Mammary epithelial-mesenchymal interaction regulates fibronectin alternative splicing via phosphatidylinositol 3-kinase

Impacts and mechanisms of alternative mRNA splicing in cancer metabolism, immune response, and therapeutics

Alternative pre-mRNA splicing (AS) provides the potential to produce diversity at RNA and protein levels. Disruptions in the regulation of pre-mRNA splicing can lead to diseases. With the development of transcriptome and genome sequencing technology, increasing diseases have been identified to be associated with abnormal splicing of mRNAs. In tumors, abnormal alternative splicing frequently plays critical roles in cancer pathogenesis and may be considered as new biomarkers and therapeutic targets for cancer intervention. Metabolic abnormalities and immune disorders are important hallmarks of cancer. AS produces multiple different isoforms and diversifies protein expression, which is utilized by the immune and metabolic reprogramming systems to expand gene functions. The abnormal splicing events contributed to tumor progression, partially due to effects on immune response and metabolic reprogramming. Herein, we reviewed the vital role of alternative splicing in regulating cancer metabolism and immune response. We discussed how alternative splicing regulates metabolic reprogramming of cancer cells and antitumor immune response, and the possible strategies to targeting alternative splicing pathways or splicing-regulated metabolic pathway in the context of anticancer immunotherapy. Further, we highlighted the challenges and discuss the perspectives for RNA-based strategies for the treatment of cancer with abnormally alternative splicing isoforms.

Identification of splice regulators of fibronectin-EIIIA and EIIIB by direct measurement of exon usage in a flow-cytometry based CRISPR screen

The extracellular matrix protein fibronectin (FN) is alternatively spliced in a variety of inflammatory conditions, resulting in increased inclusion of alternative exons EIIIA and EIIIB. Inclusion of these exons affects fibril formation, fibrosis, and inflammation. To define upstream regulators of alternative splicing in FN, we have developed an in vitro flow-cytometry based assay, using RNA-binding probes to determine alternative exon inclusion level in aortic endothelial cells. This approach allows us to detect exon inclusion in the primary transcripts themselves, rather than in surrogate splicing reporters. We validated this assay in cells with and without FN-EIIIA and -EIIIB expression. In a small-scale CRISPR KO screen of candidate regulatory splice factors, we successfully detected known regulators of EIIIA and EIIIB splicing, and detected several novel regulators. Finally, we show the potential in this approach to broadly interrogate upstream signaling pathways in aortic endothelial cells with a genome-wide CRISPR-KO screen, implicating the TNFalpha and RIG-I-like signaling pathways and genes involved in the regulation of fibrotic responses. Thus, we provide a novel means to screen the regulation of splicing of endogenous transcripts, and predict novel pathways in the regulation of FN-EIIIA inclusion.

Hallmarks of Splicing Defects in Cancer: Clinical Applications in the Era of Personalized Medicine

Alternative splicing promotes proteome diversity by using limited number of genes, a key control point of gene expression. Splicing is carried out by large macromolecular machineries, called spliceosome, composed of small RNAs and proteins. Alternative splicing is regulated by splicing regulatory cis-elements in RNA and trans-acting splicing factors that are often tightly regulated in a tissue-specific and developmental stage-specific manner. The biogenesis of ribonucleoprotein (RNP) complexes is strictly regulated to ensure that correct complements of RNA and proteins are coordinated in the right cell at the right time to support physiological functions. Any perturbations that impair formation of functional spliceosomes by disrupting the cis-elements, or by compromising RNA-binding or function of trans-factors can be deleterious to cells and result in pathological consequences. The recent discovery of oncogenic mutations in splicing factors, and growing evidence of the perturbed splicing in multiple types of cancer, underscores RNA processing defects as a critical driver of oncogenesis. These findings have resulted in a growing interest in targeting RNA splicing as a therapeutic approach for cancer treatment. This review summarizes our current understanding of splicing alterations in cancer, recent therapeutic efforts targeting splicing defects in cancer, and future potentials to develop novel cancer therapies.

Splicing-factor alterations in cancers

Tumor-associated alterations in RNA splicing result either from mutations in splicing-regulatory elements or changes in components of the splicing machinery. This review summarizes our current understanding of the role of splicing-factor alterations in human cancers. We describe splicing-factor alterations detected in human tumors and the resulting changes in splicing, highlighting cell-type-specific similarities and differences. We review the mechanisms of splicing-factor regulation in normal and cancer cells. Finally, we summarize recent efforts to develop novel cancer therapies, based on targeting either the oncogenic splicing events or their upstream splicing regulators.

Abnormal regulation of fibronectin production by fibroblasts in psoriasis

BACKGROUND

Data indicate that in psoriasis, abnormalities are already present in nonlesional skin. Transforming growth factor-β and keratinocyte growth factor (KGF), together with fibronectin and α5β1 integrin, were suggested to play a crucial role in the pathogenesis of psoriasis by influencing inflammation and keratinocyte hyperproliferation.

OBJECTIVES

To investigate the expression of KGF, fibroblast growth factor receptor (FGFR)2, fibronectin (FN) and extra domain A (EDA)-positive FN in healthy and nonlesional psoriatic skin, and to study the effect of KGF on the regulation of FN and EDA(+) FN production by fibroblasts.

METHODS

Healthy, nonlesional psoriatic skin and lesional psoriatic skin were immunostained for α5 integrin, KGF, FGFR2, EDA(+) FN and signal transducer and activator of transcription (STAT)1. KGF-treated cell cultures were analysed for FN and EDA(+) FN mRNA and protein by real-time reverse-transcriptase polymerase chain reaction and flow cytometry, respectively. The major downstream signalling of KGF was investigated by blocking experiments using inhibitors of mitogen-activated protein kinase (MAPK) kinase (MEK1), AKT1/2, STAT1 and STAT3.

RESULTS

The expression of α5 integrin, EDA(+) FN, KGF and its receptor FGFR2 is elevated in psoriatic nonlesional skin compared with healthy skin. KGF mildly induced EDA(+) FN, but not FN expression in healthy fibroblasts through MAPK signalling. Fibroblasts express the FGFR2-IIIc splice variant. STAT1 negatively regulates both FN and EDA(+) FN expression in healthy fibroblasts, and this regulation is compromised in fibroblasts derived from nonlesional psoriatic dermis. We detected active STAT1 in healthy and lesional skin, similarly to a previous report. However, in the nonlesional skin STAT1 activation was absent in tissues far away from lesions.

