Interaction between Sam68 and Src family tyrosine kinases, Fyn and Lck, in T cell receptor signaling

A non-catalytic role of IPMK is required for PLCγ1 activation in T cell receptor signaling by stabilizing the PLCγ1-Sam68 complex

BACKGROUND

Phospholipase C gamma 1 (PLCγ1) is an important mediator of the T cell receptor (TCR) and growth factor signaling. PLCγ1 is activated by Src family kinases (SFKs) and produces inositol 1,4,5-triphosphate (InsP3) from phosphatidylinositol 4,5-bisphosphate (PIP2). Inositol polyphosphate multikinase (IPMK) is a pleiotropic enzyme with broad substrate specificity and non-catalytic activities that mediate various functional protein-protein interactions. Therefore, IPMK plays critical functions in key biological events such as cell growth. However, the contribution of IPMK to the activation of PLCγ1 in TCR signaling remains mostly unelucidated. The current study aimed to elucidate the functions of IPMK in TCR signaling and to uncover the mode of IPMK-mediated signaling action in PLCγ1 activation.

METHODS

Concanavalin A (ConA)-induced acute hepatitis model was established in CD4+ T cell-specific IPMK knockout mice (IPMKΔCD4). Histological analysis was performed to assess hepatic injury. Primary cultures of naïve CD4+ T cells were used to uncover the role of mechanisms of IPMK in vitro. Western blot analysis, quantitative real-time PCR, and flow cytometry were performed to analyze the TCR-stimulation-induced PLCγ1 activation and the downstream signaling pathway in naïve CD4+ T cells. Yeast two-hybrid screening and co-immunoprecipitation were conducted to identify the IPMK-binding proteins and protein complexes.

RESULTS

IPMKΔCD4 mice showed alleviated ConA-induced acute hepatitis. CD4+ helper T cells in these mice showed reduced PLCγ1 Y783 phosphorylation, which subsequently dampens calcium signaling and IL-2 production. IPMK was found to contribute to PLCγ1 activation via the direct binding of IPMK to Src-associated substrate during mitosis of 68 kDa (Sam68). Mechanistically, IPMK stabilizes the interaction between Sam68 and to PLCγ1, thereby promoting PLCγ1 phosphorylation. Interfering this IPMK-Sam68 binding interaction with IPMK dominant-negative peptides impaired PLCγ1 phosphorylation.

CONCLUSIONS

Our results demonstrate that IPMK non-catalytically promotes PLCγ1 phosphorylation by stabilizing the PLCγ1-Sam68 complex. Targeting IPMK in CD4+ T cells may be a promising strategy for managing immune diseases caused by excessive stimulation of TCR.

Role of Sam68 as an adaptor protein in inflammatory signaling

Sam68 is a ubiquitously expressed KH-domain containing RNA-binding protein highly studied for its involvement in regulating multiple steps of RNA metabolism. Sam68 also contains multiple protein-protein interaction regions such as proline-rich regions, tyrosine phosphorylation sites, and arginine methylation sites, all of which facilitate its participation as an adaptor protein in multiple signaling pathways, likely independent of its RNA-binding role. This review focuses on providing a comprehensive report on the adaptor roles of Sam68 in inflammatory signaling and inflammatory diseases. The insights presented here have the potential to open new avenues in inflammation research and justify targeting Sam68 to control aberrant inflammatory responses.

Role of c-Src in Carcinogenesis and Drug Resistance

The aberrant transformation of normal cells into cancer cells, known as carcinogenesis, is a complex process involving numerous genetic and molecular alterations in response to innate and environmental stimuli. The Src family kinases (SFK) are key components of signaling pathways implicated in carcinogenesis, with c-Src and its oncogenic counterpart v-Src often playing a significant role. The discovery of c-Src represents a compelling narrative highlighting groundbreaking discoveries and valuable insights into the molecular mechanisms underlying carcinogenesis. Upon oncogenic activation, c-Src activates multiple downstream signaling pathways, including the PI3K-AKT pathway, the Ras-MAPK pathway, the JAK-STAT3 pathway, and the FAK/Paxillin pathway, which are important for cell proliferation, survival, migration, invasion, metastasis, and drug resistance. In this review, we delve into the discovery of c-Src and v-Src, the structure of c-Src, and the molecular mechanisms that activate c-Src. We also focus on the various signaling pathways that c-Src employs to promote oncogenesis and resistance to chemotherapy drugs as well as molecularly targeted agents.

Structure of PD1 and its mechanism in the treatment of autoimmune diseases

PD-1 and CTLA-4 can play an important role in addressing the issue of autoimmune diseases. PD-1 is a transmembrane glycoprotein expressed on T, B, and Dentric cells. This molecule functions as a checkpoint in T cell proliferation. Ligation of PD-1 with its ligands inhibits the production of IL-2, IL-7, IL-10, and IL-12 as well as other cytokines by macrophages, natural killer (NK) cells, and T cells, which can suppress cell proliferation and inflammation. Today, scientists attempt to protect against autoimmune diseases by PD-1 inhibitory signals. In this review, we discuss the structure, expression, and signaling pathway of PD-1. In addition, we discuss the importance of PD-1 in regulating several autoimmune diseases, reflecting how manipulating this molecule can be an effective method in the immunotherapy of some autoimmune diseases.

GRB2 dimerization mediated by SH2 domain-swapping is critical for T cell signaling and cytokine production

GRB2 is an adaptor protein required for facilitating cytoplasmic signaling complexes from a wide array of binding partners. GRB2 has been reported to exist in either a monomeric or dimeric state in crystal and solution. GRB2 dimers are formed by the exchange of protein segments between domains, otherwise known as "domain-swapping". Swapping has been described between SH2 and C-terminal SH3 domains in the full-length structure of GRB2 (SH2/C-SH3 domain-swapped dimer), as well as between α-helixes in isolated GRB2 SH2 domains (SH2/SH2 domain-swapped dimer). Interestingly, SH2/SH2 domain-swapping has not been observed within the full-length protein, nor have the functional influences of this novel oligomeric conformation been explored. We herein generated a model of full-length GRB2 dimer with an SH2/SH2 domain-swapped conformation supported by in-line SEC-MALS-SAXS analyses. This conformation is consistent with the previously reported truncated GRB2 SH2/SH2 domain-swapped dimer but different from the previously reported, full-length SH2/C-terminal SH3 (C-SH3) domain-swapped dimer. Our model is also validated by several novel full-length GRB2 mutants that favor either a monomeric or a dimeric state through mutations within the SH2 domain that abrogate or promote SH2/SH2 domain-swapping. GRB2 knockdown and re-expression of selected monomeric and dimeric mutants in a T cell lymphoma cell line led to notable defects in clustering of the adaptor protein LAT and IL-2 release in response to TCR stimulation. These results mirrored similarly-impaired IL-2 release in GRB2-deficient cells. These studies show that a novel dimeric GRB2 conformation with domain-swapping between SH2 domains and monomer/dimer transitions are critical for GRB2 to facilitate early signaling complexes in human T cells.

