Urokinase-type plasminogen activator receptor (uPAR or CD87) is a glycosyl-phosphatidyl-inositol anchored (GPI) membrane protein. The uPAR primary ligand is the serine protease urokinase (uPA), converting plasminogen into plasmin, a broad spectrum protease, active on most extracellular matrix components. Besides uPA, the uPAR binds specifically also to the matrix protein vitronectin and, therefore, is regarded also as an adhesion receptor. Complex formation of the uPAR with diverse transmembrane proteins, including integrins, formyl peptide receptors, G protein-coupled receptors and epidermal growth factor receptor results in intracellular signalling. Thus, the uPAR is a multifunctional receptor coordinating surface-associated pericellular proteolysis and signal transduction, thereby affecting physiological and pathological mechanisms. The uPAR-initiated signalling leads to remarkable cellular effects, that include increased cell migration, adhesion, survival, proliferation and invasion. Although this is beyond the scope of this review, the uPA/uPAR system is of great interest to cancer research, as it is associated to aggressive cancers and poor patient survival. Increasing evidence links the uPA/uPAR axis to epithelial to mesenchymal transition, a highly dynamic process, by which epithelial cells can convert into a mesenchymal phenotype. Furthermore, many reports indicate that the uPAR is involved in the maintenance of the stem-like phenotype and in the differentiation process of different cell types. Moreover, the levels of anchor-less, soluble form of uPAR, respond to a variety of inflammatory stimuli, including tumorigenesis and viral infections. Finally, the role of uPAR in virus infection has received increasing attention, in view of the Covid-19 pandemics and new information is becoming available. In this review, we provide a mechanistic perspective, via the detailed examination of consolidated and recent studies on the cellular responses to the multiple uPAR activities.

The urokinase‐type plasminogen activator (uPA) receptor (uPAR) focuses uPA proteolytic activity on the cell membrane, promoting localized degradation of extracellular matrix (ECM), and binds vitronectin (VN), mediating cell adhesion to the ECM. uPAR‐bound uPA and VN induce proteolysis‐independent intracellular signalling, regulating cell adhesion, migration, survival and proliferation. uPAR cross‐talks with CXCR4, the receptor for the stroma‐derived factor 1 chemokine. CXCR4 is crucial in the trafficking of hematopoietic stem cells from/to the bone marrow, which involves also uPAR. Both uPAR and CXCR4 are expressed in acute myeloid leukaemia (AML), with a lower expression in undifferentiated and myeloid subsets, and higher expression in myelomonocytic and promyelocytic subsets. We hypothesized a microRNA (miR)‐mediated co‐regulation of uPAR and CXCR4 expression, which could allow their cross‐talk at the cell surface. We identified three miRs, miR‐146a, miR‐335 and miR‐622, regulating the expression of both uPAR and CXCR4 in AML cell lines. Indeed, these miRs directly target the 3′untranslated region of both uPAR‐ and CXCR4‐mRNAs; accordingly, uPAR/CXCR4 expression is reduced by their overexpression in AML cells and increased by their specific inhibitors. Overexpression of all three miRs impairs migration, invasion and proliferation of myelomonocytic cells. Interestingly, we observed an inverse relationship between uPAR/CXCR4 expression and miR‐146a and miR‐335 levels in AML blasts, suggesting their possible role in the regulation of uPAR/CXCR4 expression also in vivo.

Phosphoprotein enriched in diabetes/phosphoprotein enriched in astrocytes‐15 kD (PED/PEA‐15) is an anti‐apoptotic protein whose expression is increased in several human cancers. In addition to apoptosis, PED/PEA‐15 is involved in the regulation of other major cellular functions, including cell adhesion, migration, proliferation and glucose metabolism. To further understand the functions of this protein, we performed a yeast two‐hybrid screening using PED/PEA‐15 as a bait and identified the 67 kD high‐affinity laminin receptor (67LR) as an interacting partner. 67 kD laminin receptor is a non‐integrin cell‐surface receptor for the extracellular matrix (ECM), derived from the dimerization of a 37 kD cytosolic precursor (37LRP). The 67LR is highly expressed in human cancers and widely recognized as a molecular marker of metastatic aggressiveness. The molecular interaction of PED/PEA‐15 with 67LR was confirmed by pull‐down experiments with recombinant His‐tagged 37LRP on lysates of PED/PEA‐15 transfected HEK‐293 cells. Further, overexpressed or endogenous PED/PEA‐15 was co‐immunoprecipitated with 67LR in PED/PEA‐15‐transfected HEK‐293 cells and in U‐373 glioblastoma cells, respectively. PED/PEA‐15 overexpression significantly increased 67LR‐mediated HEK‐293 cell adhesion and migration to laminin that, in turn, determined PED/PEA‐15 phosphorylation both in Ser‐104 and Ser‐116, thus enabling cell proliferation and resistance to apoptosis. PED/PEA‐15 ability to induce cell responses to ECM‐derived signals through interaction with 67LR may be of crucial importance for tumour cell survival in a poor microenvironment, thus favouring the metastatic spread and colonization.

