Adipogenesis is the formation of adipocytes (fat cells) from stem cells. [1] It involves 2 phases, determination, and terminal differentiation. Determination is mesenchymal stem cells committing to the adipocyte precursor cells, also known as lipoblasts or preadipocytes which lose the potential to differentiate to other types of cells such as chondrocytes, myocytes, and osteoblasts. [2] Terminal differentiation is that preadipocytes differentiate into mature adipocytes. Adipocytes can arise either from preadipocytes resident in adipose tissue, or from bone-marrow derived progenitor cells that migrate to adipose tissue. [3]
Adipocytes play a vital role in energy homeostasis and process the largest energy reserve as triglycerol in the body of animals. [4] Adipocytes stay in a dynamic state, they start expanding when the energy intake is higher than the expenditure and undergo mobilization when the energy expenditure exceeds the intake. This process is highly regulated by counter regulatory hormones to which these cells are very sensitive. The hormone insulin promotes expansion whereas the counter hormones epinephrine, glucagon, and ACTH promote mobilization. Adipogenesis is a tightly regulated cellular differentiation process, in which mesenchymal stem cells committing to preadipocytes and preadipocytes differentiating into adipocytes. Cellular differentiation is a change of gene expression patterns which multipotent gene expression alters to cell type specific gene expression. Therefore, transcription factors are crucial for adipogenesis. Transcription factors, peroxis proliferator-activated receptor γ (PPARγ) and CCAAT enhancer-binding proteins (C/EBPs) are main regulators of adipogenesis. [5] Comparing with cells from other lineage, the in vitro differentiation of fat cells is authentic and recapitulates most of the characteristic feature of in vivo differentiation. The key features of differentiated adipocytes are growth arrest, morphological change, high expression of lipogenic genes and production of adipokines like adiponectin, leptin, resistin (in the mouse, not in humans) and TNF-alpha.
In vitro studies on differentiation have used the pre-committed preadipocyte lineage, such as 3T3-L1 and 3T3-F442A cell line, or preadipocytes isolated from the stromal-vascular fraction of white adipose tissue. In vitro differentiation is a highly ordered process. Firstly, proliferating preadipocytes arrest growth usually by contact inhibition. The growth arrest followed by the earliest events, including a morphological change of preadipocyte from the fibroblast-shape to the round-shape and the induction of transcription factors C/EBPβ, and C/EBPδ. The second phase of growth arrest is the expression of two key transcription factors PPARγ and C/EBPα which promote expression of genes that confer the characteristics of mature adipocytes. These genes include adipocyte protein (aP2), insulin receptor, glycerophosphate dehydrogenase, fatty acid synthase, acetyl CoA carboxylase, glucose transporter type 4 (Glut 4) and so on. [6] Through this process, lipid droplets accumulate in the adipocyte. However, preadipocytes cell lines have difficult to different to differentiate into adipocytes. Preadipocytes display CD45− CD31− CD34+ CD29+ SCA1+ CD24+ surface markers can proliferate and differentiate to adipocytes in vivo. [7]
Cell Line | Origin | Differentiation Protocol | ||
---|---|---|---|---|
Committed Pre-adipocytes | ||||
3T3-L1 | Sub-clone of Swiss 3T3 [8] | FBS+ I+ D+ M | ||
3T3-F442A | Sub-clone of Swiss 3T3 [9] | FBS + I | ||
Ob17 | Differentiated adipocyte from epididymal fat pad of C57BL/6J ob/ob mice [10] | FBS+ I+ T3 | ||
TA1 | Subclone of C3H10T1/2 [11] | FBS + D + I | ||
30A5 | Subclone of C3H10T1/2 [12] | FBS + D + M + I | ||
1246 | Adipogenic Subclone of CH3 mouse teratocarcinoma cell line T984 [13] | D + M + I | ||
Non-committed with adipogenic potential | ||||
NIH3T3 | NIH Swiss mouse embryo cells [14] | Ectopic expression of PPAR-gamma, C/EBP-alpha or C/EBP-beta + D+ M+ I | ||
Swiss 3T3 | Swiss mouse embryo cells [15] | Ectopic expression of C/EBP-alpha | ||
Balb/3T3 | Balb/c mouse embryo cells [16] | Ectopic expression of C/EBP-alpha | ||
C3H 10T1/2 | C3H mouse embryo cells [17] | PPAR-gamma ligand | ||
Kusa 4b10 | mouse bone marrow stromal cell line [18] | FBS + I + D + M | ||
C2C12 | Thigh muscles of C3H mice [19] | Thiazolidinediones | ||
G8 | Hind limb muscles of fetal Swiss webster mouse [20] | Ectopic expression of PPAR-gamma + CEBP/alpha +D + I | ||
FBS = Fetal Bovine Serum, D = Dexamethasone, I = Insulin, M = Methylisobutylxanthine T3 = Triiodothyronine |
PPARγ is a member of the nuclear-receptor superfamily and is the master regulator of adipogenesis. PPARγ heterodimerizes with retinoid X receptor (RXR) and then binds to DNA, which activates the promoters of the downstream genes. PPARγ induces fat-cell specific genes, including aP2, adiponectin and phosphoenolpyruvate carboxykinase (PEPCK). PPARg activation has effects on several aspects of the mature adipocyte characteristics such as morphological changes, lipid accumulation, and the acquisition of insulin sensitivity. [21] PPARγ is necessary and sufficient to promote fat cell differentiation. PPARγ is required for embryonic stem cells (ES cells) differentiation to adipocytes. [22] The expression of PPARγ itself is sufficient to convert fibroblast into adipocytes in vitro. [23] Other pro-adipogenic factors like C/EBPs and Krüppel-like factors (KLFs) have been shown to induce the PPARγ promoter. Moreover, PPARγ is also required to maintain the expression of genes that characterize the mature adipocyte. [24] Thiazolidinediones (TZDs), antidiabetic agents which well used differentiation cocktail in vitro, promoting the activity of PPARγ.
