RASSF6 expression in adipocytes is down-regulated by interaction with macrophages

Characterization of mouse embryonic fibroblasts derived from Rassf6 knockout mice shows the implication of Rassf6 in the regulation of NF-κB signaling

RASSF6 is a member of the tumor suppressor Ras association domain family (RASSF) proteins. We have reported using human cancer cell lines that RASSF6 induces apoptosis and cell cycle arrest via p53 and plays tumor suppressive roles. In this study, we generated Rassf6 knockout mice by CRISPR/Cas technology. Contrary to our expectation, Rassf6 knockout mice were apparently healthy. However, Rassf6-null mouse embryonic fibroblasts (MEF) were resistant against ultraviolet (UV)-induced apoptosis/cell cycle arrest and senescence. UV-induced p53-target gene expression was compromised, and DNA repair was delayed in Rassf6-null MEF. More importantly, KRAS active mutant promoted the colony formation of Rassf6-null MEF but not the wild-type MEF. RNA sequencing analysis showed that NF-κB signaling was enhanced in Rassf6-null MEF. Consistently, 7,12-dimethylbenz(a)anthracene (DMBA) induced skin inflammation in Rassf6 knockout mice more remarkably than in the wild-type mice. Hence, Rassf6 deficiency not only compromises p53 function but also enhances NF-κB signaling to lead to oncogenesis.

Vitamin B6 and Diabetes: Relationship and Molecular Mechanisms

Vitamin B6 is a cofactor for approximately 150 reactions that regulate the metabolism of glucose, lipids, amino acids, DNA, and neurotransmitters. In addition, it plays the role of antioxidant by counteracting the formation of reactive oxygen species (ROS) and advanced glycation end-products (AGEs). Epidemiological and experimental studies indicated an evident inverse association between vitamin B6 levels and diabetes, as well as a clear protective effect of vitamin B6 on diabetic complications. Interestingly, by exploring the mechanisms that govern the relationship between this vitamin and diabetes, vitamin B6 can be considered both a cause and effect of diabetes. This review aims to report the main evidence concerning the role of vitamin B6 in diabetes and to examine the underlying molecular and cellular mechanisms. In addition, the relationship between vitamin B6, genome integrity, and diabetes is examined. The protective role of this vitamin against diabetes and cancer is discussed.

GDE5 inhibition accumulates intracellular glycerophosphocholine and suppresses adipogenesis at a mitotic clonal expansion stage

Mammalian glycerophosphodiesterases (GDEs) were recently shown to be involved in multiple cellular signaling pathways. This study showed that decreased GDE5 expression results in accumulation of intracellular glycerophosphocholine (GPC), showing that GDE5 is actively involved in GPC/choline metabolism in 3T3-L1 adipocytes. Using 3T3-L1 adipocytes, we further studied the biological significance of GPC/choline metabolism during adipocyte differentiation. Inhibition of GDE5 suppressed the formation of lipid droplets, which is accompanied by the decreased expression of adipocyte differentiation markers. We further showed that the decreased GDE5 expression suppressed mitotic clonal expansion (MCE) of preadipocytes. Decreased expression of CTP: phosphocholine cytidylyltransferase (CCTβ), a rate-limiting enzyme for phosphatidylcholine (PC) synthesis, is similarly able to inhibit MCE and PC synthesis; however, the decreased GDE5 expression resulted in accumulation of intracellular GPC but did not affect PC synthesis. Furthermore, we showed that mRNAs of proteoglycans and transporters for organic osmolytes are significantly upregulated and that intracellular amino acids and urea levels are altered in response to GDE5 inhibition. Finally, we showed that reduction of GDE5 expression increased lactate dehydrogenase release from preadipocytes. These observations indicate that decreased GDE5 expression can suppress adipocyte differentiation not through the PC pathway but possibly by intracellular GPC accumulation. These results provide insight into the roles of mammalian GDEs and their dependence upon osmotic regulation by altering intracellular GPC levels.

Role of murine macrophage in temporal regulation of cortisol- and serotonin-induced adipogenesis in pre-adipocytes when grown together

Regulation of adipogenesis, the root cause for obesity, is very poorly understood. However, studies have presented evidence of immuno-metabolic regulation of adipose tissue during periods of chronic psychological stress, leading to adverse conditions related to stress manifestation, including visceral obesity and atherosclerosis. Despite pronounced association of hormonal markers of stress with dys-regulated metabolic states, the contributing signalling events are yet to be established. It is apparent that to understand contributing signalling events we need a model. Although an in vivo model is preferred, it is difficult to establish. The current report, therefore, presents an in vitro model system for the simulation of adipose tissue in a chronic stress micro-environment by growing pre-adipocytes with macrophages in the presence and absence of stress hormones. In this report, effects of cortisol and serotonin on the kinetics of immune and metabolic changes in adipocytes and macrophage (alone and co-cultured) was studied through whole genome transcriptome profiling. A transition from pro- to anti-inflammatory response in the immune profile of pre-adipocytes, with increasing time in co-culture with macrophages, was observed. This transition was reversed by stress hormones cortisol and/or serotonin.

