The primary function of the endothelial cells (EC) lining the inner surface of all vessels is to regulate permeability of vascular walls and to control exchange between circulating blood and tissue fluids of organs. The EC actin cytoskeleton plays a crucial role in maintaining endothelial barrier function. Actin cytoskeleton reorganization result in EC contraction and provides a structural basis for the increase in vascular permeability, which is typical for many diseases. Actin cytoskeleton in non-muscle cells presented two actin isoforms: non-muscle β-cytoplasmic and γ-cytoplasmic actins (β-actins and γ-actins), which are encoded by ACTB and ACTG1 genes, respectively. They are ubiquitously expressed in the different cells in vivo and in vitro and the β/γ-actin ratio depends on the cell type. Both cytoplasmic actins are essential for cell survival, but they perform various functions in the interphase and cell division and play different roles in neoplastic transformation. In this review, we briefly summarize the research results of recent years and consider the features of the cytoplasmic actins: The spatial organization in close connection with their functional activity in different cell types by focusing on endothelial cells.

Inhibition of KIT-signaling is a major molecular target for gastrointestinal stromal tumor (GIST) therapy, and imatinib mesylate (IM) is known as the most effective first-line treatment option for patients with advanced, unresectable, and/or metastatic GISTs. We show here for the first time that the inhibition of KIT-signaling in GISTs induces profound changes in the cellular secretome, leading to the release of multiple chemokines, including FGF-2. IM increased migration, invasion, and colony formation of IM-resistant GISTs in an FGF2-dependent manner, whereas the use of blocking anti-FGF2 antibodies or BGJ398, a selective FGFR inhibitor, abolished these effects, thus suggesting that the activation of FGF2-mediated signaling could serve as a compensatory mechanism of KIT-signaling inhibited in GISTs. Conversely, FGF-2 rescued the growth of IM-naive GISTs treated by IM and protected them from IM-induced apoptosis, consistent with the possible involvement of FGF-2 in tumor response to IM-based therapy. Indeed, increased FGF-2 levels in serum and tumor specimens were found in IM-treated mice bearing IM-resistant GIST xenografts, whereas BGJ398 used in combination with IM effectively inhibited their growth. Similarly, increased FGF-2 expression in tumor specimens from IM-treated patients revealed the activation of FGF2-signaling in GISTs in vivo. Collectively, the continuation of IM-based therapy for IM-resistant GISTs might facilitate disease progression by promoting the malignant behavior of tumors in an FGF2-dependent manner. This provides a rationale to evaluate the effectiveness of the inhibitors of FGF-signaling for IM-resistant GISTs.

We have shown previously that two cytoplasmic actin isoforms play different roles in neoplastic cell transformation. Namely, β-cytoplasmic actin acts as a tumor suppressor, whereas γ-cytoplasmic actin enhances malignant features of tumor cells. The distinct participation of each cytoplasmic actin in the cell cycle driving was also observed. The goal of this study was to describe the diverse roles of cytoplasmic actins in the progression of chromosomal instability of MDA-MB-231 basal-like human carcinoma cell line. We performed traditional methods of chromosome visualization, as well as 3D-IF microscopy and western blotting for CENP-A detection/quantification, to investigate chromosome morphology. Downregulation of cytoplasmic actin isoforms alters the phenotype and karyotype of MDA-MB-231 breast cancer cells. Moreover, β-actin depletion leads to the progression of chromosomal instability with endoreduplication and aneuploidy increase. On the contrary, γ-actin downregulation results not only in reduced percentage of mitotic carcinoma cells, but leads to chromosome stability, reduced polyploidy, and aneuploidy.