Sam68 contributes to intestinal inflammation in experimental and human colitis

Sam68 is an RNA-binding protein with an adaptor role in signal transduction. Our previous work identified critical proinflammatory and apoptotic functions for Sam68, downstream of the TNF/TNFR1 and TLR2/3/4 pathways. Recent studies have shown elevated Sam68 in inflamed tissues from rheumatoid arthritis and ulcerative colitis (UC) patients, suggesting that Sam68 contributes to chronic inflammatory diseases. Here, we hypothesized that deletion of Sam68 is protective against experimental colitis in vivo, via reductions in TNF-associated inflammatory signaling. We used Sam68 knockout (KO) mice to study the role of Sam68 in experimental colitis, including its contributions to TNF-induced inflammatory gene expression in three-dimensional intestinal organoid cultures. We also studied the expression of Sam68 and inflammatory genes in colon tissues of UC patients. Sam68 KO mice treated with an acute course of DSS exhibited significantly less weight loss and histopathological inflammation compared to wild-type controls, suggesting that Sam68 contributes to experimental colitis. Bone marrow transplants showed no pathologic role for hematopoietic cell-specific Sam68, suggesting that non-hematopoietic Sam68 drives intestinal inflammation. Gene expression analyses showed that Sam68 deficiency reduced the expression of proinflammatory genes in colon tissues from DSS-treated mice, as well as TNF-treated three-dimensional colonic organoids. We also found that inflammatory genes, such as TNF, CCR2, CSF2, IL33 and CXCL10, as well as Sam68 protein, were upregulated in inflamed colon tissues of UC patients. This report identifies Sam68 as an important inflammatory driver in response to intestinal epithelial damage, suggesting that targeting Sam68 may hold promise to treat UC patients.

Prunin suppresses viral IRES activity and is a potential candidate for treating enterovirus A71 infection

Human enterovirus A71 (HEVA71) causes hand, foot, and mouth disease (HFMD) in young children and is considered a major neurotropic pathogen but lacks effective antivirals. To identify potential therapeutic agents against HFMD, we screened a 502-compound flavonoid library for compounds targeting the HEVA71 internal ribosome entry site (IRES) that facilitates translation of the HEVA71 genome and is vital for the production of HEVA71 viral particles. We validated hits using cell viability and viral plaque assays and found that prunin was the most potent inhibitor of HEVA71. Downstream assays affirmed that prunin disrupted viral protein and RNA synthesis and acted as a narrow-spectrum antiviral against enteroviruses A and B, but not enterovirus C, rhinovirus A, herpes simplex 1, or chikungunya virus. Continuous HEVA71 passaging with prunin yielded HEVA71-resistant mutants with five mutations that mapped to the viral IRES. Knockdown studies showed that the mutations allowed HEVA71 to overcome treatment-induced suppression by differentially regulating recruitment of the IRES trans-acting factors Sam68 and hnRNPK without affecting the hnRNPA1-IRES interaction required for IRES translation. Furthermore, prunin effectively reduced HEVA71-associated clinical symptoms and mortality in HEVA71-infected BALB/c mice and suppressed hepatitis C virus at higher concentrations, suggesting a similar mechanism of prunin-mediated IRES inhibition for both viruses. These studies establish prunin as a candidate for further development as a HEVA71 therapeutic agent.

The RNA binding protein Sam68 controls T helper 1 differentiation and anti-mycobacterial response through modulation of miR-29

Polarization of naive T cells into interferon (IFN)-γ-producing T helper 1 (Th1) cells is an essential event in the inflammatory response to pathogens. Herein, we identify the RNA binding protein Sam68 as a specific modulator of Th1 differentiation. Sam68-knockout (ko) naive T cells are strongly defective in IL-12-mediated Th1 polarization and express low levels of T-bet and Eomes. Consequently, Sam68-ko Th1 cells are significantly impaired in IFN-γ production. Moreover, we found that Sam68 is required for the induction of an inflammatory Th1 response during Mycobacterium bovis Bacillus Calmette-Guerin (BCG) infection, thus limiting bacterial dissemination in the lungs. Mechanistically, Sam68 directly binds to the microRNA miR-29, a negative regulator of Th1 response, and inhibits its expression during BCG infection. These findings uncover a novel post-transcriptional mechanism required for the Th1-mediated defense against intracellular pathogens and identify a new function for Sam68 in the regulation of the immune response.

Design, Synthesis, and Biological Evaluation of Novel 1,3,4-Thiadiazole Derivatives as Potential Antitumor Agents against Chronic Myelogenous Leukemia: Striking Effect of Nitrothiazole Moiety

In an attempt to develop potent antitumor agents, new 1,3,4-thiadiazole derivatives were synthesized and evaluated for their cytotoxic effects on multiple human cancer cell lines, including the K562 chronic myelogenous leukemia cell line that expresses the Bcr-Abl tyrosine kinase. N-(5-Nitrothiazol-2-yl)-2-((5-((4-(trifluoromethyl)phenyl)amino)-1,3,4-thiadiazol-2-yl)thio)acetamide (2) inhibited the Abl protein kinase with an IC₅₀ value of 7.4 µM and showed selective activity against the Bcr-Abl positive K562 cell line. Furthermore, a Bcr-Abl-compound 2 molecular modelling simulation highlighted the anchoring role of the nitrothiazole moiety in bonding and hydrophobic interaction with the key amino acid residues. These results provide promising starting points for further development of novel kinase inhibitors.

Sam68 Mediates the Activation of Insulin and Leptin Signalling in Breast Cancer Cells

Obesity is a well-known risk factor for breast cancer development in postmenopausal women. High insulin and leptin levels seem to have a role modulating the growth of these tumours. Sam68 is an RNA-binding protein with signalling functions that has been found to be overexpressed in breast cancer. Moreover, Sam68 may be recruited to insulin and leptin signalling pathways, mediating its effects on survival, growth and proliferation in different cellular types. We aimed to study the expression of Sam68 and its phosphorylation level upon insulin and leptin stimulation, and the role of Sam68 in the proliferative effect and signalling pathways that are activated by insulin or leptin in human breast adenocarcinoma cells. In the human breast adenocarcinoma cell lines MCF7, MDA-MB-231 and BT-474, Sam68 protein quantity and gene expression were increased upon leptin or insulin stimulation, as it was checked by qPCR and immunoblot. Moreover, both insulin and leptin stimulation promoted an increase in Sam68 tyrosine phosphorylation and negatively regulated its RNA binding capacity. siRNA was used to downregulate Sam68 expression, which resulted in lower proliferative effects of both insulin and leptin, as well as a lower activation of MAPK and PI3K pathways promoted by both hormones. These effects may be partly explained by the decrease in IRS-1 expression by down-regulation of Sam68. These results suggest the participation of Sam68 in both leptin and insulin receptor signaling in human breast cancer cells, mediating the trophic effects of these hormones in proliferation and cellular growth.

The nuclear protein Sam68 is recruited to the cytoplasmic stress granules during enterovirus 71 infection

Our previous study found that the nuclear protein, 68-kDa Src-associated in mitosis protein (Sam68), is translocated to the cytoplasm and forms punctate pattern during enterovirus 71 (EV71) infection [Virus Research, 180 (2014), 1-11]. However, the exact function of this punctate pattern in cytoplasm during EV71 infection remains unknown. In this study, we firstly have examined this punctate pattern of Sam68 re-localization in the cytoplasm, and observed the obvious recruitments of Sam68 to the EV71-induced stress granules (SGs). Sam68, belongs to the KH domain family of RNA binding proteins (RBPs), was then confirmed that its KH domain was essential for this recruitment. Nevertheless, Knockdown of Sam68 expression using ShRNA had no effects on SGs assembly, indicating that Sam68 is not a constitutive component of the SGs during EV71 infection. Lastly, we investigated the importance of microtubulin transport to SGs aggregation, and revealed that microtubule depolymerization inhibited SGs formation, suggesting that EV71-induced SGs move throughout the cytoplasm in a microtubule-dependent manner. Taken together, these results illuminated that EV71 infections can induce SGs formation, and Sam68, as a SGs component, migrates alone with SGs dependent on intact microtubule upon the viral infections. These findings may provide novel underlying mechanism for delineating the role of SGs during EV71 infection.

