipose tissue (BAT)], in developing novel antiobesity therapies. It is based on a research article published in the July 20 2012 issue of Cell by Dr. Bruce Spiegelman (Dana-Farber Cancer Institute and Harvard Medical School, Boston MA) and his colleagues.
As we discussed in our May 23, 2012 blog article, Dr. Spiegelman, a leading metabolic disease researcher, is a founder of Ember Therapeutics. Ember is working to develop novel metabolic disease therapeutics based on Dr. Spiegelman’s work on BAT physiology and on novel insulin sensitizers.
In the work described in the new Cell article, Dr. Spiegelman and his colleagues showed that the white adipose tissue (WAT)-derived “brown fat-like cells” that they had been studying are actually a new type of cells known as “beige adipocytes”.
Other researchers had previously described a class of “brite” or “beige” adipocytes, which were induced in WAT depots that have been chronically treated with the peroxisome proliferator-activated receptor γ (PPARγ) agonist rosiglitazone. Brite/beige adipocytes expressed UCP1 (uncoupling protein 1) and had high levels of mitochondria as do brown adipocytes but lacked expression of certain characteristic brown fat-specific genes. UCP1 is the key mitochondrial protein that makes the process of thermogenesis (i.e., production of heat by oxidizing fat rather than storing it) possible. As shown previously by Dr. Spiegelman and his colleagues, BAT is derived from a myf-5 muscle-like cell lineage, via the action of the transcriptional regulator PRDM16. However, beige cells are not.
Beige precursor cells in murine subcutaneous white fat depots
In the new Cell publication, Dr. Spiegelman and his colleagues carry these earlier studies further by cloning murine beige fat cells and describing their unique gene expression signature. They first isolated adipocyte precursors from mouse subcutaneous WAT (specifically, the inguinal fat depots). These precursors are found in the stromal-vascular fraction of the adipose tissue. The researchers immortalized stromal vascular fraction (SVF) cells from subcutaneous WAT of the 3T3 mouse by passaging them in culture, similar to the classic derivation of 3T3 fibroblast cell lines by Todaro and Green 49 years ago. They cloned the resulting immortalized cells via limiting dilution, and selected the 20-some odd cell lines that could be readily differentiated (via standard differentiation protocols) to adipocytes. For purposes of comparison, the researchers also derived multiple adipogenic clones from the SVF cells of the interscapular brown fat depot.
They then differentiated the WAT-derived adiopogenic clones to adipocytes, and treated them with forskolin [an agent that raises intracellular levels of cyclic AMP (cAMP)]–increases in cAMP levels activate expression of UCP1 in the mitochondria of brown fat cells. They then determined the gene expression pattern of the differentiated and forskolin-stimulated WAT derived clones. The clones clustered into two groups, one of which had a gene expression pattern more similar to BAT cells than the other group. These two groups appeared to represent beige and white adipocytes, respectively. Comparison of these two groups of cells to BAT cells revealed that the presumptive beige adipocytes had gene expression patterns that were similar, but not identical to, those of brown adipocytes.
Further characterization of the three types of cells indicated that beige cells have characteristics of both white and brown adipocytes. Both white and beige adipocytes have low basal levels of UCP1, while brown adipocytes have higher levels. Upon cAMP stimulation, however, the beige adipocyte lines responded with a very large induction of UCP1 gene expression, reaching similar UCP1 levels to that observed in the brown adipocyte lines. White adipocyte cell lines showed little UCPI induction. These characteristics of beige and white adipocyte cell lines also were seen in in vivo studies.
Still further characterization of the three types of cell lines revealed that beige and brown fat cells have related but distinct gene expression profiles. These include a set of beige-selective genes that can distinguish beige fat cells from both brown fat cells and white fat cells. Protein expression was highly concordant with mRNA expression. Since some of the beige-selective markers are cell surface proteins, and since antibodies to these proteins are commercially available, this allowed the researchers to use fluorescence-activated cell sorting (FACS) to isolate primary beige precursor cells from the SVF of mouse inguinal fat.
Murine beige fat precursors–not white fat or brown fat–are targets of the hormone irisin
In our May 23, 2012 blog article, we discussed the myokine hormone irisin, which was recently discovered by the Spiegelman group. As we discussed, irisin is produced by muscle cells and increases with exercise. It has little or no effect on classic brown fat found in the interscapular depot. However, It acts on subcutaneous white adipose cells in culture and in vivo to stimulate what appears to be development into brown fat-like cells. Specifically, irisin stimulates expression of UCP1 and an array of other brown fat genes, producing a thermogenic effect.
