Growth-inhibitory activity and downregulation of the class II tumor-suppressor gene H-rev107 in tumor cell lines and experimental tumors

Structural basis of the oncogenic KRAS mutant and GJ101 complex

KRAS mutations occur in a quarter of all human cancers. When activated in its GTP-bound form, RAS stimulates diverse cellular systems, such as cell division, differentiation, growth, and apoptosis through the activations of various signaling pathways, which include mitogen-activated protein kinase (MAPK), phosphoinositide 3 kinases (PI3K), and RAL-GEFs pathways. We found that GJ101 (⁶⁵LYDVA⁶⁹) binds directly to the KRAS mutant (G12V) and showed tumor-suppressive activity. In addition, the GJ101 peptide inhibited KRAS mutant as determined by a [α-³²P] guanosine triphosphate (GTP) binding assay and suppressed pancreatic cell line in a cell proliferation assay. Herein, the complex structure of KRAS and GJ101 was clarified by X-ray crystallography. Isothermal titration calorimetry showed that GJ101 binds highly with KRAS mutant and the complex structure of KRAS G12V^.GJ101 complex presented that the residue of Q61 directly interacted with L65 of GJ101. Overall, the results suggest GJ101 be considered a developmental starting point for KRAS G12V inhibitor.

Silencing effects of mutant RAS signalling on transcriptomes

Mutated genes of the RAS family encoding small GTP-binding proteins drive numerous cancers, including pancreatic, colon and lung tumors. Besides the numerous effects of mutant RAS gene expression on aberrant proliferation, transformed phenotypes, metabolism, and therapy resistance, the most striking consequences of chronic RAS activation are changes of the genetic program. By performing systematic gene expression studies in cellular models that allow comparisons of pre-neoplastic with RAS-transformed cells, we and others have estimated that 7 percent or more of all transcripts are altered in conjunction with the expression of the oncogene. In this context, the number of up-regulated transcripts approximates that of down-regulated transcripts. While up-regulated transcription factors such as MYC, FOSL1, and HMGA2 have been identified and characterized as RAS-responsive drivers of the altered transcriptome, the suppressed factors have been less well studied as potential regulators of the genetic program and transformed phenotype in the breadth of their occurrence. We therefore have collected information on downregulated RAS-responsive factors and discuss their potential role as tumor suppressors that are likely to antagonize active cancer drivers. To better understand the active mechanisms that entail anti-RAS function and those that lead to loss of tumor suppressor activity, we focus on the tumor suppressor HREV107 (alias PLAAT3 [Phospholipase A and acyltransferase 3], PLA2G16 [Phospholipase A2, group XVI] and HRASLS3 [HRAS-like suppressor 3]). Inactivating HREV107 mutations in tumors are extremely rare, hence epigenetic causes modulated by the RAS pathway are likely to lead to down-regulation and loss of function.

Crystal structure of the phospholipase A and acyltransferase 4 (PLAAT4) catalytic domain

Phospholipase A and Acyltransferase 4 (PLAAT4) is a class II tumor suppressor, that also plays a role as a restrictor of intracellular Toxoplasma gondii infection through restriction of parasitic vacuole size. The catalytic N-terminal domain (NTD) interacts with the C-terminal domain (CTD), which is important for sub-cellular targeting and enzymatic function. The dynamics of the NTD main (L1) loop and the L2(B6) loop adjacent to the active site, have been shown to be important regulators of enzymatic activity. Here, we present the crystal structure of PLAAT4 NTD, determined from severely intergrown crystals using automated, laser-based crystal harvesting and data reduction technologies. The structure showed the L1 loop in two distinct conformations, highlighting a complex network of interactions likely influencing its conformational flexibility. Ensemble refinement of the crystal structure recapitulates the major correlated motions observed in solution by NMR. Our analysis offers useful insights on millisecond dynamics based on the crystal structure, complementing NMR studies which preclude structural information at this time scale.

Phospholipase A/acyltransferase 4 is a prognostic biomarker and correlated with immune infiltrates in pancreatic cancer

Background

Phospholipase A/acyltransferase (PLAAT) family exhibits O- and N-acyltransferase activity and biosynthesize N-acylated ethanolamine phospholipids. Previously, PLAAT4 was seen as a tumor suppressor, but the exact function of PLAAT4 in pancreatic cancer was still unknown. In this study, we investigated the relationship of PLAAT4 and pancreatic cancer.

Methods

Using the data from the cancer genome atlas (TCGA), Genotype-Tissue Expression (GTEx) database and Gene Expression Omnibus (GEO) datasets we compared the expression of PLAAT4 in normal and tumor tissues and analyzed the connections between PLAAT4 and several clinicopathological factors. Further, we conducted Gene ontology (GO) analysis, Gene set enrichment analysis (GSEA), single sample gene set enrichment analysis (ssGSEA) and estimate analysis to explore the association between PLAAT4 and biological function and immune infiltration. In addition, Kaplan-Meier (KM) analysis, univariate and multivariate Cox analysis were used to explore the association between PLAAT4 and prognosis. In addition, we plotted a nomogram according to the multivariate cox analysis visualizing the predictive ability of PLAAT4 on prognosis. In addition, we explore the influence of PLAAT4 on malignant behaviors of the pancreatic cancer cells in vitro.

Results

After comparing the expression of PLAAT4 in normal and tumor tissues, we found that the expression of PLAAT4 was significantly high in pancreatic ductal adenocarcinoma (PDAC) samples. In addition, the results of GO and GSEA found that the expression of PLAAT4 was related to cell cycle checkpoints, M phase, regulation by p53, cell cycle mitotic and etc. Further, ssGSEA has shown that PLAAT4 was positively related to the abundance of aDC, Th1 cells, Th2 cells and negatively related to the Th17 cells. Subsequently, KM analysis, univariate and multivariate Cox analysis were used to analyze the correlation between PLAAT4 and prognosis. Additionally, we found that higher expression of PLAAT4 was related to T stage, N stage, histologic grade, etc (P < 0.05) and has a significant correlation with poor Overall Survival (OS), Disease-Specific Survival (DSS) and Progression-Free Interval (PFI). At last, we proved that PLAAT4 contributed to the malignant behaviors of the pancreatic cancer cells.

Conclusion

This study indicated PLAAT4 as a novel prognostic biomarker and an important molecular that mediated immune response in pancreatic cancer.

PLAAT1 promotes p53 degradation via autophagy-lysosome pathway in zebrafish

PLAAT1 belongs to the PLAAT family and plays regulatory roles in cell growth, tumor suppression and phospholipid metabolism. However, whether PLAAT1 is involved in p53 mediated signaling has not been investigated. Here, we report that PLAAT1 promotes degradation of p53 in zebrafish. We found that the plaat1 gene was constitutively expressed in tissues including liver, kidney, spleen, intestine, eye and brain, with relative higher expression levels detected in the brain and eye. Overexpression of plaat1 led to inhibition of p53 and tnfα mRNA expression. Furthermore, it was shown that PLAAT1 interacted with p53 to facilitate p53 degradation via autophagy-lysosome dependent pathway. Our work indicates that PLAAT1 is involved in the interplay between p53 mediated cellular responses and autophagy.

