Early endosomes, regarded as the main sorting station on endocytic pathway, are characterized by high frequency of homotypic fusions mediated by tethering protein EEA1. Despite intensive investigations, biogenesis of endosomes, boundaries between early and late endosomes, and process of cargo transition though them remain obscure. Here, using EGF/EGFR endocytosis as a model and confocal microscopy of fixed and live cells, we provide evidence favoring EEA1-vesicles being pre-existed vesicular compartment, that maintains its resident proteins’ level and is sensitive to biosynthetic, but not endocytic pathway disturbance. EEA1-vesicles directly fuse with incoming EGF/EGFR-vesicles into hybrid endosomes with separated EEA1- and EGFR-domains, thus providing a platform for rapid achievement of an excess of surface-derived membrane that is used to form intraluminal vesicles (ILVs). Thus, multivesicular structures colocalized with EEA1 are still early endosomes. “EEA1-cycle” ends by exclusion of EGFR-containing domains with ILVs inside that turns into MVE and restoration of initial EEA1-vesicles population.

The prototypical receptor tyrosine kinase epidermal growth factor receptor (EGFR) is regulated by a set of its ligands, which determines the specificity of signaling and intracellular fate of the receptor. The EGFR signaling system is well characterized in immortalized cell lines such as HeLa derived from tumor tissues, but much less is known about EGFR function in untransformed multipotent stromal/stem cells (MSCs). We compared the effect of epidermal growth factor (EGF), transforming growth factor-$\alpha$ (TGF-$\alpha$) and amphiregulin (AREG) on physiological responses in endometrial MSCs (enMSC) and HeLa cells. In addition, using Western blotting and confocal microscopy, we studied the internalization and degradation of EGFR stimulated by the three ligands in these cell lines. We demonstrated that unlike HeLa, EGF and TGF-$\alpha$, but not AREG, stimulated enMSC proliferation and prevented decidual differentiation in an EGFR-dependent manner. In HeLa cells, EGF targeted EGFR for degradation, while TGF-$\alpha$ stimulated its recycling. Surprisingly, in enMSC, both ligands caused EGFR degradation. In both cell lines, AREG-EGFR internalization was not registered. In HeLa cells, EGFR was degraded within 2 h, restoring its level in 24 h, while in enMSC, degradation took more than 4–8 h, and the low EGFR level persisted for several days. This indicates that EGFR homeostasis in MSCs may differ significantly from that in immortalized cell lines.

The c-ErbB receptor family is a fundamental cell signaling system that regulates cell proliferation, motility, apoptosis, differentiation, and other key cellular functions. Overexpressed and mutated in some tumors, c-ErbB receptors play a pivotal role in their progression but are also present in many non-malignant cells, including those that are promising from the point of view of regenerative medicine, such as mesenchymal stromal cells (MSCs). The role of c-ErbB receptors in these cells is not clearly understood, and the data on their expression are sporadic. Therefore, the systemic characterization of c-ErbB receptor family expression in MSCs from a wide range of tissues is of high priority. Here, using RT-qPCR and Western blotting analysis, we evaluated the c-ErbB receptors expression pattern at the mRNA and protein levels in human MSCs isolated from six different tissues. We found that MSCs possess considerable EGFR and HER2 mRNA levels comparable to those in some malignant cells while showing trace HER3 and HER4 expression. However, EGFR but not HER2 was detected in MSCs at the protein level. We also show that the absence of HER2 protein is not associated with its rapid lysosomal degradation. We conclude that c-ErbB signaling in human MSCs is exclusively mediated by EGFR.

Background: Real-time reverse transcription quantitative polymerase chain reaction (RT-qPCR) is a powerful tool for analysing target gene expression in biological samples. To achieve reliable results by RT-qPCR, the most stable reference genes must be selected for proper data normalisation, particularly when comparing cells of different types. We aimed to choose the least variable candidate reference genes among eight housekeeping genes tested within a set of human cancer cell lines (HeLa, MCF-7, SK-UT-1B, A549, A431, SK-BR-3), as well as four lines of normal, non-malignant mesenchymal stromal cells (MSCs) of different origins. Methods: The reference gene stability was evaluated using four algorithms (BestKeeper, NormFinder, geNorm and the comparative ΔCt method) and ranked with the RefFinder web-based tool. Results: We found increased variability in the housekeeping genes’ expression in the cancer cell lines compared to that in normal MSCs. POP4 and GAPDH were identified as the most suitable reference genes in cancer cells, while 18S and B2M were the most suitable in MSCs. POP4 and EIF2B1 were shown to be the least variable genes when analysing normal and cancer cell lines together. Epidermal growth factor receptor (EGFR) mRNA relative expression was normalised by the three most stable or three least stable reference genes to demonstrate the reliability of reference genes validation. Conclusion: We analysed and selected stable reference genes for RT-qPCR analysis in the wide panel of cancer cell lines and MSCs. The study provides a reliable tool for future research concerning the expression of genes involved in various intracellular signalling pathways and emphasises the need for careful selection of suitable references before analysing target gene expression.