The ability of brown fat to suppress obesity through increased energy expenditure has caused an explosion of interest in the development and function of brown adipocytes. The studies indicating that the brown fat cells that emerge in white fat depots come from a completely different cell lineage than those in the classical brown fat depots first suggested the possible existence of a new type of fat cell, termed beige or brite cells (Seale et al., 2008
). This view has been strengthened by recent studies of primary cultures derived from epididymal adipose tissue, which showed that the thermogenic gene program expressed in response to PPARγ agonists was distinct from that stimulated in cultures of classical brown fat cells from the interscapular depot (Petrovic et al., 2010
). Alternatively, this apparent morphological “transdifferentiation” from white fat to brown fat could, at least theoretically, represent a fundamental switch in cell identity (Himms-Hagen et al., 2000
). To date, definitive evidence supporting either hypothesis has been lacking due to the heterogeneity of tissues in vivo
and in primary stromal-vascular cultures.
Stem/progenitor cells have been reported to be isolated from different fat depots and skeletal muscle by FACS and their cell-intrinsic adipogenic capacities have been evaluated in the presence of various environmental cues (Joe et al., 2009
; Joe et al., 2010
; Lee et al., 2012
; Rodeheffer et al., 2008
; Schulz et al., 2011
; Uezumi et al., 2010
). Notably, Sca-1+/CD45-/Mac1- (referred as ScaPCs; (Schulz et al., 2011
) and PDGFα+/CD34+/Sca-1+ (referred as PDGFRα+ cells; (Lee et al., 2012
) have been suggested to have “brown” adipogenic potential. These studies use cell-surface markers to enrich for stem/progenitor cells that later commit for adipose lineage; however, knowledge about the molecular identity and functional phenotypes of these cells remain limited due to the heterogeneity of the cellular populations.
The studies presented here demonstrate that a subset of the precursor cells within subcutaneous adipose tissue give rise to beige cells, which are capable of expressing abundant UCP1 and a broad gene program that is distinct from either white or classical brown adipocytes. Roughly 40% of the differentiation-competent preadipocyte clones that we isolated from the subcutaneous cultures had the characteristics of beige cells. While 129 mice are rather prone to the “browning” of their white fat compared to certain other murine strains, these data certainly suggest that the beige adipocytes are not a rare cell type in the subcutaneous depots of mice. Further studies will help to answer important questions regarding the presence and/or abundance of beige precursor cells in various white fat depots of inbred mice and whether/how the beige precursor pool contributes to the metabolic status of these animals. It is tempting to speculate that at basal state, the beige precursor cells represent a higher percentage of adipose precursors in the subcutaneous depots than in the visceral depots. When animals are exposed to cold or receive chronic β-adrenergic stimulation, the pre-existing beige adipocytes (which may appear unilocular at the basal state) will go through phenotypic “transdifferentiation” and “browning” will appear morphologically and histochemically. . On the other hand, for the depots more resistant to “browning” (such as the abdominal WAT depots), beige precursor cells may have to go through a proliferation step before robust browning can take place. This notion is consistent with the observations that BrdU+, UCP1+ cells are abundant in the epididymal depot but not in the inguinal or retroperitoneal depots upon adrenergic stimulation (Himms-Hagen et al., 2000
; Lee et al., 2012
One important issue is how the beige cells differ functionally from the classical brown fat cells; the gene expression studies presented here give a first indication. The beige cells express very little of the thermogenic gene program, including UCP1, in the basal (unstimulated) state. In this regard they resemble bona fide white adipocytes. On the other hand, once stimulated, these cells activate expression of UCP1 to levels that are similar to those of the classic brown fat cells. Thus, the beige cells have the capability to switch between an energy storage and energy dissipation phenotype in a manner that other fat cells lack. It is worth noting that primary preadipose cells isolated by sorting for CD137 have a higher basal level of UCP1 than the beige immortalized cells. Whether this is a consequence of the immortalization procedure itself or is simply a function of the time apart from the β-adrenergic stimulation that takes place in vivo is not known. Further optimization of this protocol for isolation and purification of primary beige cells should provide a powerful tool for investigations of this cell type.
Infant humans have brown fat in the same interscapular location as the classical brown fat of rodents. This tissue disappears as humans mature and it is only recently that brown fat was fully recognized in adult humans (Cypess et al., 2009
; Orava et al., 2011
; Ouellet et al., 2012
; van Marken Lichtenbelt et al., 2009
; Virtanen et al., 2009
). This brown fat is observed primarily in the supraclavicular and neck region, and along the spine. The cloning and subsequent gene expression analyses of the beige and brown rodent fat cells done here allowed us to critically determine the nature of the adult human BAT. Our data show that the selective molecular markers of beige fat cells, rather than those of the classical brown fat cells, are enriched in the human UCP1-positive tissues. If these UCP1-positive cells are functionally similar to the rodent beige cells, this might explain why a relatively low proportion of humans show PET-positive fat deposits until activated with a brief cold exposure. The existence of beige fat cells may represent an evolutionarily conserved cellular mechanism to provide flexibility in adaptive thermogenesis. It is likely that these beige adipocytes, rather than the classical brown fat cells, remain present in the adult state of larger mammals where hypothermia is a less frequent threat than in rodents.
The therapeutic potential of activating brown fat-mediated thermogenesis in human has yet to be fulfilled. Trials of drugs that increase the β-adrenergic activation of BAT have not been successful in humans, due to either lack of efficacy or to intolerable side-effects due to activation of β-adrenergic receptors in other tissues (Collins et al., 2004
; Whittle et al., 2012
). Clearly there is a need to develop more specific means to activate brown fat in humans in more specific ways. Since irisin is an endogenous circulating molecule that mediates some of the benefits of exercise, and activates beige fat cells in rodents, it could represent one way to do this in humans. Other polypeptide hormones that brown white adipose tissues in rodents, such as FGF21(Fisher et al., 2012
) and ANF(Bordicchia et al., 2012
), might also prove useful in humans with obesity and diabetes.