Summary: Stress-induced obesity is poorly understood in vivo at the molecular level. The current report established a novel molecular basis of adipogenesis in vitro.

Serum Amyloid A3 Gene Expression in Adipocytes is an Indicator of the Interaction with Macrophages

The infiltration of macrophages into adipose tissue and their interaction with adipocytes are essential for the chronic low-grade inflammation of obese adipose tissue. In this study, we identified the serum amyloid A3 (Saa3) gene as a key adipocyte-derived factor that is affected by interaction with macrophages. We showed that the Saa3 promoter in adipocytes actually responds to activated macrophages in a co-culture system. Decreasing C/EBPβ abundance in 3T3-L1 adipocytes or point mutation of C/EBPβ elements suppressed the increased promoter activity in response to activated macrophages, suggesting an essential role of C/EBPβ in Saa3 promoter activation. Bioluminescence based on Saa3 promoter activity in Saa3-luc mice was promoted in obese adipose tissue, showing that Saa3 promoter activity is most likely related to macrophage infiltration. This study suggests that the level of expression of the Saa3 gene could be utilized for the number of infiltrated macrophages in obese adipose tissue.

Metabolomics uncovers the role of adipose tissue PDXK in adipogenesis and systemic insulin sensitivity

AIMS/HYPOTHESIS

We aimed to investigate the potential mechanisms involved in the compromised adipogenesis of visceral (VAT) vs subcutaneous adipose tissue (SAT) using comparative metabolomics. Based on the differentially identified metabolites, we focused on the relationship between the active form of vitamin B6 (pyridoxal 5-phosphate [PLP]), known to be generated through pyridoxal kinase (PDXK), and adipogenesis.

METHODS

Non-targeted metabolomics analyses were performed in paired VAT and SAT (n = 14, discovery cohort). PDXK gene expression was evaluated in two validation cohorts of paired SAT and VAT samples in relation to obesity status and insulin sensitivity, and mechanistically after weight loss in vivo and in 3T3-L1 cells in vitro.

RESULTS

Comparative metabolomics showed that PLP was significantly decreased in VAT vs SAT. Concordantly, PDXK mRNA levels were significantly decreased in VAT vs SAT, specifically in adipocytes. The decrease was specially marked in obese individuals. PDXK mRNA levels showed a strong association with adipogenic, lipid-droplet-related and lipogenic genes. At a functional level, systemic insulin sensitivity positively associated with PDXK expression, and surgically-induced weight loss (improving insulin sensitivity) led to increased SAT PDXK mRNA levels in parallel with adipogenic genes. In human pre-adipocytes, PDXK mRNA levels increased during adipocyte differentiation and after administration of peroxisome proliferator-activated receptor-γ agonists, and decreased under inflammatory stimuli. Mechanistic studies in 3T3-L1 cells showed that PLP administration resulted in increased adipogenic mRNA markers during early adipogenesis, whereas the PLP antagonist 4-deoxypyridoxine exerted opposite effects.

CONCLUSIONS/INTERPRETATION

Overall, these results support the notion that in situ production of PLP is required for physiological adipogenesis.

RASSF6; the Putative Tumor Suppressor of the RASSF Family

Humans have 10 genes that belong to the Ras association (RA) domain family (RASSF). Among them, RASSF7 to RASSF10 have the RA domain in the N-terminal region and are called the N-RASSF proteins. In contradistinction to them, RASSF1 to RASSF6 are referred to as the C-RASSF proteins. The C-RASSF proteins have the RA domain in the middle region and the Salvador/RASSF/Hippo domain in the C-terminal region. RASSF6 additionally harbors the PSD-95/Discs large/ZO-1 (PDZ)-binding motif. Expression of RASSF6 is epigenetically suppressed in human cancers and is generally regarded as a tumor suppressor. RASSF6 induces caspase-dependent and -independent apoptosis. RASSF6 interacts with mammalian Ste20-like kinases (homologs of Drosophila Hippo) and cross-talks with the Hippo pathway. RASSF6 binds MDM2 and regulates p53 expression. The interactions with Ras and Modulator of apoptosis 1 (MOAP1) are also suggested by heterologous protein-protein interaction experiments. RASSF6 regulates apoptosis and cell cycle through these protein-protein interactions, and is implicated in the NF-κB and JNK signaling pathways. We summarize our current knowledge about RASSF6 and discuss what common and different properties RASSF6 and the other C-RASSF proteins have.

Low RASSF6 expression in pancreatic ductal adenocarcinoma is associated with poor survival

AIM: To analyze RASSF6 expression in pancreatic ductal adenocarcinoma (PDAC) and to determine whether RASSF6 has an independent prognostic value in PDAC.

