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

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.

Sam68 is a druggable vulnerability point in cancer stem cells

Sam68 (Src associated in mitosis of 68 kDa) is an RNA-binding and multifunctional protein extensively characterized in numerous cellular functions, such as RNA processing, cell cycle regulation, kinase- and growth factor signaling. Recent investigations highlighted Sam68 as a primary target of a class of reverse-turn peptidomimetic drugs, initially developed as inhibitors of Wnt/β-catenin mediated transcription. Further investigations on such compounds revealed their capacity to selectively eliminate cancer stem cell (CSC) activity upon engaging Sam68. This work highlighted previously unappreciated roles for Sam68 in the maintenance of neoplastic self-renewal and tumor-initiating functions. Here, we discuss the implication of Sam68 in tumorigenesis, where central findings support its contribution to chromatin regulation processes essential to CSCs. We also review advances in CSC-targeting drug discovery aiming to modulate Sam68 cellular distribution and protein-protein interactions. Ultimately, Sam68 constitutes a vulnerability point of CSCs and an attractive therapeutic target to impede neoplastic stemness in human tumors.

DNA Damage Regulates the Functions of the RNA Binding Protein Sam68 through ATM-Dependent Phosphorylation

Simple Summary

Alterations of the complex network of interactions between the DNA damage response pathway and RNA metabolism have been described in several tumors, and increasing efforts are devoted to the elucidation of the molecular mechanisms involved in this network. Previous large-scale proteomic studies identified the RNA binding protein Sam68 as a putative target of the ATM kinase. Herein, we demonstrate that ATM phosphorylates Sam68 upon DNA damage induction, and this post-translational modification regulates both the signaling function of Sam68 in the initial phase of the DNA damage response and its RNA processing activity. Thus, our study uncovers anew crosstalk between ATM and Sam68, which may represent a paradigm for the functional interaction between the DDR pathway and RNA binding proteins, and a possible actionabletarget in human cancers.

Abstract

Cancer cells frequently exhibit dysregulation of the DNA damage response (DDR), genomic instability, and altered RNA metabolism. Recent genome-wide studies have strongly suggested an interaction between the pathways involved in the cellular response to DDR and in the regulation of RNA metabolism, but the molecular mechanism(s) involved in this crosstalk are largely unknown. Herein, we found that activation of the DDR kinase ATM promotes its interaction with Sam68, leading to phosphorylation of this multifunctional RNA binding protein (RBP) on three residues: threonine 61, serine 388 and serine 390. Moreover, we demonstrate that ATM-dependent phosphorylation of threonine 61 promotes the function of Sam68 in the DDR pathway and enhances its RNA processing activity. Importantly, ATM-mediated phosphorylation of Sam68 in prostate cancer cells modulates alternative polyadenylation of transcripts that are targets of Sam68, supporting the notion that the ATM–Sam68 axis exerts a multifaceted role in the response to DNA damage. Thus, our work validates Sam68 as an ATM kinase substrate and uncovers an unexpected bidirectional interplay between ATM and Sam68, which couples the DDR pathway to modulation of RNA metabolism in response to genotoxic stress.

pncCCND1_B Engages an Inhibitory Protein Network to Downregulate CCND1 Expression upon DNA Damage

Promoter-associated noncoding RNAs (pancRNAs) represent a class of noncoding transcripts driven from the promoter region of protein-coding or non-coding genes that operate as cis-acting elements to regulate the expression of the host gene. PancRNAs act by altering the chromatin structure and recruiting transcription regulators. PncCCND1_B is driven by the promoter region of CCND1 and regulates CCND1 expression in Ewing sarcoma through recruitment of a multi-molecular complex composed of the RNA binding protein Sam68 and the DNA/RNA helicase DHX9. In this study, we investigated the regulation of CCND1 expression in Ewing sarcoma cells upon exposure to chemotherapeutic drugs. Pan-inhibitor screening indicated that etoposide, a drug used for Ewing sarcoma treatment, promotes transcription of pncCCND1_B and repression of CCND1 expression. RNA immunoprecipitation experiments showed increased binding of Sam68 to the pncCCND1_B after treatment, despite the significant reduction in DHX9 protein. This effect was associated with the formation of DNA:RNA duplexes at the CCND1 promoter. Furthermore, Sam68 interacted with HDAC1 in etoposide treated cells, thus contributing to chromatin remodeling and epigenetic changes. Interestingly, inhibition of the ATM signaling pathway by KU 55,933 treatment was sufficient to inhibit etoposide-induced Sam68-HDAC1 interaction without rescuing DHX9 expression. In these conditions, the DNA:RNA hybrids persist, thus contributing to the local chromatin inactivation at the CCND1 promoter region. Altogether, our results show an active role of Sam68 in DNA damage signaling and chromatin remodeling on the CCND1 gene by fine-tuning transitions of epigenetic complexes on the CCND1 promoter.

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.

