Association of leupaxin with Src in osteoclasts

The Non-receptor Tyrosine Kinase Pyk2 in Brain Function and Neurological and Psychiatric Diseases

Pyk2 is a non-receptor tyrosine kinase highly enriched in forebrain neurons. Pyk2 is closely related to focal adhesion kinase (FAK), which plays an important role in sensing cell contacts with extracellular matrix and other extracellular signals controlling adhesion and survival. Pyk2 shares some of FAK’s characteristics including recruitment of Src-family kinases after autophosphorylation, scaffolding by interacting with multiple partners, and activation of downstream signaling pathways. Pyk2, however, has the unique property to respond to increases in intracellular free Ca²⁺, which triggers its autophosphorylation following stimulation of various receptors including glutamate NMDA receptors. Pyk2 is dephosphorylated by the striatal-enriched phosphatase (STEP) that is highly expressed in the same neuronal populations. Pyk2 localization in neurons is dynamic, and altered following stimulation, with post-synaptic and nuclear enrichment. As a signaling protein Pyk2 is involved in multiple pathways resulting in sometimes opposing functions depending on experimental models. Thus Pyk2 has a dual role on neurites and dendritic spines. With Src family kinases Pyk2 participates in postsynaptic regulations including of NMDA receptors and is necessary for specific types of synaptic plasticity and spatial memory tasks. The diverse functions of Pyk2 are also illustrated by its role in pathology. Pyk2 is activated following epileptic seizures or ischemia-reperfusion and may contribute to the consequences of these insults whereas Pyk2 deficit may contribute to the hippocampal phenotype of Huntington’s disease. Pyk2 gene, PTK2B, is associated with the risk for late-onset Alzheimer’s disease. Studies of underlying mechanisms indicate a complex contribution with involvement in amyloid toxicity and tauopathy, combined with possible functional deficits in neurons and contribution in microglia. A role of Pyk2 has also been proposed in stress-induced depression and cocaine addiction. Pyk2 is also important for the mobility of astrocytes and glioblastoma cells. The implication of Pyk2 in various pathological conditions supports its potential interest for therapeutic interventions. This is possible through molecules inhibiting its activity or increasing it through inhibition of STEP or other means, depending on a precise evaluation of the balance between positive and negative consequences of Pyk2 actions.

Leupaxin Expression Is Dispensable for B Cell Immune Responses

The generation of a potent humoral immune response by B cells relies on the integration of signals induced by the B cell receptor, toll-like receptors and both negative and positive co-receptors. Several reports also suggest that integrin signaling plays an important role in this process. How integrin signaling is regulated in B cells is however still partially understood. Integrin activity and function are controlled by several mechanisms including regulation by molecular adaptors of the paxillin family. In B cells, Leupaxin (Lpxn) is the most expressed member of the family and in vitro studies suggest that it could dampen BCR signaling. Here, we report that Lpxn expression is increased in germinal center B cells compared to naïve B cells. Moreover, Lpxn deficiency leads to decreased B cell differentiation into plasma cells in vitro. However, Lpxn seems dispensable for the generation of a potent B cell immune response in vivo. Altogether our results suggest that Lpxn is dispensable for T-dependent and T-independent B cell immune responses.

A kindlin-3-leupaxin-paxillin signaling pathway regulates podosome stability

Kindlin-3 regulates podosome stability by recruiting leupaxin to podosomes, which in turn controls PTP-PEST activity and paxillin phosphorylation. Kindlin-3 deficiency allows formation of initial adhesion patches containing talin, vinculin, and paxillin, whereas paxillin family proteins are dispensable for podosome formation.

Binding of kindlins to integrins is required for integrin activation, stable ligand binding, and subsequent intracellular signaling. How hematopoietic kindlin-3 contributes to the assembly and stability of the adhesion complex is not known. Here we report that kindlin-3 recruits leupaxin into podosomes and thereby regulates paxillin phosphorylation and podosome turnover. We demonstrate that the activity of the protein tyrosine phosphatase PTP-PEST, which controls paxillin phosphorylation, requires leupaxin. In contrast, despite sharing the same binding mode with leupaxin, paxillin recruitment into podosomes is kindlin-3 independent. Instead, we found paxillin together with talin and vinculin in initial adhesion patches of kindlin-3–null cells. Surprisingly, despite its presence in these early adhesion patches, podosomes can form in the absence of paxillin or any paxillin member. In conclusion, our findings show that kindlin-3 not only activates and clusters integrins into podosomes but also regulates their lifetime by recruiting leupaxin, which controls PTP-PEST activity and thereby paxillin phosphorylation and downstream signaling.

