Association of p62, a multifunctional SH2- and SH3-domain-binding protein, with src family tyrosine kinases, Grb2, and phospholipase C gamma-1

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.

Research progress on RNA‑binding proteins in breast cancer (Review)

Breast cancer is the most common malignancy among women, and the abnormal regulation of gene expression serves an important role in its occurrence and development. However, the molecular mechanisms underlying gene expression are highly complex and heterogeneous, and RNA-binding proteins (RBPs) are among the key regulatory factors. RBPs bind targets in an environment-dependent or environment-independent manner to influence mRNA stability and the translation of genes involved in the formation, progression, metastasis and treatment of breast cancer. Due to the growing interest in these regulators, the present review summarizes the most influential studies concerning RBPs associated with breast cancer to elucidate the role of RBPs in breast cancer and to assess how they interact with other key pathways to provide new molecular targets for the diagnosis and treatment of breast cancer.

The Quaking RNA-binding proteins as regulators of cell differentiation

The RNA-binding protein Quaking (QKI) has emerged as a potent regulator of cellular differentiation in developmental and pathological processes. The QKI gene is itself alternatively spliced to produce three major isoforms, QKI-5, QKI-6, and QKI-7, that possess very distinct functions. Here, we highlight roles of the different QKI isoforms in neuronal, vascular, muscle, and monocyte cell differentiation, and during epithelial-mesenchymal transition in cancer progression. QKI isoforms control cell differentiation through regulating alternative splicing, mRNA stability and translation, with activities in gene transcription now also becoming evident. These diverse functions of the QKI isoforms contribute to their broad influences on RNA metabolism and cellular differentiation. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Processing > Splicing Regulation/Alternative Splicing RNA in Disease and Development > RNA in Development.

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 regulates tumor cell viability and hepatic carcinogenesis by inhibiting the transcriptional activity of FOXOs

Src associated in mitosis (Sam68; 68 kDa) is a KH domain RNA-binding protein that belongs to the signal transduction and activation of RNA family, and has been implicated in the oncogenesis and progression of several human cancers. Our study aimed to investigated the clinicopathologic significance of Sam68 expression and its role in cell proliferation and the underlying molecular mechanism in hepatocellular carcinoma (HCC). We demonstrated that Sam68 expression was significantly increased in HCC and high expression of Sam68 was significantly associated with Edmondson grade, tumor size, tumor nodule number, HBsAg status and Ki-67 expression. The Kaplan-Meier survival curves showed that increased expression of Sam68 was correlated with poor prognosis in HCC patients and served as an independent prognostic marker of overall survival in a multivariable analysis. In addition, through serum starvation and refeeding assay, we demonstrated that Sam68 was lowly expressed in serum-starved HCC cells, and was progressively increased after serum-additioning. Furthermore, siRNA knockdown of endogenous Sam68 inhibited cell proliferation and tumourigenicity of HCC cells in vitro, through blocking the G1 to S phase transition. Moreover, we reported that the anti-proliferative effect of silencing Sam68 was accompanied with up-regulated expression of cyclin-dependent kinase inhibitors, p21(Cip1) and p27(Kip1), enhanced transactivation of FOXO factors (FOXO4), and dysreuglation of Akt/GSK-3β signaling. Taken together, these findings provide a rational framework for the progression of HCC and thereby indicated that Sam68 might be a novel and useful prognostic marker and a potential target for human HCC treatment.

SAM68: Signal Transduction and RNA Metabolism in Human Cancer

Alterations in expression and/or activity of splicing factors as well as mutations in cis-acting splicing regulatory sequences contribute to cancer phenotypes. Genome-wide studies have revealed more than 15,000 tumor-associated splice variants derived from genes involved in almost every aspect of cancer cell biology, including proliferation, differentiation, cell cycle control, metabolism, apoptosis, motility, invasion, and angiogenesis. In the past decades, several RNA binding proteins (RBPs) have been implicated in tumorigenesis. SAM68 (SRC associated in mitosis of 68 kDa) belongs to the STAR (signal transduction and activation of RNA metabolism) family of RBPs. SAM68 is involved in several steps of mRNA metabolism, from transcription to alternative splicing and then to nuclear export. Moreover, SAM68 participates in signaling pathways associated with cell response to stimuli, cell cycle transitions, and viral infections. Recent evidence has linked this RBP to the onset and progression of different tumors, highlighting misregulation of SAM68-regulated splicing events as a key step in neoplastic transformation and tumor progression. Here we review recent studies on the role of SAM68 in splicing regulation and we discuss its contribution to aberrant pre-mRNA processing in cancer.