Coordination of RNA Processing Regulation by Signal Transduction Pathways

Signal transduction pathways transmit the information received from external and internal cues and generate a response that allows the cell to adapt to changes in the surrounding environment. Signaling pathways trigger rapid responses by changing the activity or localization of existing molecules, as well as long-term responses that require the activation of gene expression programs. All steps involved in the regulation of gene expression, from transcription to processing and utilization of new transcripts, are modulated by multiple signal transduction pathways. This review provides a broad overview of the post-translational regulation of factors involved in RNA processing events by signal transduction pathways, with particular focus on the regulation of pre-mRNA splicing, cleavage and polyadenylation. The effects of several post-translational modifications (i.e., sumoylation, ubiquitination, methylation, acetylation and phosphorylation) on the expression, subcellular localization, stability and affinity for RNA and protein partners of many RNA-binding proteins are highlighted. Moreover, examples of how some of the most common signal transduction pathways can modulate biological processes through changes in RNA processing regulation are illustrated. Lastly, we discuss challenges and opportunities of therapeutic approaches that correct RNA processing defects and target signaling molecules.

Sam68 splicing regulation contributes to motor unit establishment in the postnatal skeletal muscle

Sam68 ensures the establishment of neuromuscular junctions (NMJs) and motor unit integrity by orchestrating a neuronal splicing program.

RNA-binding proteins orchestrate the composite life of RNA molecules and impact most physiological processes, thus underlying complex phenotypes. The RNA-binding protein Sam68 regulates differentiation processes by modulating splicing, polyadenylation, and stability of select transcripts. Herein, we found that Sam68^−/− mice display altered regulation of alternative splicing in the spinal cord of key target genes involved in synaptic functions. Analysis of the motor units revealed that Sam68 ablation impairs the establishment of neuromuscular junctions and causes progressive loss of motor neurons in the spinal cord. Importantly, alterations of neuromuscular junction morphology and properties in Sam68^−/− mice correlate with defects in muscle and motor unit integrity. Sam68^−/− muscles display defects in postnatal development, with manifest signs of atrophy. Furthermore, fast-twitch muscles in Sam68^−/− mice show structural features typical of slow-twitch muscles, suggesting alterations in the metabolic and functional properties of myofibers. Collectively, our data identify a key role for Sam68 in muscle development and suggest that proper establishment of motor units requires timely expression of synaptic splice variants.

Emerging Contribution of PancRNAs in Cancer

“Cancer” includes a heterogeneous group of diseases characterized by abnormal growth beyond natural boundaries. Neoplastic transformation of cells is orchestrated by multiple molecular players, including oncogenic transcription factors, epigenetic modifiers, RNA binding proteins, and coding and noncoding transcripts. The use of computational methods for global and quantitative analysis of RNA processing regulation provides new insights into the genomic and epigenomic features of the cancer transcriptome. In particular, noncoding RNAs are emerging as key molecular players in oncogenesis. Among them, the promoter-associated noncoding RNAs (pancRNAs) are noncoding transcripts acting in cis to regulate their host genes, including tumor suppressors and oncogenes. In this review, we will illustrate the role played by pancRNAs in cancer biology and will discuss the latest findings that connect pancRNAs with cancer risk and progression. The molecular mechanisms involved in the function of pancRNAs may open the path to novel therapeutic opportunities, thus expanding the repertoire of targets to be tested as anticancer agents in the near future.

The Role of RNA Splicing Factors in Cancer: Regulation of Viral and Human Gene Expression in Human Papillomavirus-Related Cervical Cancer

The spliceosomal complex components, together with the heterogeneous nuclear ribonucleoproteins (hnRNPs) and serine/arginine-rich (SR) proteins, regulate the process of constitutive and alternative splicing, the latter leading to the production of mRNA isoforms coding multiple proteins from a single pre-mRNA molecule. The expression of splicing factors is frequently deregulated in different cancer types causing the generation of oncogenic proteins involved in cancer hallmarks. Cervical cancer is caused by persistent infection with oncogenic human papillomaviruses (HPVs) and constitutive expression of viral oncogenes. The aberrant activity of hnRNPs and SR proteins in cervical neoplasia has been shown to trigger the production of oncoproteins through the processing of pre-mRNA transcripts either derived from human genes or HPV genomes. Indeed, hnRNP and SR splicing factors have been shown to regulate the production of viral oncoprotein isoforms necessary for the completion of viral life cycle and for cell transformation. Target-therapy strategies against hnRNPs and SR proteins, causing simultaneous reduction of oncogenic factors and inhibition of HPV replication, are under development. In this review, we describe the current knowledge of the functional link between RNA splicing factors and deregulated cellular as well as viral RNA maturation in cervical cancer and the opportunity of new therapeutic strategies.