CONCLUSIONS

The production of FN and EDA(+) FN by fibroblasts and the signalling of STAT1 are abnormally regulated in psoriatic nonlesional skin.

The Regulation of TGFβ1 Induced Fibronectin EDA Exon Alternative Splicing in Human Renal Proximal Tubule Epithelial Cells

The EDA+ splice variant of fibronectin (Fn) is an early and important component of the extracellular matrix in renal fibrosis. In this work, we investigate cellular mechanisms of EDA+Fn production in human primary proximal tubule epithelial cells (PTECs). TGFβ1-induced EDA+Fn production was assessed by immunocytochemistry, PCR, and Western blotting. SRp40 knockdown was achieved by siRNA. The role of the PI3 kinase-AKT signalling and splicing regulatory protein SRp40 in the production of EDA+Fn was studied by using the chemical inhibitor LY294002 and siRNA targeted to SRp40 respectively. Interaction between PI3 kinase-AKT signalling and SRp40 were assessed by immunofluorescence and immunoprecipitation. To assess the specificity of SRp40 in regulating the splicing of EDA+ exon, we studied the effect of SRp40 knockdown on TGFβ1 induced splicing of FGF receptor 2. Primary human PTECs expressed EDA+ and EDA- Fn. TGFβ1 treatment resulted in increases in the production and deposition of EDA+ Fn as well as an increase in the ratio of EDA+/EDA- Fn mRNA. The TGFβ1 induced EDA+ production was dependent on PI3 kinase-AKT signalling and SRp40 expression. Immunoprecipitation experiments demonstrated direct binding between AKT and SRp40 with an increase in the amount of SRp40 bound to AKT upon TGFβ1 treatment. TGFβ1 treatment resulted in reduction in the FGF receptor2 IIIb splice variant which was unaffected by SRp40 knockdown. In this work, we have presented the first evidence for the regulation of Fn pre-mRNA splicing by PI3 kinase-AKT signalling and SRp40 in human PTECs. Targeting the splicing of Fn pre-mRNA to skip the EDA exon is an attractive option to combat fibrosis.

Regulation of epithelial-mesenchymal transition in breast cancer cells by cell contact and adhesion

Epithelial-mesenchymal transition (EMT) is a physiological program that is activated during cancer cell invasion and metastasis. We show here that EMT-related processes are linked to a broad and conserved program of transcriptional alterations that are influenced by cell contact and adhesion. Using cultured human breast cancer and mouse mammary epithelial cells, we find that reduced cell density, conditions under which cell contact is reduced, leads to reduced expression of genes associated with mammary epithelial cell differentiation and increased expression of genes associated with breast cancer. We further find that treatment of cells with matrix metalloproteinase-3 (MMP-3), an inducer of EMT, interrupts a defined subset of cell contact-regulated genes, including genes encoding a variety of RNA splicing proteins known to regulate the expression of Rac1b, an activated splice isoform of Rac1 known to be a key mediator of MMP-3-induced EMT in breast, lung, and pancreas. These results provide new insights into how MMPs act in cancer progression and how loss of cell-cell interactions is a key step in the earliest stages of cancer development.

The PI3K pathway acting on alternative HIV-1 pre-mRNA splicing

HIV-1 mediates pro-survival signals and prevents apoptosis via the phosphatidylinositol-3-kinase (PI3K) pathway. This pathway, however, also affects phosphorylation of serine-arginine (SR) proteins, a family of splicing regulatory factors balancing splice site selection. We now show that pharmacologic inhibition of PI3K signalling alters the HIV-1 splicing pattern of both minigene- and provirus-derived mRNAs. This indicates that HIV-1 might also promote PI3K signalling to balance processing of its transcripts by regulating phosphorylation of splicing regulatory proteins.

Phosphorylation of SRSF1 by SRPK1 regulates alternative splicing of tumor-related Rac1b in colorectal cells

The premessenger RNA of the majority of human genes can generate various transcripts through alternative splicing, and different tissues or disease states show specific patterns of splicing variants. These patterns depend on the relative concentrations of the splicing factors present in the cell nucleus, either as a consequence of their expression levels or of post-translational modifications, such as protein phosphorylation, which are determined by signal transduction pathways. Here, we analyzed the contribution of protein kinases to the regulation of alternative splicing variant Rac1b that is overexpressed in certain tumor types. In colorectal cells, we found that depletion of AKT2, AKT3, GSK3β, and SRPK1 significantly decreased endogenous Rac1b levels. Although knockdown of AKT2 and AKT3 affected only Rac1b protein levels suggesting a post-splicing effect, the depletion of GSK3β or SRPK1 decreased Rac1b alternative splicing, an effect mediated through changes in splicing factor SRSF1. In particular, the knockdown of SRPK1 or inhibition of its catalytic activity reduced phosphorylation and subsequent translocation of SRSF1 to the nucleus, limiting its availability to promote the inclusion of alternative exon 3b into the Rac1 pre-mRNA. Altogether, the data identify SRSF1 as a prime regulator of Rac1b expression in colorectal cells and provide further mechanistic insight into how the regulation of alternative splicing events by protein kinases can contribute to sustain tumor cell survival.

Alternative RNA splicing and cancer

Alternative splicing of pre-messenger RNA (mRNA) is a fundamental mechanism by which a gene can give rise to multiple distinct mRNA transcripts, yielding protein isoforms with different, even opposing, functions. With the recognition that alternative splicing occurs in nearly all human genes, its relationship with cancer-associated pathways has emerged as a rapidly growing field. In this review, we summarize recent findings that have implicated the critical role of alternative splicing in cancer and discuss current understandings of the mechanisms underlying dysregulated alternative splicing in cancer cells.

Modification of Akt by SUMO conjugation regulates alternative splicing and cell cycle

Akt/PKB is a key signaling molecule in higher eukaryotes and a crucial protein kinase in human health and disease. Phosphorylation, acetylation, and ubiquitylation have been reported as important regulatory post-translational modifications of this kinase. We describe here that Akt is modified by SUMO conjugation, and show that lysine residues 276 and 301 are the major SUMO attachment sites within this protein. We found that phosphorylation and SUMOylation of Akt appear as independent events. However, decreasing Akt SUMOylation levels severely affects the role of this kinase as a regulator of fibronectin and Bcl-x alternative splicing. Moreover, we observed that the Akt mutant (Akt E17K) found in several human tumors displays increased levels of SUMOylation and also an enhanced capacity to regulate fibronectin splicing patterns. This splicing regulatory activity is completely abolished by decreasing Akt E17K SUMO conjugation levels. Additionally, we found that SUMOylation controls Akt regulatory function at G₁/S transition during cell cycle progression. These findings reveal SUMO conjugation as a novel level of regulation for Akt activity, opening new areas of exploration related to the molecular mechanisms involved in the diverse cellular functions of this kinase.