Current Knowledge on Mammalian Phospholipase A1, Brief History, Structures, Biochemical and Pathophysiological Roles

Phospholipase A₁ (PLA₁) is an enzyme that cleaves an ester bond at the sn-1 position of glycerophospholipids, producing a free fatty acid and a lysophospholipid. PLA₁ activities have been detected both extracellularly and intracellularly, which are well conserved in higher eukaryotes, including fish and mammals. All extracellular PLA₁s belong to the lipase family. In addition to PLA₁ activity, most mammalian extracellular PLA₁s exhibit lipase activity to hydrolyze triacylglycerol, cleaving the fatty acid and contributing to its absorption into the intestinal tract and tissues. Some extracellular PLA₁s exhibit PLA₁ activities specific to phosphatidic acid (PA) or phosphatidylserine (PS) and serve to produce lysophospholipid mediators such as lysophosphatidic acid (LPA) and lysophosphatidylserine (LysoPS). A high level of PLA₁ activity has been detected in the cytosol fractions, where PA-PLA₁/DDHD1/iPLA₁ was responsible for the activity. Many homologs of PA-PLA₁ and PLA₂ have been shown to exhibit PLA₁ activity. Although much has been learned about the pathophysiological roles of PLA₁ molecules through studies of knockout mice and human genetic diseases, many questions regarding their biochemical properties, including their genuine in vivo substrate, remain elusive.

Rapid testing of candidate oncogenes and tumour suppressor genes in signal transduction and neoplastic transformation

The COSMIC database (version 94) lists 576 genes in the Cancer Gene Census which have a defined function as drivers of malignancy (oncogenes) or as tumour suppressors (Tier 1). In addition, there are 147 genes with similar functions, but which are less well characterised (Tier 2). Furthermore, next-generation sequencing projects in the context of precision oncology activities are constantly discovering new ones. Since cancer genes differ from their wild-type precursors in numerous molecular and biochemical properties and exert significant differential effects on downstream processes, simple assays that can uncover oncogenic or anti-oncogenic functionality are desirable and may precede more sophisticated analyses. We describe simple functional assays for PTPN11 (protein-tyrosine phosphatase, non-receptor-type 11)/SHP2 mutants, which are typically found in RASopathies and exhibit potential oncogenic activity. We have also designed a functional test for lysyl oxidase (LOX), a prototypical class II tumour suppressor gene whose loss of function may contribute to neoplastic transformation by RAS oncogenes. Moreover, we applied this test to analyse three co-regulated, RAS-responsive genes for transformation-suppressive activity. The integration of these tests into systems biology studies will contribute to a better understanding of cellular networks in cancer.

Protein Dynamics Influence the Enzymatic Activity of Phospholipase A/Acyltransferases 3 and 4

Phospholipase A/acyltransferase 3 (PLAAT3) and PLAAT4 are enzymes involved in the synthesis of bioactive lipids. Despite sequential and structural similarities, the two enzymes differ in activity and specificity. The relation between the activity and dynamics of the N-terminal domains of PLAAT3 and PLAAT4 was studied. PLAAT3 has a much higher melting temperature and exhibits less nanosecond and millisecond dynamics in the active site, in particular in loop L2(B6), as shown by NMR spectroscopy and molecular dynamics calculations. Swapping the L2(B6) loops between the two PLAAT enzymes results in strongly increased phospholipase activity in PLAAT3 but no reduction in PLAAT4 activity, indicating that this loop contributes to the low activity of PLAAT3. The results show that, despite structural similarity, protein dynamics differ substantially between the PLAAT variants, which can help to explain the activity and specificity differences.

The rise and fall of anandamide: processes that control synthesis, degradation, and storage

Anandamide is an endocannabinoid derived from arachidonic acid-containing membrane lipids and has numerous biological functions. Its effects are primarily mediated by the cannabinoid receptors CB1 and CB2, and the vanilloid TRPV1 receptor. Anandamide is known to be involved in sleeping and eating patterns as well as pleasure enhancement and pain relief. This manuscript provides a review of anandamide synthesis, degradation, and storage and hence the homeostasis of the anandamide signaling system.

Omega-3FAs Can Inhibit the Inflammation and Insulin Resistance of Adipose Tissue Caused by HHcy Induced Lipids Profile Changing in Mice

The adipose Nod-like receptor protein 3 (NLRP3) inflammasome initiates insulin resistance; however, the mechanism of inflammasome activation in adipose tissue remains elusive. In this study, homocysteine (Hcy) was found to participate in insulin resistance via a NLRP3 inflammasome-related process. Hcy-induced activation of NLRP3 inflammasomes were observed in adipose tissue during the generation of insulin resistance in vivo. This animal model suggests that diets high in omega-3 fatty acids alter serum and adipose lipid profiles, and in this way, omega-3 fatty acids may reduce adipose tissue inflammation and attenuate insulin resistance.

Skeletal muscle expression of adipose-specific phospholipase in peripheral artery disease

Flow-limiting atherosclerotic lesions of arteries supplying the limbs are a cause of symptoms in patients with peripheral artery disease (PAD). Musculoskeletal metabolic factors also contribute to the pathophysiology of claudication, which is manifest as leg discomfort that impairs walking capacity. Accordingly, we conducted a case-control study to determine whether skeletal muscle metabolic gene expression is altered in PAD. Calf skeletal muscle gene expression of patients with PAD and healthy subjects was analyzed using microarrays. The top-ranking gene differentially expressed between PAD and controls (FDR < 0.001) was PLA2G16, which encodes adipose-specific phospholipase A2 (AdPLA) and is implicated in the maintenance of insulin sensitivity and regulation of lipid metabolism. Differential expression was confirmed by qRT-PCR; PLA2G16 was downregulated by 68% in patients with PAD (p < 0.001). Expression of Pla2g16 was then measured in control (db/+) and diabetic (db/db) mice that underwent unilateral femoral artery ligation. There was significantly reduced expression of Pla2g16 in the ischemic leg of both control and diabetic mice (by 51%), with significantly greater magnitude of reduction in the diabetic mice (by 79%). We conclude that AdPLA is downregulated in humans with PAD and in mice with hindlimb ischemia. Reduced AdPLA may contribute to impaired walking capacity in patients with PAD via its effects on skeletal muscle metabolism. Further studies are needed to fully characterize the role of AdPLA in PAD and to investigate its potential as a therapeutic target for alleviating symptoms of claudication.

A H-REV107 Peptide Inhibits Tumor Growth and Interacts Directly with Oncogenic KRAS Mutants

Kirsten-RAS (KRAS) has been the target of drugs because it is the most mutated gene in human cancers. Because of the low affinity of drugs for KRAS mutations, it was difficult to target these tumor genes directly. We found a direct interaction between KRAS G12V and tumor suppressor novel H-REV107 peptide with high binding affinity. We report the first crystal structure of an oncogenic mutant, KRAS G12V-H-REV107. This peptide was shown to interact with KRAS G12V in the guanosine diphosphate (GDP)-bound inactive state and to form a stable complex, blocking the activation function of KRAS. We showed that the peptide acted as an inhibitor of mutant KRAS targets by [α-³²P] guanosine triphosphate (GTP) binding assay. The H-REV107 peptide inhibited pancreatic cancer and colon cancer cell lines in cell proliferation assay. Specially, the H-REV107 peptide can suppress pancreatic tumor growth by reduction of tumor volume and weight in xenotransplantation mouse models. Overall, the results presented herein will facilitate development of novel drugs for inhibition of KRAS mutations in cancer patients.