Leptin, Adiponectin, and Sam68 in Bone Metastasis from Breast Cancer

The most serious aspect of neoplastic disease is the spread of cancer cells to secondary sites. Skeletal metastases can escape detection long after treatment of the primary tumour and follow-up. Bone tissue is a breeding ground for many types of cancer cells, especially those derived from the breast, prostate, and lung. Despite advances in diagnosis and therapeutic strategies, bone metastases still have a profound impact on quality of life and survival and are often responsible for the fatal outcome of the disease. Bone and the bone marrow environment contain a wide variety of cells. No longer considered a passive filler, bone marrow adipocytes have emerged as critical contributors to cancer progression. Released by adipocytes, adipokines are soluble factors with hormone-like functions and are currently believed to affect tumour development. Src-associated in mitosis of 68 kDa (Sam68), originally discovered as a protein physically associated with and phosphorylated by c-Src during mitosis, is now recognised as an important RNA-binding protein linked to tumour onset and progression of disease. Sam68 also regulates splicing events and recent evidence reports that dysregulation of these events is a key step in neoplastic transformation and tumour progression. The present review reports recent findings on adipokines and Sam68 and their role in breast cancer progression and metastasis.

The Promoter-Associated Noncoding RNA pncCCND1_B Assembles a Protein-RNA Complex to Regulate Cyclin D1 Transcription in Ewing Sarcoma

Most Ewing sarcomas are characterized by the in-frame chromosomal translocation t(11;22) generating the EWS-FLI1 oncogene. EWS-FLI1 protein interacts with the RNA helicase DHX9 and affects transcription and processing of genes involved in neoplastic transformation, including CCND1 (the cyclin D1 gene), which contributes to cell-cycle dysregulation in cancer. In this study, we found that CCND1 expression is significantly higher in patients with Ewing sarcoma compared with other sarcomas and that the pncCCND1_B RNA, a previously uncharacterized CCND1 promoter-associated noncoding (pnc) transcript, is expressed in Ewing sarcoma cells. PncCCND1_B interacted with the RNA-binding protein Sam68 and repressed CCND1 expression. Notably, knockdown of Sam68 affected pncCCND1_B subcellular localization and cyclin D1 expression. Pharmacologic impairment of DHX9/EWS-FLI1 interaction promoted RNA-dependent association of Sam68 with DHX9 and recruitment of Sam68 to the CCND1 promoter, thus repressing it. Conversely, mitogenic stimulation of Ewing sarcoma cells with IGF1 impaired Sam68/DHX9 interaction and positively regulated CCND1 expression. These studies uncover a fine-tuned modulation of the proto-oncogene CCND1 in Ewing sarcoma cells via alternative complexes formed by DHX9 with either EWS-FLI1 or pncCCND1_B-Sam68. SIGNIFICANCE: A pncRNA-based mechanism represses expression of CCND1 through the formation of a protein-RNA complex and provides new therapeutic opportunities for patients with Ewing sarcoma.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/79/14/3570/F1.large.jpg.

The stem cell factor (SCF)/c-KIT signalling in testis and prostate cancer

The stem cell factor (SCF) is a cytokine that specifically binds the tyrosine kinase receptor c-KIT. The SCF/c-KIT interaction leads to receptor dimerization, activation of kinase activity and initiation of several signal transduction pathways that control cell proliferation, apoptosis, differentiation and migration in several tissues. The activity of SCF/c-KIT system is linked with the phosphatidylinositol 3-kinase (PI3-K), the Src, the Janus kinase/signal transducers and activators of transcription (JAK/STAT), the phospholipase-C (PLC-γ) and the mitogen-activated protein kinase (MAPK) pathways. Moreover, it has been reported that cancer cases display an overactivation of c-KIT due to the presence of gain-of-function mutations or receptor overexpression, which renders c-KIT a tempting target for cancer treatment. In the case of male cancers the most documented activated pathways are the PI3-K and Src, both enhancing abnormal cell proliferation. It is also known that the Src activity in prostate cancer cases depends on the presence of tr-KIT, the cytoplasmic truncated variant of c-KIT that is specifically expressed in tumour tissues and, thus, a very interesting target for drug development. The present review provides an overview of the signalling pathways activated by SCF/c-KIT and discusses the potential application of c-KIT inhibitors for treatment of testicular and prostatic cancers.

Expression of Sam68 Correlates With Cell Proliferation and Survival in Epithelial Ovarian Cancer

Src associated in mitosis, 68 kDa (Sam68) is a KH domain RNA-binding protein that belongs to the signal transduction and activation of RNA family. It is a multifunctional protein known to regulate cellular signal transduction, transcription, RNA metabolism, proliferation, and apoptosis, thus implicated in tumor growth. Herein, we investigated the clinical significance of Sam68 in human epithelial ovarian cancer (EOC). Western blot and immunohistochemical staining demonstrated that Sam68 expression was upregulated in EOC tissues and cell lines. Statistical analysis showed that high expression of Sam68 correlated with poor prognosis of patients with EOC. In vitro, serum starvation-refeeding experiment was primarily performed to confirm that Sam68 participated in the cell cycle progression of EOC cell lines. Then knocking down Sam68 level with small interfering RNA, cell cycle was arrested at G1 phase and cell proliferation impaired. Furthermore, we demonstrated that the antiproliferative effect of silencing Sam68 in EOC cells was associated with the upregulation of cyclin-dependent kinase inhibitors p21^Cip1 and p27^Kip1, along with the downregulation of p-FOXO3a, p-Akt, and p-GSK-3β. Taken together, our findings uncovered that Sam68 played an important role in promoting the proliferation of human ovarian cancer, thereby might be a novel therapeutic target for EOC.