EMT-Inducing Molecular Factors in Gynecological Cancers

Gynecologic cancers are the unregulated growth of neoplastic cells that arise in the cervix, ovaries, fallopian tubes, uterus, vagina, and vulva. Although gynecologic cancers are characterized by different signs and symptoms, studies have shown that they share common risk factors, such as smoking, obesity, age, exposure to certain chemicals, infection with human immunodeficiency virus (HIV), and infection with human papilloma virus (HPV). Despite recent advancements in the preventative, diagnostic, and therapeutic interventions for gynecologic cancers, many patients still die as a result of metastasis and recurrence. Since mounting evidence indicates that the epithelial-mesenchymal transition (EMT) process plays an essential role in metastatic relapse of cancer, understanding the molecular aberrations responsible for the EMT and its underlying signaling should be given high priority in order to reduce cancer morbidity and mortality.

Role of Sam68 in post-transcriptional gene regulation

The STAR family of proteins links signaling pathways to various aspects of post-transcriptional regulation and processing of RNAs. Sam68 belongs to this class of heteronuclear ribonucleoprotein particle K (hnRNP K) homology (KH) single domain-containing family of RNA-binding proteins that also contains some domains predicted to bind critical components in signal transduction pathways. In response to phosphorylation and other post-transcriptional modifications, Sam68 has been shown to have the ability to link signal transduction pathways to downstream effects regulating RNA metabolism, including transcription, alternative splicing or RNA transport. In addition to its function as a docking protein in some signaling pathways, this prototypic STAR protein has been identified to have a nuclear localization and to take part in the formation of both nuclear and cytosolic multi-molecular complexes such as Sam68 nuclear bodies and stress granules. Coupling with other proteins and RNA targets, Sam68 may play a role in the regulation of differential expression and mRNA processing and translation according to internal and external signals, thus mediating important physiological functions, such as cell death, proliferation or cell differentiation.

Role of adipokines signaling in the modulation of T cells function

The field that links immunity and metabolism is rapidly expanding. Apparently non-immunological disorders such as obesity and type 2 diabetes have been linked to immune dysregulation, suggesting that metabolic alterations can be induced by or be consequence of an altered self-immune tolerance. In this context, adipose tissue produces and releases a variety of pro-inflammatory and anti-inflammatory factors, termed "adipokines," which can be considered as the bridge between obesity-related exogenous factors, such as nutrition and lifestyle, and the molecular events leading to metabolic syndrome, inflammatory, and/or autoimmune conditions. In obesity, increased production of most adipokines impacts on multiple functions such as appetite and energy balance, modulation of immune responses, insulin sensitivity, angiogenesis, blood pressure, lipid metabolism, and so on. This report aims to discuss some of the recent topics of adipocytokine research and their related signaling pathways, that may be of particular importance as could lead to effective therapeutic strategies for obesity-associated diseases.

High expression level and nuclear localization of Sam68 are associated with progression and poor prognosis in colorectal cancer

BACKGROUND

Src-associated in mitosis (Sam68; 68 kDa) has been implicated in the oncogenesis and progression of several human cancers. The aim of this study was to investigate the clinicopathologic significance of Sam68 expression and its subcellular localization in colorectal cancer (CRC).

METHODS

Sam68 expression was examined in CRC cell lines, nine matched CRC tissues and adjacent noncancerous tissues using reverse transcription (RT)-PCR, quantitative RT-PCR and Western blotting. Sam68 protein expression and localization were determined in 224 paraffin-embedded archived CRC samples using immunohistochemistry. Statistical analyses were applied to evaluate the clinicopathologic significance.

RESULTS

Sam68 was upregulated in CRC cell lines and CRC, as compared with normal tissues; high Sam68 expression was detected in 120/224 (53.6%) of the CRC tissues. High Sam68 expression correlated significantly with poor differentiation (P = 0.033), advanced T stage (P < 0.001), N stage (P = 0.023) and distant metastasis (P = 0.033). Sam68 nuclear localization correlated significantly with poor differentiation (P = 0.002) and T stage (P =0.021). Patients with high Sam68 expression or Sam68 nuclear localization had poorer overall survival than patients with low Sam68 expression or Sam68 cytoplasmic localization. Patients with high Sam68 expression had a higher risk of recurrence than those with low Sam68 expression.

CONCLUSIONS

Overexpression of Sam68 correlated highly with cancer progression and poor differentiation in CRC. High Sam68 expression and Sam68 nuclear localization were associated with poorer overall survival.

Expression and activity of Fyn mediate proliferation and blastic features of chronic myelogenous leukemia

The BCR-ABL1 oncogene is a tyrosine kinase that activates many signaling pathways, resulting in the induction of chronic myeloid leukemia (CML). Kinase inhibitors, such as imatinib, have been developed for the treatment of CML; however, the terminal, blast crisis phase of the disease remains a clinical challenge. Blast crisis CML is difficult to treat due to resistance to tyrosine kinase inhibitors, increased genomic instability and acquired secondary mutations. Our recent studies uncovered a role for Fyn in promoting BCR-ABL1 mediated cell growth and sensitivity to imatinib. Here we demonstrate that Fyn contributes to BCR-ABL1 induced genomic instability, a feature of blast crisis CML. Bone marrow cells and mouse embryonic fibroblasts derived from Fyn knockout mice transduced with BCR-ABL1 display slowed growth and clonogenic potential as compared to Fyn wild-type BCR-ABL1 expressing counterparts. K562 cells overexpressing constitutively active Fyn kinase were larger in size and displayed an accumulation of genomic abnormalities such as chromosomal aberrations and polyploidy. Importantly, loss of Fyn protected mouse embryonic fibroblast cells from increased number of chromosomal aberrations and fragments induced by BCR-ABL1. Together, these results reveal a novel role for Fyn in regulating events required for genomic maintenance and suggest that Fyn kinase activity plays a role in the progression of CML to blast crisis.

Sam68 guest STARs in TNF-α signaling

In this issue of Molecular Cell, Ramakrishnan and Baltimore (2011) identify Sam68 as a TNFR1 docking protein required for proper activation of TNF-α signaling.

The RNA-binding protein Sam68 is a multifunctional player in human cancer

Src associated in mitosis, of 68 kDa (Sam68) is a KH domain RNA-binding protein that belongs to the signal transduction and activation of RNA family. Although ubiquitously expressed, Sam68 plays very specialized roles in different cellular environments. In most cells, Sam68 resides in the nucleus and is involved in several steps of mRNA processing, from transcription, to alternative splicing, to nuclear export. In addition, Sam68 translocates to the cytoplasm upon cell stimulation, cell cycle transitions or viral infections, where it takes part to signaling complexes and associates with the mRNA translation machinery. Recent evidence has linked Sam68 function to the onset and progression of endocrine tumors, such as prostate and breast carcinomas. Notably, all the biochemical activities reported for Sam68 seem to be implicated in carcinogenesis. Herein, we review the recent advancement in the knowledge of Sam68 function and regulation and discuss it in the frame of its participation to neoplastic transformation and tumor progression.

