Human adipose tissue-derived preadipocytes are multipotent precursor cells that form adipocytes. They are involved in lipid metabolism and storage. Adipose tissue preadipocytes also demonstrate immune and hemostatic functions. Preadipocytes are important regulators of metabolic health with implications in obesity and the related comorbidities. Therefore, their biochemical processes in health and diseases are under investigation with a focus on the differentiation of preadipocytes.
Human Adipose Tissue-Derived preadipocytes and adipocytes
Preadipocytes are precursors to adipocytes and exhibit fibroblast morphology. They differentiate into adipocytes, which are mature fat cells. Adipocytes store nutrients in the form of lipids within the lipid droplet. These droplets encompass the majority of the cell pushing the organelles to one side. Upon requirement, adipocytes release these lipids by lipolysis. In addition to their role in metabolism, they also display endocrine function by the secretion of adipokines such as leptin, adiponectin, and resistin.
Types of adipocytes
Adipocytes are broadly classified as white and brown depending on their visual appearance. White adipocytes execute the functions of nutrient storage, endocrine secretion, and insulin sensitivity. On the other hand, brown adipocytes maintain body temperature. They are typically present in infants as well as thoracic and supraclavicular regions in humans. Morphologically, white adipocytes are spherical, containing a single large droplet of lipid, whereas ellipsoidal brown adipocytes have multiple lipid droplets, imparting the brownish color.
Beige adipocytes are other distinct adipocytes that possess the properties of both brown and white adipocytes. Research suggests that they arise from the differentiation of white adipocytes in response to cold, exercise, diet, adipokines, etc. Their occurrence, however, has only been noticed in mice. Pink adipocytes were discovered in 2014 during the pregnancy and lactation period. Studies have indicated that white adipocytes undergo reversible transdifferentiating into epithelial morphology with cytoplasmic extensions for milk secretion.
Research Models for studying differentiation of Preadipocytes
The differentiation of preadipocytes forms a pool of adipocytes. The quantity of adipocytes has serious implications in obesity, and therefore, differentiation needs to be delineated. While studying the differentiation process, in vivo models were taken into account. However, their use was challenging owing to the existence of preadipocytes in different stages of development. Therefore, the next accurate representation, the two-dimensional culture of primary preadipocytes, was chosen for the studies. The differentiation process was also called adipogenesis. Two transcription factors- CCAAT/enhancer binding protein-α (C/EBP-α) and peroxisome-proliferator activating receptor-γ (PPARγ) are key regulators of the differentiation process. The deletion of PPARγ in humans not only reduces adipose tissue mass but also insulin sensitivity.
Describing in detail how human adipose tissue-derived preadipocytes differentiate
Early Stage: During the differentiation phase, human adipose tissue-derived preadipocytes withdraw from the cell cycle, leading to growth arrest. Early stages of differentiation show the expression of lipoprotein lipase mRNA. This enzyme is responsible for lipid accumulation. Unlike PPAR-γ, it is not specific to adipocytes and also demonstrates induction by external agents, thus not establishing it as the early differentiation marker. In addition to C/EBP-α and PPARγ, early differentiation stages also witness the induction of sterol regulatory element binding protein-1c (SREBP-1c). It is a transcription factor containing a basic helix-loop-helix (bHLH) leucine zipper motif and participates in cholesterol metabolism. The differentiation changes the cell morphology, cytoskeleton, and extracellular matrix. Actin and tubulin content declines before the morphological change. Collagen shifts from type 1 and III to type IV along with the production of entactin. There is also a decrease in pref-1 (preadipocyte factor-1) that maintains the preadipocyte phenotype.
Late Stage: It marks the increase in lipid metabolism proteins such as fatty-acid synthase, acetyl-CoA carboxylase, glyceraldehyde 3-phosphate dehydrogenase, etc. Additionally, glucose transporters and insulin receptors also rise in number. The loss of β1 type of adrenergic receptors and gain in β2 and β3 types is seen. The formation of adipose tissue-specific products like adipocyte-fatty acid binding protein, perilipin, fatty acid transporter, etc., occurs. Low levels of leptin are also detected. The terminal differentiation of cells leads to the loss of dedifferentiation and cell proliferation ability. However, researchers have reported dedifferentiation and cell division in adipocytes after terminal differentiation. Although dedifferentiation did not completely form the preadipocytes, as evident by the lack of markers. Thus, more studies are needed to delineate the phenomenon.
Elucidating the factors influencing the differentiation of human adipose tissue-derived preadipocytes
Numerous factors regulate the differentiation process. For example, insulin-like growth factor-1 (IGF1) promotes the differentiation of preadipocytes through Ras. Preadipocytes express insulin-like growth factor receptor proteins in a differentiation-dependent manner. Raf and Akt are the downstream effectors of the adipocyte differentiation pathway. Transforming growth factor- β (TGFβ) inhibits differentiation via extracellular matrix synthesis. Cytokines like IL11, IFNγ, and IL1β also suppress differentiation.