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.

Paradoxical and contradictory effects of imatinib in two cell line models of hormone-refractory prostate cancer

BACKGROUND

Imatinib mesylate is a chemotherapeutic drug that inhibits the tyrosine kinase activity of c-KIT and has been successfully used to treat leukemias and some solid tumors. However, its application for treatment of hormone-refractory prostate cancer (HRPC) has shown modest effectiveness and did not follow the outcomes in cultured cells or animal models. Moreover, the molecular pathways by which imatinib induces cytotoxicity in prostate cancer cells are poorly characterized.

METHODS

Two cell line models of HRPC (DU145 and PC3) were exposed to 20 μM of imatinib for 6-72 hr. MTS assay was used to assess cell viability during the course of experiment. Gene expression analysis of c-KIT, cell-cycle and apoptosis regulators, and angiogenic factors was determined by means of real-time PCR, western blot, and/or immunocytochemistry. The enzymatic activity of the apoptosis effector, caspase-3, was determined by a colorimetric assay.

RESULTS

Imatinib significantly decreased the viability of DU145 cells but paradoxically augmented the viability of PC3 cells. DU145 cells displayed diminished expression of anti-apoptotic Bcl-2 protein and augmented levels of caspase-8 and -9, as well as, increased enzymatic activity of caspase-3 in response to imatinib. No differences existed on the expression levels of apoptosis-related proteins in PC3 cells treated with imatinib, though the activity of caspase-3 was decreased. The mRNA levels of angiogenic factor VEGF were decreased in DU145-treated cells, whereas an opposite effect was seen in PC3. In addition, it was shown that DU145 and PC3 cells present a differential expression of c-KIT protein variants.

CONCLUSION

DU145 and PC3 cells displayed a contradictory behavior in response to imatinib, which was underpinned by a distinct expression pattern (or activity) of target regulators of cell-cycle, apoptosis, and angiogenesis. The paradoxical effect of imatinib in PC3 cells may be related with the differential expression of c-KIT protein variants. Moreover, the present findings helped to understand the discrepancies in the efficacy of imatinib as therapeutic option in HRPC.

The SCF/c-KIT system in the male: Survival strategies in fertility and cancer

Maintaining the delicate balance between cell survival and death is of the utmost importance for the proper development of germ cells and subsequent fertility. On the other hand, the fine regulation of tissue homeostasis by mechanisms that control cell fate is a factor that can prevent carcinogenesis. c-KIT is a type III receptor tyrosine kinase activated by its ligand, stem cell factor (SCF). c-KIT signaling plays a crucial role in cell fate decisions, specifically controlling cell proliferation, differentiation, survival, and apoptosis. Indeed, deregulating the SCF/c-KIT system by attenuation or overactivation of its signaling strength is linked to male infertility and cancer, and rebalancing its activity via c-KIT inhibitors has proven beneficial in treating human tumors that contain gain-of-function mutations or overexpress c-KIT. This review addresses the roles of SCF and c-KIT in the male reproductive tract, and discusses the potential application of c-KIT target therapies in disorders of the reproductive system.

Regulation of alternative pre-mRNA splicing

Alternative splicing plays a prevalent role in generating functionally diversified proteomes from genomes with a more limited repertoire of protein-coding genes. Alternative splicing is frequently regulated with cell type or developmental specificity and in response to signaling pathways, and its mis-regulation can lead to disease. Co-regulated programs of alternative splicing involve interplay between a host of cis-acting transcript features and trans-acting RNA-binding proteins. Here, we review the current state of understanding of the logic and mechanism of regulated alternative splicing and indicate how this understanding can be exploited to manipulate splicing for therapeutic purposes.

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.

Protein-tyrosine kinase signaling in the biological functions associated with sperm

In sexual reproduction, two gamete cells (i.e., egg and sperm) fuse (fertilization) to create a newborn with a genetic identity distinct from those of the parents. In the course of these developmental processes, a variety of signal transduction events occur simultaneously in each of the two gametes, as well as in the fertilized egg/zygote/early embryo. In particular, a growing body of knowledge suggests that the tyrosine kinase Src and/or other protein-tyrosine kinases are important elements that facilitate successful implementation of the aforementioned processes in many animal species. In this paper, we summarize recent findings on the roles of protein-tyrosine phosphorylation in many sperm-related processes (from spermatogenesis to epididymal maturation, capacitation, acrosomal exocytosis, and fertilization).

