GATA-3 as a potential therapeutic target for insulin resistance and type 2 diabetes mellitus

Background: Impaired adipogenesis plays an important role in the development of obesity-associated insulin resistance and type 2 diabetes as it leads to ectopic fat deposition. Discussion: The anti-adipogenic transcription factor GATA-3 was identified as one of the potential molecular targets responsible for impairment of adipogenesis. The expression of GATA-3 is higher in insulin resistant obese individuals compared to BMI-matched insulin sensitive counterparts. Adipose tissue inflammation is a crucial mediator of this process. Hyperglycemia mediates the activation of immune system, partially through upregulation of GATA-3, causing exacerbation of the inflammatory state associated with obesity. Conclusion: This review discusses evidence supporting the inhibition of GATA-3 as a useful therapeutic strategy in obesity-associated insulin resistance and type 2 diabetes, through up-regulation adipogenesis and amelioration of the immune response.

Interplay of BAF and MLL4 promotes cell type-specific enhancer activation

Cell type-specific enhancers are activated by coordinated actions of lineage-determining transcription factors (LDTFs) and chromatin regulators. The SWI/SNF complex BAF and the histone H3K4 methyltransferase MLL4 (KMT2D) are both implicated in enhancer activation. However, the interplay between BAF and MLL4 in enhancer activation remains unclear. Using adipogenesis as a model system, we identified BAF as the major SWI/SNF complex that colocalizes with MLL4 and LDTFs on active enhancers and is required for cell differentiation. In contrast, the promoter enriched SWI/SNF complex PBAF is dispensable for adipogenesis. By depleting BAF subunits SMARCA4 (BRG1) and SMARCB1 (SNF5) as well as MLL4 in cells, we showed that BAF and MLL4 reciprocally regulate each other’s binding on active enhancers before and during adipogenesis. By focusing on enhancer activation by the adipogenic transcription factor C/EBPβ without inducing cell differentiation, we provide direct evidence for an interdependent relationship between BAF and MLL4 in activating cell type-specific enhancers. Together, these findings reveal a positive feedback between BAF and MLL4 in promoting LDTF-dependent activation of cell type-specific enhancers.

Targeting senescent cells enhances adipogenesis and metabolic function in old age

Senescent cells accumulate in fat with aging. We previously found genetic clearance of senescent cells from progeroid INK-ATTAC mice prevents lipodystrophy. Here we show that primary human senescent fat progenitors secrete activin A and directly inhibit adipogenesis in non-senescent progenitors. Blocking activin A partially restored lipid accumulation and expression of key adipogenic markers in differentiating progenitors exposed to senescent cells. Mouse fat tissue activin A increased with aging. Clearing senescent cells from 18-month-old naturally-aged INK-ATTAC mice reduced circulating activin A, blunted fat loss, and enhanced adipogenic transcription factor expression within 3 weeks. JAK inhibitor suppressed senescent cell activin A production and blunted senescent cell-mediated inhibition of adipogenesis. Eight weeks-treatment with ruxolitinib, an FDA-approved JAK1/2 inhibitor, reduced circulating activin A, preserved fat mass, reduced lipotoxicity, and increased insulin sensitivity in 22-month-old mice. Our study indicates targeting senescent cells or their products may alleviate age-related dysfunction of progenitors, adipose tissue, and metabolism.

Programmed regulation of rat offspring adipogenic transcription factor (PPARγ) by maternal nutrition

We determined the protein expression of adipogenic transcription factor, peroxisome proliferator-activated receptor gamma (PPARγ) and its co-repressor and co-activator complexes in adipose tissue from the obese offspring of under- and over-nourished dams. Female rats were fed either a high-fat (60% kcal) or control (10% kcal) diet before mating, and throughout pregnancy and lactation (Mat-OB). Additional dams were 50% food-restricted from pregnancy day 10 to term [intrauterine growth-restricted (IUGR)]. Adipose tissue protein expression was analyzed in newborn and adult male offspring. Normal birth weight Mat-OB and low birth weight IUGR newborns had upregulated PPARγ with variable changes in co-repressors and co-activators. As obese adults, Mat-OB and IUGR offspring had increased PPARγ with decreased co-repressor and increased co-activator expression. Nutritionally programmed increased PPARγ expression is associated with altered expression of its co-regulators in the newborn and adult offspring. Functional studies of PPARγ co-regulators are necessary to establish their role in PPARγ-mediated programmed obesity.

Fyn Regulates Adipogenesis by Promoting PIKE-A/STAT5a Interaction

Fyn is a tyrosine kinase with multiple roles in a variety of cellular processes. Here we report that Fyn is a new kinase involved in adipocyte differentiation. Elevated Fyn protein is detected specifically in the adipocytes of obese mice. Moreover, Fyn expression increases progressively in 3T3-L1 cells during in vitro adipogenesis, which correlates with its kinase activity. Inhibition of Fyn by either genetic or pharmacological manipulation restrains the 3T3-L1 preadipocytes from fully differentiating into mature adipocytes. Mechanistically, Fyn regulates the activity of the adipogenic transcription factor signal transducer and activator of transcription 5a (STAT5a) through enhancing its interaction with the GTPase phosphoinositide 3-kinase enhancer A (PIKE-A). The STAT5a activity is therefore reduced in Fyn - or PIKE -ablated adipose tissues, leading to diminished expression of adipogenic markers and adipocyte differentiation. Our data thus demonstrate a novel functional interaction between Fyn, PIKE-A, and STAT5a in mediating adipogenesis.

