Multiple Signal Transduction Pathways Interact to Regulate Adipocyte Differentiation Through PPARγ, the Master Transcriptional Regulator

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Insulin is also a major inducer of the formation of white adipocytes, commonly called “fat cells.” Adipocytes are the body’s major fat storage depots; mature adipocytes contain a few triglyceride droplets that occupy the bulk of the cell. Adipocytes are also endocrine cells and secrete several signaling proteins that affect the metabolic functions of muscle, liver, and other organs. Adipocytes are the one type of cell in the body that can increase in number almost without limit. Readers in every country do not need to be reminded that obesity is a growing public health problem, and that it is a major risk factor not only for diabetes but also for cardiovascular diseases, such as heart attacks and stroke, and for certain cancers.

As we discuss in Chapter 21, several types of stem cells in vertebrates are used to generate specific types of differentiated cells. The mesenchymal stem cell, which resides in the bone marrow and other organs, gives rise to progenitor cells that in turn can form either adipocytes, cartilage-producing cells, or bone-forming osteoblasts. The adipocyte progenitor, called the preadipocyte, has lost the potential to differentiate into other cell types. When treated with insulin and other hormones, preadipocytes undergo terminal differentiation; they acquire the proteins that are necessary for lipid transport and synthesis, insulin responsiveness, and the secretion of adipocyte-specific proteins. Several lines of cultured preadipocytes can differentiate into adipocytes and express adipocyte-specific mRNAs and proteins, such as enzymes required for triglyceride synthesis.

The transcription factor PPARγ, a member of the nuclear hormone receptor family, is the master transcriptional regulator of adipocyte differentiation. As evidence, recombinant expression of PPARγ in many fibroblast lines has been found sufficient to trigger the differentiation of these cells into adipocytes. Conversely, knocking down the gene for PPARγ in preadipocytes prevents their differentiation into adipocytes. Most hormones, such as insulin, that promote adipogenesis do so at least in part by activating expression of PPARγ. PPARγ, in turn, binds to the promoters of most adipocyte-specific genes, including genes encoding proteins needed in the insulin-signaling pathway, such as the insulin receptor and GLUT4, and induces their expression. Like other members of the nuclear hormone receptor family, such as steroid hormone receptors (see Chapter 9), which become activated when they bind their ligand, PPARγ is also thought to bind a ligand, possibly an oxidized derivative of a fatty acid.

Another transcription factor, C/EBPα, is induced during adipocyte differentiation and also directly induces many adipocyte genes. Importantly, C/EBPα induces expression of the PPARγ gene, and PPARγ induces expression of C/EBPα, leading to a rapid increase in both proteins during the first two days of differentiation. Together, PPARγ and C/EBPα induce expression of all genes required for the differentiation of preadipocytes into mature fat cells.

Many signaling proteins, such as Wnt and TGF-β, oppose the action of insulin and prevent preadipocyte differentiation into adipocytes. As Figure 16-41 shows, transcription factors activated by receptors for these hormones prevent expression of the PPARγ gene, in part by blocking the ability of C/EBPα to induce PPARγ gene expression. Thus multiple extracellular signals act in concert to regulate adipogenesis, and the signal transduction pathways activated by them intersect at the regulation of expression of one key “master” gene, encoding PPARγ.

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FIGURE 16-41 Multiple signal transduction pathways interact to regulate adipocyte differentiation. The transcription factor PPARγ (purple) is the master regulator of adipocyte differentiation; together with C/EBPα, it induces expression of all genes required for differentiation of preadipocytes into mature fat cells. Both PPARγ and C/EBPα are induced early in adipogenesis; each of them enhances the transcription of the gene encoding the other (an arrow at the end of a line means enhancement of expression of target genes), leading to a rapid increase in expression of both proteins during the first two days of differentiation. Signals from hormones such as insulin and from growth factors such as Wnt and TGF-β that activate or repress adipogenesis are integrated in the nucleus by transcription factors that regulate—directly or indirectly—expression of the PPARγ and C/EBPα genes. (A T shape at the end of a line indicates inhibition of expression of the target gene.) (a) Insulin activates adipogenesis by several pathways leading to activation of PPARγ expression, two of which are depicted here. Activation of protein kinase B (PKB) downstream of the IGF-1 and insulin receptor tyrosine kinases leads to repression of Necdin expression; Necdin, by modulating other transcription factors, would otherwise repress expression of the PPARγ gene. PKB also phosphorylates, and thus inactivates, the transcription factor GATA2, which when nonphosphorylated binds to the C/EBPα protein and prevents it from activating expression of the PPARγ gene. By inhibiting two repressors of the PPARγ gene, insulin thus stimulates PPARγ expression. (b) Wnt and TGF-β inhibit adipogenesis by reducing expression of the PPARγ gene. Wnt signaling triggers release of β-catenin from a cytoplasmic complex, and free β-catenin binds the transcription factor TCF (see Figure 16-30). Active TCF blocks expression of the PPARγ and C/EBPα genes, probably by binding to their regulatory sequences. (c) Smad3, activated by phosphorylation following TGF-β binding to the types I and II TGF-β receptors, binds to the C/EBPα protein and prevents it from activating expression of the PPARγ gene. See E. Rosen and O. MacDougald, 2006, Nat. Rev. Mol. Cell Biol. 7:885.