Stem Cell Factor Receptor/c-Kit: From Basic Science to Clinical Implications

Stem cell factor (SCF) is a dimeric molecule that exerts its biological functions by binding to and activating the receptor tyrosine kinase c-Kit. Activation of c-Kit leads to its autophosphorylation and initiation of signal transduction. Signaling proteins are recruited to activated c-Kit by certain interaction domains (e.g., SH2 and PTB) that specifically bind to phosphorylated tyrosine residues in the intracellular region of c-Kit. Activation of c-Kit signaling has been found to mediate cell survival, migration, and proliferation depending on the cell type. Signaling from c-Kit is crucial for normal hematopoiesis, pigmentation, fertility, gut movement, and some aspects of the nervous system. Deregulated c-Kit kinase activity has been found in a number of pathological conditions, including cancer and allergy. The observation that gain-of-function mutations in c-Kit can promote tumor formation and progression has stimulated the development of therapeutics agents targeting this receptor, e.g., the clinically used inhibitor imatinib mesylate. Also other clinically used multiselective kinase inhibitors, for instance, sorafenib and sunitinib, have c-Kit included in their range of targets. Furthermore, loss-of-function mutations in c-Kit have been observed and shown to give rise to a condition called piebaldism. This review provides a summary of our current knowledge regarding structural and functional aspects of c-Kit signaling both under normal and pathological conditions, as well as advances in the development of low-molecular-weight molecules inhibiting c-Kit function.

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.

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.

The splicing regulator Sam68 binds to a novel exonic splicing silencer and functions in SMN2 alternative splicing in spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neurodegenerative disease caused by loss of motor neurons in patients with null mutations in the SMN1 gene. An almost identical SMN2 gene is unable to compensate for this deficiency because a single C-to-T transition at position +6 in exon-7 causes skipping of the exon by a mechanism not yet fully elucidated. We observed that the C-to-T transition in SMN2 creates a putative binding site for the RNA-binding protein Sam68. RNA pull-down assays and UV-crosslink experiments showed that Sam68 binds to this sequence. In vivo splicing assays showed that Sam68 triggers SMN2 exon-7 skipping. Moreover, mutations in the Sam68-binding site of SMN2 or in the RNA-binding domain of Sam68 completely abrogated its effect on exon-7 skipping. Retroviral infection of dominant-negative mutants of Sam68 that interfere with its RNA-binding activity, or with its binding to the splicing repressor hnRNP A1, enhanced exon-7 inclusion in endogenous SMN2 and rescued SMN protein expression in fibroblasts of SMA patients. Our results thus indicate that Sam68 is a novel crucial regulator of SMN2 splicing.

Genotoxic stress causes the accumulation of the splicing regulator Sam68 in nuclear foci of transcriptionally active chromatin

DNA-damaging agents cause a multifaceted cellular stress response. Cells set in motion either repair mechanisms or programmed cell death pathways, depending on the extent of the damage and on their ability to withstand it. The RNA-binding protein (RBP) Sam68, which is up-regulated in prostate carcinoma, promotes prostate cancer cell survival to genotoxic stress. Herein, we have investigated the function of Sam68 in this cellular response. Mitoxantrone (MTX), a topoisomerase II inhibitor, induced relocalization of Sam68 from the nucleoplasm to nuclear granules, together with several other RBPs involved in alternative splicing, such as TIA-1, hnRNP A1 and the SR proteins SC35 and ASF/SF2. Sam68 accumulation in nuclear stress granules was independent of signal transduction pathways activated by DNA damage. Using BrU labelling and immunofluorescence, we demonstrate that MTX-induced nuclear stress granules are transcriptionally active foci where Sam68 and the phosphorylated form of RNA polymerase II accumulate. Finally, we show that MTX-induced relocalization of Sam68 correlates with changes in alternative splicing of its mRNA target CD44, and that MTX-induced CD44 splicing depends on Sam68 expression. These results strongly suggest that Sam68 is part of a RNA-mediated stress response of the cell that modulates alternative splicing in response to DNA damage.

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.

Alternative splicing modulates autoinhibition and SH3 accessibility in the Src kinase Fyn

Src family kinases are central regulators of a large number of signaling pathways. To adapt to the idiosyncrasies of different cell types, these kinases may need a fine-tuning of their intrinsic molecular control mechanisms. Here, we describe on a molecular level how the Fyn kinase uses alternative splicing to adapt to different cellular environments. Using structural analysis, site-directed mutagenesis, and functional analysis, we show how the inclusion of either exon 7A or 7B affects the autoinhibition of Fyn and how this changes the SH3-dependent interaction and tyrosine phosphorylation of Sam68, with functional consequences for the Sam68-regulated survival of epithelial cells. Our results illustrate a novel mechanism of evolution that may contribute to the complexity of Src kinase regulation.

Sam68 regulates a set of alternatively spliced exons during neurogenesis

Sam68 (Src-associated in mitosis, 68 kDa) is a KH domain RNA binding protein implicated in a variety of cellular processes, including alternative pre-mRNA splicing, but its functions are not well understood. Using RNA interference knockdown of Sam68 expression and splicing-sensitive microarrays, we identified a set of alternative exons whose splicing depends on Sam68. Detailed analysis of one newly identified target exon in epsilon sarcoglycan (Sgce) showed that both RNA elements distributed across the adjacent introns and the RNA binding activity of Sam68 are necessary to repress the Sgce exon. Sam68 protein is upregulated upon neuronal differentiation of P19 cells, and many Sam68 RNA targets change in expression and splicing during this process. When Sam68 is knocked down by short hairpin RNAs, many Sam68-dependent splicing changes do not occur and P19 cells fail to differentiate. We also found that the differentiation of primary neuronal progenitor cells from embryonic mouse neocortex is suppressed by Sam68 depletion and promoted by Sam68 overexpression. Thus, Sam68 controls neurogenesis through its effects on a specific set of RNA targets.