Activity-Based Protein Profiling Identifies α-Ketoamides as Inhibitors for Phospholipase A2 Group XVI

Phospholipase A2, group XVI (PLA2G16) is a thiol hydrolase from the HRASLS family that regulates lipolysis in adipose tissue and has been identified as a host factor enabling the cellular entry of picornaviruses. Chemical tools are essential to visualize and control PLA2G16 activity, but they have not been reported to date. Here, we show that MB064, which is a fluorescent lipase probe, also labels recombinant and endogenously expressed PLA2G16. Competitive activity-based protein profiling (ABPP) using MB064 enabled the discovery of α-ketoamides as the first selective PLA2G16 inhibitors. LEI110 was identified as a potent PLA2G16 inhibitor ( Ki = 20 nM) that reduces cellular arachidonic acid levels and oleic acid-induced lipolysis in human HepG2 cells. Gel-based ABPP and chemical proteomics showed that LEI110 is a selective pan-inhibitor of the HRASLS family of thiol hydrolases (i.e., PLA2G16, HRASLS2, RARRES3 and iNAT). Molecular dynamic simulations of LEI110 in the reported crystal structure of PLA2G16 provided insight in the potential ligand-protein interactions to explain its binding mode. In conclusion, we have developed the first selective inhibitor that can be used to study the cellular role of PLA2G16.

Adipocyte-derived Lysophosphatidylcholine Activates Adipocyte and Adipose Tissue Macrophage Nod-Like Receptor Protein 3 Inflammasomes Mediating Homocysteine-Induced Insulin Resistance

The adipose Nod-like receptor protein 3 (NLRP3) inflammasome senses danger-associated molecular patterns (DAMPs) and initiates insulin resistance, but the mechanisms of adipose inflammasome activation remains elusive. In this study, Homocysteine (Hcy) is revealed to be a DAMP that activates adipocyte NLRP3 inflammasomes, participating in insulin resistance. Hcy-induced activation of NLRP3 inflammasomes were observed in both adipocytes and adipose tissue macrophages (ATMs) and mediated insulin resistance. Lysophosphatidylcholine (lyso-PC) acted as a second signal activator, mediating Hcy-induced adipocyte NLRP3 inflammasome activation. Hcy elevated adipocyte lyso-PC generation in a hypoxia-inducible factor 1 (HIF1)-phospholipase A2 group 16 (PLA2G16) axis-dependent manner. Lyso-PC derived from the Hcy-induced adipocyte also activated ATM NLRP3 inflammasomes in a paracrine manner. This study demonstrated that Hcy activates adipose NLRP3 inflammasomes in an adipocyte lyso-PC-dependent manner and highlights the importance of the adipocyte NLRP3 inflammasome in insulin resistance.

Phospholipases: An Overview

Phospholipases are lipolytic enzymes that hydrolyze phospholipid substrates at specific ester bonds. Phospholipases are widespread in nature and play very diverse roles from aggression in snake venom to signal transduction, lipid mediator production, and metabolite digestion in humans. Phospholipases vary considerably in structure, function, regulation, and mode of action. Tremendous advances in understanding the structure and function of phospholipases have occurred in the last decades. This introductory chapter is aimed at providing a general framework of the current understanding of phospholipases and a discussion of their mechanisms of action and emerging biological functions.

An involvement of phospholipase A/acyltransferase family proteins in peroxisome regulation and plasmalogen metabolism

The H-Ras-like suppressor (HRASLS) is a protein family consisting of five members in humans. Despite their discovery as tumor suppressors, we demonstrated that all these proteins are phospholipid-metabolizing enzymes, such as phospholipase (PL) A₁ /A₂ and acyltransferase. We thus proposed to rename HRASLS1-5 as PLA/acyltransferase (PLAAT)-1-5. Notably, PLAATs exhibit N-acyltransferase activity to biosynthesize N-acylated ethanolamine phospholipids, including N-acyl-plasmalogen, which serve as precursors of bioactive N-acylethanolamines. Furthermore, the overexpression of PLAAT-3 in animal cells causes disappearance of peroxisomes and a remarkable reduction in plasmalogen levels. This finding might be related to the inhibitory effect of PLAAT-3 on the chaperone activity of the peroxin PEX19. In this article, we will review our recent findings about PLAAT proteins, with special reference to their roles in peroxisome biogenesis and plasmalogen metabolism.

High expression of PLA2G16 is associated with a better prognosis in HER2-positive breast cancer

BACKGROUND

PLA2G16 functions as a phosphatase in metabolism and its abnormal expression is closely associated with tumor progression. The aim of this study is to investigate the prognosis value of PLA2G16 in breast cancer.

METHODS

A tissue microarray including 200 invasive ductal carcinoma specimens was constructed. Immunohistochemistry was performed to determine the PLA2G16 expression status. The Kaplan-Meier analysis and log-rank test were used to evaluate the prognostic value of PLA2G16. The Multivariate Cox regression analysis was performed to identify whether PLA2G16 was an independent prognostic factor.

RESULTS

In our retrospective study, Kaplan-Meier analysis showed that elevated PLA2G16 expression was correlated with improved DFS (P=0.032) in the whole breast cancer patients. In further subgroup analysis, PLA2G16 overexpression was found to be associated with prolonged DFS (P=0.018) in HER2-positive breast cancer patients. More importantly, Multivariate analysis suggested that PLA2G16 was a significant independent prognostic factor in HER2-enriched patients [hazard ratio (HR) =0.151; 95% confidence interval (CI) =0.034-0.672; P=0.013].

CONCLUSIONS

Our study evaluated the prognostic significance of PLA2G16 in patients with HER2-positive breast cancer and confirmed the relevance of this metabolism-related gene in patient outcome.

PLA2G16 promotes osteosarcoma metastasis and drug resistance via the MAPK pathway

The prognosis of metastatic osteosarcoma is dismal and a better understanding of the mechanisms underlying disease progression is essential to improve treatment options and patient outcomes. We previously demonstrated Pla2g16 overexpression in mouse osteosarcoma contributes to metastasis phenotypes and increased expression of PLA2G16 is associated with metastasis and poor prognosis in human tumors. To further examine the mechanisms through which PLA2G16 contributes to human osteosarcoma metastasis and explore the potential of PLA2G16 as a therapeutic target in osteosarcoma, we generated a panel of human osteosarcoma cell lines expressing different levels of PLA2G16. The functional analyses of these cell lines demonstrated high levels of PLA2G16 expression increased osteosarcoma cell migration, invasion, clonogenic survival, and anchorage-independent colony formation. Importantly, this activity was dependent on the phospholipase activity of PLA2G16. Additionally, PLA2G16 overexpression decreased the sensitivity of cells to a panel of chemotherapeutic agents. Analysis of downstream pathways revealed the pro-metastasis functions of PLA2G16 were mediated through the MAPK pathway, as knockdown of PLA2G16 decreased ERK1/2 phosphorylation and pharmacological inhibition of MEK significantly repressed PLA2G16 mediated cell migration and clonogenic survival. Furthermore, PLA2G16 overexpression promoted xenograft tumor growth in vivo, and these tumors exhibit increased ERK1/2 phosphorylation. Lastly, the expression of PLA2G16 is strongly correlated with the increased ERK1/2 phosphorylation in human osteosarcoma samples, and the combined lesions are associated with reduced overall and metastasis-free survival. Collectively, these results demonstrate increased PLA2G16 expression activates the MAPK pathway to enhance osteosarcoma metastasis and may be a novel therapeutic target for these cancers.