Clinical significance of Sam68 expression in endometrial carcinoma

Sam68 (Src-associated in mitosis of 68 kDa) is a substrate for tyrosine kinase c-Src during mitosis. The nuclear protein level has been found to be associated with progression and prognosis in various human malignant tumors. The aim of this study is to investigate the clinical value of Sam68 in endometrial carcinoma (EC). Sam68 expression was confirmed by real-time PCR, Western blot, and immunofluorescent assay in primary normal endometrial epithelial cells, endometrial carcinoma cell lines, as well as seven pairs of EC and matched adjacent noncancerous endometrial tissues. Moreover, the protein level of Sam68 was evaluated by immunohistochemistry in a cohort of surgical specimens derived from 131 patients including primary endometrial carcinoma (n = 95), endometrial atypical hyperplasia (precancerous lesions, n = 26), and normal endometria (n = 10). In endometrial cancer cell lines, RNA interfering approach was employed to downregulate Sam68 expression to determine its role in proliferation. Clinicopathological relevance and prognostic associations were examined by statistical analyses. Compared with normal endometrial and endometrial atypical hyperplasia tissues, Sam68 significantly elevated in endometrial cancer samples (P < 0.01), which was negative or low in 37 cases (38.9 %) and high in 58 cases (61.1 %). The high expression of Sam68 was associated with histological grade (P < 0.001), FIGO stage (P = 0.039), and myometrial invasion (P = 0.002). Kaplan-Meier analysis demonstrated that overexpression of Sam68 correlated with shorter overall survival. It is confirmed by univariate and multivariate analysis (P < 0.001 and P = 0.048, respectively). Additionally, we found that Sam68 was highly expressed at both the transcriptional and translational levels in endometrial cancer cell lines (Ishikawa, HEC-1B, AN3CA, KLE, and RL95-2) and siRNA knockdown of Sam68 remarkably inhibited cellular proliferation in in vitro models. Sam68 may be useful prognostic marker for EC, and it plays an important role in promoting the cellular proliferation. Further investigation of Sam68 as a potential therapeutic target for EC patients could be of interest.

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.

Effect of newly identified hTERT-interacting proteins on telomerase activity

There is a close relationship between telomeres-telomerase and age-related disease. Human telomerase reverse transcriptase (hTERT) is both the catalytic component of human telomerase and the rate-limiting determinant of telomerase activity. Its transcriptional regulation is the primary mode of control of telomerase activity. It is critical to find the proteins interacting with hTERT for exploring the regulatory mechanisms of the hTERT expression and the telomerase activity. In this study, the yeast two-hybrid system was used to screen the potential interactive proteins of hTERT. Six proteins were obtained, among which T-STAR, LOXL3, HKR3, and Par-4 were further confirmed as the interacting proteins of hTERT by co-immunoprecipitation. Then the sense and antisense gene eukaryotic expression vectors containing these four genes were constructed and transfected into tumor cell lines. The correlations among the expression levels of these four proteins, the expression level of hTERT, and the telomerase activity were analyzed. Results showed that the up-regulation of T-STAR expression and down-regulation of HKR3 expression led to the increase of hTERT expression and telomerase activity, while the up- and down-regulation of LOXL3 and Par-4 expressions had no obvious effect. Our results suggested that T-STAR has a positive correlation with the telomerase activity while HKR3 may be a negative regulator. This conclusion is important to further explore the influencing factors or regulation pathways of human telomerase activity, which may be of great importance for the potential clinical application.

Stay lean without dieting: Lose Sam68

Alternative splicing is well known to be tissue-specific. Although several genes have been shown to undergo alternative splicing in adipocytes, little is known about the mechanism that regulates alternative splicing during adipogenesis. We recently reported that Sam68^−/− mice exhibit a lean phenotype and are protected against diet-induced obesity. Our genome-wide exon array analysis in white adipose tissue (WAT) from wild-type and Sam68^−/− mice revealed that Sam68 deficiency leads to an abnormal splicing of the mTOR gene. This has been shown to reduce the overall mTOR protein content and activity during in vitro adipose differentiation. In Sam68^−/− mice, this situation leads to an increased energy expenditure, decreased adipogenesis and WAT formation.

Mechanisms of HGF/Met signaling to Brk and Sam68 in breast cancer progression

Signal transduction pathways downstream of receptor tyrosine kinases (RTKs) are often deregulated during oncogenesis, tumor progression, and metastasis. In particular, the peptide growth factor hormone, hepatocyte growth factor (HGF), and its specific receptor, Met tyrosine kinase, regulate cancer cell migration, thereby conferring an aggressive phenotype (Nakamura et al., J Clin Invest 106(12):1511-1519, 2000; Huh et al., Proc Natl Acad Sci U S A 101:4477-4482, 2004). Additionally, overexpression of Met is associated with enhanced invasiveness of breast cancer cells (Edakuni et al., Pathol Int 51(3):172-178, 2001; Jin et al., Cancer 79(4):749-760, 1997; Tuck et al., Am J Pathol 148(1):225-232, 1996). Here, we review the regulation of recently identified novel downstream mediators of HGF/Met signaling, Breast tumor kinase (Brk/PTK6), and Src-associated substrate during mitosis of 68 kDa (Sam68), and discuss their relevance to mechanisms of breast cancer progression.

Comprehensive analysis of interactions between the Src-associated protein in mitosis of 68 kDa and the human Src-homology 3 proteome