Alternative splicing of tumor suppressors and oncogenes

Alternative splicing is a fundamental mechanism to modulate gene expression programs in response to different growth and environmental stimuli. There is now ample evidence that alternative splicing errors, caused by mutations in cis-acting elements and defects and/or imbalances in trans-acting factors, may be causatively associated to cancer progression. Recent work indicates the existence of an intricate network of interactions between alternative splicing events and signal transduction pathways. In this network, splicing factors occupy a central position and appear to function both as targets and effectors of regulatory circuits. Thus, a change in their activity deeply affects alternative splicing profiles and hence the cell behavior. Here, we discuss a number of cases that exemplify the involvement of deregulated alternative splicing in tumor progression.

Cancer-Associated Perturbations in Alternative Pre-messenger RNA Splicing

For most of our 25,000 genes, the removal of introns by pre-messenger RNA (pre-mRNA) splicing represents an essential step toward the production of functional messenger RNAs (mRNAs). Alternative splicing of a single pre-mRNA results in the production of different mRNAs. Although complex organisms use alternative splicing to expand protein function and phenotypic diversity, patterns of alternative splicing are often altered in cancer cells. Alternative splicing contributes to tumorigenesis by producing splice isoforms that can stimulate cell proliferation and cell migration or induce resistance to apoptosis and anticancer agents. Cancer-specific changes in splicing profiles can occur through mutations that are affecting splice sites and splicing control elements, and also by alterations in the expression of proteins that control splicing decisions. Recent progress in global approaches that interrogate splicing diversity should help to obtain specific splicing signatures for cancer types. The development of innovative approaches for annotating and reprogramming splicing events will more fully establish the essential contribution of alternative splicing to the biology of cancer and will hopefully provide novel targets and anticancer strategies. Metazoan genes are usually made up of several exons interrupted by introns. The introns are removed from the pre-mRNA by RNA splicing. In conjunction with other maturation steps, such as capping and polyadenylation, the spliced mRNA is then transported to the cytoplasm to be translated into a functional protein. The basic mechanism of splicing requires accurate recognition of each extremity of each intron by the spliceosome. Introns are identified by the binding of U1 snRNP to the 5' splice site and the U2AF65/U2AF35 complex to the 3' splice site. Following these interactions, other proteins and snRNPs are recruited to generate the complete spliceosomal complex needed to excise the intron. While many introns are constitutively removed by the spliceosome, other splice junctions are not used systematically, generating the phenomenon of alternative splicing. Alternative splicing is therefore the process by which a single species of pre-mRNA can be matured to produce different mRNA molecules (Fig. 1). Depending on the number and types of alternative splicing events, a pre-mRNA can generate from two to several thousands different mRNAs leading to the production of a corresponding number of proteins. It is now believed that the expression of at least 70 % of human genes is subjected to alternative splicing, implying an enormous contribution to proteomic diversity, and by extension, to the development and the evolution of complex animals. Defects in splicing have been associated with human diseases (Caceres and Kornblihtt, Trends Genet 18(4):186-93, 2002, Cartegni et al., Nat Rev Genet 3(4):285-98, 2002, Pagani and Baralle, Nat Rev Genet 5(5):389-96, 2004), including cancer (Brinkman, Clin Biochem 37(7):584-94, 2004, Venables, Bioessays 28(4):378-86, 2006, Srebrow and Kornblihtt, J Cell Sci 119(Pt 13):2635-2641, 2006, Revil et al., Bull Cancer 93(9):909-919, 2006, Venables, Transworld Res Network, 2006, Pajares et al., Lancet Oncol 8(4):349-57, 2007, Skotheim and Nees, Int J Biochem Cell Biol 39:1432-1449, 2007). Numerous studies have now confirmed the existence of specific differences in the alternative splicing profiles between normal and cancer tissues. Although there are a few cases where specific mutations are the primary cause for these changes, global alterations in alternative splicing in cancer cells may be primarily derived from changes in the expression of RNA-binding proteins that control splice site selection. Overall, these cancer-specific differences in alternative splicing offer an immense potential to improve the diagnosis and the prognosis of cancer. This review will focus on the functional impact of cancer-associated alternative splicing variants, the molecular determinants that alter the splicing decisions in cancer cells, and future therapeutic strategies.

RNA metabolism and ubiquitin/ubiquitin-like modifications collide

Alternative splicing and post-translational modifications are key events for the generation of proteome diversity in eukaryotes. The study of the molecular mechanisms governing these processes, and every other step of gene expression, has underscored the existing interconnectedness among them. Therefore, molecules that could concertedly regulate different stages from transcription to pre-mRNA processing, translation and even protein activity have called our attention. Serine/arginine-rich proteins, initially identified as splicing regulators, are involved in diverse aspects of gene expression. Although most of the roles exerted by members of this family are related to mRNA biogenesis and metabolism, few recently uncovered ones link these proteins to other regulatory steps along gene expression, particularly the regulation of post-translational modification by conjugation of the small ubiquitin-related modifier. This along with the established link between ubiquitin, transcription and pre-mRNA processing points to a general mechanism of interaction between different cellular machineries, such as ubiquitin/ubiquitin-like conjugation pathways, transcription apparatus and the spliceosome.

Type III and V collagens modulate the expression and assembly of EDA(+) fibronectin in the extracellular matrix of defective Ehlers-Danlos syndrome fibroblasts

BACKGROUND

Alternative splicing of EDA fibronectin (FN) region is a cell type- and development-regulated mechanism controlled by pathological processes, growth factors and extracellular matrix (ECM). Classic and vascular Ehlers-Danlos syndrome (cEDS and vEDS) are connective tissue disorders caused by COL5A1/COL5A2 and COL3A1 gene mutations, leading to an in vivo abnormal collagen fibrillogenesis and to an in vitro defective organisation in the ECM of type V (COLLV) and type III collagen (COLLIII). These defects induce the FN-ECM disarray and the decrease of COLLs and FN receptors, the α2β1 and α5β1 integrins. Purified COLLV and COLLIII restore the COLL-FN-ECMs in both EDS cell strains.