Effect of 2,4-Thiazolidinedione on Limousin Cattle Growth and on Muscle and Adipose Tissue Metabolism

The main adipogenic transcription factor PPARγpossesses high affinity to 2,4-TZD, a member of the Thiazolidinedione family of insulin-sensitizing compounds used as adipogenic agents. We evaluated 2,4-TZD’s effect on bovine growth and PPAR tissue expression. Seventeen Limousin bulls (18 month-old; 350 kg body weight (BW)) were assigned into 2 treatments: control and 2,4-TZD (8 mg/70 kg BW) and were fed until bulls reached 500 kg BW. They were weighed and their blood was sampled. DNA, RNA, and protein were determined in liver; skeletal muscle; subcutaneous (SC), omental, perirenal adipose tissues (AT) to determine protein synthesis rate and cellular size. Expression of PPAR mRNA was measured in liver and muscle (PPARα, -δ, and -γ) and SC adipose tissue (γ) by real-time PCR. No significant differences were found (P>0.1) in weight gain, days on feed, and carcass quality. Muscle synthesis was greater in controls (P<0.05); cell size was larger with 2,4-TZD (P<0.05). PPARα, -δ, and -γexpressions with 2,4-TZD in liver were lower (P<0.01) than in muscle. No differences were found for PPARγmRNA expression in SCAT. The results suggest the potential use of 2,4-TZD in beef cattle diets, because it improves AT differentiation, liver, and muscle fatty acid oxidation that, therefore, might improve energy efficiency.

Abundance of two human preadipocyte subtypes with distinct capacities for replication, adipogenesis, and apoptosis varies among fat depots

Fat depots vary in function and size. The preadipocytes that fat cells develop from exhibit distinct regional characteristics that persist in culture. Human abdominal subcutaneous cultured preadipocytes undergo more extensive lipid accumulation, higher adipogenic transcription factor expression, and less TNF-α-induced apoptosis than omental preadipocytes. We found higher replicative potential in subcutaneous and mesenteric than in omental preadipocytes. In studies of colonies arising from single preadipocytes, two preadipocyte subtypes were found, one capable of more extensive replication, differentiation, and adipogenic transcription factor expression and less apoptosis in response to TNF-α than the other. The former was more abundant in subcutaneous and mesenteric than in omental preadipocyte populations, potentially contributing to regional variation in replication, differentiation, and apoptosis. Both subtypes were found in strains derived from single human preadipocytes stably expressing telomerase, confirming that both subtypes are of preadipocyte lineage. After subcloning of cells of either subtype, both subtypes were found, indicating that switching can occur between subtypes. Thus proportions of preadipocyte subtypes with distinct cell-dynamic properties vary among depots, potentially permitting tissue plasticity through subtype selection during development. Furthermore, mesenteric preadipocyte cell-dynamic characteristics are distinct from omental cells, indicating that visceral fat depots are not functionally uniform.

T-Cell Factor 4N (TCF-4N), a Novel Isoform of Mouse TCF-4, Synergizes with β-Catenin To Coactivate C/EBPα and Steroidogenic Factor 1 Transcription Factors

ABSTRACT We have cloned T-cell factor 4N (TCF-4N), an alternative isoform of TCF-4, from developing pituitary and 3T3-L1 preadipocytes. This protein contains the N-terminal interaction domain for β-catenin but lacks the DNA binding domain. While TCF-4N inhibited coactivation by β-catenin of a TCF/lymphoid-enhancing factor (LEF)-dependent promoter, TCF-4N potentiated coactivation by β-catenin of several non-TCF/LEF-dependent promoters. For example, TCF-4N synergized with β-catenin to activate the α-inhibin promoter through functional and physical interactions with the orphan nuclear receptor steroidogenic factor 1 (SF-1). In addition, TCF-4N and β-catenin synergized with the adipogenic transcription factor CCAAT/enhancer binding protein α (C/EBPα) to induce leptin promoter activity. The mechanism by which β-catenin and TCF-4N coactivated C/EBPα appeared to involve p300, based upon synergy between these important transcriptional regulators. Consistent with TCF-4N′s redirecting the actions of β-catenin in cells, ectopic expression of TCF-4N in 3T3-L1 preadipocytes partially relieved the block of adipogenesis caused by β-catenin. Thus, we propose that TCF-4N inhibits coactivation by β-catenin of TCF/LEF transcription factors and potentiates the coactivation by β-catenin of other transcription factors, such as SF-1 and C/EBPα.