The protein Sam68 is involved in many cellular processes such as cell-cycle regulation, RNA metabolism, or signal transduction. Sam68 comprises a central RNA-binding domain flanked by unstructured tails containing docking sites for signalling proteins including seven proline-rich sequences (denoted P0 to P6) as potential SH3-domain binding motifs. To comprehensively assess Sam68-SH3-interactions, we applied a phage-display screening of a library containing all approx. 300 human SH3 domains. Thereby we identified five new (from intersectin 2, the osteoclast stimulating factor OSF, nephrocystin, sorting nexin 9, and CIN85) and seven already known high-confidence Sam68-ligands (mainly from the Src-kinase family), as well as several lower-affinity binders. Interaction of the high-affinity Sam68-binders was confirmed in independent assays in vitro (phage-ELISA, GST-pull-down) and in vivo (FACS-based FRET-analysis with CFP- and YFP-tagged proteins). Fine-mapping analyses with peptides established P0, P3, P4, and P5 as exclusive docking-sites for SH3 domains, which showed varying preferences for these motifs. Mutational analyses identified individual residues within the proline-rich motifs being crucial for the interactions. Based on these data, we generated a Sam68-mutant incapable of interacting with SH3 domains any more, as subsequently demonstrated by FRET-analyses. In conclusion, we present a thorough characterization of Sam68's interplay with the SH3 proteome. The observed interaction between Sam68 and OSF complements the known Sam68-Src and OSF-Src interactions. Thus, we propose, that Sam68 functions as a classical scaffold protein in this context, assembling components of an osteoclast-specific signalling pathway.

Crystal structures of the armadillo repeat domain of adenomatous polyposis coli and its complex with the tyrosine-rich domain of Sam68

Adenomatous polyposis coli (APC) is a tumor suppressor protein commonly mutated in colorectal tumors. APC plays important roles in Wnt signaling and other cellular processes. Here, we present the crystal structure of the armadillo repeat (Arm) domain of APC, which facilitates the binding of APC to various proteins. APC-Arm forms a superhelix with a positively charged groove. We also determined the structure of the complex of APC-Arm with the tyrosine-rich (YY) domain of the Src-associated in mitosis, 68 kDa protein (Sam68), which regulates TCF-1 alternative splicing. Sam68-YY forms numerous interactions with the residues on the groove and is thereby fixed in a bent conformation. We assessed the effects of mutations and phosphorylation on complex formation between APC-Arm and Sam68-YY. Structural comparisons revealed different modes of ligand recognition between the Arm domains of APC and other Arm-containing proteins.

Sam68 interacts with IRS1

Sam68 (Src associated in mitosis) is a RNA binding protein that links cellular signaling to RNA processing. In previous studies we found that insulin promotes Sam68 relocalization in the cytoplasm allowing Sam68 to associate with p85PI3K, Grb2, GAP and probably the insulin receptor (IR), modulating insulin action positively. In the present work, we wanted to define the role of Sam68 in the first stages of IR signaling. Both BRET and co-immunoprecipitation assays have been used for the study of Sam68 binding to IR, IRS1 and p85-PI3K. BRET saturation experiments indicated, for the first time, that Sam68 associates with IRS1 in basal condition. To map the region of Sam68 implicated in the interaction with IRS1, different Sam68 mutants deleted in the proline-rich domains were used. The deletion of P0, P1 and P2 proline rich domains in N-terminus as well as P4 and P5 in C-terminus of Sam68 increased BRET(50), thus indicating that the affinity of Sam68 for IRS1 is lower when these domains are missing. Moreover, in IR-transfected HEK-293 cells, BRET saturation experiment indicated that insulin increases the affinity between Sam68-Rluc and IRS1-YFP. In conclusion, our data indicate that Sam68 interacts with IRS-1 in basal conditions, and insulin increases the affinity between these two partners.

The Cdc42-associated kinase ACK1 is not autoinhibited but requires Src for activation

The non-RTK (receptor tyrosine kinase) ACK1 [activated Cdc42 (cell division cycle 42)-associated kinase 1] binds a number of RTKs and is associated with their endocytosis and turnover. Its mode of activation is not well established, but models have suggested that this is an autoinhibited kinase. Point mutations in its SH3 (Src homology 3)- or EGF (epidermal growth factor)-binding domains have been reported to activate ACK1, but we find neither of the corresponding W424K or F820A mutations do so. Indeed, deletion of the various ACK1 domains C-terminal to the catalytic domain are not associated with increased activity. A previous report identified only one major tyrosine phosphorylated protein of 60 kDa co-purified with ACK1. In a screen for new SH3 partners for ACK1 we found multiple Src family kinases; of these c-Src itself binds best. The SH2 and SH3 domains of Src interact with ACK1 Tyr518 and residues 623-652 respectively. Src targets the ACK1 activation loop Tyr284, a poor autophosphorylation site. We propose that ACK1 fails to undergo significant autophosphorylation on Tyr284 in vivo because it is basophilic (whereas Src is acidophilic). Subsequent ACK1 activation downstream of receptors such as EGFR (EGF receptor) (and Src) promotes turnover of ACK1 in vivo, which is blocked by Src inhibitors, and is compromised in the Src-deficient SYF cell line. The results of the present study can explain why ACK1 is responsive to so many external stimuli including RTKs and integrin ligation, since Src kinases are commonly recruited by multiple receptor systems.

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 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.