The HRASLS (PLA/AT) subfamily of enzymes

The H-RAS-like suppressor (HRASLS) subfamily consists of five enzymes (1–5) in humans and three (1, 3, and 5) in mice and rats that share sequence homology with lecithin:retinol acyltransferase (LRAT). All HRASLS family members possess in vitro phospholipid metabolizing abilities including phospholipase A_1/2 (PLA_1/2) activities and O-acyltransferase activities for the remodeling of glycerophospholipid acyl chains, as well as N-acyltransferase activities for the production of N-acylphosphatidylethanolamines. The in vivo biological activities of the HRASLS enzymes have not yet been fully investigated. Research to date indicates involvement of this subfamily in a wide array of biological processes and, as a consequence, these five enzymes have undergone extensive rediscovery and renaming within different fields of research. This review briefly describes the discovery of each of the HRASLS enzymes and their role in cancer, and discusses the biochemical function of each enzyme, as well as the biological role, if known. Gaps in current understanding are highlighted and suggestions for future research directions are discussed.

H-rev107 Regulates Cytochrome P450 Reductase Activity and Increases Lipid Accumulation

H-rev107 is a member of the HREV107 type II tumor suppressor gene family and acts as a phospholipase to catalyze the release of fatty acids from glycerophospholipid. H-rev107 has been shown to play an important role in fat metabolism in adipocytes through the PGE2/cAMP pathway, but the detailed molecular mechanism underlying H-rev107-mediated lipid degradation has not been studied. In this study, the interaction between H-rev107 and cytochrome P450 reductase (POR), which is involved in hepatic lipid content regulation, was determined by yeast two-hybrid screen and confirmed by using in vitro pull down assays and immunofluorescent staining. The expression of POR in H-rev107-expressing cells enhanced the H-rev107-mediated release of arachidonic acid. However, H-rev107 inhibited POR activity and relieved POR-mediated decreased triglyceride content in HtTA and HeLa cervical cells. The inhibitory effect of H-rev107 will be abolished when POR-expressing cells transfected with PLA₂-lacking pH-rev107 or treated with PLA₂ inhibitor. Silencing of H-rev107 using siRNA resulted in increased glycerol production and reversion of free fatty acid-mediated growth suppression in Huh7 hepatic cells. In summary, our results revealed that H-rev107 is also involved in lipid accumulation in liver cells through the POR pathway via its PLA₂ activity.

Structural and functional characterization of tumor suppressors TIG3 and H-REV107

H-REV107-like family proteins TIG3 and H-REV107 are class II tumor suppressors. Here we report that the C-terminal domains (CTDs) of TIG3 and H-REV107 can induce HeLa cell death independently. The N-terminal domain (NTD) of TIG3 enhances the cell death inducing ability of CTD, while NTD of H-REV107 plays an inhibitory role. The solution structure of TIG3 NTD is very similar to that of H-REV107 in overall fold. However, the CTD binding regions on NTD are different between TIG3 and H-REV107, which may explain their functional difference. As a result, the flexible main loop of H-REV107, but not that of TIG3, is critical for its NTD to modulate its CTD in inducing cell death.

Pla2g16 phospholipase mediates gain-of-function activities of mutant p53

p53(R172H/+) mice inherit a p53 mutation found in Li-Fraumeni syndrome and develop metastatic tumors at much higher frequency than p53(+/-) mice. To explore the mutant p53 metastatic phenotype, we used expression arrays to compare primary osteosarcomas from p53(R172H/+) mice with metastasis to osteosarcomas from p53(+/-) mice lacking metastasis. For this study, 213 genes were differentially expressed with a P value <0.05. Of particular interest, Pla2g16, which encodes a phospholipase that catalyzes phosphatidic acid into lysophosphatidic acid and free fatty acid (both implicated in metastasis), was increased in p53(R172H/+) osteosarcomas. Functional analyses showed that Pla2g16 knockdown decreased migration and invasion in mutant p53-expressing cells, and vice versa: overexpression of Pla2g16 increased the invasion of p53-null cells. Furthermore, Pla2g16 levels were increased upon expression of mutant p53 in both mouse and human osteosarcoma cell lines, indicating that Pla2g16 is a downstream target of the mutant p53 protein. ChIP analysis revealed that several mutant p53 proteins bind the Pla2g16 promoter at E26 transformation-specific (ETS) binding motifs and knockdown of ETS2 suppressed mutant p53 induction of Pla2g16. Thus, our study identifies a phospholipase as a transcriptional target of mutant p53 that is required for metastasis.

Enzymatic activity of Lecithin:retinol acyltransferase: a thermostable and highly active enzyme with a likely mode of interfacial activation

Lecithin:retinol acyltransferase (LRAT) plays a major role in the vertebrate visual cycle. Indeed, it is responsible for the esterification of all-trans retinol into all-trans retinyl esters, which can then be stored in microsomes or further metabolized to produce the chromophore of rhodopsin. In the present study, a detailed characterization of the enzymatic properties of truncated LRAT (tLRAT) has been achieved using in vitro assay conditions. A much larger tLRAT activity has been obtained compared to previous reports and to an enzyme with a similar activity. In addition, tLRAT is able to hydrolyze phospholipids bearing different chain lengths with a preference for micellar aggregated substrates. It therefore presents an interfacial activation property, which is typical of classical phospholipases. Furthermore, given that stability is a very important quality of an enzyme, the influence of different parameters on the activity and stability of tLRAT has thus been studied in detail. For example, storage buffer has a strong effect on tLRAT activity and high enzyme stability has been observed at room temperature. The thermostability of tLRAT has also been investigated using circular dichroism and infrared spectroscopy. A decrease in the activity of tLRAT was observed beyond 70°C, accompanied by a modification of its secondary structure, i.e. a decrease of its α-helical content and the appearance of unordered structures and aggregated β-sheets. Nevertheless, residual activity could still be observed after heating tLRAT up to 100°C. The results of this study highly improved our understanding of this enzyme.

Phospholipase A/Acyltransferase enzyme activity of H-rev107 inhibits the H-RAS signaling pathway

BACKGROUND

H-rev107, also called HRASLS3 or PLA2G16, is a member of the HREV107 type II tumor suppressor gene family. Previous studies showed that H-rev107 exhibits phospholipase A/acyltransferase (PLA/AT) activity and downregulates H-RAS expression. However, the mode of action and the site of inhibition of H-RAS by H-rev107 are still unknown.

RESULTS

Our results indicate that H-rev107 was co-precipitated with H-RAS and downregulated the levels of activated RAS (RAS-GTP) and ELK1-mediated transactivation in epidermal growth factor-stimulated and H-RAS-cotransfected HtTA cervical cancer cells. Furthermore, an acyl-biotin exchange assay demonstrated that H-rev107 reduced H-RAS palmitoylation. H-rev107 has been shown to be a PLA/AT that is involved in phospholipid metabolism. Treating cells with the PLA/AT inhibitor arachidonyl trifluoromethyl ketone (AACOCF3) or methyl arachidonyl fluorophosphate (MAFP) alleviated H-rev107-induced downregulation of the levels of acylated H-RAS. AACOCF3 and MAFP also increased activated RAS and ELK1-mediated transactivation in H-rev107-expressing HtTA cells following their treatment with epidermal growth factor. In contrast, treating cells with the acyl-protein thioesterase inhibitor palmostatin B enhanced H-rev107-mediated downregulation of acylated H-RAS in H-rev107-expressing cells. Palmostatin B had no effect on H-rev107-induced suppression of RAS-GTP levels or ELK1-mediated transactivation. These results suggest that H-rev107 decreases H-RAS activity through its PLA/AT activity to modulate H-RAS acylation.