Sam68 is required for both NF-κB activation and apoptosis signaling by the TNF receptor

The RNA-binding protein Sam68 is implicated in various cellular processes including RNA metabolism, apoptosis, and signal transduction. Here we identify a role of Sam68 in TNF-induced NF-κB activation and apoptosis. We found that Sam68 is recruited to the TNF receptor, and its deficiency dramatically reduces RIP recruitment and ubiquitylation. It also impairs cIAP1 recruitment and maintenance of recruited TRAF2 at the TNF receptor. In its absence, activation of the TAK1-IKK kinase complex is defective, greatly reducing signal transduction. Sam68 is also found as a part of the TNF-induced cytoplasmic caspase-8-FADD complex. RIP is not recruited to this complex in Sam68 knockout cells, and caspase activation is virtually absent. These findings delineate previously unknown functions for Sam68 in the TNF signaling pathway, where it acts as a signaling adaptor both in the membrane-associated complex I and in the cytoplasmic complex II, regulating both NF-κB activation and apoptosis.

Sam68 mediates leptin-stimulated growth by modulating leptin receptor signaling in human trophoblastic JEG-3 cells

BACKGROUND

Sam68, a member of the signal transduction and activation of RNA metabolism (STAR) family of RNA-binding proteins, has been previously implicated as an adaptor molecule in different signaling systems, including leptin receptor (LEPR) signaling. LEPR activation is known to stimulate JAK-STAT, MAPK and PI3K signaling pathways, thus mediating the biological effects of leptin in different cell types, including trophoblastic cells. We have recently found that leptin stimulation also promotes the overexpression and tyrosine phosphorylation of Sam68 in human trophoblastic JEG-3 cells, suggesting a role for Sam68 in leptin signaling and action in these cells. In the present work, we have studied the participation of Sam68 in the main signaling pathways activated by LEPR to increase growth and proliferation in trophoblastic JEG-3 cells.

METHODS

We used an antisense strategy to down-regulate Sam68 expression in these cells, and we studied LEPR signaling by immunoprecipitation and poly-U affinity precipitation and by analyzing phosphorylation levels of signaling proteins by immunoblot. The effect of leptin on protein synthesis and proliferation was studied by ³[H]-leucine and ³[H]-thymidine incorporation.

RESULTS

Sam68 knockdown impaired leptin activation of JAK-STAT, PI3K and MAPK signaling pathways in JEG-3 cells. We have also found that leptin-stimulated Sam68 tyrosine phosphorylation is dependent on JAK-2 activity, since the pharmacological inhibitor AG490 prevents the phosphorylation of Sam68 in JEG-3 cells. Finally, the trophic and proliferative effect of leptin in trophoblastic cells is dependent on Sam68 expression, since its down-regulation impaired the leptin-stimulated DNA and protein synthesis.

CONCLUSIONS

These data demonstrate that Sam68 participates in the main signaling pathways of LEPR to mediate the trophic and proliferative effect of leptin in human trophoblastic cells.

Regulation of SRC family kinases in human cancers

The nonreceptor protein tyrosine kinase Src plays a crucial role in the signal transduction pathways involved in cell division, motility, adhesion, and survival in both normal and cancer cells. Although the Src family kinases (SFKs) are activated in various types of cancers, the exact mechanisms through which they contribute to the progression of individual tumors remain to be defined. The activation of Src in human cancers may occur through a variety of mechanisms that include domain interaction and structural remodeling in response to various activators or upstream kinases and phosphatastes. Because of Src's prominent roles in invasion and tumor progression, epithelial-to-mesenchymal transition, angiogenesis, and the development of metastasis, Src is a promising target for cancer therapy. Several small molecule inhibitors of Src are currently being investigated in clinical trials. In this article, we will summarize the mechanisms regulating Src kinase activity in normal and cancer cells and discuss the status of Src inhibitor development against various types of cancers.

T cell receptor (TCR)-induced tyrosine phosphorylation dynamics identifies THEMIS as a new TCR signalosome component

Stimulation of the T cell antigen receptor (TCR) induces formation of a phosphorylation-dependent signaling network via multiprotein complexes, whose compositions and dynamics are incompletely understood. Using stable isotope labeling by amino acids in cell culture (SILAC)-based quantitative proteomics, we investigated the kinetics of signal propagation after TCR-induced protein tyrosine phosphorylation. We confidently assigned 77 proteins (of 758 identified) as a direct or indirect consequence of tyrosine phosphorylation that proceeds in successive "signaling waves" revealing the temporal pace at which tyrosine kinases activate cellular functions. The first wave includes thymocyte-expressed molecule involved in selection (THEMIS), a protein recently implicated in thymocyte development but whose signaling role is unclear. We found that tyrosine phosphorylation of THEMIS depends on the presence of the scaffold proteins Linker for activation of T cells (LAT) and SH2 domain-containing lymphocyte protein of 76 kDa (SLP-76). THEMIS associates with LAT, presumably via the adapter growth factor receptor-bound protein 2 (Grb2) and with phospholipase Cγ1 (PLC-γ1). RNAi-mediated THEMIS knock-down inhibited TCR-induced IL-2 gene expression due to reduced ERK and nuclear factor of activated T cells (NFAT)/activator protein 1 (AP-1) signaling, whereas JNK, p38, or nuclear factor κB (NF-κB) activation were unaffected. Our study reveals the dynamics of TCR-dependent signaling networks and suggests a specific role for THEMIS in early TCR signalosome function.

Post-translational regulation of star proteins and effects on their biological functions

STAR (Signal Transduction and Activation of RNA) proteins owed their name to the presence in their structure ofa RNA-binding domain and several hallmarks of their involvement in signal transduction pathways. In many members of the family, the STAR RNA-binding domain (also named GSG, an acronym for GRP33/Sam68/ GLD-1) is flanked by regulatory regions containing proline-rich sequences, which serve as docking sites for proteins containing SH3 and WW domains and also a tyrosine-rich region at the C-terminus, which can mediateprotein-protein interactions with partners through SH2 domains. These regulatory regions contain consensus sequences for additional modifications, including serine/threonine phosphorylation, methylation, acetylation and sumoylation. Since their initial description, evidence has been gathered in different cell types and model organisms that STAR proteins can indeed integrate signals from external and internal cues with changes in transcription and processing of target RNAs. The most striking example of the high versatility of STAR proteins is provided by Sam68 (KHDRBS1), whose function, subcellular localization and affinity for RNA are strongly modulated by several signaling pathways through specific modifications. Moreover, the recent development of genetic knockout models has unveiled the physiological function of some STAR proteins, pointing to a crucial role of their post-translational modifications in the biological processes regulated by these RNA-binding proteins. This chapter offers an overview of the most updated literature on the regulation of STAR proteins by post-translational modifications and illustrates examples of how signal transduction pathways can modulate their activity and affect biological processes.

Leptin signaling: A key pathway in immune responses

Leptin is a hormone whose central role is to regulate endocrine functions and to control energy expenditure. After the discovery that leptin can also have pro-inflammatory effects, several studies have tried to address - at the molecular level - the pathways involved in leptin-induced modulation of the immune functions in normal and pathologic conditions. The signaling events influenced by leptin after its binding to the leptin receptor have been under scrutiny in the past few years, and considerable experimental work has elucidated the consequences of leptin effects on immune cells. This review examines the biochemistry, function and regulation of leptin signaling in view of possible intervention on this molecule for a better management and therapy of immune-mediated diseases.