Met receptors induce Sam68-dependent cell migration by activation of alternate extracellular signal-regulated kinase family members

The hepatocyte growth factor (HGF)/Met receptor signaling pathway is deregulated in diverse human malignancies and plays a central role in oncogenesis, tumor progression, and invasive cancer growth. Similarly, altered expression and splicing (i.e. inclusion of variant exon 5, "v5") of the cell adhesion marker, CD44, is associated with advanced cancer phenotypes. We sought to further understand how HGF regulates CD44v5 expression. Immortalized nontumorigenic keratinocyte (HaCaT) cells abundantly express both Met receptors and CD44v5 transmembrane glycoproteins. HGF stimulated CD44v5 protein expression and HaCaT cell migration; these events required activation of the ERK1/2 MAPK module and Sam68, a protein involved in RNA processing, splicing, and v5 inclusion. Similar to HaCaT cells, highly migratory MDA-MB-231 breast cancer cells also required Sam68 expression for HGF-induced migration. However, MDA-MB-231 cell migration occurred independently of ERK1/2 and CD44v5 expression and instead required ERK5 signaling to Sam68. Phospho-mutant, but not WT-Sam68, blocked HGF-induced cell migration in both cell types; MDA-MB-435 cells behaved similarly. These results suggest that Sam68 acts as a convergence point for ERK signaling to cell migration; blockade of phospho-Sam68 may provide a new avenue for therapeutic inhibition of metastatic cancers.

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.

Sam68 up-regulation correlates with, and its down-regulation inhibits, proliferation and tumourigenicity of breast cancer cells

The biosynthesis and metabolism of RNA play important roles in regulating gene expression. On the other hand, it has been shown that RNA expression profiling is differentially distinct between cancer and normal cells, suggesting the possibility that aberrant regulation of RNA metabolism might be associated with the development and progression of cancer. In the current study, we found that Sam68, an RNA-binding protein that links cellular signalling to RNA processing, was markedly overexpressed in breast cancer cells and tissues. Immunohistochemical analysis showed that the expression and cytoplasmic localization of Sam68 significantly correlated with clinical characteristics of patients, including clinical stage, tumour-nodule-metastasis (TNM) classification, histological grade, and ER expression. Univariate and multivariate analyses showed that the expression level and cytoplasmic localization of Sam68 were identified as independent prognostic factors. Furthermore, we found that siRNA knockdown of endogenous Sam68 inhibited cell proliferation and tumourigenicity of breast cancer cells in vitro, through blocking the G1 to S phase transition. Moreover, we demonstrated that the anti-proliferative effect of silencing Sam68 on breast cancer cells was associated with up-regulation of cyclin-dependent kinase inhibitor p21(Cip1) and p27(Kip1), enhanced transactivation of FOXO factors, and attenuation of Akt/GSK-3β signalling. Taken together, our results suggest that Sam68 might play an important role in promoting the proliferation and carcinogenesis of human breast cancer, and thereby might be a novel and useful prognostic marker and a potential target for human breast cancer treatment.

Structural basis for homodimerization of the Src-associated during mitosis, 68-kDa protein (Sam68) Qua1 domain

Sam68 (Src-associated during mitosis, 68 kDa) is a prototypical member of the STAR (signal transducer and activator of RNA) family of RNA-binding proteins. STAR proteins bind mRNA targets and modulate cellular processes such as cell cycle regulation and tissue development in response to extracellular signals. Sam68 has been shown to modulate alternative splicing of the pre-mRNAs of CD44 and Bcl-xL, which are linked to tumor progression and apoptosis. Sam68 and other STAR proteins recognize bipartite RNA sequences and are thought to function as homodimers. However, the structural and functional roles of the self-association are not known. Here, we present the solution structure of the Sam68 Qua1 homodimerization domain. Each monomer consists of two antiparallel alpha-helices connected by a short loop. The two subunits are arranged perpendicular to each other in an unusual four-helix topology. Mutational analysis of Sam68 in vitro and in a cell-based assay revealed that the Qua1 domain and residues within the dimerization interface are essential for alternative splicing of a CD44 minigene. Together, our results indicate that the Qua1 homodimerization domain is required for regulation of alternative splicing by Sam68.

Differential phosphoproteomics of fibroblast growth factor signaling: identification of Src family kinase-mediated phosphorylation events

Activation of signal transduction by the receptor tyrosine kinase, fibroblast growth factor receptor (FGFR), results in a cascade of protein-protein interactions that rely on the occurrence of specific tyrosine phosphorylation events. One such protein recruited to the activated receptor complex is the nonreceptor tyrosine kinase, Src, which is involved in both initiation and termination of further signaling events. To gain a further understanding of the tyrosine phosphorylation events that occur during FGF signaling, with a specific focus on those that are dependent on Src family kinase (SFK) activity, we have applied SILAC combined with chemical inhibition of SFK activity to search for phosphorylation events that are dependent on SFK activity in FGF stimulated cells. In addition, we used a more targeted approach to carry out high coverage phosphopeptide mapping of one Src substrate protein, the multifunctional adaptor Dok1, and to identify SFK-dependent Dok1 binding partners. From these analyses we identify 80 SFK-dependent phosphorylation events on 40 proteins. We further identify 18 SFK-dependent Dok1 interactions and 9 SFK-dependent Dok1 phosphorylation sites, 6 of which had not previously been known to be SFK-dependent.

Expression and functions of the star proteins Sam68 and T-STAR in mammalian spermatogenesis

Spermatogenesis is one of the few major developmental pathways which are still ongoing in the adult. In this chapter we review the properties of Sam68 and T-STAR, which are the STAR proteins functionally implicated in mammalian spermatogenesis. Sam68 is a ubiquitously expressed member of the STAR family, but has an essential role in spermatogenesis. Sam68 null mice are male infertile and at least in part this is due to a failure in important translational controls that operate during and after meiosis. The homologous T-STAR protein has a much more restricted anatomic expression pattern than Sam68, with highest levels seen in the testis and the developing brain. The focus of this chapter is the functional role of Sam68 and T-STAR proteins in male germ cell development. Since these proteins are known to have many cellular functions we extrapolate from other cell types and tissues to speculate on each of their likely functions within male germ cells, including control of alternative pre-mRNA splicing patterns in male germ cells.