CONCLUSIONS

We made the novel observation that H-rev107 decrease in the steady state levels of H-RAS palmitoylation through the phospholipase A/acyltransferase activity. H-rev107 is likely to suppress activation of the RAS signaling pathway by reducing the levels of palmitoylated H-RAS, which decreases the levels of GTP-bound H-RAS and also the activation of downstream molecules. Our study further suggests that the PLA/AT activity of H-rev107 may play an important role in H-rev107-mediated RAS suppression.

Pericentrosomal localization of the TIG3 tumor suppressor requires an N-terminal hydrophilic region motif

Tazarotene-induced gene 3 (TIG3) is a tumor suppressor protein that has a key role in controlling cell proliferation. TIG3 is observed at reduced levels in epidermal squamous cell carcinoma, and the restoration of expression in skin cancer cells reduces cell survival. TIG3 suppresses cell survival through mechanisms that involve localization at the plasma membrane and at the centrosome. TIG3 interacts at the plasma membrane to activate enzymes involved in keratinocyte terminal differentiation, and at the centrosome to inhibit daughter centrosome separation during mitosis leading to cessation of cell proliferation and induction of apoptosis. An important goal is identifying the motifs required for TIG3 localization at these intracellular sites as a method to understand the function of TIG3 at each location. TIG3 encodes an N-terminal hydrophilic region (amino acids 1-135) and a C-terminal membrane-anchoring domain (amino acids 135-164). We show that the C-terminal hydrophobic domain targets intact TIG3 to the plasma membrane, but when isolated as an independent element localizes at the mitochondria. We further demonstrate that a segment of the N-terminal hydrophilic region targets the centrosome. These studies provide important insights regarding the mechanisms that guide subcellular localization of this keratinocyte survival regulator.

H-rev107 regulates prostaglandin D2 synthase-mediated suppression of cellular invasion in testicular cancer cells

Background

H-rev107 is a member of the HREV107 type II tumor suppressor gene family which includes H-REV107, RIG1, and HRASLS. H-REV107 has been shown to express at high levels in differentiated tissues of post-meiotic testicular germ cells. Prostaglandin D2 (PGD2) is conjectured to induce SRY-related high-mobility group box 9 (SOX9) expression and subsequent Sertoli cell differentiation. To date, the function of H-rev107 in differentiated testicular cells has not been well defined.

Results

In the study, we found that H-rev107 was co-localized with prostaglandin D2 synthase (PTGDS) and enhanced the activity of PTGDS, resulting in increase of PGD2 production in testis cells. Furthermore, when H-rev107 was expressed in human NT2/D1 testicular cancer cells, cell migration and invasion were inhibited. Also, silencing of PTGDS would reduce H-rev107-mediated increase in PGD2, cAMP, and SOX9. Silencing of PTGDS or SOX9 also alleviated H-rev107-mediated suppression of cell migration and invasion.

Conclusions

These results revealed that H-rev107, through PTGDS, suppressed cell migration and invasion. Our data suggest that the PGD2-cAMP-SOX9 signal pathway might play an important role in H-rev107-mediated cancer cell invasion in testes.

Involvement of the prostaglandin D2 signal pathway in retinoid-inducible gene 1 (RIG1)-mediated suppression of cell invasion in testis cancer cells

Retinoid-inducible gene 1 (RIG1), also called tazarotene-induced gene 3, belongs to the HREV107 gene family, which contains five members in humans. RIG1 is expressed in high levels in well-differentiated tissues, but its expression is decreased in cancer tissues and cancer cell lines. We found RIG1 to be highly expressed in testicular cells. When RIG1 was expressed in NT2/D1 testicular cancer cells, neither cell death nor cell viability was affected. However, RIG1 significantly inhibited cell migration and invasion in NT2/D1 cells. We found that prostaglandin D2 synthase (PTGDS) interacted with RIG1 using yeast two-hybrid screens. Further, we found PTGDS to be co-localized with RIG1 in NT2/D1 testis cells. In RIG1-expressing cells, elevated levels of prostaglandin D2 (PGD2), cAMP, and SRY-related high-mobility group box 9 (SOX9) were observed. This indicated that RIG1 can enhance PTGDS activity. Silencing of PTGDS expression significantly decreased RIG1-mediated cAMP and PGD2 production. Furthermore, silencing of PTGDS or SOX9 alleviated RIG1-mediated suppression of migration and invasion. These results suggest that RIG1 will suppress cell migration/invasion through the PGD2 signaling pathway. In conclusion, RIG1 can interact with PTGDS to enhance its function and to further suppress NT2/D1 cell migration and invasion. Our study suggests that RIG1-PGD2 signaling might play an important role in cancer cell suppression in the testis.

Structure/function relationships of adipose phospholipase A2 containing a cys-his-his catalytic triad

Adipose phospholipase A(2) (AdPLA or Group XVI PLA(2)) plays an important role in the onset of obesity by suppressing adipose tissue lipolysis. As a consequence, AdPLA-deficient mice are resistant to obesity induced by a high fat diet or leptin deficiency. It has been proposed that AdPLA mediates its antilipolytic effects by catalyzing the release of arachidonic acid. Based on sequence homology, AdPLA is part of a small family of acyltransferases and phospholipases related to lecithin:retinol acyltransferase (LRAT). To better understand the enzymatic mechanism of AdPLA and LRAT-related proteins, we solved the crystal structure of AdPLA. Our model indicates that AdPLA bears structural similarity to proteins from the NlpC/P60 family of cysteine proteases, having its secondary structure elements configured in a circular permutation of the classic papain fold. Using both structural and biochemical evidence, we demonstrate that the enzymatic activity of AdPLA is mediated by a distinctive Cys-His-His catalytic triad and that the C-terminal transmembrane domain of AdPLA is required for the interfacial catalysis. Analysis of the enzymatic activity of AdPLA toward synthetic and natural substrates indicates that AdPLA displays PLA(1) in addition to PLA(2) activity. Thus, our results provide insight into the enzymatic mechanism and biochemical properties of AdPLA and LRAT-related proteins and lead us to propose an alternate mechanism for AdPLA in promoting adipose tissue lipolysis that is not contingent on the release of arachidonic acid and that is compatible with its combined PLA(1)/A(2) activity.

Phospholipases: an overview

Phospholipids are present in all living organisms. They are a major component of all biological membranes, along with glycolipids and cholesterol. Enzymes aimed at cleaving the various bonds in phospholipids, namely phospholipases, are consequently widespread in nature, playing very diverse roles from aggression in snake venom to signal transduction, lipid mediators production, and digestion in humans. Although all phospholipases target phospholipids as substrates, they vary in the site of action on the phospholipids molecules, physiological function, mode of action, and their regulation. Significant studies on phospholipases characterization, physiological role, and industrial potential have been conducted worldwide. Some of them have been directed for biotechnological advances, such as gene discovery and functional enhancement by protein engineering. Others reported phospholipases as virulence factors and major causes of pathophysiological effects. In this introductory chapter, we provide brief details of different phospholipases.