C/EBPs, transcription factors, are members of the basic-leucine zipper class. cAMP, an inducer of adipogenesis, can promote expression of C/EBPβ and C/EBPδ. [25] At the early stage of differentiation, the transient increase of C/EBPβ and C/EBPδ mRNA and protein levels are thought to activate the adipogenic transcription factors, PPARγ and C/EBPα. PPARγ and C/EBPα can feedback to induce the expression of each other as well as their downstream genes. [26] C/EBPα also plays an important role in the insulin sensitivity of adipocytes. [27] However, C/EBPγ suppresses differentiation which might due to inactivation by C/EBPβ.
Although PPARγ and C/EBPα are master regulators of adipogenesis, other transcription factors function in the progression of differentiation. Adipocyte determination and differentiation factor 1 (ADD1) and sterol regulatory element binding protein 1 (SREBP1) can activate PPARγ by the production of an endogenous PPARγ ligand or directly promote the expression of PPARγ. cAMP-responsive element binding protein promotes differentiation, while the activation of PPARγ and C/EBPα is also responsive to negative regulation. T-cell factor/lymphoid enhancer-binding factor (TCF/LEF), [28] GATA2/3, [29] retinoic acid receptor α, [30] and SMAD6/7 [31] don't affect the expression of C/EBPβ and C/EBPδ but inhibit the induction of PPARγ and C/EBPα.
Products of endocrine system such as insulin, IGF-1, cAMP, glucocorticoid, and triiodothyronine effectively induce adipogenesis in preadipocytes. [32] [33] [34]
Insulin regulates adipogenesis through insulin-like growth factor 1 (IGF1) receptor signaling. Insulin/IGF1 promotes the induction transcription factors regulating terminal differentiation.
Wnt/β-catenin signaling suppresses adipogenesis, by promoting the differentiation of mesenchymal stem cells into myocytes and osteocytes but blocking the commitment to the adipocytic lineage. [35] Wnt/β-catenin inhibits the differentiation of preadipocytes by inhibiting the induction of PPARγ and C/EBPα.
Bone morphogenetic proteins (BMPs) are transforming growth factor β (TGFβ) superfamily members. BMP2 can either stimulates the determination of multipotent cells or induce osteogenesis through different receptor heteromers. [36] BMPs also promotes the differentiation of preadipocytes.
Senescent adipose progenitor cells in subcutaneous adipose tissue has been shown to suppress adipogenic differentiation. [37] Reduced adipogenesis in obese persons is due to increased senescent cells in adipose tissue rather than reduced numbers of stem/progenitor cells. [38]
Adipose tissue is a loose connective tissue composed mostly of adipocytes. It also contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular endothelial cells and a variety of immune cells such as adipose tissue macrophages. Its main role is to store energy in the form of lipids, although it also cushions and insulates the body.
Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue, specialized in storing energy as fat. Adipocytes are derived from mesenchymal stem cells which give rise to adipocytes through adipogenesis. In cell culture, adipocyte progenitors can also form osteoblasts, myocytes and other cell types.
Adiponectin is a protein hormone and adipokine, which is involved in regulating glucose levels and fatty acid breakdown. In humans, it is encoded by the ADIPOQ gene and is produced primarily in adipose tissue, but also in muscle and even in the brain.
In molecular biology, a CCAAT box is a distinct pattern of nucleotides with GGCCAATCT consensus sequence that occur upstream by 60–100 bases to the initial transcription site. The CAAT box signals the binding site for the RNA transcription factor, and is typically accompanied by a conserved consensus sequence. It is an invariant DNA sequence at about minus 70 base pairs from the origin of transcription in many eukaryotic promoters. Genes that have this element seem to require it for the gene to be transcribed in sufficient quantities. It is frequently absent from genes that encode proteins used in virtually all cells. This box along with the GC box is known for binding general transcription factors. Both of these consensus sequences belong to the regulatory promoter. Full gene expression occurs when transcription activator proteins bind to each module within the regulatory promoter. Protein specific binding is required for the CCAAT box activation. These proteins are known as CCAAT box binding proteins/CCAAT box binding factors.