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.

Identification of a Sam68 ribonucleoprotein complex regulated by epidermal growth factor

Sam68, Src associated in mitosis of 68 kDa, is a known RNA-binding protein and a signaling adaptor protein for tyrosine kinases. However, the proteins associated with Sam68 and the existence of a Sam68 complex, its mass, and regulation are, however, unknown. Herein we identify a large Sam68 complex with a mass >1 MDa in HeLa cells that is composed of approximately 40 proteins using an immunoprecipitation followed by a mass spectrometry approach. Many of the proteins identified are RNA-binding proteins and are known components of a previously identified structure termed the spreading initiation center. The large Sam68 complex is a ribonucleoprotein complex, as treatment with RNases caused a shift in the molecular mass of the complex to 200-450 kDa. Moreover, treatment of HeLa cells with phorbol 12-myristate 13-acetate or epidermal growth factor induced the disassociation of Sam68 from the large complex and the appearance of Sam68 within the smaller complex. Actually, in certain cell lines such as breast cancer cell lines MCF-7 and BT-20, Sam68 exists in equilibrium between a large and a small complex. The appearance of the small Sam68 complex in cells correlates with the ability of Sam68 to promote the alternative splicing of CD44 and cell migration. Our findings show that Sam68 exists in equilibrium in transformed cells between two complexes and that extracellular signals, such as epidermal growth factor stimulation, promote alternative splicing by modulating the composition of the Sam68 complex.

An adaptor role for cytoplasmic Sam68 in modulating Src activity during cell polarization

The Src-associated substrate during mitosis with a molecular mass of 68 kDa (Sam68) is predominantly nuclear and is known to associate with proteins containing the Src homology 3 (SH3) and SH2 domains. Although Sam68 is a Src substrate, little is known about the signaling pathway that link them. Src is known to be activated transiently after cell spreading, where it modulates the activity of small Rho GTPases. Herein we report that Sam68-deficient cells exhibit loss of cell polarity and cell migration. Interestingly, Sam68-deficient cells exhibited sustained Src activity after cell attachment, resulting in the constitutive tyrosine phosphorylation and activation of p190RhoGAP and its association with p120rasGAP. Consistently, we observed that Sam68-deficient cells exhibited deregulated RhoA and Rac1 activity. By using total internal reflection fluorescence microscopy, we observed Sam68 near the plasma membrane after cell attachment coinciding with phosphorylation of its C-terminal tyrosines and association with Csk. These findings show that Sam68 localizes near the plasma membrane during cell attachment and serves as an adaptor protein to modulate Src activity for proper signaling to small Rho GTPases.

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.

The evolutionarily conserved arrangement of domains in SRC family kinases is important for substrate recognition

The SH3-SH2-kinase domain arrangement in nonreceptor tyrosine kinases has been conserved throughout evolution. For Src family kinases, the relative positions of the domains are important for enzyme regulation; they permit the assembly of Src kinases into autoinhibited conformations. The SH3 and SH2 domains of Src family kinases have an additional role in determining the substrate specificity of the kinase. We addressed the question of whether the domain arrangement of Src family kinases has a role in substrate specificity by producing mutants with alternative arrangements. Our results suggest that changes in the positions of domains can lead to specific changes in the phosphorylation of Sam68 and Cas by Src. Phosphorylation of Cas by several mutants triggers downstream signaling leading to cell migration. The placement of the SH2 domain with respect to the catalytic domain of Src appears to be especially important for proper substrate recognition, while the placement of the SH3 domain is more flexible. The results suggest that the involvement of the SH3 and SH2 domains in substrate recognition is one reason for the strict conservation of the SH3-SH2-kinase architecture.

Interactions between the Fyn SH3-domain and adaptor protein Cbp/PAG derived ligands, effects on kinase activity and affinity

Csk-binding protein/phosphoprotein associated with glycosphingolipid-enriched domains is a transmembrane adaptor protein primarily involved in negative regulation of T-cell activation by recruitment of C-terminal Src kinase (Csk), a protein tyrosine kinase which represses Src kinase activity through C-terminal phosphorylation. Recruitment of Csk occurs via SH2-domain binding to PAG pTyr317, thus, the interaction is highly dependent on phosphorylation performed by the Src family kinase Fyn, which docks onto PAG using a dual-domain binding mode involving both SH3- and SH2-domains of Fyn. In this study, we investigated Fyn SH3-domain binding to 14-mer peptide ligands derived from Cbp/PAG-enriched microdomains sequence using biochemical, biophysical and computational techniques. Interaction kinetics and dissociation constants for the various ligands were determined by SPR. The local structural impact of ligand association has been evaluated using CD, and molecular modelling has been employed to investigate details of the interactions. We show that data from these investigations correlate with functional effects of ligand binding, assessed experimentally by kinase assays using full-length PAG proteins as substrates. The presented data demonstrate a potential method for modulation of Src family kinase tyrosine phosphorylation through minor changes of the substrate SH3-interacting motif.