Regulation of peroxisomal lipid metabolism by catalytic activity of tumor suppressor H-rev107

H-rev107 is a mammalian protein belonging to the HRAS-like suppressor family. Although the protein was originally found as a tumor suppressor, currently it is receiving considerable attention as a regulator of adipocyte lipolysis. We recently revealed that purified recombinant H-rev107 has phospholipase A(1/2) activity, releasing free fatty acids from glycerophospholipids with a preference for esterolysis at the sn-1 position. In the present study, we constitutively expressed H-rev107 in cloned HEK293 cells to examine its biological function in living cells. Initially, the cells accumulated free fatty acids. We also found a remarkable decrease in the levels of ether-type lipids, including plasmalogen and ether-type triglyceride, with a concomitant increase in fatty alcohols, substrates for the biosynthesis of ether-type lipids. Considering that peroxisomes are involved in the ether-type lipid biosynthesis, we next focused on peroxisomes and found that the peroxisomal markers 70-kDa peroxisomal membrane protein and catalase were abnormally distributed in the transfected cells. These biochemical and morphological abnormalities were not seen in HEK293 cells stably expressing a catalytically inactive mutant of H-rev107. When H-rev107 or its fusion protein with enhanced green fluorescence protein was transiently expressed in mammalian cells, both proteins were associated with peroxisomes in some of the observed cells. These results suggest that H-rev107 interferes with the biosynthesis of ether-type lipids and is responsible for the dysfunction of peroxisomes in H-rev107-expressing cells.

Enzymological analysis of the tumor suppressor A-C1 reveals a novel group of phospholipid-metabolizing enzymes

A-C1 protein is the product of a tumor suppressor gene negatively regulating the oncogene Ras and belongs to the HRASLS (HRAS-like suppressor) subfamily. We recently found that four members of this subfamily expressed in human tissues function as phospholipid-metabolizing enzymes. Here we examined a possible enzyme activity of A-C1. The homogenates of COS-7 cells overexpressing recombinant A-C1s from human, mouse, and rat showed a phospholipase A½ (PLA½) activity toward phosphatidylcholine (PC). This finding was confirmed with the purified A-C1. The activity was Ca²⁺ independent, and dithiothreitol and Nonidet P-40 were indispensable for full activity. Phosphatidylethanolamine (PE) was also a substrate and the phospholipase A₁ (PLA₁) activity was dominant over the PLA₂ activity. Furthermore, the protein exhibited acyltransferase activities transferring an acyl group of PCs to the amino group of PEs and the hydroxyl group of lyso PCs. As for tissue distribution in human, mouse, and rat, A-C1 mRNA was abundantly expressed in testis, skeletal muscle, brain, and heart. These results demonstrate that A-C1 is a novel phospholipid-metabolizing enzyme. Moreover, the fact that all five members of the HRASLS subfamily, including A-C1, show similar catalytic properties strongly suggests that these proteins constitute a new class of enzymes showing PLA½ and acyltransferase activities.

Structural analysis of papain-like NlpC/P60 superfamily enzymes with a circularly permuted topology reveals potential lipid binding sites

NlpC/P60 superfamily papain-like enzymes play important roles in all kingdoms of life. Two members of this superfamily, LRAT-like and YaeF/YiiX-like families, were predicted to contain a catalytic domain that is circularly permuted such that the catalytic cysteine is located near the C-terminus, instead of at the N-terminus. These permuted enzymes are widespread in virus, pathogenic bacteria, and eukaryotes. We determined the crystal structure of a member of the YaeF/YiiX-like family from Bacillus cereus in complex with lysine. The structure, which adopts a ligand-induced, “closed” conformation, confirms the circular permutation of catalytic residues. A comparative analysis of other related protein structures within the NlpC/P60 superfamily is presented. Permutated NlpC/P60 enzymes contain a similar conserved core and arrangement of catalytic residues, including a Cys/His-containing triad and an additional conserved tyrosine. More surprisingly, permuted enzymes have a hydrophobic S1 binding pocket that is distinct from previously characterized enzymes in the family, indicative of novel substrate specificity. Further analysis of a structural homolog, YiiX (PDB 2if6) identified a fatty acid in the conserved hydrophobic pocket, thus providing additional insights into possible function of these novel enzymes.

The DNA binding factor Hmg20b is a repressor of erythroid differentiation

BACKGROUND

In erythroblasts, the CoREST repressor complex is recruited to target promoters by the transcription factor Gfi1b, leading to repression of genes mainly involved in erythroid differentiation. Hmg20b is a subunit of CoREST, but its role in erythropoiesis has not yet been established.

DESIGN AND METHODS

To study the role of Hmg20b in erythropoiesis, we performed knockdown experiments in a differentiation-competent mouse fetal liver cell line, and in primary mouse fetal liver cells. The effects on globin gene expression were determined. We used microarrays to investigate global gene expression changes induced by Hmg20b knockdown. Functional analysis was carried out on Hrasls3, an Hmg20b target gene.

RESULTS

We show that Hmg20b depletion induces spontaneous differentiation. To identify the target genes of Hmg20b, microarray analysis was performed on Hmg20b knockdown cells and controls. In line with its association to the CoREST complex, we found that 85% (527 out of 620) of the deregulated genes are up-regulated when Hmg20b levels are reduced. Among the few down-regulated genes was Gfi1b, a known repressor of erythroid differentiation. Among the consistently up-regulated targets were embryonic β-like globins and the phospholipase HRAS-like suppressor 3 (Hrasls3). We show that Hrasls3 expression is induced during erythroid differentiation and that knockdown of Hrasls3 inhibits terminal differentiation of proerythroblasts.

CONCLUSIONS

We conclude that Hmg20b acts as an inhibitor of erythroid differentiation, through the down-regulation of genes involved in differentiation such as Hrasls3, and activation of repressors of differentiation such as Gfi1b. In addition, Hmg20b suppresses embryonic β-like globins.

Recent progress in phospholipase A₂ research: from cells to animals to humans

Mammalian genomes encode genes for more than 30 phospholipase A₂s (PLA₂s) or related enzymes, which are subdivided into several classes including low-molecular-weight secreted PLA₂s (sPLA₂s), Ca²+-dependent cytosolic PLA₂s (cPLA₂s), Ca²+-independent PLA₂s (iPLA₂s), platelet-activating factor acetylhydrolases (PAF-AHs), lysosomal PLA₂s, and a recently identified adipose-specific PLA. Of these, the intracellular cPLA₂ and iPLA₂ families and the extracellular sPLA₂ family are recognized as the "big three". From a general viewpoint, cPLA₂α (the prototypic cPLA₂ plays a major role in the initiation of arachidonic acid metabolism, the iPLA₂ family contributes to membrane homeostasis and energy metabolism, and the sPLA₂ family affects various biological events by modulating the extracellular phospholipid milieus. The cPLA₂ family evolved along with eicosanoid receptors when vertebrates first appeared, whereas the diverse branching of the iPLA₂ and sPLA₂ families during earlier eukaryote development suggests that they play fundamental roles in life-related processes. During the past decade, data concerning the unexplored roles of various PLA₂ enzymes in pathophysiology have emerged on the basis of studies using knockout and transgenic mice, the use of specific inhibitors, and information obtained from analysis of human diseases caused by mutations in PLA₂ genes. This review focuses on current understanding of the emerging biological functions of PLA₂s and related enzymes.

The Role of Phospholipase A(2)-derived Mediators in Obesity

Obesity has become an epidemic and its prevalence is increasing exponentially. A great deal of focus has been given to understanding the molecular processes that regulate obesity. The characterization of phospholipase A(2)s, especially adipose-specific PLA(2), have lead to a proposed role of their downstream products in the progression of obesity and obesity related disorders. This review summarizes recent developments in the role of PLA(2) and their downstream effects in the development of metabolic disorders.

