The bicellular tensile force sorts the localization of LSRs in bicellular and tricellular junctions

Atypical Macropinocytosis Contributes to Malignant Progression: A Review of Recent Evidence in Endometrioid Endometrial Cancer Cells

Simple Summary

A novel type of macropinocytosis has been identified as a trigger for the malignant progression of endometrial cancer. Transiently reducing epithelial barrier homeostasis leads to macropinocytosis by splitting between adjacent cells in endometrioid endometrial cancer. Macropinocytosis causes morphological changes in well-differentiated to poorly differentiated cancer cells. Inhibition of macropinocytosis promotes a persistent dormant state in the intrinsic KRAS-mutated cancer cell line Sawano. This review focuses on the mechanisms of atypical macropinocytosis and its effects on cellular function, and it describes the physiological processes involved in inducing resting conditions in endometrioid endometrial cancer cells.

Abstract

Macropinocytosis is an essential mechanism for the non-specific uptake of extracellular fluids and solutes. In recent years, additional functions have been identified in macropinocytosis, such as the intracellular introduction pathway of drugs, bacterial and viral infection pathways, and nutritional supplement pathway of cancer cells. However, little is known about the changes in cell function after macropinocytosis. Recently, it has been reported that macropinocytosis is essential for endometrial cancer cells to initiate malignant progression in a dormant state. Macropinocytosis is formed by a temporary split of adjacent bicellular junctions of epithelial sheets, rather than from the apical surface or basal membrane, as a result of the transient reduction of tight junction homeostasis. This novel type of macropinocytosis has been suggested to be associated with the malignant pathology of endometriosis and endometrioid endometrial carcinoma. This review outlines the induction of malignant progression of endometrial cancer cells by macropinocytosis based on a new mechanism and the potential preventive mechanism of its malignant progression.

LSR antibody promotes apoptosis and disrupts epithelial barriers via signal pathways in endometrial cancer

Lipolysis-stimulated lipoprotein receptor (LSR), a lipid metabolism-related factor localized in tricellular tight junctions (tTJs), plays an important role in maintaining the epithelial barrier. LSR is highly expressed in well-differentiated endometrial endometrioid carcinoma (EEC), and its expression decreases during malignancy. Angubindin-1, a novel LSR ligand peptide, regulates tTJs without cytotoxicity, enhances paracellular permeability, and regulates epithelial barrier via c-Jun N-terminal kinase (JNK)/cofilin. In this study, we investigated the immune-modulatory roles of an anti-LSR antibody in the treatment of EEC in vitro compared to those of angubindin-1. We prepared an antibody against the extracellular N-terminal domain of human LSR (LSR-N-ab) and angubindin-1. EEC cell-line Sawano cells in 2D and 2.5D cultures were treated with 100 μg/ml LSR-N-ab or 2.5 μg/ml angubindin-1 with or without protein tyrosine kinase 2β inhibitor PF431396 (PF43) and JNK inhibitor SP600125 (SP60) at 10 μM. Treatment with LSR-N-ab and angubindin-1 decreased LSR at the membranes of tTJs and the activity of phosphorylated LSR and phosphorylated cofilin in 2D culture. Treatment with LSR-N-ab and angubindin-1 decreased the epithelial barrier measured as TEER values in 2D culture and enhanced the epithelial permeability of FD-4 in 2.5D culture. Treatment with LSR-N-ab, but not angubindin-1, induced apoptosis in 2D culture. Pretreatment with PF43 and SP60 prevented all the changes induced by treatment with LSR-N-ab and angubindin-1. Treatment with LSR-N-ab and angubindin-1 enhanced the cell metabolism measured as the mitochondrial respiration levels in 2D culture. LSR-N-ab and angubindin-1 may be useful for therapy of human EEC via enhanced apoptosis or drug absorption.

Effects of Lipolysis-Stimulated Lipoprotein Receptor on Tight Junctions of Pancreatic Ductal Epithelial Cells in Hypertriglyceridemic Acute Pancreatitis

Objective

We investigated the effects of lipolysis-stimulated lipoprotein receptor (LSR) on the tight junctions (TJs) of pancreatic ductal epithelial cells (PDECs) in hypertriglyceridemic acute pancreatitis (HTGAP).

Methods

Sprague-Dawley rats were fed standard rat chow or a high-fat diet and injected with sodium taurocholate to obtain normal and HTGAP rats, respectively. Serum triglyceride (TG) levels, pathological changes, TJ proteins in the pancreas, and TJ ultrastructure of PDECs were assessed. LSR overexpression (OE) and knockdown (KD) HPDE6-C7 models were designed and cultured in a high-fat environment. Protein levels were quantified by Western blotting. Cell monolayer permeability was detected using FITC-Dextran.

Results

Serum TG concentration and pancreatic scores were higher in the HTGAP group than in the normal group. Among the TJ proteins, LSR protein expression was significantly lower in the HTGAP group than in the acute pancreatitis (AP) group. Tricellulin (TRIC) expression in the pancreatic ductal epithelia was higher in the HTGAP group than in the AP group. The HTGAP group had lower TJ protein levels, wider intercellular space, and widespread cellular necrosis with disappearance of cell junction structures. In the cell study, TJ proteins were downregulated and the cellular barrier was impaired by palmitic acid (PA), which was reversed by LSR-OE, whereas LSR-KD downregulated the TJ proteins and aggravated PA-induced cellular barrier impairment.

Conclusions

Hypertriglyceridemia downregulates the TJ proteins in PDECs, which may impair the pancreatic ductal mucosal barrier function. LSR regulation can change the effects of HTG on cellular barrier function by upregulating the TJ proteins.

Translocation of LSR from tricellular corners causes macropinocytosis at cell-cell interface as a trigger for breaking out of contact inhibition

Withdrawal from contact inhibition is necessary for epithelial cancer precursor cells to initiate cell growth and motility. Nevertheless, little is understood about the mechanism for the sudden initiation of cell growth under static conditions. We focused on cellular junctions as one region where breaking out of contact inhibition occurs. In well-differentiated endometrial cancer cells, Sawano, the ligand administration for tricellular tight junction protein LSR, which transiently decreased the robust junction property, caused an abrupt increase in cell motility and consequent excessive multilayered cell growth despite being under contact inhibition conditions. We observed that macropinocytosis essentially and temporarily occurred as an antecedent event for the above process at intercellular junctions without disruption of the junction apparatus but not at the apical plasma membrane. Collectively, we concluded that the formation of macropinocytosis, which is derived from tight junction-mediated signaling, was triggered for the initiation of cell growth in static precancerous epithelium.

Role of Tricellular Tight Junction Protein Lipolysis-Stimulated Lipoprotein Receptor (LSR) in Cancer Cells

Maintaining a robust epithelial barrier requires the accumulation of tight junction proteins, LSR/angulin-1 and tricellulin, at the tricellular contacts. Alterations in the localization of these proteins temporarily cause epithelial barrier dysfunction, which is closely associated with not only physiological differentiation but also cancer progression and metastasis. In normal human endometrial tissues, the endometrial cells undergo repeated proliferation and differentiation under physiological conditions. Recent observations have revealed that the localization and expression of LSR/angulin-1 and tricellulin are altered in a menstrual cycle-dependent manner. Moreover, it has been shown that endometrial cancer progression affects these alterations. This review highlights the differences in the localization and expression of tight junction proteins in normal endometrial cells and endometrial cancers and how they cause functional changes in cells.