Early events of basal cell carcinoma (BCC) tumorigenesis are triggered by inappropriate activation of SHH signaling, via the loss of Patched1 (Ptch1) or by activating mutations of Smoothened (Smo). TBX1 is a key regulator of pharyngeal development, mainly through expression in multipotent progenitor cells of the cardiopharyngeal lineage. This transcription factor is connected to several major signaling systems, such as FGF, WNT, and SHH, and it has been linked to cell proliferation and to the regulation of cell shape and cell dynamics. Here, we show that TBX1 was expressed in all of the 51 BCC samples that we have tested, while in healthy human skin it was only expressed in the hair follicle. Signal intensity and distribution was heterogeneous among tumor samples. Experiments performed on a cellular model of mouse BCC showed that Tbx1 is downstream to GLI2, a factor in the SHH signaling, and that, in turn, it regulates the expression of Dvl2, which encodes an adaptor protein that is necessary for the transduction of WNT signaling. Consistently, Tbx1 depletion in the cellular model significantly reduced cell migration. These results suggest that TBX1 is part of a core transcription network that promotes BCC tumorigenesis.

The T-box transcription factor TBX1 is expressed in the cardio-pharyngeal mesoderm. The correct cell fate decisions of cardio-pharyngeal mesoderm cells are critical, as any defect in this process can alter second heart field morphogenesis and lead to cardiac outflow tract and pharyngeal apparatus defects. The second heart field plays a crucial role in cardiac development by incorporating cardiac progenitors into the heart. It is also the major gene implicated in 22q11.2 deletion (or DiGeorge) syndrome, a primary genetic cause of congenital heart defects associated with hypoplasia of the cardiac outflow tract. The murine model recapitulates the heart phenotype and shows anomalies in the ECM–integrin–focal adhesion pathway. Here, we used a cell culture model to manipulate Tbx1 levels in order to molecularly and functionally characterize the defective focal adhesions (FAs) caused by Tbx1 loss and to analyse their dynamics on the ECM. Intriguingly, we found that Tbx1 regulates FA dynamics by influencing the FA disassembly process. Furthermore, Tbx1 is required for the paxillin (PXN) signalling pathway and controls cell spreading primarily through Pxn regulation. In fact, consistent with this observation, the ectopic expression of PXN rescued the cell spreading and signalling defects caused by Tbx1 depletion. Finally, our study revealed that, at least in vitro, TBX1 is a critical regulator of cell adhesion by affecting FA turnover.

SUMMARY: Tbx1, the major candidate gene for DiGeorge or 22q11.2 deletion syndrome, is required for efficient incorporation of cardiac progenitors (CPs) of the second heart field (SHF) into the heart. However, the mechanisms by which TBX1 regulates this process are still unclear. Here, we have used two independent models, mouse embryos and cultured cells, to define the role of TBX1 in establishing morphological and dynamic characteristics of SHF in the mouse. We found that loss of TBX1 impairs extra cellular matrix (ECM)-integrin-focal adhesion (FA) signaling in both models. Mosaic analysis in embryos showed that this function is non-cell autonomous and, in cultured cells, loss of TBX1 impairs cell migration and focal adhesions. Additionally, we found that ECM-mediated outside-in integrin signaling is disrupted upon loss of TBX1. Finally, we show that interfering with the ECM-integrin-FA axis between E8.5 and E9.5 in mouse embryos, corresponding to the time window within which TBX1 is required in the SHF, causes outflow tract dysmorphogenesis. Our results demonstrate that TBX1 is required to maintain the integrity of ECM-cell interactions in the SHF, and that this interaction is critical for cardiac outflow tract development. More broadly, our data identifies a novel TBX1 downstream pathway as an important player in SHF tissue architecture and cardiac morphogenesis.