1 Department of Medicine, University of California, Los Angeles
2 Veterans Affairs Greater Los Angeles Healthcare System, Los Angeles, California 90073
3 Deputy editor, Physiological Genomics
ADIPOSE TISSUE plays an important role in obesity, diabetes, and insulin resistance. Mammals possess two types of adipose tissue, white and brown. These two tissues are both equipped to synthesize and store lipid but have contrary roles in energy metabolism: white adipose tissue serves an energy storage function, whereas brown adipose tissue functions in energy dissipation. The basis for this distinction is the capacity of brown adipose tissue for expression of the mitochondrial uncoupling protein 1 (UCP1) for thermogenesis, which is critical for maintenance of body temperature in small animals and for burning excess calories for body weight regulation. Although white and brown adipocytes occur in specific depots in the body and are phenotypically distinct in cell culture, white and brown preadipocytes are morphologically indistinguishable. There exist well-characterized models detailing the molecular basis of the differentiation of white preadipocytes into adipocytes, based upon cell lines such as the widely studied mouse 3T3-L1 preadipocyte line. However, the process whereby fibroblast-like precursors commit to either a white or brown preadipocyte lineage is not yet well understood.
In this online release of Physiological Genomics, Boeuf et al. (Ref. 1; see page 15 in this release) describe the differential expression of genes in brown and white preadipocytes, which may play a role in the differentiation of these two cell types. They combined cDNA representational difference analysis (RDA) with microarray screening to identify such genes. Preadipocytes were isolated from Siberian dwarf hamsters, which possess abundant active brown adipose tissue and from which parallel cultures of white and brown preadipocytes have been established and characterized. Using RNA from primary cultures of white and brown preadipocytes, the investigators performed RDA, which combines subtractive hybridization with PCR, to amplify transcripts that are differentially expressed between the two cell types. Approximately 250 of the resulting RDA products plus about 50 known adipocyte-related genes were spotted onto a cDNA microarray. The microarray was hybridized to mRNA isolated from white and brown preadipocytes at different stages of differentiation. Genes with a greater than twofold difference in expression level between samples were subsequently subjected to Northern blot analysis to confirm the microarray results.
The number of differentially expressed genes detected between white and brown preadipocytes was quite small, with only 11 genes showing greater than twofold differences. The expression differences included higher expression in white preadipocytes of complement factor genes and higher expression in brown preadipocytes of genes encoding proteins involved in cell adhesion, cytoskeletal organization, and cellular proliferation. The expression levels of each of these genes decreased as differentiation progressed, confirming that their expression is specific to preadipocytes. Although the relationship between these gene expression patterns and cellular phenotype are not clear, these results provide perhaps the first glimpse at molecular differences between white and brown preadipocytes.
The identification of gene expression differences between brown and white preadipocytes is significant in light of current views concerning the relationship between the two cell types. Based on studies demonstrating that development of a mature, lipid-filled brown adipocyte morphology can be dissociated from development of thermogenic capacity, the possibility existed that brown adipocytes represent white adipocytes that have differentiated a step further to express the capacity for thermogenesis. Results from the current study, however, refute this interpretation by documenting differences between the two cell types at the preadipocyte state, before lipid accumulation or expression of thermogenic function. Future studies that investigate the consequences of differential expression of complement factors, cytoskeletal and adhesion molecules, and the other genes that were identified by Boeuf et al. (1) should further define the distinctions between brown and white preadipocytes.
The small number of differences detected between white and brown preadipocytes contrasts with the large number of differences detected between white preadipocytes and mature adipocytes in a recent study by Soukas et al. (2). These investigators utilized microarrays containing 11,000 gene elements to examine expression patterns at several time points during in vitro differentiation of the 3T3-L1 cell line and during in vivo differentiation by comparing expression in preadipocytes vs. mature adipocytes isolated from C57BL/6 wild-type and ob/ob obese mice. They found that the expression profile of differentiating 3T3-L1 cells is considerably more complex than anticipated. More than 1,250 genes showed altered expression levels during differentiation, and these could be grouped into 27 distinct kinetic profiles. Furthermore, substantial differences existed between gene expression changes occurring in adipose tissue in vivo and in the 3T3-L1 cell line in vitro. Thus it appears that nonadipocyte factors present in vivo, such as hormones and cell interactions, are required to express the fully differentiated adipocyte phenotype.
The two studies described here illustrate the value of using large scale gene expression profiling approaches to identify new players in complex cellular processes, such as adipocyte determination and differentiation. Because of limitations in the techniques (i.e., the presence of only a portion of all genes on the microarrays used, and biases against certain mRNA abundance classes introduced by RDA), neither study can be considered to present a comprehensive catalog of gene expression changes. Nevertheless, these and other recent gene expression studies begin to paint a more detailed picture of the intricate molecular changes that occur during adipocyte differentiation. The full value of these findings will be realized only after the functional consequences of these gene expression changes are elucidated.
FOOTNOTES
Article published online before print. See web site for date of publication (http://physiolgenomics.physiology.org).
Address for reprint requests and other correspondence: K. Reue, UCLA, 11301 Wilshire Blvd., Bldg. 113, Rm. 312, Los Angeles, CA 90073 (E-mail: Reuek{at}ucla.edu).
REFERENCES
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