METHODS: We studied RASSF6 expression in 96 histologically confirmed PDAC samples and 20 chronic pancreatitis specimens using immunohistochemistry and real-time quantitative reverse transcription-PCR. PDAC issues were then classified as RASSF6 strongly positive, weakly positive or negative. RASSF6 mRNA and protein expression in PDAC samples with strong positive staining was further evaluated using real-time PCR and Western blot analysis. Lastly, correlations between RASSF6 staining and patients’ clinicopathological variables and outcomes were assessed.

RESULTS: RASSF6 was negatively expressed in 51 (53.1%) PDAC samples, weakly positively expressed in 29 (30.2%) and strongly positively expressed in 16 (16.7%), while its expression was much higher in para-tumor tissues and chronic pancreatitis tissues. Positive relationships between RASSF6 expression and T-stage (P = 0.047) and perineural invasion (P = 0.026) were observed. The median survival time of strongly and weakly positive and negative RASSF6 staining groups was 33 mo, 15 mo and 11 mo, respectively. Cox multivariate analysis indicated that RASSF6 was an independent prognostic indicator of overall survival in patients with PDAC. A survival curve analysis revealed that increased RASSF6 expression was correlated with better overall survival (P = 0.009).

CONCLUSION: RASSF6 expression is an independent biomarker of an unfavorable prognosis in patients with PDAC.

New members of the mammalian glycerophosphodiester phosphodiesterase family: GDE4 and GDE7 produce lysophosphatidic acid by lysophospholipase D activity

The known mammalian glycerophosphodiester phosphodiesterases (GP-PDEs) hydrolyze glycerophosphodiesters. In this study, two novel members of the mammalian GP-PDE family, GDE4 and GDE7, were isolated, and the molecular basis of mammalian GP-PDEs was further explored. The GDE4 and GDE7 sequences are highly homologous and evolutionarily close. GDE4 is expressed in intestinal epithelial cells, spermatids, and macrophages, whereas GDE7 is particularly expressed in gastro-esophageal epithelial cells. Unlike other mammalian GP-PDEs, GDE4 and GDE7 cannot hydrolyze either glycerophosphoinositol or glycerophosphocholine. Unexpectedly, both GDE4 and GDE7 show a lysophospholipase D activity toward lysophosphatidylcholine (lyso-PC). We purified the recombinant GDE4 and GDE7 proteins and show that these enzymes can hydrolyze lyso-PC to produce lysophosphatidic acid (LPA). Further characterization of purified recombinant GDE4 showed that it can also convert lyso-platelet-activating factor (1-O-alkyl-sn-glycero-3-phosphocholine; lyso-PAF) to alkyl-LPA. These data contribute to our current understanding of mammalian GP-PDEs and of their physiological roles via the control of lyso-PC and lyso-PAF metabolism in gastrointestinal epithelial cells and macrophages.

IκB kinase epsilon expression in adipocytes is upregulated by interaction with macrophages

Macrophage infiltration in the adipose tissue, and the interaction with adipocytes, is well documented to be involved in fat inflammation and obesity-associated complications. In this study, we isolated IκB kinase ε (IKKε) as a key adipocyte factor that is potentially affected by interaction with macrophages in adipose tissue in vivo. We showed that IKKε mRNA expression levels in white adipose tissue were increased in both genetic and diet-induced obese mouse. Furthermore, IKKε mRNA expression was decreased by the administration of vitamin B6, an anti-inflammatory vitamin, and that IKKε expression levels in adipose tissue were closely correlated with the numbers of infiltrating macrophages. In a co-culture system, we showed that IKKε expression in adipocytes was upregulated by interaction with activated macrophages. This study provides novel insight into IKKε, which is involved in adipose tissue inflammation during the development of obesity.

PLCɛ and the RASSF family in tumour suppression and other functions

Not all proteins implicated in direct binding to Ras appear to have a positive role in the generation and progression of tumours; examples include Phospholipase C epsilon (PLCɛ) and some members of the Ras-association domain family (RASSF). The RASSF family comprises of ten members, known as RASSF1 to RASSF10. PLCɛ and RASSF members carry a common Ras-association domain (RA) that can potentially bind Ras oncoproteins and mediate Ras-regulated functions. RASSF1 to RASSF6 also share a common SARAH domain that facilitates protein-protein interactions with other SARAH domain proteins. The majority of the family are frequently downregulated by epigenetic silencing in cancers. They are implicated in various important biological processes including apoptosis, microtubule stabilisation and cell cycle regulation. Recent studies have reinforced the tumour suppressive properties of the RASSF family, with new evidence of emerging pathways and novel functions that suggest a wider role for these proteins. This review will first describe an emerging role of PLCɛ in tumour suppression and then focus on and summarise the new findings on the RASSF family in the last five years to consolidate their well-established functions, and highlight the new regulatory roles of specific RASSF members.