Leupaxin binds to PEST domain tyrosine phosphatase PEP

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.

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.

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.

Nonreceptor Protein-Tyrosine Phosphatases in Immune Cell Signaling

Tyrosyl phosphorylation plays a critical role in multiple signaling pathways regulating innate and acquired immunity. Although tyrosyl phosphorylation is a reversible process, we know much more about the functions of protein-tyrosine kinases (PTKs) than about protein-tyrosine phosphatases (PTPs). Genome sequencing efforts have revealed a large and diverse superfamily of PTPs, which can be subdivided into receptor-like (RPTPs) and nonreceptor (NRPTPs). The role of the RPTP CD45 in immune cell signaling is well known, but those of most other PTPs remain poorly understood. Here, we review the mechanism of action, regulation, and physiological functions of NRPTPs in immune cell signaling. Such an analysis indicates that PTPs are as important as PTKs in regulating the immune system.

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.

Association of leupaxin with Src in osteoclasts

Leupaxin (LPXN), which belongs to the paxillin extended family of adaptor proteins, was previously identified as a component of the sealing zone in osteoclasts. LPXN was found to associate with several podosomal proteins, such as the protein tyrosine kinase Pyk2, the protein-tyrosine phosphatase-PEST (PTP-PEST), actin-binding proteins, and regulators of actin cytoskeletal reorganization. It was previously demonstrated that inhibition of LPXN expression resulted in reduced osteoclast-mediated resorption. In the current study, overexpression of LPXN in murine osteoclasts resulted in both enhanced resorptive activity and cell adhesion, as assessed by in vitro resorption assays. The overexpression of LPXN resulted in an increased association of Pyk2 with LPXN. In an attempt to determine an additional biochemical basis for the observed phenomenon in increased osteoclast activity, a coimmunoprecipitation screen for additional binding partners revealed that Src, a protein tyrosine kinase that is critical to both podosome formation and osteoclast function, was also associated with LPXN. After exposure to the pro-inflammatory and osteoclastogenic cytokine TNF-alpha, there was an increase in the level of Src that coimmunoprecipitated with LPXN. Our data indicate that association of the scaffold protein LPXN with Src adds further complexity to the organization of the podosomal signaling complex in osteoclasts.