A novel negative regulatory function of the phosphoprotein associated with glycosphingolipid-enriched microdomains: blocking Ras activation

In primary human T cells, anergy induction results in enhanced p59Fyn activity. Because Fyn is the kinase primarily responsible for the phosphorylation of PAG (the phosphoprotein associated with glycosphingolipid-enriched microdomains), which negatively regulates Src-kinase activity by recruiting Csk (the C-terminal Src kinase) to the membrane, we investigated whether anergy induction also affects PAG. Analysis of anergic T cells revealed that PAG is hyperphosphorylated at the Csk binding site, leading to enhanced Csk recruitment and inhibitory tyrosine phosphorylation within Fyn. This together with enhanced phosphorylation of a tyrosine within the SH2 domain of Fyn leads to the formation of a hyperactive conformation, thus explaining the enhanced Fyn kinase activity. In addition, we have also identified the formation of a multiprotein complex containing PAG, Fyn, Sam68, and RasGAP in stimulated T cells. We demonstrate that PAG-Fyn overexpression is sufficient to suppress Ras activation in Jurkat T cells and show that this activity is independent of Csk binding. Thus, in addition to negatively regulating Src family kinases by recruiting Csk, PAG also negatively regulates Ras by recruiting RasGAP to the membrane. Finally, by knocking down PAG, we demonstrate both enhanced Src kinase activity and Ras activation, thereby establishing PAG as an important negative regulator of T-cell activation.

Insights into human Lck SH3 domain binding specificity: different binding modes of artificial and native ligands

We analyzed the ligand binding specificity of the lymphocyte specific kinase (Lck) SH3 domain. We identified artificial Lck SH3 ligands using phage display. In addition, we analyzed Lck SH3 binding sites within known natural Lck SH3 binding proteins using an Lck specific binding assay on membrane-immobilized synthetic peptides. On one hand, from the phage-selected peptides, representing mostly special class I' ligands, a well-defined consensus sequence was obtained. Interestingly, a histidine outside the central polyproline motif contributes significantly to Lck SH3 binding affinity and specificity. On the other hand, we confirmed previously mapped Lck SH3 binding sites in ADAM15, HS1, SLP76, and NS5A, and identified putative Lck SH3 binding sites of Sam68, FasL, c-Cbl, and Cbl-b. Without exception, the comparatively diverse Lck SH3 binding sites of all analyzed natural Lck SH3 binding proteins emerged as class II proteins. Possible explanations for the observed variations between artificial and native ligands-which are not due to significant K(D) value differences as shown by calculating Lck SH3 affinities of artificial peptide PD1-Y(-3)R as well as for peptides comprising putative Lck SH3 binding sites of NS5A, Sos, and Sam68-are discussed. Our data suggest that phage display, a popular tool for determining SH3 binding specificity, must-at least in the case of Lck-not irrevocably mirror physiologically relevant protein-ligand interactions.

Regulation of Sam68 activity by small heat shock protein 22

Sam68 associates with c-Src kinase during mitosis. We previously demonstrated that Sam68 functionally replaces and/or synergizes with HIV-1 Rev in rev response element (RRE)-mediated gene expression and virus production. Furthermore, we reported that knockdown of Sam68 inhibited Rev-mediated RNA export and it is absolutely required for HIV-1 production. In the present study, we identified small heat shock protein, hsp22, as a novel interacting partner of Sam68. Hsp22 binds to Sam68 in vitro and in vivo. Overexpression of hsp22 significantly inhibits Sam68-mediated RRE- as well as CTE (constitutive transport element)-dependent reporter gene expression. Furthermore, exposing 293T cells to heat shock inhibits Sam68/RRE function by virtue of elevating hsp22. The critical domain of hsp22 that interacts with Sam68 resides between amino acids 62 and 133. Our studies provide evidence for the first time that hsp22 specifically binds to Sam68 and modulates its activity, thus playing a role in the post-transcriptional regulation of gene expression.

A rare mRNA variant of the human lymphocyte-specific protein tyrosine kinase LCK gene with intron B retention and exon 7 skipping encodes a putative protein with altered SH3-dependent molecular interactions

A rare mRNA variant of the human lymphocyte-specific protein tyrosine kinase LCK gene that retains intron B and excludes exon 7 (B+7-) due to alternative splicing of the canonical LCK transcripts was identified and characterized. LCK B+7- mRNA is detected in all tested peripheral blood T lymphocytes total RNA samples but is apparently sequestered in the nucleus. The presence of intron B sequence does not disrupt the reading frame and results in the insertion of 58 aminoacids, containing a proline-rich region just upstream of p56lck SH3 domain. This putative isoform encodes an unstable 516 aminoacids protein (LckB+7-) which can be expressed in transfected COS-7 cells. Furthermore in Jurkat T cell extracts, a recombinant intron B plus SH3 p56lck domain fails to interact with some TCR-induced tyrosine phosphorylated polypeptides and known p56lck partners such as Sam68 and c-Cbl. The biological function of this rare messenger remains to be elucidated.

Sam68 is tyrosine phosphorylated and recruited to signalling in peripheral blood mononuclear cells from HIV infected patients

Human immunodeficiency virus (HIV) codes for a protein, Rev, that mediates the viral RNA export from the nucleus to the cytoplasm. Recently, it has been found that Sam68, the substrate of Src associated in mitosis, is a functional homologue of Rev, and a synergistic activator of Rev activity. Thus, it has been suggested that Sam68 may play an important role in the post-transcriptional regulation of HIV. Sam68 contains an RNA binding motif named KH [homology to the nuclear ribonucleoprotein (hnRNP) K]. Tyrosine phosphorylation of Sam68 and binding to SH3 domains have been found to negatively regulate its RNA binding capacity. Besides, tyrosine phosphorylation of Sam68 allows the formation of signalling complexes with other proteins containing SH2 and SH3 domains, suggesting a role in signal transduction of different systems in human lymphocytes, such as the T cell receptor, and leptin receptor, or the insulin receptor in other cell types. In the present work, we have found that Sam68 is tyrosine phosphorylated in peripheral blood mononuclear cells (PBMC) from HIV infected subjects, leading to the formation of signalling complexes with p85 the regulatory subunit of PI3K, GAP and STAT-3, and decreasing its RNA binding capacity. In contrast, PBMC from HIV infected subjects have lower expression levels of Sam68 compared with controls. These results suggest that Sam68 may play some role in the immune function of lymphocytes in HIV infection.

The role of potential splicing factors including RBMY, RBMX, hnRNPG-T and STAR proteins in spermatogenesis

Investigations into the RBM gene family are uncovering networks of protein interactions which regulate RNA processing, and which might operate downstream of signal transduction pathways. Similar pathways likely operate in germ cells and somatic cells, with RBMY, hnRNPGT and T-STAR proteins providing germ cell-specific components. These pathways may be important for normal germ cell development, and might be compromised in men with Y chromosome deletions affecting RBMY gene expression. The STAR proteins have multiple functions in pre-mRNA splicing, signalling and cell cycle control. These processes might have to be very finely regulated during germ cell development, which involves both two sequential meiotic divisions (meiosis I and II) as well as mitotic (spermatogonial) cell divisions, and which is controlled by paracrine signalling within the testis from Sertoli cells.

