Adipogenesis is the formation of (fat cells) from stem cells.
Introduction
Adipocytes play a vital role in energy homeostasis and process the largest energy reserve as
triglycerol in the body of animals.
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.
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
like
adiponectin,
leptin,
resistin (in the mouse, not in humans) and
TNF-alpha.
Differentiation
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
CEBPB, and
CEBPD. The second phase of growth arrest is the expression of two key transcription factors PPARγ and
CEBPA 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.
Through this process, lipid droplets accumulate in the
adipocyte. However, preadipocytes cell lines have difficulty differentiating into adipocytes. Preadipocytes display CD45
− CD31
− CD34
+ CD29
+ SCA1
+ CD24
+ surface markers can proliferate and differentiate to adipocytes in vivo.
Models of in vitro differentiation
|
|
| |
| FBS+ I+ D+ M |
| FBS + I |
| FBS+ I+ T3 |
| FBS + D + I |
| FBS + D + M + I |
| D + M + I |
| |
| Ectopic expression of PPAR-gamma, C/EBP-alpha or C/EBP-beta + D+ M+ I |
| Ectopic expression of C/EBP-alpha |
| Ectopic expression of C/EBP-alpha |
| PPAR-gamma ligand |
| FBS + I + D + M |
| Thiazolidinediones |
| Ectopic expression of PPAR-gamma + CEBP/alpha +D + I |
|
Transcriptional regulations
PPARγ
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.
PPARγ is necessary and sufficient to promote fat cell differentiation. PPARγ is required for embryonic stem cells (ES cells) differentiation to
.
The expression of PPARγ itself is sufficient to convert fibroblast into
in vitro.
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.
Thiazolidinediones (TZDs), antidiabetic agents which well used differentiation cocktail in vitro, promoting the activity of PPARγ.
C/EBPs
C/EBPs, transcription factors, are members of the basic-leucine zipper class. cAMP, an inducer of adipogenesis, can promote expression of
CEBPB and
CEBPD.
At the early stage of differentiation, the transient increase of
CEBPB and
CEBPD mRNA and protein levels are thought to activate the adipogenic transcription factors, PPARγ and
CEBPA. PPARγ and
CEBPA can feedback to induce the expression of each other as well as their downstream genes.
also plays an important role in the insulin sensitivity of adipocytes.
However,
CEBPG suppresses differentiation which might due to inactivation by
CEBPB.
Transcriptional cascade
Although PPARγ and
CEBPA 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
CEBPA is also responsive to negative regulation. T-cell factor/lymphoid enhancer-binding factor (TCF/LEF),
GATA2/3,
retinoic acid receptor α,
and SMAD6/7
don't affect the expression of
CEBPB and
CEBPD but inhibit the induction of PPARγ and
CEBPA.
Epigenetic regulation: TET1
TET1 plays an important role in the early stages of adipogenesis. During this process, TET1 increases its expression and promotes DNA demethylation. This epigenetic modification facilitates the activation of genes required for adipocyte formation. In this way, TET1 helps progenitor cells change their identity and acquire the characteristics of mature adipocytes.
Other regulations
Products of endocrine system such as
insulin, IGF-1, cAMP,
glucocorticoid, and
triiodothyronine effectively induce adipogenesis in preadipocytes.
Insulin and IGF1
Insulin regulates adipogenesis through insulin-like growth factor 1 (IGF1) receptor signaling. Insulin/IGF1 promotes the induction transcription factors regulating terminal differentiation.
Wnt signaling
Wnt/β-catenin signaling suppresses adipogenesis, by promoting the differentiation of mesenchymal stem cells into
and
but blocking the commitment to the adipocytic lineage.
Wnt/β-catenin inhibits the differentiation of preadipocytes by inhibiting the induction of PPARγ and
CEBPA.
BMPs
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.
BMPs also promotes the differentiation of preadipocytes.
Senescent cells
Senescent adipose progenitor cells in subcutaneous adipose tissue has been shown to suppress adipogenic differentiation.
Reduced adipogenesis in obese persons is due to increased senescent cells in adipose tissue rather than reduced numbers of stem/progenitor cells.