Murine hematopoietic stem cells and multipotent progenitors express truncated intracellular form of c-kit receptor

The c-kit receptor plays a vital role in self-renewal and differentiation of hematopoietic stem cells (HSCs) and multipotent progenitors (MPPs). We have discovered that besides c-kit, the murine multipotent HSC/MPP-like cell line EML expresses the transcript and protein for a truncated intracellular form of c-kit receptor, called tr-kit. Notably, the tr-kit transcript and protein levels were down-regulated during cytokine-induced differentiation of the HSC/MPP-like cell line EML into myeloerythroid lineages. These findings prompted us to analyze tr-kit expression in purified murine fetal liver and bone marrow cell populations containing long-term repopulating (LTR) HSCs, short-term repopulating (STR) HSCs, MPPs, lineage-committed progenitors, and immature blood cells. Remarkably, these studies have revealed that in contrast to more widespread expression of c-kit, tr-kit is transcribed solely in cell populations enriched for LTR-HSCs, STR-HSCs, and MPPs. On the other hand, cell populations in which HSCs and MPPs are either present at a much lower frequency or are absent altogether, cells representing more advanced stages of differentiation into lymphoid and myeloid lineages do not express tr-kit. The observation that tr-kit is co-expressed with c-kit only in more primitive HSC- and MPP-enriched cell populations raises an exciting possibility that tr-kit functions either as a new component of the stem cell factor (SCF)/c-kit pathway or is involved in a novel signaling pathway, present exclusively in HSC and MPPs. Taken together, these findings necessitate functional characterization of tr-kit and analysis of its potential role in the self-renewal, proliferation, and/or differentiation of HSC and multipotent progenitors.

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.

Dynamic expression of the RNA-binding protein Sam68 during mouse pre-implantation development

The STAR protein Sam68 (KHDRBS1) is involved in several aspects of post-transcriptional mRNA metabolism. Herein, we have investigated the expression and subcellular localization of Sam68 during early mouse embryogenesis. We found that mouse oocytes express high levels of Sam68 mRNA, low levels of the transcript for Khdrbs2 (current symbol for Slm-1) and no Khdrbs3 (current symbol for Slm-2), two highly homologous STAR genes. Sam68 protein is expressed throughout oocyte meiotic maturation and early embryogenesis. It is released in the cytoplasm upon meiotic resumption and it slowly accumulates in the nucleus after fertilization. Unlike what was observed for other RNA-binding proteins, nuclear accumulation of Sam68 was independent of de novo mRNA transcription. However, we found that inhibition of mRNA translation by either cycloheximide or puromycin in one-cell embryos caused the accumulation of Sam68 in cytoplasmic granules. Analysis of these granules by deconvolution microscopy demonstrated that they are sites of accumulation for proteins involved in the initiation of mRNA translation, such as eIF4A1, eIF4E and eIF4G. These granules contained RNA and were dissolved by treatment with RNase A. Other proteins expressed by the zygote, like the splicing factor SC35 or the cytoplasmic kinase ERK2, did not accumulate in such structures after treatment with inhibitors of mRNA translation, indicating that the localization of Sam68 and of the translation initiation factors in these granules is a specific event. These results indicate that Sam68 is involved in translational regulation of maternal mRNAs in the zygote and in the early signaling events triggered by fertilization.

The RNA-binding protein Sam68 modulates the alternative splicing of Bcl-x

The RNA-binding protein Sam68 is involved in apoptosis, but its cellular mRNA targets and its mechanism of action remain unknown. We demonstrate that Sam68 binds the mRNA for Bcl-x and affects its alternative splicing. Depletion of Sam68 by RNA interference caused accumulation of antiapoptotic Bcl-x(L), whereas its up-regulation increased the levels of proapoptotic Bcl-x(s). Tyrosine phosphorylation of Sam68 by Fyn inverted this effect and favored the Bcl-x(L) splice site selection. A point mutation in the RNA-binding domain of Sam68 influenced its splicing activity and subnuclear localization. Moreover, coexpression of ASF/SF2 with Sam68, or fusion with an RS domain, counteracted Sam68 splicing activity toward Bcl-x. Finally, Sam68 interacted with heterogenous nuclear RNP (hnRNP) A1, and depletion of hnRNP A1 or mutations that impair this interaction attenuated Bcl-x(s) splicing. Our results indicate that Sam68 plays a role in the regulation of Bcl-x alternative splicing and that tyrosine phosphorylation of Sam68 by Src-like kinases can switch its role from proapoptotic to antiapoptotic in live cells.

