Identification and characterization of a splice variant of the catalytic domain of mouse NTE-related esterase

Hypermethylation reduces the expression of PNPLA7 in hepatocellular carcinoma

Liver cancer has a high morbidity and mortality rate, and is one of the most common types of cancer in men. PNPLA7 is a member of the patatin-like phospholipase domain-containing protein family which is involved in triglyceride hydrolysis, energy metabolism and lipid droplet metabolism. The liver is the most important energy metabolism organ; whether PNPLA7 is deregulated in liver cancer has not been previously reported. In the present study, reverse transcription-quantitative polymerase chain reaction and subsequent methylation analysis provided evidence that PNPLA7 is down-regulated in hepatocellular carcinoma (HCC) cell lines and tissue samples, via the mechanism of transcriptional silencing by promoter hypermethylation. These results may provide novel insights for HCC diagnosis.

Identification of two novel splicing variants of murine NTE-related esterase

NTE-related esterase (NRE) is an insulin-regulated lysophospholipase with homology to neuropathy target esterase (NTE), which plays a role in energy metabolism. Here, we reported two alternative splicing variants of the murine NRE (mNRE) gene, termed mNREV1 and mNREV2. Genomic organization analysis indicated that 5' splice site of mNRE intron 33 was changed in both mNREV1 and mNREV2, and mNRE exon 21 was deleted in mNREV2. mNREV1 had the same protein domains with mNRE, while mNREV2 lacked the patatin domain in the C-terminal catalytic region. Green fluorescent protein-mNREV1 or mNREV2 fusion proteins localized to the endoplasmic reticulum. mNREV1 and mNRE exhibited equal hydrolytic activity to the substrate phenyl valerate, whereas mNREV2 did not have any catalytic activity. The expression profiles of mNRE and its splicing isoforms in white adipose tissue, cardiac muscle, skeletal muscle, and testis tissues were further analyzed by real time quantitative-PCR in fed and fasted states, which indicated that the major isoform of mNRE mRNA generated switched from mNREV2 to mNREV1 during fasting. Thus there was a nutritional regulation of mNRE expression at the mRNA levels via alternative splicing.

Degradation of mouse NTE-related esterase by macroautophagy and the proteasome

NTE-related esterase (NRE) is a novel endoplasmic reticulum-anchored lysophospholipase with high homology to neuropathy target esterase (NTE). However, little is known about the regulation of NRE protein. In the current study, we investigated the degradation pathways of mouse NRE (mNRE) in mammalian cells. Based on experiments with inhibitors and inducer of protein degradation pathways, we provide here the first evidence that mNRE is degraded by macroautophagy as well as by the proteasome. Moreover, the contribution of protein domains to the degradation of mNRE was investigated, which showed that the transmembrane and regulatory domain played a role in the degradation of mNRE by macroautophagy and the proteasome respectively. In contrast the C-terminal catalytic domain was not involved in both degradation pathways of mNRE. These findings showed for the first time that the degradation pathways in controlling mNRE quantity and may provide further insight into structure and regulation of mNRE.

Protein domains, catalytic activity, and subcellular distribution of mouse NTE-related esterase

A mammalian family of lipid hydrolases, designated "patatin-like phospholipase domain containing (PNPLA)" recently has attracted attention. NTE-related esterase (NRE) as a member of PNPLA is an insulin-regulated lysophospholipase with homology to neuropathy target esterase (NTE). Mouse NRE (mNRE) has a predicted amino-terminal transmembrane region (TM), a putative regulatory (R) domain, and a hydrophobic catalytic (C) domain. In the current study, we described the expression of green fluorescent protein (GFP)-tagged constructs of mNRE and mutant proteins lacking the specific protein domains. Esterase assays indicated that neither the TM nor R-domain was essential for mNRE esterase activity, but the TM significantly contributed to its activity. Subcellular distribution showed that mNRE was anchored in ER via its TM domain and that its C-domain was associated with ER. Furthermore, experiments involving proteinase treatment revealed that most of mNRE molecule was exposed on the cytoplasmic face of ER membranes. Collectively, our results for the first time revealed the protein domains, catalytic activity, and subcellular location of mNRE and a simplified model for mNRE was proposed.