In the study reported in the July 20 2012 Cell paper, the researchers used FACS to sort primary inguinal precursor cells into white and beige preadipocytes, and studied the effects of two forms of recombinant irisin on these cells during adipogenic differentiation. All cells treated either with vehicle or irisin showed good adipocyte differentiation. Both forms of irisin, but not vehicle, induced the expression of brown fat-like genes such as UCP-1 in beige cells, but had little effect on white cells. This suggests that irisin works on white fat depots in vivo by inducing brown-like gene expression in the beige cell component of preadipocytes in these depots.
Adult human “brown fat” is really beige fat
As we discussed in our blog articles of November 17, 2010 and May 23, 2012, and as illustrated in the figures at the top of each of these articles, adult humans possess what appears to be reservoirs of brown fat in the neck region and other areas of the upper body as well as in skeletal muscle.
In the study reported in the July 20 2012 Cell paper, the researchers preformed BAT biopsies from two independent cohorts of human subjects, and analyzed their gene expression signatures based on the findings of the studies of mouse brown, beige, and white adipocytes. They found that the UCP1-positive “BAT” cells from the human biopsies had gene expression signatures that resembled those of murine beige adipocytes more closely than they resemble classic brown fat or white fat. As a result of this finding, several popular articles written about the new Cell paper are entitled “Beige is the New Brown”.
Although additional research is needed to fully characterize beige fat physiology, the picture that emerges from the above study is that beige fat cells exist in subcutaneous fat mainly in a basal, unstimulated state, in which their phenotype resembles that of white fat. Once stimulated, however, beige cells activate expression of a brown fat-like thermogenic program, including expression of levels of UCP1 similar to those of brown fat cells. Thermogenesis of beige fat cells is induced by such stimuli as 1. β-adrenergic activation; 2. the myokine irisin; and 3. other polypeptide hormones, such as fibroblast growth factor 21 (FGF21), and atrial natriuretic peptide (ANP). The role of FGF21 in inducing UCP1 and the thermogenic program in adipose tissues was elucidated by Dr. Spiegelman and his colleagues. This was described in a February 2012 report in Genes & Development.
The above scenario is mainly based on studies in mice. However, as discussed earlier, humans also have what appear to be beige cells, which may be amenable to induction by irisin or other agents, thus giving rise to a thermogenic program that might be utilized to combat obesity and type 2 diabetes. Classical interscapular brown fat in humans, however, although it is present in infants, disappears as humans mature. This it is likely that beige fat will be the target such agents as irisin, which are aimed at overcoming metabolic disease via increasing energy expenditure.
Long before researchers obtained evidence for “browning” of white fat as a potential mechanism for induction of a thermogenic program, the pharmaceutical industry had a long history of attempting to develop β3-adrenergic agonists as therapies for metabolic diseases. Many agents were entered into clinical trials by numerous companies, but all failed, either due to lack of efficacy or to averse effects due to activation of β-adrenergic receptors in other tissues. An important underlying factor in the failure of these studies was the lack of understanding of brown (or beige) fat physiology in humans, including whether brown fat existed in adult humans at all. Of course, beige fat was completely unknown.
In our May 23, 2012 blog article, we discussed several young companies that are working to develop novel approaches to treating obesity based on brown-fat physiology and/or other non-CNS pathways involved in increasing energy expenditure. Among these companies are Ember Therapeutics. As we discussed in the May 2012 article, Ember entered into an exclusive license agreement with Dana-Farber Cancer Institute for the irisin technology, and is optimizing and developing a proprietary molecule based on this technology. This research constitutes the company’s lead BAT biology program.
More recently, Ember completed a licensing agreement with the Dana-Farber for intellectual property related to Dr. Spiegelman’s beige fat discovery. As discussed earlier, beige fat cells are specifically targeted by irisin, which induces the thermogenic program in these cells. Especially in adult humans, which appear to lack classic brown fat, beige adipocytes and/or their precursors are the true target of irisin, and any program to develop irisin-like protein drugs for metabolic disease will need to focus on the effect of such products on beige cells. Moreover, Ember’s programs to develop small molecules via screening for compounds that activate pathways specific to the “brown fat” cell lineage will need to focus specifically on pathways involved in induction of the thermogenic program in beige adipocytes.
In contrast to Ember’s R&D programs, which must focus on beige adipocytes, Energesis Pharmaceuticals’ R&D programs focus on brown fat “stem cells” from skeletal muscle. This research thus has to do with classical BAT, which is derived from a skeletal muscle-like lineage, as opposed to beige adipocytes, which are not. Thus Ember’s and Energesis’ R&D programs represent completely different approaches to developing antiobesity agents that work to increase energy expenditure. Among the other companies mentioned in our May 2012 article, Zafgen’s R&D programs represent still another completely different approach, involving targeting the liver. Acceleron Pharma’s approach, however, probably involves targeting beige fat. In fact, Dr. Spiegelman has been a collaborator in some of their research.
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