Tumor necrosis factor-α (TNFα) decreases the synthesis of lipoprotein lipase synthesis. It is linked to the downregulation of C/EBPα, decreased levels of PPARγ, and increased c-myc expression. Adipocytes also express high amounts of steroid hormone receptors. However, studies in the case of estrogen and androgen have been contradictory at best, showing the differentiation-promoting and inhibiting activities by hormones. Glucocorticoids promote differentiation by inducing C/EBPβ, which along with C/EBPδ, induces PPARγ expression. Aldosterone also modulates adipogenesis by its mineralocorticoid receptor. Vitamin D suppresses adipogenesis by inhibiting C/EBPβ and PPARγ.
How pathways affect the differentiation of human adipose tissue-derived preadipocytes?
A complex interplay occurs between various signaling molecules to cause differentiation of preadipocytes.
MAPK signaling: MAPK signaling mediates cell proliferation and differentiation. It exerts its effects via downstream effectors like ERK, c-Jun, and p38 MAPK. Both ERK and p38 MAPK participate in adipogenesis. Studies have emphasized the significance of ERK in early-stage differentiation but also show PPARγ inhibition on continuous ERK activation. The research on p38 MAPK has yielded contradictory results for its role in adipogenesis.
Wnt Signaling: It facilitates the determination of cell fate. The signaling elicits a response with β-catenin (canonical pathway) and without β-catenin (non-canonical pathway). In adipogenesis, it uses a canonical pathway and affects adipogenesis in a negative manner. Activation of Wnt suppresses both C/EBP-α and PPARγ as well as adipogenic genes.
Hedgehog signaling: This signaling pathway drives cell differentiation and embryogenesis. It promotes chondrogenesis and osteogenesis while inhibiting adipogenesis. Through its cell surface receptors Patched and Smoothened, it activates the Gli family of transcription factors. Hedgehog signaling inhibits C/EBP-α and PPARγ and promotes pref-1 expression, thus reducing adipogenesis and lipid accumulation.
AMPK signaling: AMPK plays a role in cell cycle and energy homeostasis. Exposure to cold, low nutrient levels, exercise, and fasting stimulates its activation. Low levels of adenosine triphosphate (ATP) due to ischemia, lack of nutrients, and hypoxia activate AMPK. It stimulates absorption of fatty acids and glucose along with fatty acid oxidation while inhibiting fatty acid synthesis. AMPK activation suppresses adipogenesis by blocking clonal expansion that precedes growth arrest. The reports also show reduced expression of C/EBP-α, PPARγ, and adipogenic factors.
Role of differentiation of human adipose tissue-derived preadipocytes in obesity
During obesity, size, and number of adipocytes increase, and their functioning is disrupted. The adipose tissue expandability hypothesis explains the underlying reasons. It states that the lack of adipogenesis leads to low numbers of adipocytes and limits the nutrient storage capacity of adipocytes despite their enlarged size. It is evident with better insulin sensitivity and lipid profiles resulting from an increasing number of adipocytes. However, there have been contradictory studies indicating that a high number of adipocytes having limited expansion ability can also impair metabolic health.
Research has revealed that fat deposition in visceral and abdominal subcutaneous regions is more susceptible to obesity-related comorbidities rather than in femoral and subcutaneous gluteal regions. With limited storage capacity of adipocytes, fats deposit ectopically on non-adipose tissues like the liver, kidney, and skeletal muscles. Intracellular fat deposition in skeletal muscle causes insulin resistance while deposition in the liver results in fatty liver and impaired insulin-based glucose production.
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The Final Outlook
Obesity poses a major health challenge. It has an association with cardiovascular disorders and type II diabetes mellitus. Differentiation of human adipose tissue-derived preadipocytes into adipocytes has serious implications in lipid metabolism and obesity. Research has been going on to understand the underlying mechanism, its genetic regulation, and the proteins that govern the process.
Kosheeka offers primary preadipocytes and stromal vascular fraction, which includes both preadipocytes and adipocytes. We also offer comprehensive solutions for tailoring donor characteristics. To guarantee cell viability and purity, the team at Kosheeka characterizes them and conducts reliable testing.
FAQs
Q: What are preadipocytes and adipocytes?
Both are the cells of adipose tissue. Preadipocytes are the precursors that differentiate to form adipocytes.
Q: What happens to preadipocytes and adipocytes in obesity?
To store lipids, preadipocytes form more adipocytes. The increase in number and size of adipocytes is seen during obesity to enhance the storage capacity of adipose tissue.
Q: How do preadipocytes and adipocytes contribute to insulin resistance?
Less differentiation of preadipocytes and limited expansion of adipocytes lead to low storage capacity. When the fats exceed the adipose tissue storage, they deposit on other organs, causing insulin resistance.
Q: What are the transcription factors involved in the differentiation of preadipocytes?
CCAAT/enhancer binding protein-α (C/EBP-α) and peroxisome-proliferator activating receptor-γ (PPARγ) are two major transcription factors regulating differentiation. Additionally, sterol regulatory element binding protein-1c (SREBP-1c) also governs the differentiation process.