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.

The role of c-KIT in tumorigenesis: evaluation in canine cutaneous mast cell tumors

The c-KIT proto-oncogene has been implicated in the pathogenesis of several neoplastic diseases, including gastrointestinal stromal tumors and mastocytosis in humans, and mast cell tumors (MCTs) in canines. Cutaneous MCTs are common neoplasms in dogs and have a variable biologic behavior. The goal of this study was to define the prognostic significance of c-KIT mutations identified in canine MCTs and the associations between c-KIT mutations, KIT localization, and KIT expression levels. Microdissection and polymerase chain reaction were performed on 60 MCTs to identify c-KIT mutations. Anti-KIT antibodies were used for immunohistochemical evaluation of KIT localization. Forty-two MCTs were included in a tissue microarray, and KIT expression was quantified using immunofluorescence. Canine MCTs with c-KIT mutations were significantly associated with an increased incidence of recurrent disease and death. c-KIT mutations were also significantly associated with aberrant protein localization; however, the level of KIT expression did not correlate with either c-KIT mutations or changes in protein localization. Considering the high prevalence of canine MCTs and the central role of c-KIT in the tumorigenesis of certain tumors, canine MCTs are an excellent model for characterizing the role of c-KIT in neoplastic diseases and is a potential target for novel therapeutic agents in clinical trials.

Expression of T-STAR gene is associated with regulation of telomerase activity in human colon cancer cell line HCT-116

AIM: To investigate the effects on telomerase activity of transfection of human T-STAR gene full-length sense cDNA or partial antisense cDNA into human colon cancer cell line HCT-116.

METHODS: mRNA and protein expression levels of T-STAR gene were determined by RT-PCR and western blot, and telomerase activity was measured by PCR-ELISA, after transfection of T-STAR sense or antisense gene into HCT-116 cells with lipofectamine.

RESULTS: T-STAR gene expression was enhanced or knocked down both at mRNA and protein levels, and telomerase activity was significantly increased or decreased.

CONCLUSION: The T-STAR gene may participate in regulation of telomerase activity in human colon cancer HCT-116 cells in a parallel fashion.

The tumour-suppressor protein ASPP1 is nuclear in human germ cells and can modulate ratios of CD44 exon V5 spliced isoforms in vivo

The ASPP1 (Apoptosis Stimulating Protein of p53) protein is an important tumour-suppressor. We have detected a novel protein interaction between the human ASPP1 (hASPP1) protein and the predominantly nuclear adaptor protein SAM68. In the human testis, full-length endogenous hASPP1 protein is located in the nucleus like SAM68, predominantly within meiotic and postmeiotic cells. Mouse ASPP1 (mASPP1) protein is mainly expressed in the brain and testis. The interaction with nuclear SAM68 is likely to be restricted to human germ cells, since endogenous mASPP1 protein is exclusively cytoplasmic. The C-terminal region of hASPP1 efficiently targeted a fused GFP molecule to the nucleus, whereas the N-terminus of hASPP1 targeted GFP to the cytoplasm. In the context of the full-length molecule this cytoplasmic targeting sequence is dominant in HEK293 and Saos-2 cells, since full-length hASPP1-GFP is almost exclusively cytoplasmic. Despite its predominantly cytoplasmic location, we show that ASPP1-GFP expression in HEK293 cells can regulate the ratio of alternative spliced isoforms derived from a pre-mRNA regulated downstream of cytoplasmic signalling pathways, and our data suggest that ASPP1 may operate in this case downstream or parallel to RAS signalling pathways.

The nuclear RNA-binding protein Sam68 translocates to the cytoplasm and associates with the polysomes in mouse spermatocytes

Translational control plays a crucial role during gametogenesis in organisms as different as worms and mammals. Mouse knockout models have highlighted the essential function of many RNA-binding proteins during spermatogenesis. Herein we have investigated the expression and function during mammalian male meiosis of Sam68, an RNA-binding protein implicated in several aspects of RNA metabolism. Sam68 expression and localization within the cells is stage specific: it is expressed in the nucleus of spermatogonia, it disappears at the onset of meiosis (leptotene/zygotene stages), and it accumulates again in the nucleus of pachytene spermatocytes and round spermatids. During the meiotic divisions, Sam68 translocates to the cytoplasm where it is found associated with the polysomes. Translocation correlates with serine/threonine phosphorylation and it is blocked by inhibitors of the mitogen activated protein kinases ERK1/2 and of the maturation promoting factor cyclinB-cdc2 complex. Both kinases associate with Sam68 in pachytene spermatocytes and phosphorylate the regulatory regions upstream and downstream of the Sam68 RNA-binding motif. Molecular cloning of the mRNAs associated with Sam68 in mouse spermatocytes reveals a subset of genes that might be posttranscriptionally regulated by this RNA-binding protein during spermatogenesis. We also demonstrate that Sam68 shuttles between the nucleus and the cytoplasm in secondary spermatocytes, suggesting that it may promote translation of specific RNA targets during the meiotic divisions.

