Accumulated phosphatidylcholine (16:0/16:1) in human colorectal cancer; possible involvement of LPCAT4.

The identification of cancer biomarkers is critical for target-linked cancer therapy. The overall level of phosphatidylcholine (PC) is elevated in colorectal cancer (CRC). To investigate which species of PC is overexpressed in colorectal cancer, an imaging mass spectrometry was performed using a panel of non-neoplastic mucosal and CRC tissues. In the present study, we identified a novel biomarker, PC(16:0/16:1), in CRC using imaging mass spectrometry. Specifically, elevated levels of PC(16:0/16:1) expression were observed in the more advanced stage of CRC. Our data further showed that PC(16:0/16:1) was specifically localized in the cancer region when examined using imaging mass spectrometry. Notably, because the ratio of PC(16:0/16:1) to lyso-PC(16:0) was higher in CRC, we postulated that lyso-PC acyltransferase (LPCAT) activity is elevated in CRC. In an in vitro analysis, we showed that LPCAT4 is involved in the deregulation of PC(16:0/16:1) in CRC. In an immunohistochemical analysis, LPCAT4 was shown to be overexpressed in CRC. These data indicate the potential usefulness of PC(16:0/16:1) for the clinical diagnosis of CRC and implicate LPCAT4 in the elevated expression of PC(16:0/16:1) in CRC.

AGPAT1 as a Novel Colonic Biomarker for Discriminating Between Ulcerative Colitis With and Without Primary Sclerosing Cholangitis.

INTRODUCTION: Ulcerative colitis (UC) associated with primary sclerosing cholangitis (PSC-UC) is considered a unique inflammatory bowel disease (IBD) entity. PSC diagnosis in an IBD individual entails a significantly higher risk of gastrointestinal cancer; however, biomarkers for identifying patients with UC at risk for PSC are lacking. We, therefore, performed a thorough PSC-UC biomarker study, starting from archived colonic tissue. METHODS: Proteins were extracted out of formalin-fixed paraffin-embedded proximal colon samples from PSC-UC (n = 9), UC (n = 7), and healthy controls (n = 7). Patients with IBD were in clinical and histological remission, and all patients with UC had a history of pancolitis. Samples were processed by the multienzyme digestion FASP and subsequently analyzed by liquid chromatography-tandem mass spectrometry. Candidate proteins were replicated in an independent cohort (n: PSC-UC = 16 and UC = 21) and further validated by immunohistochemistry. RESULTS: In the discovery step, 7,279 unique proteins were detected. The top 5 most differentiating proteins (PSC-UC vs UC) based on linear regression analysis were selected for replication. Of these, 1-acetylglycerol-3-phosphate O-acyltransferase 1 (AGPAT1) was verified as higher in PSC-UC than UC (P = 0.009) in the replication cohort. A difference on the group level was also confirmed by immunohistochemistry, showing more intense AGPAT1 staining in patients with PSC-UC compared with UC. DISCUSSION: We present AGPAT1 as a potential colonic biomarker for differentiating PSC-UC from UC. Our findings have possible implication for future PSC-IBD diagnostics and surveillance.

Functional characterization of human 1-acylglycerol-3-phosphate-O-acyltransferase isoform 9: cloning, tissue distribution, gene structure, and enzymatic activity.

Most cells synthesize their glycerophospholipids and triglycerides (TG) to maintain the cellular integrity and to provide energy for cellular functions. The phospholipids are synthesized de novo in cells through an evolutionary conserved process involving serial acylations of glycerol-3-phosphate. Several isoforms of the enzyme 1-acylglycerol-3-phosphate-O-acyltransferase (AGPAT) acylate lysophosphatidic acid at the sn-2 position to produce phosphatidic acid. We cloned a cDNA predicted to be an AGPAT isoform and designated it AGPAT9. The human AGPAT9 gene spans across 14 exons and encodes for a polypeptide of 534 amino acids. AGPAT9 is highly expressed in the lung and spleen, followed by leukocyte, omental adipose tissue, and placenta. In the Chinese Hamster Ovary (CHO), cell lysates overexpressing AGPAT9, we observed AGPAT activity but not the lysophosphatidylcholine acyltransferase activity. When AGPAT9 is coexpressed with AGPAT1 in CHO cells, both the isoforms localize to the endoplasmic reticulum (ER) and occupy the same ER domain as AGPAT1. Despite substitution of asparagine with proline in the NHX(4)D motif and arginine with cysteine in the EGTR motif, AGPAT9 retains AGPAT activity suggesting that residues asparagine and arginine in the NHX(4)D and EGTR motifs respectively are not essential for the enzymatic activity. Based on the X-ray crystallographic structure of a related acyltransferase, squash gpat, a model is proposed in which a hydrophobic pocket in AGPAT9 accommodates fatty acyl chains of both substrates in an orientation, whereas the HX(4)D motif participates in catalysis. Based on the activity and expression pattern of AGPAT9 in the lung and spleen, this novel isoform could be implicated in the biosynthesis of phospholipids and TG in these tissues.

Cellular localization and associations of the major lipolytic proteins in human skeletal muscle at rest and during exercise.

Lipolysis involves the sequential breakdown of fatty acids from triacylglycerol and is increased during energy stress such as exercise. Adipose triglyceride lipase (ATGL) is a key regulator of skeletal muscle lipolysis and perilipin (PLIN) 5 is postulated to be an important regulator of ATGL action of muscle lipolysis. Hence, we hypothesized that non-genomic regulation such as cellular localization and the interaction of these key proteins modulate muscle lipolysis during exercise. PLIN5, ATGL and CGI-58 were highly (>60%) colocated with Oil Red O (ORO) stained lipid droplets. PLIN5 was significantly colocated with ATGL, mitochondria and CGI-58, indicating a close association between the key lipolytic effectors in resting skeletal muscle. The colocation of the lipolytic proteins, their independent association with ORO and the PLIN5/ORO colocation were not altered after 60 min of moderate intensity exercise. Further experiments in cultured human myocytes showed that PLIN5 colocation with ORO or mitochondria is unaffected by pharmacological activation of lipolytic pathways. Together, these data suggest that the major lipolytic proteins are highly expressed at the lipid droplet and colocate in resting skeletal muscle, that their localization and interactions appear to remain unchanged during prolonged exercise, and, accordingly, that other post-translational mechanisms are likely regulators of skeletal muscle lipolysis.