PFKFB3-dependent glucose metabolism regulates 3T3-L1 adipocyte development

Mechanistic studies of PFKFB2 reveals a novel inhibitor of its kinase activity

The 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase (PFKFB) family of proteins are bifunctional enzymes that are of clinical relevance because of their roles in regulating glycolysis in insulin sensitive tissues and cancer. Here, we sought to express recombinant PFKFB2 and develop a robust protocol to measure its kinase activity. These studies resulted in the unexpected finding that bacterially expressed PFKFB2 is phosphorylated in situ on Ser483 but is not a result of autophosphorylation. Recombinant PFKFB2 was used to develop an enzymatic assay to test a library of molecules selected by the Atomwise AtomNet® AI platform. This resulted in the identification of a new inhibitor, B2, that inhibits PFKFB2 (IC50 3.29 μM) and PFKFB3 (IC50 11.89 μM). A-498 cells, which express both PFKFB2 and PFKFB3, were treated with B2. Seahorse XFe analysis revealed B2 inhibited cellular glycolysis and glycolytic capacity. Targeted LC/MS analysis showed B2 decreased fructose-1,6-bisphosphate and downstream glycolytic intermediates but increased fructose-6-phosphate levels, which is consistent with an inhibitory effect on PFK-1 activity. The LC/MS metabolic profile of A-498 cells treated under identical conditions with the known PFKFB3 inhibitor, PFK158, was distinct from that induced by B2. These results thus demonstrate the identification and validation of a new PFKFB kinase inhibitor.

PFKFB3 protein in adipose tissue contributes to whole body glucose homeostasis

Age-dependent changes in adipose tissue are thought to play a role in development of insulin resistance. A major age-dependent change in adipose tissue is the downregulation of key proteins involved in carbohydrate metabolism. In the current study, we investigate the role of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3) a key governor of the rate of glycolysis in adipocytes via the synthesis of fructose-2,6-bisphosphate that was significantly downregulated in aged mice. We employed an adipocyte-specific PFKFB3 mouse line to investigate the role of PFKFB3 on adipocyte function. In both aged mice and PFKFB3-knockout mice, we observed an increase in O-glcNAcylated proteins consistent with a shift in glucose metabolism toward the hexosamine biosynthetic pathway. Under chow-fed conditions, PFKFB3 knockout resulted in significantly smaller adipocyte area, but no difference in total fat mass. While glucose tolerance was unchanged under chow conditions, when mice were challenged with a 4 weeks high-fat feeding, PFKFB3 deletion led to a greater decrease in glucose tolerance as well as a significant increase in macrophage infiltration. These results indicate that perturbation of the glycolytic pathway in adipose tissue has multiple effects of adipocyte biology and may play a significant role in metabolic changes associated with aging. Results of this student support the notion that changes in glucose metabolism in adipose tissue impact whole-body metabolism.

Disruption of nucleotide biosynthesis reprograms mitochondrial metabolism to inhibit adipogenesis

A key organismal response to overnutrition involves the development of new adipocytes through the process of adipogenesis. Preadipocytes sense changes in the systemic nutrient status and metabolites can directly modulate adipogenesis. We previously identified a role of de novo nucleotide biosynthesis in adipogenesis induction, whereby inhibition of nucleotide biosynthesis suppresses the expression of the transcriptional regulators PPARγ and C/EBPα. Here, we set out to identify the global transcriptomic changes associated with the inhibition of nucleotide biosynthesis. Through RNA sequencing (RNAseq), we discovered that mitochondrial signatures were the most altered in response to inhibition of nucleotide biosynthesis. Blocking nucleotide biosynthesis induced rounded mitochondrial morphology, and altered mitochondrial function, and metabolism, reducing levels of tricarboxylic acid cycle intermediates, and increasing fatty acid oxidation (FAO). The loss of mitochondrial function induced by suppression of nucleotide biosynthesis was rescued by exogenous expression of PPARγ. Moreover, inhibition of FAO restored PPARγ expression, mitochondrial protein expression, and adipogenesis in the presence of nucleotide biosynthesis inhibition, suggesting a regulatory role of nutrient oxidation in differentiation. Collectively, our studies shed light on the link between substrate oxidation and transcription in cell fate determination.