The RNA-binding and adaptor protein Sam68 modulates signal-dependent splicing and transcriptional activity of the androgen receptor

The RNA-binding protein Sam68 has been reported to be up-regulated in clinical cases of prostate cancer (PCa), where it is thought to contribute to cell proliferation and survival. Consistent with this, we observed over-expression of Sam68 in a panel of clinical prostate tumours as compared with benign controls. Since Sam68 is implicated in a number of signalling pathways, we reasoned that its role in PCa may involve modulation of the androgen receptor (AR) signalling cascade, which drives the onset and progression of PCa. We found that Sam68 interacts with the AR in vivo in LNCaP cells, and is dynamically recruited to androgen response elements within the promoter region of the prostate-specific antigen (PSA) gene. Based on its known functions and nuclear location, Sam68 might either: (a) co-regulate AR-dependent transcription positively or negatively; or (b) modulate AR-dependent alternative splicing by enhancing incorporation of a Sam68-responsive exon transcribed under the control of an androgen-responsive promoter. We tested these possibilities using functional assays. Both wild-type Sam68 protein and the Sam68(V229F) mutant, which is impaired in RNA binding, functioned as a ligand-dependent AR co-activator on an androgen-regulated reporter gene. In contrast, splicing of a Sam68-responsive variable exon, transcribed under control of an androgen-responsive promoter, was strongly repressed in the presence of AR and androgens. This splicing inhibition was reversed by ectopic expression of Sam68 but enhanced by Sam68(V229F). These results demonstrate that Sam68 has separable effects on AR-regulated transcriptional activity and alternative splicing, both of which may affect PCa phenotypes.

Targeting the RNA-binding protein Sam68 as a treatment for cancer?

The contradictory properties of RNA-binding proteins (RBPs) have mystified their roles in human diseases including cancer. Are certain RBPs oncogenes or tumor suppressors? In the case of the signal transduction activator of RNA metabolism (STAR) family of hnRNP K homology (KH)-domain-containing RBPs, the dominant view with loose experimental evidence is that these proteins are tumor suppressors. However, recent developments support a pro-oncogenic role for archetypical STAR protein Sam68. Sam68-null mice are not prone to cancer, but instead display pronounced defects in mammary gland ductal development, and haploinsufficiency of Sam68 impedes mammary tumor onset and tumor multiplicity in mouse models expressing the mammary-targeted polyoma middle T antigen oncogene. These advances have increased the interest in the role of Sam68 as a positive regulator of cancer progression and position Sam68 as a viable therapeutic target. Retrospective and perspective implications of Sam68 in cancer are discussed.

The association of Sam68 with Vav1 contributes to tumorigenesis

Vav1 functions in the hematopoietic system as a specific GDP/GTP nucleotide exchange factor regulated by tyrosine phosphorylation. An intact C-terminal SH3 domain of Vav1 (Vav1SH3C) was shown to be necessary for Vav1-induced transformation, yet the associating protein(s) necessary for this activity have not yet been identified. Using a proteomics approach, we identified Sam68 as a Vav1SH3C-associating protein. Sam68 (Src-associated in mitosis of 68 kD) belongs to the heteronuclear ribonucleoprotein particle K (hnRNP-K) homology (KH) domain family of RNA-binding proteins. The Vav1/Sam68 interaction was observed in vitro and in vivo. Mutants of Vav1SH3C previously shown to lose their transforming potential did not associate with Sam68. Co-expression of Vav1 and Sam68 in Jurkat T cells led to increased localization of Vav1 in the nucleus and changes in cell morphology. We then tested the contribution of Sam68 to known functions of Vav1, such as focus-forming in NIH3T3 fibroblasts and NFAT stimulation in T cells. Co-expression of oncogenic Vav1 with Sam68 in NIH3T3 fibroblasts resulted in a dose-dependent increase in foci, yet no further enhancement of NFAT activity was observed in Jurkat T cells, as compared to cells overexpressing only Vav1 or Sam68. Our results strongly suggest that Sam68 contributes to transformation by oncogenic Vav1.

Cooperative activation of Src family kinases by SH3 and SH2 ligands

Src family nonreceptor tyrosine kinases are kept in a repressed state by intramolecular interactions involving the SH3 and SH2 domains of the enzymes. Ligands for these domains can displace the intramolecular associations and activate the kinases. Here, we carried out in vitro activation experiments with purified, down-regulated hematopoietic cell kinase (Hck), a Src family kinase. We show that SH3 and SH2 ligands act cooperatively to activate Src family kinases: the presence of one ligand lowers the concentration of the second ligand necessary for activation. To confirm the findings in intact cells, we studied Cas, a Src substrate that possesses SH2 and SH3 ligands. In contrast to wild-type Cas, mutant forms of Cas lacking the SH3 or SH2 ligands were unable to stimulate Src autophosphorylation when expressed in Cas-deficient fibroblasts. Cells expressing the Cas mutants also showed decreased amounts of activated Src at focal adhesions. The results suggest that proteins containing ligands for both SH3 and SH2 domains can produce a synergistic activation of Src family kinases.