Expression of a truncated form of the c-Kit tyrosine kinase receptor and activation of Src kinase in human prostatic cancer

A truncated form of the c-Kit tyrosine kinase receptor, originally identified in mouse haploid germ cells, is aberrantly expressed in human cancer cell lines of various origin. This alternative transcript originates in the 15th intron of the human c-kit gene. We have previously demonstrated that sperm-carried mouse truncated c-Kit (tr-Kit) is a strong activator of the Src-family tyrosine kinases both in transfected cells and in mouse oocytes. In the present work, we report that human tr-Kit mRNA and protein are expressed in LNCaP prostatic cancer cells. We have identified two regions in the 15th and 16th introns of the human c-kit gene that show homology with sequences in the spermatid-specific tr-Kit promoter within the 16th intron of mouse c-kit. We also show that nuclear factors present in LNCaP cells bind to discrete sequences of the mouse tr-Kit promoter. Moreover, Western blot analysis of 23 primary prostate cancers indicated that tr-Kit was expressed in approximately 28% of the tumors at less advanced stages (Gleason grade 4 to 6) and in 66% of those at more advanced stages (Gleason grade 7 to 9), whereas it was not expressed in benign prostatic hypertrophies. Sequencing of the cDNA for the truncated c-Kit, amplified from both LNCaP cells and neoplastic tissues, confirmed the existence in prostate cancer cells of a transcript arising from the 15th intron of human c-kit. We also show that tr-Kit-expressing LNCaP cells and prostatic tumors have higher levels of phosphorylated/activated Src than tr-Kit-negative PC3 cells or prostatic tumors, and that transfection of tr-Kit in PC3 cells caused a dramatic increase in Src activity. Interestingly, we found that Sam68, a RNA-binding protein phosphorylated by Src in mitosis, is phosphorylated only in prostate tumors expressing tr-Kit. Indeed, both activation of Src and phosphorylation of Sam68 were observed in all of the three grade 7 to 9 tumors analyzed that expressed tr-Kit. Our data describe for the first time the existence of a truncated c-Kit protein in primary tumors and show a correlation between tr-Kit expression and activation of the Src pathway in the advanced stages of the disease. Thus, these results might pave the way for the elucidation of a novel pathway in neoplastic transformation of prostate cells.

Depolarization-induced translocation of the RNA-binding protein Sam68 to the dendrites of hippocampal neurons

The traffic and expression of mRNAs in neurons are modulated by changes in neuronal activity. The regulation of neuronal RNA-binding proteins is therefore currently receiving attention. Sam68 is a ubiquitous nuclear RNA-binding protein implicated in post-transcriptional processes such as signal-dependent splice site selection. We show that Sam68 undergoes activity-responsive translocation to the soma and dendrites of hippocampal neurons in primary culture. In unstimulated neurons transiently expressing a GFP-Sam68 fusion protein, 90% of the cells accumulated the protein exclusively in the nucleus, and 4% showed extension of GFP-Sam68 to the dendrites. This nuclear expression pattern required the integrity of the Sam68 N-terminus. When present, the dendritic GFP-Sam68 formed granules, 26% of which were colocalized with ethidium bromide-stained RNA clusters. Most of the GFP-Sam68 granules were completely stationary, but a few moved in either a retrograde or anterograde direction. Following depolarization by 25 mM KCl, 50% of neurons displayed dendritic GFP-Sam68. GFP-Sam68 invaded the dendrites after 2 hours with high KCl, and returned to the nucleus within 3 hours after termination of the KCl treatment. A control GFP fusion derived from the SC-35 splicing factor remained fully nuclear during depolarization. No significant change was observed in the phosphorylation of Sam68 after depolarization. Translocation of Sam68 to the distal dendrites was microtubule dependent. Blockade of calcium channels with nimodipine abolished the translocation. Furthermore, inhibition of CRM-1-mediated nuclear export by leptomycin B partially prevented the depolarization-induced nuclear efflux of GFP-Sam68. These results support the possible involvement of Sam68 in the activity-dependent regulation of dendritic mRNAs.

Somatodendritic localization and mRNA association of the splicing regulatory protein Sam68 in the hippocampus and cortex

The RNA-binding protein Sam68 has been implicated in the signal-dependent processing of pre-mRNA and in the utilization of intron-containing retroviral mRNAs. Sam68 is predominantly nuclear but exhibits remarkable binding affinity for signalling proteins located at the membrane. We have investigated the subcellular distribution of Sam68 in adult rat cortex and hippocampus. Subcellular fractionation showed that the protein was most abundant in nuclei but also was present at a significant level in the cytosol and membrane fractions, including light and synaptic membranes derived from crude synaptosomes. Sam68 extracted from the synaptosomal fraction cosedimented with polysomes on sucrose gradients. In agreement with these findings, immunohistochemical staining indicated that Sam68 was concentrated in neuronal nuclei but was also detectable in the soma and dendrites. Sam68 immunoreactivity examined at the ultrastructural level was found to associate with dendritic microtubules, endoplasmic reticulum, and free polyribosomes, sometimes close to synapses. A combination of immunoprecipitation and RT-PCR directly confirmed that Sam68 was bound to polyadenylated mRNA in cortical lysates. The alphaCaMKII mRNA was identified as one of the coprecipitated transcripts; in contrast, the gephyrin and NR1-1 mRNAs were not coprecipitated, indicating a certain degree of sequence specificity in the association. In electrophoretic mobility shift assays, recombinant GST-Sam68 as well as brain-derived Sam68 bound with high affinity to the alphaCaMKII 3' untranslated region. These results suggest that Sam68 may accompany and, conceivably, regulate mature mRNAs during nuclear export, somatodendritic transport, and translation.

Sam68, the KH domain-containing superSTAR

Sam68 is one of the most studied members of the STAR family of RNA-binding proteins since its identification over a decade ago. Since its ascension into prominence, enormous progress has been made to unmask the link between the RNA-binding properties of Sam68 and the regulation of cellular processes including signal transduction, cell cycle regulation and tumorigenesis and RNA biogenesis in general. In this review we provide a detailed description of the functional domains of Sam68 and the possible biological roles that justify its superSTAR status.

SAM68: a downstream target of angiotensin II signaling in vascular smooth muscle cells in genetic hypertension

We investigated whether phosphatidylinositol 3-kinase (PI3K) and 68-kDa Src associated during mitosis (SAM68) are involved in angiotensin II (ANG II) growth signaling in vascular smooth muscle cells (VSMCs) from spontaneously hypertensive rats (SHR). PI3K activity was assessed by measuring the phosphorylation of the regulatory subunit p85alpha and kinase activity of the catalytic 110-kDa subunit of PI3K. The PI3K-SAM68 interaction was assessed by coimmunoprecipitation, and SAM68 activity was evaluated by poly(U) binding. SAM68 expression was manipulated by SAM68 antisense oligonucleotide transfection. VSMC growth was evaluated by measuring [3H]leucine and [3H]thymidine incorporation as indexes of protein and DNA synthesis, respectively. ANG II increased the phosphorylation of p85alpha and kinase activity of the 110-kDa PI3K subunit in VSMCs from SHR and transiently increased p85alpha-SAM68 association. In Wistar-Kyoto (WKY) rat cells, ANG II increased SAM68 phosphorylation without influencing poly(U) binding. In SHR, ANG II did not influence SAM68 phosphorylation but increased SAM68 binding to poly(U). ANG II stimulated phosphoinositol phosphate synthesis by PI3K in SAM68 immunoprecipitates in both groups, with significantly enhanced effects in SHR. Inhibition of PI3K, using the selective inhibitor LY-294002, and downregulation of SAM68, by antisense oligonucleotides, significantly decreased ANG II-stimulated incorporation of [3H]leucine and [3H]thymidine in VSMCs, showing the functional significance of PI3K and SAM68. Our data demonstrate that PI3K and SAM68 are involved in ANG II signaling and that SAM68 is differentially regulated in VSMCs from SHR. These processes may contribute to the enhanced ANG II signaling and altered VSMC growth in SHR.