Klf9 plays a critical role in GR -dependent metabolic adaptations in cardiomyocytes

Glucocorticoids through activation of the Glucocorticoid receptor (GR) play an essential role in cellular homeostasis during physiological variations and in response to stress. Our genomic GR binding and transcriptome data from Dexamethasone (Dex) treated cardiomyocytes showed an early differential regulation of mostly transcription factors, followed by sequential change in genes involved in downstream functional pathways. We examined the role of Krüppel-like factor 9 (Klf9), an early direct target of GR in cardiomyocytes. Klf9-ChIPseq identified 2150 genes that showed an increase in Klf9 binding in response to Dex. Transcriptome analysis of Dex treated cardiomyocytes with or without knockdown of Klf9 revealed differential regulation of 1777 genes, of which a reversal in expression is seen in 1640 genes with knockdown of Klf9 compared to Dex. Conversely, only 137 (∼8%) genes show further dysregulation in expression with siKLf9, as seen with Dex treated cardiomyocytes. Functional annotation identified genes of metabolic pathways on the top of differentially expressed genes, including those involved in glycolysis and oxidative phosphorylation. Knockdown of Klf9 in cardiomyocytes inhibited Dex induced increase in glycolytic function and mitochondrial spare respiratory capacity, as measured by glycolysis and mito stress tests, respectively. Thus, we conclude that cyclic, diurnal GR activation, through Klf9 -dependent feedforward signaling plays a central role in maintaining cellular homeostasis through metabolic adaptations in cardiomyocytes.

Expression of Pfkfb isoenzymes during in vitro differentiation of mouse embryonic stem cells into insulin-producing cells

Background/aim

Type 1 diabetes mellitus (T1DM) is caused by the autoimmune-mediated destruction of insulin-producing cells (IPCs) and still has no effective cure. Better understanding of the molecular mechanisms involved in the differentiation of embryonic stem cells (ESCs) into IPCs may help us improve the therapeutic strategies for treating T1DM. 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatases (Pfkfb1-4) are key regulators of glucose metabolism. Although Pfkfb3 has been shown to be required for the growth of early differentiated mouse ESCs (mESCs), more studies are needed to further assess the roles of Pfkfb isoenzymes in embryonic development and differentiation, particularly into specific cell types. In this study, we aimed to elucidate the changes in the expression of Pfkfb isoenzymes on the differentiation of mESCs into IPCs.

Materials and methods

A 3-step protocol was used to differentiate R1 and J1 mESCs into IPCs. The changes in the gene expression of MafA, MafB, Ins2, and Nkx6.1 (IPC specific markers) and Pfkfb1-4 were analyzed using real-time quantitative polymerase chain reaction (qPCR). Insulin expression and secretion were determined by immunofluorescence (IF) staining and the enzyme linked immunosorbent assay (ELISA), respectively.

Results

Upon differentiation, the IPC specific markers in differentiated cells were upregulated. Continued differentiation was confirmed by the development of insulin-positive islet-like clusters that secreted insulin in response to glucose uptake. Expressions of the Pfkfb2 and Pfkfb3 isoenzymes were markedly increased in various stages of differentiation.

Conclusion

These findings suggest that Pfkfb2 and Pfkfb3 may impact the differentiation of mESCs into IPCs and the regulation of the insulin response to glucose levels. This study also lays a foundation for researchers to further probe the roles of Pfkfb isoenzymes on the differentiation of mESCs into IPCs and may open new avenues for regenerative medicine.

Increased expression of 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase-3 is required for growth of mouse embryonic stem cells that are undergoing differentiation

The unlimited proliferation capacity of embryonic stem cells (ESCs) coupled with their capability to differentiate into several cell types makes them an attractive candidate for studying the molecular mechanisms regulating self-renewal and transition from pluripotent state. Although the roles of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase family (PFKFB1-4) in cell survival, proliferation, and differentiation in tumor cells have been studied, their role in mouse ESC (mESC) biology is currently unkown. In the current study, Pfkfb isoenzyme expressions were analyzed in R1 and J1 mESCs that were cultured in the presence and absence of leukemia inhibitory factor (LIF). We report that expression of the Pfkfb3 isoenzyme was markedly increased when mESCs were promoted to differentiate upon LIF removal. We then demonstrated that Pfkfb3 silencing induced the differentiation marker Brachyury suggesting that Pfkfb3 may be required for the regulation of mesodermal differentiation of mESCs. Furthermore, we show that the increase in Pfkfb3 expression is required for the growth of early differentiated mESCs. Although these results provide important insights into the early differentiation of mESCs with regard to Pfkfb expressions, further mechanistic studies will be needed for understanding the pathways and mechanisms involved in regulation of proliferation and early differentiation of mESCs through Pfkfb3.