The widespread occurrence of breast cancer and its propensity to develop drug resistance highlight the need for a comprehensive understanding of the molecular mechanisms involved. This study investigates the intricate pathways associated with secondary resistance to taxol in triple-negative breast cancer (TNBC) cells, with a particular focus on the changes observed in the cytoplasmic actin isoforms. By studying a taxol-resistant TNBC cell line, we revealed a shift between actin isoforms towards γ-actin predominance, accompanied by increased motility and invasive properties. This was associated with altered tubulin isotype expression and reorganisation of the microtubule system. In addition, we have shown that taxol-resistant TNBC cells underwent epithelial-to-mesenchymal transition (EMT), as evidenced by Twist1-mediated downregulation of E-cadherin expression and increased nuclear translocation of β-catenin. The RNA profiling analysis revealed that taxol-resistant cells exhibited significantly increased positive regulation of cell migration, hormone response, cell–substrate adhesion, and actin filament-based processes compared with naïve TNBC cells. Notably, taxol-resistant cells exhibited a reduced proliferation rate, which was associated with an increased invasiveness in vitro and in vivo, revealing a complex interplay between proliferative and metastatic potential. This study suggests that prolonged exposure to taxol and acquisition of taxol resistance may lead to pro-metastatic changes in the TNBC cell line.

Cancer cell aggressiveness, marked by actin cytoskeleton reconfiguration critical for metastasis, may result from an imbalanced ratio favoring γ-actin. Dysfunctional p53 emerges as a key regulator of invasiveness and migration in various cancer cells, both in vitro and in vivo. P53 inactivation (via mutants R175H, R248W, R273H, or TP53 repression) significantly enhanced the migration, invasion, and proliferation of human lung adenocarcinoma A549 cells in vitro and in vivo, facilitating intrapulmonary xenograft metastasis in athymic mice. Conversely, wild-type TP53 (TP53 WT) overexpression in p53-deficient non-small- cell lung cancer (NSCLC) H1299 cells substantially reduced proliferation and migration in vitro, effectively curbing orthotopic tumorigenicity and impeding in vivo metastasis. These alterations in cell motility were closely associated with actin cytoskeleton restructuring, favoring γ-actin, and coincided with ERK1/2-mediated signaling activation, unveiling an innovative regulatory mechanism in malignancy progression. Cancer cell aggressiveness, driven by actin cytoskeleton reorganization and a shift towards γ-actin predominance, may be regulated by p53 dysfunction, thereby providing novel insight into tumor progression mechanisms.

SUMMARY: How junctional proteins regulate the mechanics of the plasma membrane and how actin and myosin isoforms are selectively localized at epithelial cell-cell junctions is poorly understood. Here we show by atomic force indentation microscopy, immunofluorescence analysis and FLIM membrane tension imaging that the tight junction (TJ) protein cingulin maintains apical surface stiffness and TJ membrane tortuosity and down-regulates apico-lateral membrane tension in MDCK cells. KO of cingulin in MDCK, mCCD and Eph4 cells results in a decrease in the juxta-membrane accumulation of labeling for cytoplasmic myosin-2B (NM2B), γ-actin, phalloidin and ARHGEF18, but no detectable effect on myosin-2A (NM2A) and β-actin. Loss of paracingulin leads to weaker mechanical phenotypes in MDCK cells, correlating with no detectable effect on the junctional accumulation of myosins and actins. Cingulin and paracingulin form biomolecular condensates, bind to the ZU5 domain of ZO-1, and are recruited as clients into ZO-1 condensates in a ZU5-dependent manner. Cingulin binding to ZO-1 promotes the unfolding of ZO-1, as determined by interaction with DbpA in cells lacking ZO-2 and in vitro. Cingulin promotes the accumulation of a pool of ZO-1 at the TJ and is required in a ZU5-dependent manner for the recruitment of phalloidin-labelled actin filaments into ZO-1 condensates, suggesting that ZU5-cingulin interaction promotes ZO-1 interaction with actin filaments. Our results indicate that cingulin tethers the juxta-membrane and apical branched γ-actin-NM2B network to TJ to modulate ZO-1 conformation and the TJ assembly of a pool of ZO-1 and fine-tune the distribution of forces to apical and TJ membranes.