Downregulation of LPXN expression by siRNA decreases the malignant proliferation and transmembrane invasion of SHI-1 cells

The aim of the present study was to investigate the effects of decreasing leupaxin (LPXN) expression on the proliferation and invasion of human acute monocytic leukemia SHI-1 cells. The transfection efficiency of fluorescein amidite (FAM)-small interfering RNA (siRNA) was determined using flow cytometry, and the protein expression levels of LPXN, phosphorylated (p)-c-Jun N-terminal kinase (JNK), p-p38 mitogen-activated protein kinase (p38 MAPK) and p-extracellular-signal-regulated kinase (ERK) were detected by western blot analysis. Proliferation was determined using the cell counting kit-8 reagent and cellular transmembrane invasion ability was determined using a Transwell chamber system. The gelatinase levels of matrix metalloproteinase (MMP)-2 and MMP-9 in the cell culture supernatant were also analyzed by gelatin zymography. In SHI-1 cells, the optimal transfection conditions of siRNA were a cell density of 4×10⁵ cells/ml and a ratio of siRNA/Lipofectamine^® 2000 of 200 pmol/1 µl. The highest transfection efficiency of FAM-siRNA was 74.5%. In the present study, L2-siRNA was selected to effectively decrease the expression of LPXN. Following downregulation of LPXN expression by L2-siRNA, proliferation inhibition rates increased to 27.043±2.051 and cell transmembrane invasion rates decreased to 25.270±2.145 (P<0.05). The results of the western blot analysis and the gelatin zymography indicated that downregulation of LPXN expression increased the expression of p-p38 MAPK and p-JNK, and attenuated the secretion levels of MMP-2 and MMP-9. However, downregulation of LPXN expression had no effect on p-ERK expression in SHI-1 cells. The results of the present study indicated that downregulation of LPXN expression decreased the malignant proliferation and transmembrane invasion of SHI-1 cells by activating JNK and p38 MAPK, and inhibiting MMP-2 and MMP-9 secretion.

[Small interfering RNA-mediated LPXN silencing suppresses proliferation and enhances drug sensitivity of human acute monocytic leukemia SHI-1 cells in vitro]

OBJECTIVE

To investigate the effect of silencing LPXN expression by RNA interference on the proliferation and drug sensitivity of human acute monocytic leukemia SHI-1 cells in vitro.

METHODS

Small interfering RNA (siRNA) sequences targeting LPXN were designed and transiently transfected in SHI-1 cells via Lipofectamine 2000, and the most efficient siRNA sequence for LPXN silencing was identified using Western blotting. The protein expression levels of LPXN, p-JNK, p-P38 MAPK and p-ERK were in the cells transfected with the selected siRNA were detected using Western blotting, and the cell proliferation changes were assessed using CCK-8 reagent.

RESULTS

LPXN silencing by siRNA transfection resulted in significant proliferation suppression in SHI-1 cells with an inhibition rate of(27.04±2.05) % (P < 0.05). Western blotting showed that treatment of the siRNA-transfected SHI-1 cells with 0-25 μmol/L curcumin or with 0-2.0 μmol/L Ara-C further increased the cell inhibition rate and obviously enhanced the expressions of p-P38 MAPK and p-JNK without significantly affecting p-ERK expression.

CONCLUSIONS

Down-regulation of LPXN expression by siRNA transfection can suppress the proliferation and increase the drug sensitivity of SHI-1 cells probably by activating JNK and P38 MAPK.

Leupaxin stimulates adhesion and migration of prostate cancer cells through modulation of the phosphorylation status of the actin-binding protein caldesmon

The focal adhesion protein leupaxin (LPXN) is overexpressed in a subset of prostate cancers (PCa) and is involved in the progression of PCa. In the present study, we analyzed the LPXN-mediated adhesive and cytoskeletal changes during PCa progression. We identified an interaction between the actin-binding protein caldesmon (CaD) and LPXN and this interaction is increased during PCa cell migration. Furthermore, knockdown of LPXN did not affect CaD expression but reduced CaD phosphorylation. This is known to destabilize the affinity of CaD to F-actin, leading to dynamic cell structures that enable cell motility. Thus, downregulation of CaD increased migration and invasion of PCa cells. To identify the kinase responsible for the LPXN-mediated phosphorylation of CaD, we used data from an antibody array, which showed decreased expression of TGF-beta-activated kinase 1 (TAK1) after LPXN knockdown in PC-3 PCa cells. Subsequent analyses of the downstream kinases revealed the extracellular signal-regulated kinase (ERK) as an interaction partner of LPXN that facilitates CaD phosphorylation during LPXN-mediated PCa cell migration. In conclusion, we demonstrate that LPXN directly influences cytoskeletal dynamics via interaction with the actin-binding protein CaD and regulates CaD phosphorylation by recruiting ERK to highly dynamic structures within PCa cells.