METHODS

Real-time PCR, immunofluorescence microscopy, and Western blotting were used to investigate the effects of COLLs on FN1 gene expression, EDA region alternative splicing, EDA(+)-FN-ECM assembly, α5β1 integrin and EDA(+)-FN-specific α9 integrin subunit organisation, α5β1 integrin and FAK co-regulation in EDS fibroblasts.

RESULTS

COLLV-treated cEDS and COLLIII-treated vEDS fibroblasts up-regulate the FN1 gene expression, modulate the EDA(+) mRNA maturation and increase the EDA(+)-FN levels, thus restoring a control-like FN-ECM, which elicits the EDA(+)-FN-specific α9β1 integrin organisation, recruits the α5β1 integrin and switches on the FAK binding and phosphorylation.

CONCLUSION

COLLs regulate the EDA(+)-FN-ECM organisation at transcriptional and post-transcriptional level and activate the α5β1-FAK complexes. COLLs also recruit the α9β1 integrin involved in the assembly of the EDA(+)-FN-ECM in EDS cells.

GENERAL SIGNIFICANCE

The knowledge of the COLLs-ECM role in FN isotype expression and in EDA(+)-FN-ECM-mediated signal transduction adds insights in the ECM remodelling mechanisms in EDS cells.

The role of the myofibroblast in tumor stroma remodeling

Since its first description in wound granulation tissue, the myofibroblast has been recognized to be a key actor in the epithelial-mesenchymal cross-talk that plays a crucial role in many physiological and pathological situations, such as regulation of prostate development, ventilation-perfusion in lung alveoli or organ fibrosis. The presence of myofibroblasts in the stroma reaction to epithelial tumors is well established and many data are accumulating which suggest that the stroma compartment is an active participant in tumor onset and/or evolution. In this review we summarize the evidence in favor of this concept, the main mechanisms that regulate myofibroblast differentiation and function, as well as the biophysical and biochemical factors possibly involved in epithelial-stroma interactions, using liver carcinoma as main model, in view of achieving a better understanding of tumor progression mechanisms and of tools directed toward stroma as eventual therapeutic target.

Compound A, a dissociated glucocorticoid receptor modulator, inhibits T-bet (Th1) and induces GATA-3 (Th2) activity in immune cells

Background

Compound A (CpdA) is a dissociating non-steroidal glucocorticoid receptor (GR) ligand which has anti-inflammatory properties exerted by down-modulating proinflammatory gene expression. By favouring GR monomer formation, CpdA does not enhance glucocorticoid (GC) response element-driven gene expression, resulting in a reduced side effect profile as compared to GCs. Considering the importance of Th1/Th2 balance in the final outcome of immune and inflammatory responses, we analyzed how selective GR modulation differentially regulates the activity of T-bet and GATA-3, master drivers of Th1 and Th2 differentiation, respectively.

Results

Using Western analysis and reporter gene assays, we show in murine T cells that, similar to GCs, CpdA inhibits T-bet activity via a transrepressive mechanism. Different from GCs, CpdA induces GATA-3 activity by p38 MAPK-induction of GATA-3 phosphorylation and nuclear translocation. CpdA effects are reversed by the GR antagonist RU38486, proving the involvement of GR in these actions. ELISA assays demonstrate that modulation of T-bet and GATA-3 impacts on cytokine production shown by a decrease in IFN-γ and an increase in IL-5 production, respectively.

Conclusions

Taken together, through their effect favoring Th2 over Th1 responses, particular dissociated GR ligands, for which CpdA represents a paradigm, hold potential for the application in Th1-mediated immune disorders.

Transplantation of a mammary stromal cell line into a mammary fat pad: development of the site-specific in vivo analysis system for mammary stromal cells

The interaction between mammary epithelial and stromal tissue is considered to be important in breast tissue development. In this study, we developed a transplantation procedure for the mammary stromal fibroblastic cell line (MSF) to examine its life in vivo. First we established MSF cells which stably expressed lacZ (lacZ/MSF) and had characteristics of mammary stromal cells. The lacZ/MSF cells were then transplanted into a cleared mammary fat pad of syngenic mice with and without mammary primary epithelial organoids. Whole mount X-gal and carmine staining of the transplants revealed that a number of undifferentiated lacZ/MSF cells survived around the mammary epithelial tissue when transplanted with organoids. These results indicate that transplantation of MSF cells into mammary fat pad was accomplished by co-transplantation with primary mammary organoids. Finally, we discuss the application of transplantation procedure for in vivo studies of the mammary stromal tissue development and stromal-epithelial interactions.

Alternative pre-mRNA splicing regulation in cancer: pathways and programs unhinged

Alternative splicing of mRNA precursors is a nearly ubiquitous and extremely flexible point of gene control in humans. It provides cells with the opportunity to create protein isoforms of differing, even opposing, functions from a single gene. Cancer cells often take advantage of this flexibility to produce proteins that promote growth and survival. Many of the isoforms produced in this manner are developmentally regulated and are preferentially re-expressed in tumors. Emerging insights into this process indicate that pathways that are frequently deregulated in cancer often play important roles in promoting aberrant splicing, which in turn contributes to all aspects of tumor biology.

Expression of alternatively spliced CD45 isoforms by channel catfish clonal T and B cells is dependent on activation state of the cell and regulated by protein synthesis and degradation

In mammals, expression of the three alternatively spliced exons of the tyrosine phosphatase CD45 is regulated by the developmental and activation state of the cell. In comparison, the channel catfish, Ictalurus punctatus, CD45 homolog contains 18 functional alternatively spliced exons. Since very little is known about CD45 regulation in ectothermic vertebrates, this study examines the regulation of catfish CD45 mRNA isoform expression in clonal T and B cells in response to stimulation. Results show that mitogenic stimulation using catfish serum or concanavalin A induced expression of mRNAs for small CD45 isoforms, and isoform message expression was growth curve dependent, i.e. cells in logarithmic phase express message for smaller CD45 isoforms, whereas stationary phase cells express message for longer CD45 isoforms. In addition, cells treated with the protein synthesis inhibitor cycloheximide expressed message for longer CD45 isoforms, and treatment with lactacystin, which blocks protein degradation, rescued smaller isoform message expression. Collectively these data suggested that expression of CD45 isoforms, in catfish, at least at the mRNA level, is "constitutively dynamic" and highly dependent on extracellular stimuli.