CCAAT-enhancer-binding proteins is a family of transcription factors composed of six members, named from C/EBPα to C/EBPζ. They promote the expression of certain genes through interaction with their promoters. Once bound to DNA, C/EBPs can recruit so-called co-activators that in turn can open up chromatin structure or recruit basal transcription factors.
White adipose tissue or white fat is one of the two types of adipose tissue found in mammals. The other kind is brown adipose tissue. White adipose tissue is composed of monolocular adipocytes.
Peroxisome proliferator-activated receptor gamma, also known as the glitazone reverse insulin resistance receptor, or NR1C3 is a type II nuclear receptor functioning as a transcription factor that in humans is encoded by the PPARG gene.
CCAAT/enhancer-binding protein beta is a protein that in humans is encoded by the CEBPB gene.
Sterol regulatory element-binding transcription factor 1 (SREBF1) also known as sterol regulatory element-binding protein 1 (SREBP-1) is a protein that in humans is encoded by the SREBF1 gene.
CCAAT/enhancer-binding protein delta is a protein that in humans is encoded by the CEBPD gene.
3T3-L1 is a sub clonal cell line derived from the original 3T3 Swiss albino cell line of 1962. The 3T3 original cell line was isolated from a mouse embryo and propagated for 3this specific line of 3T3 cells is used to study adipose tissuerelated diseases and dysfunctions. The 3T3-L1 Swiss sub clone line has been widely utilized, since its development, due to its affinity for lipid droplet deposition in vitro. 3T3-L1 cells have a fibroblast-like morphology, but, under appropriate conditions, the cells differentiate into an adipocyte-like phenotype, providing an exemplar model for white adipocytes. 3T3-L1 cells can be utilized to study a number of cellular and molecular mechanisms related to insulin-resistance, obesity, and diabetes in vitro. Aside from its usages, this cell line is widely developed and can be purchased for continuous propagation for numerous research studies. 3T3-L1 cells of the adipocyte morphology increase the synthesis and accumulation of triglycerides and acquire the signet ring appearance of adipose cells. These cells are also sensitive to lipogenic and lipolytic hormones, as well as drugs, including epinephrine, isoproterenol, and insulin.
DNA damage-inducible transcript 3, also known as C/EBP homologous protein (CHOP), is a pro-apoptotic transcription factor that is encoded by the DDIT3 gene. It is a member of the CCAAT/enhancer-binding protein (C/EBP) family of DNA-binding transcription factors. The protein functions as a dominant-negative inhibitor by forming heterodimers with other C/EBP members, preventing their DNA binding activity. The protein is implicated in adipogenesis and erythropoiesis and has an important role in the cell's stress response.
Krüppel-like Factor 2 (KLF2), also known as lung Krüppel-like Factor (LKLF), is a protein that in humans is encoded by the KLF2 gene on chromosome 19. It is in the Krüppel-like factor family of zinc finger transcription factors, and it has been implicated in a variety of biochemical processes in the human body, including lung development, embryonic erythropoiesis, epithelial integrity, T-cell viability, and adipogenesis.
Histone-lysine N-methyltransferase 2D (KMT2D), also known as MLL4 and sometimes MLL2 in humans and Mll4 in mice, is a major mammalian histone H3 lysine 4 (H3K4) mono-methyltransferase. It is part of a family of six Set1-like H3K4 methyltransferases that also contains KMT2A, KMT2B, KMT2C, KMT2F, and KMT2G.
Insulin induced gene 2, also known as INSIG2, is a protein which in humans is encoded by the INSIG2 gene.
Chemerin, also known as retinoic acid receptor responder protein 2 (RARRES2), tazarotene-induced gene 2 protein (TIG2), or RAR-responsive protein TIG2 is a protein that in humans is encoded by the RARRES2 gene.
Forkhead box protein O1 (FOXO1), also known as forkhead in rhabdomyosarcoma (FKHR), is a protein that in humans is encoded by the FOXO1 gene. FOXO1 is a transcription factor that plays important roles in regulation of gluconeogenesis and glycogenolysis by insulin signaling, and is also central to the decision for a preadipocyte to commit to adipogenesis. It is primarily regulated through phosphorylation on multiple residues; its transcriptional activity is dependent on its phosphorylation state.
Krüppel-like factor 15 is a protein that in humans is encoded by the KLF15 gene in the Krüppel-like factor family. Its former designation KKLF stands for kidney-enriched Krüppel-like factor.
miR-27 is a family of microRNA precursors found in animals, including humans. MicroRNAs are typically transcribed as ~70 nucleotide precursors and subsequently processed by the Dicer enzyme to give a ~22 nucleotide product. The excised region or, mature product, of the miR-27 precursor is the microRNA mir-27.
Phosphotyrosine interaction domain containing 1 is a protein that in humans is encoded by the PID1 gene.