tr-kit promotes the formation of a multimolecular complex composed by Fyn, PLCγ1 and Sam68

Tr-kit is a truncated form of the tyrosine kinase receptor c-kit expressed in the haploid phase of spermatogenesis. Upon microinjection, tr-kit triggers metaphase-to-anaphase transition in mouse eggs by the sequential activation of Fyn and PLCgamma1. Here, we show that tr-kit promotes the interaction of several tyrosine-phosphorylated proteins with the SH3 domain of PLCgamma1. Western blot analysis indicates that one of these proteins is Sam68, an RNA-binding protein that is known to interact with and be phosphorylated by Src-like kinases in mitosis. tr-kit promotes the association of Sam68 with PLCgamma1 and Fyn in a multimolecular complex, as demonstrated by co-immunoprecipitation of the phosphorylated forms of these proteins using antibodies directed to anyone of the partners of the complex. Expression of tr-kit potentiates the interaction of endogenous Sam68 also with the SH3 domain of Fyn. Furthermore, the subcellular localization of Sam68 is affected by tr-kit through activation of Fyn in live cells. Lastly, we show that interaction with the SH3 domain of Fyn triggers the release of Sam68 from bound RNA. Thus, our data suggest that tr-kit promotes the formation of a multimolecular complex composed of Fyn, PLCgamma1 and Sam68, which allows phosphorylation of PLCgamma1 by Fyn, and may modulate RNA metabolism.

Role of leptin as an immunomodulator of blood mononuclear cells: mechanisms of action

Leptin is a an adipocyte-secreted hormone that regulates weight centrally. However, the leptin receptor is expressed not only in the central nervous system, but also in peripheral tissues, such as haematopoietic and immune systems. Therefore, the physiological role of leptin should not be limited to the regulation of food intake and energy expenditure. Moreover, the leptin receptor bears homology to members of the class I cytokine family, and recent data have demonstrated that leptin is able to modulate the immune response. Thus, the leptin receptor is expressed in human peripheral blood mononuclear cells, mediating the leptin effect on proliferation and activation. In vitro activation and HIV infection in vivo induce the expression of the long isoform of the leptin receptor in mononuclear cells. Also, leptin stimulates the production of proinflammatory cytokines from cultured monocytes and enhances the production of Th1 type cytokines from stimulated lymphocytes. Moreover, leptin has a trophic effect on monocytes, preventing apoptosis induced by serum deprivation. Leptin stimulation activates JAK-STAT, IRS-1-PI3K and MAPK signalling pathways. Leptin also stimulates Tyr-phosphorylation of the RNA-binding protein Sam68 mediating the dissociation from RNA. In this way, leptin signalling could modulate RNA metabolism. These signal transduction pathways provide possible mechanisms whereby leptin may modulate activation of peripheral blood mononuclear cells. Therefore, these data support the hypothesis regarding leptin as a proinflammatory cytokine with a possible role as a link between the nutritional status and the immune response. Moreover, these immunoregulatory functions of leptin could have some relevance in the pathophysiology of obesity.

The influence of the src-family kinases, Lck and Fyn, on T cell differentiation, survival and activation

The src-family kinases p56lck (Lck) and p59fyn (Fyn) are expressed in T cells and are among the first signaling molecules to be activated downstream of the T cell receptor (TCR). Evidence is emerging that although closely related, these signaling molecules have discrete functions during development, maintenance and activation of peripheral T cells. For example, during thymopoiesis Lck is uniquely able to provide all the signals required for pre-TCRbeta selection, although Fyn can substitute for a subset of these. Positive selection of CD4 single-positive (SP) cells is also critically dependent on the expression of Lck but not Fyn, while differentiation of CD8 SP cells proceeds relatively efficiently in the absence of Lck. In naïve peripheral T cells either Lck or Fyn can transmit TCR-mediated survival signals, and yet only Lck is able to trigger TCR-mediated expansion signals under conditions of lymphopenia. Stimulation of naïve T cells by antigenic stimuli is also severely compromised in the absence of Lck, but more subtly impaired by the absence of Fyn. We discuss recent experiments addressing how these two src-kinase family members interface with downstream signaling pathways to regulate these diverse aspects of T cell behavior.

Sam68 associates with the SH3 domains of Grb2 recruiting GAP to the Grb2-SOS complex in insulin receptor signaling

The 68 kDa Src substrate associated during mitosis (Sam68) is an RNA binding protein with Src homology (SH) 2 and 3 domain binding sites. We have recently found that Sam68 is a substrate of the insulin receptor (IR) that translocates from the nucleus to the cytoplasm and that Tyr-phosphorylated Sam68 associates with the SH2 domains of p85 PI3K and GAP, in vivo and in vitro. In the present work, we have further demonstrated the cytoplasmic localization of Sam68, which is increased in cells overexpressing IR. Besides, we sought to further study the association of Sam68 with the Ras-GAP pathway by assessing the interactions with SH3 domains of Grb2. We employed GST-fusion proteins containing the SH3 domains of Grb2 (N or C), and recombinant Sam68 for in vitro studies. In vivo studies of protein-protein interaction were assessed by co-immunoprecipitation experiments with specific antibodies against Sam68, GAP, Grb2, SOS, and phosphotyrosine; and by affinity precipitation with the fusion proteins (SH3-Grb2). Insulin stimulation of HTC-IR cells promotes phosphorylation of Sam68 and its association with the SH2 domains of GAP. Sam68 is constitutively associated with the SH3 domains of Grb2 and it does not change upon insulin stimulation, but Sam68 is Tyr-phosphorylated and promotes the association of GAP with the Grb2-SOS complex. In vitro studies with fusion proteins showed that Sam68 association with Grb2 is preferentially mediated by the C-terminal SH3 domains of Grb2. In conclusion, Sam68 is a substrate of the IR and may have a role as a docking protein in IR signaling, recruiting GAP to the Grb2-SOS complex, and in this way it may modulate Ras activity.

Identification of cellular mRNA targets for RNA-binding protein Sam68

Sam68 (Src-associated in mitosis, 68 kDa), a nuclear RNA-binding protein, has been postulated to play a role in cell-growth control as a modulator of signal transduction and activation of RNA metabolism. Although Sam68 was demonstrated to bind to the UAAA sequences in synthetic oligoribonucleotides and poly(U) homopolymers in vitro, the legitimate cellular mRNA target remained unclear. By using the differential display and cDNA-representational difference analysis techniques, followed by reverse transcription polymerase chain reaction of RNAs co-immunoprecipitated with Sam68 from a HeLa cell lysate, we identified 10 mRNA species that bind in vivo to Sam68 in an RNA-binding domain-dependent manner. Among them, the mRNA species for hnRNP A2/B1 and beta-actin were found to bind prominently in vivo as well as in vitro, suggesting the possible involvement of Sam68 in the post- transcriptional regulation of these genes. Mapping of the Sam68-binding sequence revealed that Sam68 associates with these mRNAs through different nucleotide motifs, UAAA for hnRNP A2/B1 mRNA and UUUUUU for beta-actin mRNA, and that both binding sequences must reside in a loop structure for recognition by Sam68. The results indicated that Sam68 recognizes both the UAAA motif and poly(U) sequences in vivo for binding to cellular target mRNAs.