Sam68 regulates EMT through alternative splicing-activated nonsense-mediated mRNA decay of the SF2/ASF proto-oncogene

Expression levels of SF2/ASF are controlled by Sam68 mediated activation of splicing-induced mRNA decay.

Epithelial-to-mesenchymal transition (EMT) and its reversal (MET) are crucial cell plasticity programs that act during development and tumor metastasis. We have previously shown that the splicing factor and proto-oncogene SF2/ASF impacts EMT/MET through production of a constitutively active splice variant of the Ron proto-oncogene. Using an in vitro model, we now show that SF2/ASF is also regulated during EMT/MET by alternative splicing associated with the nonsense-mediated mRNA decay pathway (AS-NMD). Overexpression and small interfering RNA experiments implicate the splicing regulator Sam68 in AS-NMD of SF2/ASF transcripts and in the choice between EMT/MET programs. Moreover, Sam68 modulation of SF2/ASF splicing appears to be controlled by epithelial cell–derived soluble factors that act through the ERK1/2 signaling pathway to regulate Sam68 phosphorylation. Collectively, our results reveal a hierarchy of splicing factors that integrate splicing decisions into EMT/MET programs in response to extracellular stimuli.

Extracellular matrix composition reveals complex and dynamic stromal-epithelial interactions in the mammary gland

The mammary gland is an excellent model system to study the interplay between stroma and epithelial cells because of the gland's unique postnatal development and its distinct functional states. This review focuses on the contribution of the extracellular matrix (ECM) to stromal-epithelial interactions in the mammary gland. We describe how ECM physical properties, protein composition, and proteolytic state impact mammary gland architecture as well as provide instructive cues that influence the function of mammary epithelial cells during pubertal gland development and throughout adulthood. Further, based on recent proteomic analyses of mammary ECM, we describe known mammary ECM proteins and their potential functions, as well as describe several ECM proteins not previously recognized in this organ. ECM proteins are discussed in the context of the morphologically-distinct stromal subcompartments: the basal lamina, the intra- and interlobular stroma, and the fibrous connective tissue. Future studies aimed at in-depth qualitative and quantitative characterization of mammary ECM within these various subcompartments is required to better elucidate the function of ECM in normal as well as in pathological breast tissue.

The dietary isothiocyanate sulforaphane modulates gene expression and alternative gene splicing in a PTEN null preclinical murine model of prostate cancer

Background

Dietary or therapeutic interventions to counteract the loss of PTEN expression could contribute to the prevention of prostate carcinogenesis or reduce the rate of cancer progression. In this study, we investigate the interaction between sulforaphane, a dietary isothiocyanate derived from broccoli, PTEN expression and gene expression in pre malignant prostate tissue.

Results

We initially describe heterogeneity in expression of PTEN in non-malignant prostate tissue of men deemed to be at risk of prostate cancer. We subsequently use the mouse prostate-specific PTEN deletion model, to show that sulforaphane suppresses transcriptional changes induced by PTEN deletion and induces additional changes in gene expression associated with cell cycle arrest and apoptosis in PTEN null tissue, but has no effect on transcription in wild type tissue. Comparative analyses of changes in gene expression in mouse and human prostate tissue indicate that similar changes can be induced in humans with a broccoli-rich diet. Global analyses of exon expression demonstrated that sulforaphane interacts with PTEN deletion to modulate alternative gene splicing, illustrated through a more detailed analysis of DMBT1 splicing.

Conclusion

To our knowledge, this is the first report of how diet may perturb changes in transcription induced by PTEN deletion, and the effects of diet on global patterns of alternative gene splicing. The study exemplifies the complex interaction between diet, genotype and gene expression, and the multiple modes of action of small bioactive dietary components.

Impairment of pre-mRNA splicing in liver disease: Mechanisms and consequences

Pre-mRNA splicing is an essential step in the process of gene expression in eukaryotes and consists of the removal of introns and the linking of exons to generate mature mRNAs. This is a highly regulated mechanism that allows the alternative usage of exons, the retention of intronic sequences and the generation of exonic sequences of variable length. Most human genes undergo splicing events, and disruptions of this process have been associated with a variety of diseases, including cancer. Hepatocellular carcinoma (HCC) is a molecularly heterogeneous type of tumor that usually develops in a cirrhotic liver. Alterations in pre-mRNA splicing of some genes have been observed in liver cancer, and although still scarce, the available data suggest that splicing defects may have a role in hepatocarcinogenesis. Here we briefly review the general mechanisms that regulate pre-mRNA splicing, and discuss some examples that illustrate how this process is impaired in liver tumorigenesis, and may contribute to HCC development. We believe that a more thorough examination of pre-mRNA splicing is still needed to accurately draw the molecular portrait of liver cancer. This will surely contribute to a better understanding of the disease and to the development of new effective therapies.

Involvement of PTEN/Akt signaling in capsular invasive carcinomas developed in a rat two-stage thyroid carcinogenesis model after promotion with sulfadimethoxine

PURPOSE

Rat thyroid follicular cell carcinomas invading into the thyroid capsule are highly produced by promotion with sulfadimethoxine (SDM) in a rat two-stage thyroid carcinogenesis model. In this study, we investigated the participation of phosphoinositide 3-kinase (PI3K) signaling pathway that is associated with malignant phenotypes of many cancers on the development of SDM-induced capsular invasive carcinomas.

METHODS

Thyroid proliferative lesions developed 10 or 15 weeks after promotion with SDM in male F344 rats initiated with N-bis(2-hydroxypropyl)nitrosamine were immunohistochemically analyzed with regard to cellular distribution of phosphatase and tensin homolog (PTEN) and Akt isoforms, as well as their downstream molecules.

RESULTS

Increased expression of PI3K signaling molecules was evident in association with the development of lesion stages from the early focal hyperplasia to the late carcinomas. Capsular carcinomas, and the less frequent parenchymal carcinomas, exclusively expressed phosphorylated, inactive PTEN, and active Akt isoforms, as did their downstream molecules. Among the Akt isoforms, enhanced expression of Akt1 was more prominent than that of Akt2 in both capsular and parenchymal carcinomas.

CONCLUSIONS

Activation of the PI3K pathway through phosphorylation of PTEN promotes the high production of capsular carcinomas as well as the development of less frequent parenchymal carcinomas.