ETV6-LPXN fusion transcript generated by t(11;12)(q12.1;p13) in a patient with relapsing acute myeloid leukemia with NUP98-HOXA9

ETV6, which encodes an ETS family transcription factor, is frequently rearranged in human leukemias. We show here that a patient with acute myeloid leukemia with t(7;11)(p15;p15) gained, at the time of relapse, t(11;12)(q12.1;p13) with a split ETV6 FISH signal. Using 3'-RACE PCR analysis, we found that ETV6 was fused to LPXN at 11q12.1, which encodes leupaxin. ETV6-LPXN, an in-frame fusion between exon 4 of ETV6 and exon 2 of LPXN, did not transform the interleukin-3-dependent 32D myeloid cell line to cytokine independence; however, an enhanced proliferative response was observed when these cells were treated with G-CSF without inhibition of granulocytic differentiation. The 32D and human leukemia cell lines each transduced with ETV6-LPXN showed enhanced migration towards the chemokine CXCL12. We show here for the first time that LPXN is a fusion partner of ETV6 and present evidence indicating that ETV6-LPXN plays a crucial role in leukemia progression through enhancing the response to G-CSF and CXCL12.

Diverse roles for the paxillin family of proteins in cancer

The paxillin family of intracellular scaffold proteins includes paxillin, Hic-5, and leupaxin, and all have been identified as key regulators of the cellular migration machinery in both 2- and 3-dimensional microenvironments. Herein, we provide insight into the roles of these proteins during tumorigenesis and metastasis, highlighting their functions in cancer initiation as well as tumor cell dissemination and survival. Furthermore, we speculate on the potential of paxillin family proteins as both future prognostic and therapeutic targets.

Focal adhesion kinases in adhesion structures and disease

Cell adhesion to the extracellular matrix (ECM) is essential for cell migration, proliferation, and embryonic development. Cells can contact the ECM through a wide range of matrix contact structures such as focal adhesions, podosomes, and invadopodia. Although they are different in structural design and basic function, they share common remodeling proteins such as integrins, talin, paxillin, and the tyrosine kinases FAK, Pyk2, and Src. In this paper, we compare and contrast the basic organization and role of focal adhesions, podosomes, and invadopodia in different cells. In addition, we discuss the role of the tyrosine kinases, FAK, Pyk2, and Src, which are critical for the function of the different adhesion structures. Finally, we discuss the essential role of these tyrosine kinases from the perspective of human diseases.

Role of protein-tyrosine phosphatases in regulation of osteoclastic activity

Osteoclasts, the primary cell type mediating bone resorption, are multinucleated, giant cells derived from hematopoietic cells of monocyte-macrophage lineage. Osteoclast activity is, in a large part, regulated by protein-tyrosine phosphorylation. While information about functional roles of several protein-tyrosine kinases (PTK), including c-Src, in osteoclastic resorption has been accumulated, little is known about the roles of protein-tyrosine phosphatases (PTPs) in regulation of osteoclast activity. Recent evidence implicates important regulatory roles for four PTPs (SHP-1, cyt-PTP-epsilon, PTP-PEST, and PTPoc) in osteoclasts. Cyt-PTP-epsilon, PTP-PEST, and PTP-oc are positive regulators of osteoclast activity, while SHP-1 is a negative regulator. Of these PTPs in osteoclasts, only PTP-oc is a positive regulator of c-Src PTK through dephosphorylation of the inhibitory phosphotyrosine-527 residue. Although some information about mechanisms of action of these PTPs to regulate osteoclast activity is reviewed in this article, much additional work is required to provide more comprehensive details about their functions in osteoclasts.

Targeting Src signaling in metastatic bone disease

Src is a tyrosine kinase involved in the regulation of a range of cellular processes including proliferation, adhesion, motility and survival. In addition, it is a key regulator of bone metabolism. Src has been implicated in the pathogenesis of a number of cancers, and has been found to be overexpressed in breast, prostate, colorectal, pancreatic and nonsmall-cell lung tumors. There is also evidence that aberrant Src signaling may contribute to the increased osteoclastic activity associated with bone metastases. Bone metastases frequently occur in cancer patients with advanced disease. The metastasized cells disrupt normal bone remodeling pathways resulting in the release of growth factors that further promote tumor growth. Thus, a cycle of metastatic bone destruction is initiated, leading to compromised skeletal integrity and substantially reduced quality of life. Because of the role of Src in both cancer development and in bone metabolism, it may provide a therapeutic target for patients with bone metastases.