The distinct capacity of Fyn and Lck to phosphorylate Sam68 in T cells is essentially governed by SH3/SH2-catalytic domain linker interactions

Sam68 phosphorylation correlates with Fyn but not Lck expression in T cells. This substrate has been used here to explore the possible basis of the specificity of Fyn versus Lck. We show that this specificity is not based on a spatial segregation of the two kinases, since a chimeric Lck molecule containing the membrane anchoring domain of Fyn does not phosphorylate Sam68. Moreover, a Sam68 molecule targeted to the plasma membrane by the farnesylation signal of c-Ha-Ras remains poorly phosphorylated by Lck. In T cells, Fyn appears to be the active Src kinase in rafts, but Sam68 is not expressed in rafts, and its distinct phosphorylation by Fyn and Lck is not affected by raft dispersion. The Fyn/Lck specificity does not reflect a higher kinase activity of Fyn in general, as both Fyn and Lck are similarly recognized by an anti-active Src antibody. Both also strongly phosphorylate another Src substrate in vivo. Mainly, Lck phosphorylates Sam68 when the interaction between the SH3 domain and the SH2-catalytic domain linker is altered in heterologous Src molecules or after mutating key residues in the linker that increase the accessibility of the SH3 domain. Thus, the distinct potential of Fyn and Lck to phosphorylate Sam68 is likely controlled by the interaction of the kinase SH3 domain with the linker and Sam68, possibly on a competitive binding basis.

Functional interaction of Sam68 and heterogeneous nuclear ribonucleoprotein K

Sam68 is a target of the c-Src tyrosine kinase. We previously showed that overexpression of Sam68 functionally substitutes for, as well as synergies with, HIV-1 Rev in Rev-response element (RRE)-mediated gene expression and virus replication. Here we describe the identification of heterogeneous nuclear ribonucleoprotein K (hnRNP K) as a protein that specifically interacts with Sam68 in vitro and in vivo. HnRNP K did not bind to RRE-RNA directly, but formed a super complex with Sam68 and RRE in vitro. RNase treatment did not change the strength of binding of hnRNP K to Sam68. We demonstrated that hnRNP K significantly inhibited Sam68-mediated, but not Rev-mediated, RRE-dependent gene expression. We further showed that Sam68, but not a non-functional mutant Sam68p21, inhibited transcriptional activation of CT element by hnRNP K. Interestingly, the Sam68p21 with a single amino acid substitution in the nuclear localization domain exhibited less affinity for hnRNP K in vitro. We propose that the direct interaction of Sam68 and hnRNP K adversely affect the activities of both proteins in signal transduction pathways of both transcriptional and post-transcriptional events.

Retardation of the G2-M phase progression on gene disruption of RNA binding protein Sam68 in the DT40 cell line

Sam68 is an RNA binding protein that is tyrosine-phosphorylated by Src during mitosis and has been postulated to have a role in cell cycle control by modulating RNA metabolism. To elucidate the function of this protein, we isolated a Sam68-deficient DT40 cell line by gene disruption. The Sam68-deficient cells exhibited markedly decreased growth and the growth retardation was due to elongation of the G2-M phase, however, the kinase activity associated with Cdc2 remained unaltered. Our results indicate that Sam68 may play a critical role in G2-M progression in a manner independent of the control of Cyclin/Cdc2 kinase activity.

UCS15A, a novel small molecule, SH3 domain-mediated protein-protein interaction blocking drug

Protein-protein interactions play critical regulatory roles in mediating signal transduction. Previous studies have identified an unconventional, small-molecule, Src signal transduction inhibitor, UCS15A. UCS15A differed from conventional Src-inhibitors in that it did not alter the levels or the tyrosine kinase activity of Src. Our studies suggested that UCS15A exerted its Src-inhibitory effects by a novel mechanism that involved the disruption of protein-protein interactions mediated by Src. In the present study we have examined the ability of UCS15A to disrupt the interaction of Src-SH3 with Sam68, both in vivo and in vitro. This ability of UCS15A was not restricted to Src-SH3 mediated protein-protein interactions, since the drug was capable of disrupting the in vivo interactions of Sam68 with other SH3 domain containing proteins such as Grb2 and PLCgamma. In addition, UCS15A was capable of disrupting other typical SH3-mediated protein-protein interactions such as Grb2-Sos1, cortactin-ZO1, as well as atypical SH3-mediated protein-protein interactions such as Grb2-Gab1. However, UCS15A was unable to disrupt the non-SH3-mediated protein-protein interactions of beta-catenin, with E-cadherin and alpha-catenin. In addition, UCS15A had no effect on the SH2-mediated interaction between Grb2 and activated Epidermal Growth Factor receptor. Thus, the ability of UCS15A, to disrupt protein-protein interactions appeared to be restricted to SH3-mediated protein-protein interactions. In this regard, UCS15A represents the first example of a non-peptide, small molecule agent capable of disrupting SH3-mediated protein-protein interactions. In vitro analyses suggested that UCS15A did not bind to the SH3 domain itself but rather may interact directly with the target proline-rich domains.

The Lck SH3 domain negatively regulates localization to lipid rafts through an interaction with c-Cbl

Lck is a member of the Src family of protein-tyrosine kinases and is essential for T cell development and function. Lck is localized to the inner surface of the plasma membrane and partitions into lipid rafts via dual acylation on its N terminus. We have tested the role of Lck binding domains in regulating Lck localization to lipid rafts. A form of Lck containing a point mutation inactivating the SH3 domain (W97ALck) was preferentially localized to lipid rafts compared with wild type or SH2 domain-inactive (R154K) Lck when expressed in Lck-deficient J.CaM1 cells. W97ALck incorporated more of the radioiodinated version of palmitic acid, 16-[(125)I]iodohexadecanoic acid. Overexpression of c-Cbl, a ligand of the Lck SH3 domain, depleted Lck from lipid rafts in Jurkat cells. Additionally, Lck localization to lipid rafts was enhanced in c-Cbl-deficient T cells. The association of Lck with c-Cbl in vivo required a functional SH3 domain. These results suggest a model whereby the SH3 domain negatively regulates basal localization of Lck to lipid rafts via association with c-Cbl.

Sam68 is a docking protein linking GAP and PI3K in insulin receptor signaling

The 68 kDa Src substrate associated during mitosis (Sam68) is an RNA binding protein with Src homology (SH) 2 and 3 domain binding sites. We have recently found that Sam68 is a substrate of the insulin receptor (IR) and that Tyr-phosphorylated Sam68 associates with the SH2 domains of p85 PI3K. In the present work, using HTC-IR cells, we have found that insulin stimulation promotes the relocalization of Sam68 from the nucleus to the cytoplasm, and we have further studied the role of Sam68 in insulin receptor signaling complexes, by co-precipitating experiments. Thus, Sam68 is co-precipitated with p85 PI3K, IRS-1 and IR. The association of Sam68 with these complexes is mediated by the SH2 domains of PI3K. Moreover, we have found that Sam68 is a p120GAP associated protein after Tyr-phosphorylation by the IR. This association is mediated by the SH2 domains of GAP (preferentially the C-terminal SH2). Thus, Sam68 is linking p120GAP to PI3K signaling pathway. In fact, PI3K activity was increased in both anti-Sam68 and anti-GAP immmunoprecipitates upon insulin stimulation. We propose that the recruitment of the docking protein Sam68 to the PI3K pathway may serve to allow the association of other signaling molecules, i.e. p120GAP. In this way, these signaling complexes may modulate other signaling cascades of IR, such as p21Ras pathway.