Regulation of vascular endothelial growth factor (VEGF) splicing from pro-angiogenic to anti-angiogenic isoforms: a novel therapeutic strategy for angiogenesis

Vascular endothelial growth factor (VEGF) is produced either as a pro-angiogenic or anti-angiogenic protein depending upon splice site choice in the terminal, eighth exon. Proximal splice site selection (PSS) in exon 8 generates pro-angiogenic isoforms such as VEGF₁₆₅, and distal splice site selection (DSS) results in anti-angiogenic isoforms such as VEGF₁₆₅b. Cellular decisions on splice site selection depend upon the activity of RNA-binding splice factors, such as ASF/SF2, which have previously been shown to regulate VEGF splice site choice. To determine the mechanism by which the pro-angiogenic splice site choice is mediated, we investigated the effect of inhibition of ASF/SF2 phosphorylation by SR protein kinases (SRPK1/2) on splice site choice in epithelial cells and in in vivo angiogenesis models. Epithelial cells treated with insulin-like growth factor-1 (IGF-1) increased PSS and produced more VEGF₁₆₅ and less VEGF₁₆₅b. This down-regulation of DSS and increased PSS was blocked by protein kinase C inhibition and SRPK1/2 inhibition. IGF-1 treatment resulted in nuclear localization of ASF/SF2, which was blocked by SPRK1/2 inhibition. Pull-down assay and RNA immunoprecipitation using VEGF mRNA sequences identified an 11-nucleotide sequence required for ASF/SF2 binding. Injection of an SRPK1/2 inhibitor reduced angiogenesis in a mouse model of retinal neovascularization, suggesting that regulation of alternative splicing could be a potential therapeutic strategy in angiogenic pathologies.

SF2/ASF regulates proteomic diversity by affecting the balance between translation initiation mechanisms

Post-splicing activities have been described for a subset of shuttling serine/arginine-rich splicing regulatory proteins, among them SF2/ASF. We showed that growth factors activate a Ras-PI 3-kinase-Akt/PKB signaling pathway that not only modifies alternative splicing of the fibronectin EDA exon, but also alters in vivo translation of reporter mRNAs containing the EDA binding motif for SF2/ASF, providing two co-regulated levels of isoform-specific amplification. Translation of most eukaryotic mRNAs is initiated via the scanning mechanism, which implicates recognition of the m7G cap at the mRNA 5'-terminus by the eIF4F protein complex. Several viral and cellular mRNAs are translated in a cap-independent manner by the action of cis-acting mRNA elements named internal ribosome entry sites that direct internal ribosome binding to the mRNA. Here we use bicistronic reporters that generate mRNAs carrying two open reading frames, one translated in a cap-dependent manner while the other by internal ribosome entry site-dependent initiation, to show that in vivo over-expression of SF2/ASF increases the ratio between cap-dependent and internal ribosome entry site-dependent translation. Consistently, knocking-down of SF2/ASF causes the opposite effect. Changes in expression levels of SF2/ASF also affect alternative translation of an endogenous mRNA, that one coding for fibroblast growth factor-2. These results strongly suggest a role for SF2/ASF as a regulator of alternative translation, meaning the generation of different proteins by the balance among these two translation initiation mechanisms, and expand the known potential of SF2/ASF to regulate proteomic diversity to the translation field.

Aberrant RNA splicing and its functional consequences in cancer cells

Among the myriad of alterations present in cancer cells are an abundance of aberrant mRNA transcripts. Whether abnormal gene transcription is a by-product of cellular transformation or whether it represents an inherent element that contributes to the properties of cancer cells is not yet clear. Here, we present growing evidence that in many cases, aberrant mRNA transcripts contribute to essential phenotypes associated with transformed cells, suggesting that alterations in the splicing machinery are common and functionally important for cancer development. The proteins encoded by these abnormal transcripts are often truncated or missing domains, thereby altering protein function or conferring new functions altogether. Thus, aberrant splicing regulation has genome-wide effects, potentially altering gene expression in many cancer-associated pathways.

Nuclear phosphoinositide signaling regulates messenger RNA export

Messenger RNA export from the nucleus to the cytoplasm plays an essential role in linking transcription to translation and consequently regulation of protein expression. mRNA export requires a series of events: pre-mRNA processing, ribonucleoprotein targeting to the NPC (nuclear pore complexes), and translocation through nuclear pores to the cytoplasm. Interestingly, the conventional nuclear export machinery, exportins and the Ran GTPase, is not required for mRNA export. Instead, a protein complex consisting of a number of RNA binding proteins is essential for this event including the Aly/REF protein. Phosphoinositide signaling regulates a variety of cellular functions including pre-mRNA splicing and mRNA export. In fact, a phospholipase C-dependent inositol polyphosphate kinase pathway is required for efficient mRNA export. Recently, we showed that Aly is a physiological target of nuclear phosphoinositide-3-kinase (PI3K) signaling, which regulates Aly localization as well as Aly function in cell proliferation and mRNA export through nuclear Akt-mediated phosphorylation and phosphoinositide association. Hence, water-soluble inositol polyphosphates and phosphatidylinositol lipids play pivotal roles in modulating mRNA export.

The SR protein family of splicing factors: master regulators of gene expression

The SR protein family comprises a number of phylogenetically conserved and structurally related proteins with a characteristic domain rich in arginine and serine residues, known as the RS domain. They play significant roles in constitutive pre-mRNA splicing and are also important regulators of alternative splicing. In addition they participate in post-splicing activities, such as mRNA nuclear export, nonsense-mediated mRNA decay and mRNA translation. These wide-ranging roles of SR proteins highlight their importance as pivotal regulators of mRNA metabolism, and if these functions are disrupted, developmental defects or disease may result. Furthermore, animal models have shown a highly specific, non-redundant role for individual SR proteins in the regulation of developmental processes. Here, we will review the current literature to demonstrate how SR proteins are emerging as one of the master regulators of gene expression.