Arginine methylation of Sam68 and SLM proteins negatively regulates their poly(U) RNA binding activity

Sam68 (Src substrate associated during mitosis) and its homologues, SLM-1 and SLM-2 (Sam68-like mammalian proteins), are RNA binding proteins and contain the arg-gly (RG) repeats, in which arginine residues are methylated by the protein arginine methyltransferase 1 (PRMT1). However, it remains unclear whether the arginine methylation affects an RNA binding. Here, we report that methylation of Sam68 and SLM proteins markedly reduced their poly(U) binding ability in vitro. The RG repeats of Sam68 bound poly(U), but arginine methylation of the RG repeats abrogated its poly(U) binding ability in vitro. Overexpression of PRMT1 increased arginine methylation of Sam68 and SLM proteins in cells, which resulted in a decrease of their poly(U) binding ability. The results suggest that the RG repeats conserved in Sam68 and SLM proteins may function as an auxiliary RNA binding domain and arginine methylation may eliminate or reduce an RNA binding ability of the proteins.

The RNA-binding protein Sam68 contributes to proliferation and survival of human prostate cancer cells

The tyrosine kinase Src is frequently activated in advanced human prostate carcinomas and its activation correlates with tyrosine phosphorylation of the RNA-binding protein Sam68. Herein, we have investigated the expression and function of Sam68 in human prostate cancer cells. Analysis of specimens obtained from 20 patients revealed that Sam68 is upregulated at the protein level in 35% of the samples. Real-time polymerase chain reaction confirmed the results at the mRNA level in most patients. Downregulation of Sam68 by RNAi in LNCaP prostate cancer cells delayed cell cycle progression and reduced the proliferation rate. Moreover, depletion of Sam68 sensitized cells to apoptosis induced by DNA-damaging agents. Similarly, stable cell lines expressing a truncated GFP-Sam68(GSG) protein displayed reduced growth rates and higher sensitivity to cisplatin-induced apoptosis. Microarray analyses revealed that a subset of genes involved in proliferation and apoptosis were altered when Sam68 was knocked down in LNCaP cells. Our results indicate that Sam68 expression supports prostate cancer cells proliferation and survival to cytotoxic agents.

c-Src control of chloride channel support for osteoclast HCl transport and bone resorption

Bone degradation by osteoclasts depends upon active transport of hydrogen ions to solubilize bone mineral. This transport is supported by the parallel actions of a proton ATPase and a chloride channel located in the osteoclast ruffled membrane. We have previously identified a novel chloride channel, p62, which appears to be the avian counterpart to CLIC-5b and is expressed coincident with the appearance of acid secretion as avian osteoclasts differentiate in culture. In this article, we show that suppression of CLIC-5b in differentiating avian osteoclasts results in decreased acidification by vesicles derived from these cells and decreased ability of the cells to resorb bone. Acidification is rescued by the presence of valinomycin, consistent with a selective loss of chloride channel but not proton pump activity. Osteoclast bone resorption is known to be dependent on the expression of the tyrosine kinase, c-Src. We show that CLIC-5b from osteoclasts has affinity for both Src SH2 and SH3 domains. We find that suppression of expression of Src in developing osteoclasts results in decreased vesicular acidification, which is rescued by valinomycin, consistent with the loss of chloride conductance in the proton pump-containing vesicles. Suppression of c-Src causes no change in the steady state level of CLIC-5b expression, but does result in failure of proton pump and CLIC-5b to colocalize in cultured osteoclast precursors. We conclude that suppression of c-Src interferes with osteoclast bone resorption by disrupting functional co-localization of proton pump and CLIC-5b.

Physical and functional interaction of Fyn tyrosine kinase with a brain-enriched Rho GTPase-activating protein TCGAP

Fyn, a member of the Src family of tyrosine kinases, is implicated in both brain development and adult brain function. In the present study, we identified a Rho GTPase-activating protein (GAP), TCGAP (Tc10/Cdc42 GTPase-activating protein), as a novel Fyn substrate. TCGAP interacted with Fyn and was phosphorylated by Fyn, with Tyr-406 in the GAP domain as a major Fyn-mediated phosphorylation site. Fyn suppressed the GAP activity of wild-type TCGAP but not the Y406F mutant of TCGAP in a phosphorylation-dependent manner, suggesting that Fyn-mediated Tyr-406 phosphorylation negatively regulated the TCGAP activity. In situ hybridization analyses showed that TCGAP mRNA was expressed prominently in both immature and adult mouse brain, with high levels in cortex, corpus striatum, hippocampus, and olfactory bulb. Overexpression of wild-type TCGAP in PC12 cells suppressed nerve growth factor-induced neurite outgrowth, whereas a GAP-defective mutant of TCGAP enhanced the neurite outgrowth. Nerve growth factor enhanced tyrosine phosphorylation of TCGAP through activation of Src family kinases. These results suggest that TCGAP is involved in Fyn-mediated regulation of axon and dendrite outgrowth.

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.

Nuclear compartmentalization of FAK and FRNK in cardiac myocytes

Focal adhesion kinase (FAK) and FAK-related non-kinase (FRNK) accumulate in the nucleus of cardiac myocytes during hypertensive hypertrophy. Nuclear FAK and FRNK are phosphorylated on different serines and form distinct bright spots. The subnuclear distribution of serine-phosphorylated FAK and FRNK was examined in this study by double labeling with fibrillarin, a component of nucleoli, and Sam68, a constituent of Sam68 nuclear bodies. We also investigated the role of protein kinase C (PKC)-mediated phosphorylation of FAK and FRNK on nuclear translocation. PKC activation by 12-O-tetradecanoylphorbol 13-acetate treatment increased serine phosphorylation of FAK and FRNK. Specifically, FAK was phosphorylated on serine 722 but not serine 910. On the other hand, FRNK was phosphorylated on serine 217, the equivalent site of FAK serine 910, but not serine 30, the homologous site of FAK serine 722. Serine-phosphorylated FAK and FRNK redistributed into the nucleus and formed distinct patterns. FAK with phosphorylation on serine 722 colocalized with Sam68 but not fibrillarin. On the contrary, FRNK phosphorylated on 217 coexisted with fibrillarin but not Sam68. Immunoprecipitation also confirmed that FAK associated with Sam68 and FRNK interacted with fibrillarin, respectively. These results suggest that FAK and FRNK target different nuclear subdomains by their association with distinct nuclear proteins.