The LIM protein leupaxin is enriched in smooth muscle and functions as an serum response factor cofactor to induce smooth muscle cell gene transcription

Leupaxin is a LIM domain-containing adapter protein belonging to the paxillin family that has been previously reported to be preferentially expressed in hematopoietic cells. Herein, we identified leupaxin in a screen for focal adhesion kinase binding partners in aortic smooth muscle, and we show that leupaxin is enriched in human and mouse vascular smooth muscle and that leupaxin expression is dynamically regulated during development. In addition, our studies reveal that leupaxin can undergo cytoplasmic/nuclear shuttling and functions as an serum response factor cofactor in the nucleus. We found that leupaxin forms a complex with serum response factor and associates with CArG-containing regions of smooth muscle promoters and that ectopic expression of leupaxin induces smooth muscle marker gene expression in both 10T1/2 cells and rat aortic smooth muscle cells. Subsequent studies indicated that enhanced focal adhesion kinase activity (induced by fibronectin or expression of constitutively active focal adhesion kinase) attenuates the nuclear accumulation of leupaxin and limits the ability of leupaxin to enhance serum response factor-dependent gene transcription. Thus, these studies indicate that modulation of the subcellular localization of serum response factor cofactors is 1 mechanism by which extracellular matrix-dependent signals may regulate phenotypic switching of smooth muscle cells.

Leupaxin negatively regulates B cell receptor signaling

The role of the paxillin superfamily of adaptor proteins in B cell antigen receptor (BCR) signaling has not been studied previously. We show here that leupaxin (LPXN), a member of this family, was tyrosine-phosphorylated and recruited to the plasma membrane of human BJAB lymphoma cells upon BCR stimulation and that it interacted with Lyn (a critical Src family tyrosine kinase in BCR signaling) in a BCR-induced manner. LPXN contains four leucine-rich sequences termed LD motifs, and serial truncation and specific domain deletion of LPXN indicated that its LD3 domain is involved in the binding of Lyn. Of a total of 11 tyrosine sites in LPXN, we mutated Tyr(22), Tyr(72), Tyr(198), and Tyr(257) to phenylalanine and demonstrated that LPXN was phosphorylated by Lyn only at Tyr(72) and that this tyrosine site is proximal to the LD3 domain. The overexpression of LPXN in mouse A20 B lymphoma cells led to the suppression of BCR-induced activation of JNK, p38 MAPK, and, to a lesser extent, Akt, but not ERK and NFkappaB, suggesting that LPXN can selectively repress BCR signaling. We further show that LPXN suppressed the secretion of interleukin-2 by BCR-activated A20 B cells and that this inhibition was abrogated in the Y72F LPXN mutant, indicating that the phosphorylation of Tyr(72) is critical for the biological function of LPXN. Thus, LPXN plays an inhibitory role in BCR signaling and B cell function.

Interaction of Pyk2 and PTP-PEST with leupaxin in prostate cancer cells

We have identified the presence of leupaxin (LPXN), which belongs to the paxillin extended family of focal adhesion-associated adaptor proteins, in prostate cancer cells. Previous studies have demonstrated that LPXN is a component of the podosomal signaling complex found in osteoclasts, where LPXN was found to associate with the protein tyrosine kinases Pyk2 and c-Src and the cytosolic protein tyrosine phosphatase-proline-, glutamate-, serine-, and threonine-rich sequence (PTP-PEST). In the current study, LPXN was detectable as a 50-kDa protein in PC-3 cells, a bone-derived metastatic prostate cancer cell line. In PC-3 cells, LPXN was also found to associate with Pyk2, c-Src, and PTP-PEST. A siRNA-mediated inhibition of LPXN resulted in decreased in vitro PC-3 cell migration. A recombinant adenoviral-mediated overexpression of LPXN resulted in an increased association of Pyk2 with LPXN, whereas a similar adenoviral-mediated overexpression of PTP-PEST resulted in decreased association of Pyk2 and c-Src with LPXN. The overexpression of LPXN in PC-3 cells resulted in increased migration, as assessed by in vitro Transwell migration assays. On the contrary, the overexpression of PTP-PEST in PC-3 cells resulted in decreased migration. The overexpression of LPXN resulted in increased activity of Rho GTPase, which was decreased in PTP-PEST-overexpressing cells. The increase in Rho GTPase activity following overexpression of LPXN was inhibited in the presence of Y27632, a selective inhibitor of Rho GTPase. In conclusion, our data demonstrate that LPXN forms a signaling complex with Pyk2, c-Src, and PTP-PEST to regulate migration of prostate cancer cells.