Molecular signature and therapeutic perspective of the epithelial-to-mesenchymal transitions in epithelial cancers

The mechanisms involved in the epithelial to mesenchymal transition (EMT) are integrated in concert with master developmental and oncogenic pathways regulating in tumor growth, angiogenesis, metastasis, as well as the reprogrammation of specific gene repertoires ascribed to both epithelial and mesenchymal cells. Consequently, it is not unexpected that EMT has profound impacts on the neoplastic progression, patient survival, as well as the resistance of cancers to therapeutics (taxol, vincristine, oxaliplatin, EGF-R targeted therapy and radiotherapy), independent of the "classical" resistance mechanisms linked to genotoxic drugs. New therapeutic combinations using genotoxic agents and/or EMT signaling inhibitors are therefore expected to circumvent the chemotherapeutic resistance of cancers characterized by transient or sustained EMT signatures. Thus, targeting critical orchestrators at the convergence of several EMT pathways, such as the transcription pathways NF-kappaB, AKT/mTOR axis, MAPK, beta-catenin, PKC and the AP-1/SMAD factors provide a realistic strategy to control EMT and the progression of human epithelial cancers. Several inhibitors targeting these signaling platforms are already tested in preclinical and clinical oncology. In addition, upstream EMT signaling pathways induced by receptor and nonreceptor tyrosine kinases (e.g. EGF-R, IGF-R, VEGF-R, integrins/FAK, Src) and G-protein-coupled receptors (GPCR) constitute practical options under preclinical research, clinical trials or are currently used in the clinic for cancer treatment: e.g. small molecule inhibitors (Iressa: targeting selectively the EGF-R; CP-751,871, AMG479, NVP-AEW541, BMS-536924, PQIP, AG1024: IGF-R; AZD2171, ZD6474: VEGF-R; AZD0530, BMS-354825, SKI606: Src; BIM-46174: GPCR; rapamycin, CCI-779, RAD-001: mTOR) and humanized function blocking antibodies (Herceptin: ErbB2; Avastin: VEGF-A; Erbitux: EGF-R; Abegrin: alphavbeta3 integrins). We can assume that silencing RNA and adenovirus-based gene transfer of therapeutic miR and dominant interferring expression vectors targeting EMT pathways and signaling elements will bring additional ways for the treatment of epithelial cancers. Identification of the factors that initiate, modulate and effectuate EMT signatures and their underlying upstream oncogenic pathways should provide the basis of more efficient strategies to fight cancer progression as well as genetic and epigenetic forms of drug resistance. This goal can be accomplished using global screening of human clinical tumors by EMT-associated cDNA, proteome, miRome, and tissue arrays.

Akt phosphorylation and nuclear phosphoinositide association mediate mRNA export and cell proliferation activities by ALY

Nuclear PI3K and its downstream effectors play essential roles in a variety of cellular activities including cell proliferation, survival, differentiation, and pre-mRNA splicing. Aly is a nuclear speckle protein implicated in mRNA export. Here we show that Aly is a physiological target of nuclear PI3K signaling, which regulates its subnuclear residency, cell proliferation, and mRNA export activities through nuclear Akt phosphorylation and phosphoinositide association. Nuclear Akt phosphorylates Aly on threonine-219, which is required for its interaction with Akt. Aly binds phosphoinositides, and this action is regulated by Akt-mediated phosphorylation. Phosphoinositide binding but not Akt phosphorylation dictates Aly's nuclear speckle residency. Depletion of Aly results in cell growth suppression and mRNA export reduction. Inhibition of Aly phosphorylation substantially decreases cell proliferation and mRNA export. Furthermore, disruption of phosphoinositide association with Aly also significantly reduces these activities. Thus, nuclear PI3K signaling mediates both cell proliferation and mRNA export functions of Aly.

Protein kinase C-dependent control of Bcl-x alternative splicing

The alternative splicing of Bcl-x generates the proapoptotic Bcl-x(S) protein and the antiapoptotic isoform Bcl-x(L). Bcl-x splicing is coupled to signal transduction, since ceramide, hormones, and growth factors alter the ratio of the Bcl-x isoforms in different cell lines. Here we report that the protein kinase C (PKC) inhibitor and apoptotic inducer staurosporine switches the production of Bcl-x towards the x(S) mRNA isoform in 293 cells. The increase in Bcl-x(S) elicited by staurosporine likely involves signaling events that affect splicing decisions, because it requires active transcription and no new protein synthesis and is independent of caspase activation. Moreover, the increase in Bcl-x(S) is reproduced with more specific inhibitors of PKC. Alternative splicing of the receptor tyrosine kinase gene Axl is similarly affected by staurosporine in 293 cells. In contrast to the case for 293 cells, PKC inhibitors do not influence the alternative splicing of Bcl-x and Axl in cancer cell lines, suggesting that these cells have sustained alterations that uncouple splicing decisions from PKC-dependent signaling. Using minigenes, we show that an exonic region located upstream of the Bcl-x(S) 5' splice site is important to mediate the staurosporine shift in Bcl-x splicing. When transplanted to other alternative splicing units, portions of this region confer splicing modulation and responsiveness to staurosporine, suggesting the existence of factors that couple splicing decisions with PKC signaling.

Signals, pathways and splicing regulation

Alternative splicing of messenger RNA precursors is an extraordinary source of protein diversity and the regulation of this process is crucial for diverse cellular functions in both physiological and pathological situations. For many years, several signaling pathways have been implicated in alternative splicing regulation. Recent work has begun to unravel the molecular mechanisms by which extracellular stimuli activate signaling cascades that modulate the activity of the splicing machinery and therefore the splicing pattern of many different target messenger RNA precursors. These experiments are revealing unexpected aspects of the mechanism that control splicing and the consequences of the regulated splicing events. We summarize here the current knowledge about signal-induced alternative splicing regulation of Slo, NR1, CD44, CD45 and fibronectin genes, and also discuss the importance of some of these events in determination of cellular fate. Furthermore, we highlight the relevance of signal-induced changes in phosphorylation state and subcellular distribution of splicing factors as a way of regulating the splicing process. Lastly, we explore new and unexpected findings about regulated splicing in anucleated cells.

Regulation of alternative splicing by signal transduction pathways

Alternative splicing is now recognized as a ubiquitous mechanism for controlling gene expression in a tissue-specific manner. A growing body of work from the past few years as begun to also highlight the existence of networks of signal-responsive alternative splicing in a variety of cell types. While the mechanisms by which signal transduction pathways influence the splicing machinery are relatively poorly understood, a few themes have begun to emerge for how extracellular stimuli can be communicated to specific RNA-binding proteins that control splice site selection by the spliceosome. This chapter describes our current understanding of signal-induced alternative splicing with an emphasis on these emerging themes and the likely directions for future research.

The connection between splicing and cancer

Alternative splicing is a crucial mechanism for generating protein diversity. Different splice variants of a given protein can display different and even antagonistic biological functions. Therefore, appropriate control of their synthesis is required to assure the complex orchestration of cellular processes within multicellular organisms. Mutations in cis-acting splicing elements or changes in the activity of constitutive or alternative splicing could have a profound regulatory proteins that compromise the accuracy of either impact on human pathogenesis, in particular in tumor development and progression. Mutations in splicing elements, for example, have been found in genes such as LKB1, KIT, CDH17, KLF6 and BRCA1, and changes in trans-acting regulators can affect the expression of genes such as Ron, RAC1 and CD44.