1H, 13C and 15N resonance assignments of human H-REV107 N-terminal domain

Human H-REV107 protein is the representative of a novel class II tumor suppressor family, which is lost in tumor cells and can induce cell death after restoration. The NMR assignments of the H-REV107 N-terminal domain are essential for its solution structure determination.

Enzymological studies on the biosynthesis of N-acylethanolamines

Ethanolamides of different long-chain fatty acids constitute a class of endogenous lipid molecules generally called N-acylethanolamines (NAEs). They contain N-arachidonoylethanolamine (anandamide), N-palmitoylethanolamine, and N-oleoylethanolamine, which receive considerable attention because of their actions as an endogenous cannabinoid receptor ligand (endocannabinoid), an anti-inflammatory substance, and an appetite-suppressing substance, respectively. Identification of their biosynthetic routes in animal tissues and molecular characterization of the enzymes involved are essential for better understanding of physiological importance of NAEs as well as development of enzyme inhibitors as possible therapeutic drugs. In the classical "transacylation-phosphodiesterase pathway", NAEs are formed from glycerophospholipids via N-acylphosphatidylethanolamine (NAPE), an unusual derivative of phosphatidylethanolamine with a third acyl chain attached to the amino group, by sequential catalyses by Ca(2+)-dependent N-acyltransferase and NAPE-hydrolyzing phospholipase D. However, recent studies reveal that NAE-generating pathways are more complex than presumed before. In this review article, we will focus on recent findings regarding mammalian enzymes that are involved or might be involved in the biosynthesis of NAEs.

N-acylethanolamine metabolism with special reference to N-acylethanolamine-hydrolyzing acid amidase (NAAA)

N-acylethanolamines (NAEs) constitute a class of bioactive lipid molecules present in animal and plant tissues. Among the NAEs, N-arachidonoylethanolamine (anandamide), N-palmitoylethanolamine, and N-oleoylethanolamine attract much attention due to cannabimimetic activity as an endocannabinoid, anti-inflammatory and analgesic activities, and anorexic activity, respectively. In mammalian tissues, NAEs are formed from glycerophospholipids through the phosphodiesterase-transacylation pathway consisting of Ca(2+)-dependent N-acyltransferase and N-acylphosphatidylethanolamine-hydrolyzing phospholipase D. Recent studies revealed the presence of alternative pathways and enzymes responsible for the NAE formation. As for the degradation of NAEs, fatty acid amide hydrolase (FAAH), which hydrolyzes NAEs to fatty acids and ethanolamine, plays a central role. However, a lysosomal enzyme referred to as NAE-hydrolyzing acid amidase (NAAA) also catalyzes the same reaction and may be a new target for the development of therapeutic drugs. In this article we discuss recent progress in the studies on the enzymes involved in the biosynthesis and degradation of NAEs with special reference to NAAA.

Expression, purification and biochemical characterization of the N-terminal regions of human TIG3 and HRASLS3 proteins

Tarzarotene-induced gene 3 (TIG3) and HRAS-like suppressor (HRASLS3) are members of the HREV107 family of class II tumor suppressors, which are down-regulated in various cancer cells. TIG3 and HRASLS3 also exhibit phospholipase activities. Both proteins share a common domain architecture with hydrophilic N-terminal and hydrophobic C-terminal regions. The hydrophobic C-terminal region is important for tumor suppression. However, the function of the hydrophilic N-terminal region remains elusive. To facilitate biochemical characterizations of TIG3 and HRASLS3, we expressed and purified the N-terminal regions of TIG3 and HRASLS3, designated TIG3 (1-134) and HRASLS3 (1-133), in a bacterial system. We found that the N-terminal regions of TIG3 and HRASLS3 have calcium-independent phospholipase A(2) activities. Limited proteolysis revealed that TIG3 (1-132) is a structural domain in the N-terminal region of TIG3. Our data suggest that the hydrophobic C-terminal regions might be crucial for cellular localization, while the hydrophilic N-terminal regions are sufficient for the enzymatic activity of both TIG3 and HRASLS3.

A transcriptomic computational analysis of mastic oil-treated Lewis lung carcinomas reveals molecular mechanisms targeting tumor cell growth and survival

Background

Mastic oil from Pistacia lentiscus variation chia, a blend of bioactive terpenes with recognized medicinal properties, has been recently shown to exert anti-tumor growth activity through inhibition of cancer cell proliferation, survival, angiogenesis and inflammatory response. However, no studies have addressed its mechanisms of action at genome-wide gene expression level.

Methods

To investigate molecular mechanisms triggered by mastic oil, Lewis Lung Carcinoma cells were treated with mastic oil or DMSO and RNA was collected at five distinct time points (3-48 h). Microarray expression profiling was performed using Illumina mouse-6 v1 beadchips, followed by computational analysis. For a number of selected genes, RT-PCR validation was performed in LLC cells as well as in three human cancer cell lines of different origin (A549, HCT116, K562). PTEN specific inhibition by a bisperovanadium compound was applied to validate its contribution to mastic oil-mediated anti-tumor growth effects.

Results

In this work we demonstrated that exposure of Lewis lung carcinomas to mastic oil caused a time-dependent alteration in the expression of 925 genes. GO analysis associated expression profiles with several biological processes and functions. Among them, modifications on cell cycle/proliferation, survival and NF-κB cascade in conjunction with concomitant regulation of genes encoding for PTEN, E2F7, HMOX1 (up-regulation) and NOD1 (down-regulation) indicated some important mechanistic links underlying the anti-proliferative, pro-apoptotic and anti-inflammatory effects of mastic oil. The expression profiles of Hmox1, Pten and E2f7 genes were similarly altered by mastic oil in the majority of test cancer cell lines. Inhibition of PTEN partially reversed mastic oil effects on tumor cell growth, indicating a multi-target mechanism of action. Finally, k-means clustering, organized the significant gene list in eight clusters demonstrating a similar expression profile. Promoter analysis in a representative cluster revealed shared putative cis-elements suggesting a common regulatory transcription mechanism.

Conclusions

Present results provide novel evidence on the molecular basis of tumor growth inhibition mediated by mastic oil and set a rational basis for application of genomics and bioinformatic methodologies in the screening of natural compounds with potential cancer chemopreventive activities.

TIG3: a regulator of type I transglutaminase activity in epidermis

Keratinocytes undergo a process of terminal cell differentiation that results in the construction of a multilayered epithelium designed to produce a structure that functions to protect the body from dehydration, abrasion and infection. These protective properties are due to the production of a crosslinked layer of protein called the cornified envelope. Type I transglutaminase (TG1), an enzyme that catalyzes the formation of epsilon-(gamma-glutamyl)lysine bonds, is the key protein responsible for generation of the crosslinks. The mechanisms that lead to activation of transglutaminase during terminal differentiation are not well understood. We have identified a protein that interacts with TG1 and regulates its activity. This protein, tazarotene-induced gene 3 (TIG3), is expressed in the differentiated layers of the epidermis and its expression is associated with transglutaminase activation and cornified envelope formation. We describe a novel mechanism whereby TIG3 regulates TG1 activity.