Ablation of the Sam68 RNA binding protein protects mice from age-related bone loss

The Src substrate associated in mitosis of 68 kDa (Sam68) is a KH-type RNA binding protein that has been shown to regulate several aspects of RNA metabolism; however, its physiologic role has remained elusive. Herein we report the generation of Sam68-null mice by homologous recombination. Aged Sam68^−/− mice preserved their bone mass, in sharp contrast with 12-month-old wild-type littermates in which bone mass was decreased up to approximately 75%. In fact, the bone volume of the 12-month-old Sam68^−/− mice was virtually indistinguishable from that of 4-month-old wild-type or Sam68^−/− mice. Sam68^−/− bone marrow stromal cells had a differentiation advantage for the osteogenic pathway. Moreover, the knockdown of Sam68 using short hairpin RNA in the embryonic mesenchymal multipotential progenitor C3H10T1/2 cells resulted in more pronounced expression of the mature osteoblast marker osteocalcin when differentiation was induced with bone morphogenetic protein-2. Cultures of mouse embryo fibroblasts generated from Sam68^+/+ and Sam68^−/− littermates were induced to differentiate into adipocytes with culture medium containing pioglitazone and the Sam68^−/− mouse embryo fibroblasts shown to have impaired adipocyte differentiation. Furthermore, in vivo it was shown that sections of bone from 12-month-old Sam68^−/− mice had few marrow adipocytes compared with their age-matched wild-type littermate controls, which exhibited fatty bone marrow. Our findings identify endogenous Sam68 as a positive regulator of adipocyte differentiation and a negative regulator of osteoblast differentiation, which is consistent with Sam68 being a modulator of bone marrow mesenchymal cell differentiation, and hence bone metabolism, in aged mice.

Osteoporosis is a debilitating bone disease that is characterized by reduced bone mass and microarchitectural damage, which result in increased bone fragility and susceptibility to fracture. Peak bone mass, which is achieved by the age of 30 in humans, has been identified as a major determinant of resistance or susceptibility to osteoporosis. The authors generated mice deficient for the Sam68 RNA binding protein, a protein of unknown physiologic function. The mice develop normally and are protected against bone loss during aging. Age-related bone loss has long been associated with an increase in marrow adipocytes, which are derived from the same mesenchymal lineage as osteoblasts in bone marrow. The authors showed that Sam68 regulates the differentiation of this mesenchymal lineage, such that in its absence, osteoblasts continued to be generated in aging bone, leading to preservation of bone mass. This study identifies a physiologic role for Sam68 as a modulator of the bone marrow stem cell niche and hence of bone metabolism. The data identify Sam68 as a potential therapeutic target for the prevention and treatment of age-related bone loss.

Tyrosine phosphorylation of sam68 by breast tumor kinase regulates intranuclear localization and cell cycle progression

The breast tumor kinase (BRK) is a growth promoting non-receptor tyrosine kinase overexpressed in the majority of human breast tumors. BRK is known to potentiate the epidermal growth factor (EGF) response in these cells. Although BRK is known to phosphorylate the RNA-binding protein Sam68, the specific tyrosines phosphorylated and the exact role of this phosphorylation remains unknown. Herein, we have generated Sam68 phospho-specific antibodies against C-terminal phosphorylated tyrosine residues within the Sam68 nuclear localization signal. We show that BRK phosphorylates Sam68 on all three tyrosines in the nuclear localization signal. By indirect immunofluorescence we observed that BRK and EGF treatment not only phosphorylates Sam68 but also induces its relocalization. Tyrosine 440 was identified as a principal modulator of Sam68 localization and this site was phosphorylated in response to EGF treatment in human breast tumor cell lines. Moreover, this phosphorylation event was inhibited by BRK small interfering RNA treatment, consistent with Sam68 being a physiological substrate of BRK downstream of the EGF receptor in breast cancer cells. Finally, we observed that Sam68 suppressed BRK-induced cell proliferation, suggesting that Sam68 does indeed contain anti-proliferative properties that may be neutralized in breast cancer cells by phosphorylation.

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.

Structural basis of ventricular remodeling: role of the myocyte

Structural remodeling plays a major role in the progression of various heart diseases to congestive heart failure (CHF). Major contributors to this remodeling process in the heart include alterations in myocyte shape, myocyte number, and extracellular matrix. However, it is unclear as to which of these changes is most critical in the development of CHF, and this may vary by etiology. Myocyte shape alterations largely underlie the increase in chamber diameter/wall thickness characteristic of CHF. This review mainly focuses on the role of myocyte shape in ventricular remodeling. Several signaling molecules have been implicated in this process. As we learn more about the components of myocardial remodeling, new strategies to combat the progression of heart disease should arise.

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.