Cell type and culture condition-dependent alternative splicing in human breast cancer cells revealed by splicing-sensitive microarrays

Growing evidence indicates that alternative or aberrant pre-mRNA splicing takes place during the development, progression, and metastasis of breast cancer. However, which splicing changes that might contribute directly to tumorigenesis or cancer progression remain to be elucidated. We used splicing-sensitive microarrays to detect differences in alternative splicing between two breast cancer cell lines, MCF7 (estrogen receptor positive) and MDA-MB-231 (estrogen receptor negative), as well as cultured human mammary epithelial cells. Several splicing alterations in genes, including CD44, FAS, RBM9, hnRNPA/B, APLP2, and MYL6, were detected by the microarray and verified by reverse transcription-PCR. We also compared splicing in these breast cancer cells cultured in either two-dimensional flat dishes or in three-dimensional Matrigel conditions. Only a subset of the splicing differences that distinguish MCF7 cells from MDA-MB-231 cells under two-dimensional culture condition is retained under three-dimensional conditions, suggesting that alternative splicing events are influenced by the geometry of the culture conditions of these cells. Further characterization of splicing patterns of several genes in MCF7 cells grown in Matrigel and in xenograft in nude mice shows that splicing is similar under both conditions. Thus, our oligonucleotide microarray can effectively detect changes in alternative splicing in different cells or in the same cells grown in different environments. Our findings also illustrate the potential for understanding gene expression with resolution of alternative splicing in the study of breast cancer.

Matrix metalloproteinase-induced genomic instability

Increased expression of matrix metalloproteinases (MMPs) is associated with nearly every tumor type. Although many studies have shown that MMPs can promote malignancy, recent evidence has revealed that MMPs can play a causative role also in the earliest stages of cancer development. A complex story is now emerging in which MMPs not only compromise cell-cell and cell-substratum adhesion processes that impact genomic surveillance mechanisms but also act directly on molecules at the cell surface to stimulate physiological processes that cause genetic alterations. Delineating the mechanisms involved in these processes and identifying how they are coordinated in vivo could aid identification of the crucial contribution of MMPs to tumorigenesis.

Concerted regulation of nuclear and cytoplasmic activities of SR proteins by AKT

Serine/arginine-rich (SR) proteins are important regulators of mRNA splicing. Several postsplicing activities have been described for a subset of shuttling SR proteins, including regulation of mRNA export and translation. Using the fibronectin gene to study the links between signal-transduction pathways and SR protein activity, we show that growth factors not only modify the alternative splicing pattern of the fibronectin gene but also alter translation of reporter messenger RNAs in an SR protein-dependent fashion, providing two coregulated levels of isoform-specific amplification. These effects are inhibited by specific small interfering RNAs against SR proteins and are mediated by the AKT kinase, which elicits opposite effects to those evoked by overexpressing SR protein kinases Clk and SRPK. These results show how SR protein activity is modified in response to extracellular stimulation, leading to a concerted regulation of splicing and translation.

Arachidonic acid inhibits the insulin induction of glucose-6-phosphate dehydrogenase via p38 MAP kinase

Polyunsaturated fatty acids are potent inhibitors of lipogenic gene expression in liver. The lipogenic enzyme glucose-6-phosphate dehydrogenase (G6PD) is unique in this gene family, in that fatty acids inhibit at a post-transcriptional step. In this study, we have provided evidence for a signaling pathway for the arachidonic acid inhibition of G6PD mRNA abundance. Arachidonic acid decreases the insulin induction of G6PD expression; by itself, arachidonic acid does not inhibit basal G6PD mRNA accumulation. The insulin stimulation of G6PD involves the phosphoinositide 3-kinase (PI 3-kinase) pathway (Wagle, A., Jivraj, S., Garlock, G. L., and Stapleton, S. R. (1998) J. Biol. Chem. 273, 14968-14974). Incubation of hepatocytes with arachidonic acid blocks the activation of PI 3-kinase by insulin as observed by a decrease in Ser(473) phosphorylation of Akt, the downstream effector of PI 3-kinase. The decrease in PI 3-kinase activity was associated with an increase in Ser(307) phosphorylation of IRS-1. Western analysis demonstrated increased phosphorylation of p38 mitogen-activated protein kinase (MAPK) in arachidonic acid-treated cells, whereas extracellular signal-regulated kinase and c-Jun NH(2)-terminal kinase activity was not changed. Incubating the hepatocytes with the p38 MAPK inhibitor, SB203580, blocked the arachidonic acid inhibition of G6PD mRNA accumulation. Furthermore, SB203580 decreased the arachidonic acid-mediated Ser(307) phosphorylation of IRS-1 and rescued Akt activation that was otherwise decreased by arachidonic acid. Thus, arachidonic acid inhibits the insulin stimulation of G6PD mRNA accumulation by stimulating the p38 MAPK pathway, thereby inhibiting insulin signal transduction.

Cross-talk between signaling pathways regulates alternative splicing: a novel role for JNK

The regulation of alternative splicing by extracellular signals represents a key event in the control of gene expression. There is increasing evidence showing that many extracellular cues regulate alternative splicing. Nevertheless, the broad picture regarding the role of different signaling pathways and their interaction remains incomplete. Using the fibronectin gene as a model, we show that a laminin-rich basement membrane regulates the alternative splicing of two out of three regions of the transcript (extra domain I and type III connecting segment) in mammary epithelial cells, through a non-stress c-Jun N-terminal kinase (JNK) signaling pathway. We propose that dephosphorylation of the extracellular signal-regulated kinase is involved in this regulatory process. Furthermore, the laminin-rich basement membrane blocks the effect of a mammary mesenchymal cell-conditioned medium, which stimulates the inclusion of extra domain I and type III connecting segment through a phosphatidylinositol3-kinase-dependent cascade, indicating that JNK signaling can inhibit the phosphatidylinositol 3-kinase-mediated splicing regulation. These results implicate JNK in the regulation of alternative splicing and provide new evidence on how extracellular stimuli are converted into changes in splicing patterns, strengthening the view that the control of alternative splicing is as complex and relevant as transcriptional control, together accounting for the spatiotemporal requirements of gene expression.