Ethanol induces cell-cycle activity and reduces stem cell diversity to alter both regenerative capacity and differentiation potential of cerebral cortical neuroepithelial precursors

Background

The fetal cortical neuroepithelium is a mosaic of distinct progenitor populations that elaborate diverse cellular fates. Ethanol induces apoptosis and interferes with the survival of differentiating neurons. However, we know little about ethanol's effects on neuronal progenitors. We therefore exposed neurosphere cultures from fetal rat cerebral cortex, to varying ethanol concentrations, to examine the impact of ethanol on stem cell fate.

Results

Ethanol promoted cell cycle progression, increased neurosphere number and increased diversity in neurosphere size, without inducing apoptosis. Unlike controls, dissociated cortical progenitors exposed to ethanol exhibited morphological evidence for asymmetric cell division, and cells derived from ethanol pre-treated neurospheres exhibited decreased proliferation capacity. Ethanol significantly reduced the numbers of cells expressing the stem cell markers CD117, CD133, Sca-1 and ABCG2, without decreasing nestin expression. Furthermore, ethanol-induced neurosphere proliferation was not accompanied by a commensurate increase in telomerase activity. Finally, cells derived from ethanol-pretreated neurospheres exhibited decreased differentiation in response to retinoic acid.

Conclusion

The reduction in stem cell number along with a transient ethanol-driven increase in cell proliferation, suggests that ethanol promotes stem to blast cell maturation, ultimately depleting the reserve proliferation capacity of neuroepithelial cells. However, the lack of a concomitant change in telomerase activity suggests that neuroepithelial maturation is accompanied by an increased potential for genomic instability. Finally, the cellular phenotype that emerges from ethanol pre-treated, stem cell depleted neurospheres is refractory to additional differentiation stimuli, suggesting that ethanol exposure ablates or delays subsequent neuronal differentiation.

Global Phosphoproteome of HT-29 Human Colon Adenocarcinoma Cells

Phosphorylation events in cellular signaling cascades triggered by a variety of cellular stimuli modulate protein function, leading to diverse cellular outcomes including cell division, growth, death, and differentiation. Abnormal regulation of protein phosphorylation due to mutation or overexpression of signaling proteins often results in various disease states. We provide here a list of protein phosphorylation sites identified from HT-29 human colon adenocarcinoma cell line by immobilized metal affinity chromatography (IMAC) combined with liquid chromatography (LC)-tandem mass spectrometry (MS/MS) analysis. In this study, proteins extracted from HT-29 whole cell lysates were digested with trypsin and carboxylate groups on the resulting peptides were converted to methyl esters. Derivatized phosphorylated peptides were enriched using Fe(3+)-chelated metal affinity resin. Phosphopeptides retained by IMAC were separated by high performance liquid chromatography (HPLC) and analyzed by electrospray ionization-quadrupole-time-of-flight (ESI-Q-TOF) mass spectrometry. We identified 238 phosphorylation sites, 213 of which could be conclusively localized to a single residue, from 116 proteins by searching MS/MS spectra against the human protein database using MASCOT. Peptide identification and phosphorylation site assignment were confirmed by manual inspection of the MS/MS spectra. Many of the phosphorylation sites identified in our results have not been described previously in the scientific literature. We attempted to ascribe functionality to the sites identified in this work by searching for potential kinase motifs with Scansite (http://scansite.mit.edu) and obtaining information on kinase substrate selectivity from Pattern Explorer (http://scansite.mit.edu/pe). The list of protein phosphorylation sites identified in the present experiment provides broad information on phosphorylated proteins under normal (asynchronous) cell culture conditions. Sites identified in this study may be utilized as surrogate bio-markers to assess the activity of selected kinases and signaling pathways from different cell states and exogenous stimuli.

Ectopic expression of KitD814Yin spermatids of transgenic mice, interferes with sperm morphogenesis

Kit is a receptor tyrosine kinase that plays a fundamental role during the development of germ cells. Additionally, a truncated product, tr-kit, expressed in haploid spermatids and mature spermatozoa can induce parthenogenetic activation when microinjected into mouse eggs, through the activation of PLCgamma-1. In this work, we induced ectopic expression of a mutated Kit protein, Kit(D814Y) during germ cell development. The in vivo expression of this mutant in spermatids produced malformations in mature spermatozoa, and in the most severe cases, sterility. Ultrastructural analysis indicated that condensing spermatids in the transgenic mouse presented a mislocalization of the manchette; a structure that has a crucial role during the elongation steps of spermiogenesis. This morphogenetic phenotype was accompanied by an increased phosphorylation of PLCgamma-1 in spermatogenic cells. Interestingly, we also found that, in wild-type testis, PLCgamma-1 is specifically phosphorylated in condensing spermatids, coincident with the timing of expression of tr-kit in spermiogenesis. We propose that alterations of PLCgamma-1 activity artificially promoted by ectopic Kit(D814Y) expression are related to the abnormalities of spermiogenesis. Our observations suggest that PLCgamma-1 activity could be involved in the shaping of spermatozoa.

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.