A functional and regulatory network associated with PIP expression in human breast cancer

BACKGROUND

The PIP (prolactin-inducible protein) gene has been shown to be expressed in breast cancers, with contradictory results concerning its implication. As both the physiological role and the molecular pathways in which PIP is involved are poorly understood, we conducted combined gene expression profiling and network analysis studies on selected breast cancer cell lines presenting distinct PIP expression levels and hormonal receptor status, to explore the functional and regulatory network of PIP co-modulated genes.

PRINCIPAL FINDINGS

Microarray analysis allowed identification of genes co-modulated with PIP independently of modulations resulting from hormonal treatment or cell line heterogeneity. Relevant clusters of genes that can discriminate between [PIP+] and [PIP-] cells were identified. Functional and regulatory network analyses based on a knowledge database revealed a master network of PIP co-modulated genes, including many interconnecting oncogenes and tumor suppressor genes, half of which were detected as differentially expressed through high-precision measurements. The network identified appears associated with an inhibition of proliferation coupled with an increase of apoptosis and an enhancement of cell adhesion in breast cancer cell lines, and contains many genes with a STAT5 regulatory motif in their promoters.

CONCLUSIONS

Our global exploratory approach identified biological pathways modulated along with PIP expression, providing further support for its good prognostic value of disease-free survival in breast cancer. Moreover, our data pointed to the importance of a regulatory subnetwork associated with PIP expression in which STAT5 appears as a potential transcriptional regulator.

Induction of apoptosis by the retinoid inducible growth regulator RIG1 depends on the NC motif in HtTA cervical cancer cells

Background

Retinoid-inducible gene 1 (RIG1), also known as tazarotene-induced gene 3 or retinoic-acid receptor responder 3, is a growth regulator, which induces apoptosis and differentiation. RIG1 is classified into the NC protein family. This study investigated functional domains and critical amino acids associated with RIG1-mediated cell death and apoptosis.

Results

Using enhanced green fluorescence protein (EGFP)-tagged RIG1 variants, RIG1 proteins with deletion at the NC domain significantly decreased cell death induced by RIG1, and fusion variants containing only the NC domain significantly induced apoptosis of HtTA cervical cancer cells. The EGFP-RIG1-induced apoptosis was significantly decreased in cells expressing N¹¹²C¹¹³ motif double- (NC→FG) or triple- (NCR→FGE) mutated RIG1 variants. Using dodecapeptides, nuclear localization and profound cell death was observed in HtTA cells expressing wild type RIG1_111–123 or Leu¹²¹-mutated RIG1_111–123:L→ C peptide, but peptides double- or triple-mutated at the NC motif alone, RIG1_111–123:NC→FG or RIG1_111–123:NCR→FGE, were cytoplasmically localized and did not induce apoptosis. The RIG1_111–123 also induced apoptosis of A2058 melanoma cells but not normal human fibroblasts.

Conclusion

The NC domain, especially the NC motif, plays the major role in RIG1-mediated pro-apoptotic activity. The RIG1_111–123 dodecapeptide exhibited strong pro-apoptotic activity and has potential as an anticancer drug.

The tumor suppressor gene H-Rev107 functions as a novel Ca2+-independent cytosolic phospholipase A1/2 of the thiol hydrolase type

H-Rev107 is a protein that was previously cloned as a negative regulator of proto-oncogene Ras and classified as a class II tumor suppressor. Its structural similarity to lecithin retinol acyltransferase and Ca2+-independent phosphatidylethanolamine (PE) N-acyltransferase led us to analyze H-Rev107 as an enzyme involved in phospholipid metabolism. Here, we show that recombinant H-Rev107s from rat, human, and mouse possess phospholipase (PL) A1 or A2 activity toward phosphatidylcholine (PC). Further examination with purified recombinant protein revealed that H-Rev107 functions as a cytosolic Ca2+-independent PLA(1/2) for PC and PE with higher PLA1 activity than PLA2 activity. Dithiothreitol and iodoacetic acid exhibited stimulatory and inhibitory effects, respectively. Histidine-21 and cysteine-111 of rat H-Rev107 were presumed to form a catalytic dyad based on database analysis, and their single mutants were totally inactive. These results suggested that H-Rev107 is a hydrolase of the thiol type. The N-terminal proline-rich and C-terminal hydrophobic domains of H-Rev107 were earlier reported to be responsible for the regulation of cell proliferation. Analysis of deletion mutants indicated that these domains are also catalytically essential, suggesting relevance of the catalytic activity to the anti-proliferative activity.

Identification and functional characterization of adipose-specific phospholipase A2 (AdPLA)

Phospholipases A(2) (PLA(2)s) catalyze hydrolysis of fatty acids from the sn-2 position of phospholipids. Here we report the identification and characterization of a membrane-associated intracellular calcium-dependent, adipose-specific PLA(2) that we named AdPLA (adipose-specific phospholipase A(2)). We found that AdPLA was highly expressed specifically in white adipose tissue and was induced during preadipocyte differentiation into adipocytes. Clearance of AdPLA by immunoprecipitation significantly decreased PLA activity in white adipose tissue lysates but had no effect on liver lysates, where expression was hardly detectable. In characterizing AdPLA, we employed radiochemical assays with TLC analysis of the enzyme activity of lysates from COS-7 cells overexpressing AdPLA. For kinetic studies, we produced purified recombinant AdPLA for use in a lipoxidase-coupled spectrophotometric assay. AdPLA generated free fatty acid and lysophospholipid from phosphatidylcholine with a preference for hydrolysis at the sn-2 position. Although we found low but detectable lysophospholipase activity, AdPLA showed no significant activity against a variety of other lipid substrates. Calcium was found to activate AdPLA but was not essential for activity. Studies with known phospholipase inhibitors, including bromoenolactone, methyl arachidonyl fluorophosphate, AACOCF(3), 7,7-dimethyl-5,8-eicosadienoic acid, and thioetheramide, supported that AdPLA is a phospholipase. Mutational studies showed that His-23 and Cys-113 are critical for activity of AdPLA and suggested that AdPLA is likely a His/Cys PLA(2). Overall, although AdPLA is similar to other histidine phospholipases in pH and calcium dependence, AdPLA showed different characteristics in many regards, including predicted catalytic mechanism. AdPLA may therefore represent the first member of a new group of PLA(2)s, group XVI.

HRASLS3 is a PPARgamma-selective target gene that promotes adipocyte differentiation

The prevalence of obesity and its associated metabolic diseases worldwide has focused attention on understanding the mechanisms underlying adipogenesis. The nuclear receptor PPARgamma has emerged as a central regulator of adipose tissue function and formation. Despite the identification of numerous PPARgamma targets involved in a range of processes, from lipid droplet formation to adipokine secretion, information is still lacking on targets downstream of PPARgamma that directly affect fat cell differentiation. Here we identify HRASLS3 as a novel PPARgamma regulated gene with a role in adipogenesis. HRASLS3 expression increases during the differentiation of preadipocyte cell lines and is highly expressed in white and brown adipose tissue in mice. HRASLS3 expression is induced by PPARgamma ligands in preadipocyte cell lines as well in adipose tissue in vivo. We demonstrate that the HRASLS3 promoter contains a functional PPAR response element and is a direct target for regulation by PPARgamma/RXR heterodimers. Finally, we show that overexpression of HRASLS3 augments PPARgamma-driven lipid accumulation and adipogenesis, whereas siRNA-mediated knockdown of HRASLS3 expression decreases differentiation. Together, these results identify HRASLS3 as one of the downstream effectors of PPARgamma action in adipogenesis.