Retroviral expression screening of oncogenes in pancreatic ductal carcinoma

Differential localization of A-Raf regulates MST2-mediated apoptosis during epithelial differentiation

A-Raf belongs to the family of oncogenic Raf kinases that are involved in mitogenic signaling by activating the MEK-ERK pathway. Low kinase activity of A-Raf toward MEK suggested that A-Raf might have alternative functions. We recently identified A-Raf as a potent inhibitor of the proapoptotic mammalian sterile 20-like kinase (MST2) tumor suppressor pathway in several cancer entities including head and neck, colon, and breast. Independent of kinase activity, A-Raf binds to MST2 thereby efficiently inhibiting apoptosis. Here, we show that the interaction of A-Raf with the MST2 pathway is regulated by subcellular compartmentalization. Although in proliferating normal cells and tumor cells A-Raf localizes to the mitochondria, differentiated non-carcinogenic cells of head and neck epithelia, which express A-Raf at the plasma membrane. The constitutive or induced re-localization of A-Raf to the plasma membrane compromises its ability to efficiently sequester and inactivate MST2, thus rendering cells susceptible to apoptosis. Physiologically, A-Raf re-localizes to the plasma membrane upon epithelial differentiation in vivo. This re-distribution is regulated by the scaffold protein kinase suppressor of Ras 2 (KSR2). Downregulation of KSR2 during mammary epithelial cell differentiation or siRNA-mediated knockdown re-localizes A-Raf to the plasma membrane causing the release of MST2. By using the MCF7 cell differentiation system, we could demonstrate that overexpression of A-Raf in MCF7 cells, which induces differentiation. Our findings offer a new paradigm to understand how differential localization of Raf complexes affects diverse signaling functions in normal cells and carcinomas.

Splicing factor hnRNP A2 activates the Ras-MAPK-ERK pathway by controlling A-Raf splicing in hepatocellular carcinoma development

In recent years, it has become clear that splicing factors play a direct role in cancer development. We showed previously that splicing factors SRSF1, SRSF6, and hnRNP A2/B1 are up-regulated in several cancers and can act as oncogenes when up-regulated. Here we examined the role of splicing factors hnRNP A1/A1b and hnRNP A2/B1 in hepatocellular carcinoma (HCC). We show that the splicing factors hnRNP A1 and hnRNP A2 are up-regulated in HCC tumors derived from inflammation-induced liver cancer mouse model. Overexpression of hnRNP A1 or hnRNP A2, but not the splicing isoform hnRNP B1, induced tumor formation of immortalized liver progenitor cells, while knockdown of these proteins inhibited anchorage-independent growth and tumor growth of human liver cancer cell lines. In addition, we found that cells overexpressing hnRNP A2 showed constitutive activation of the Ras-MAPK-ERK pathway. In contrast, knockdown of hnRNP A2 inhibited the Ras-MAPK-ERK pathway and prevented ERK1/2 activation by EGF. Moreover, we found that hnRNP A2 regulates the splicing of A-Raf, reducing the production of a short dominant-negative isoform of A-Raf and elevating the full-length A-Raf transcript. Taken together, our data suggest that hnRNP A2 up-regulation in HCC induces an alternative splicing switch that down-regulates a dominant-negative isoform of A-Raf, leading to activation of the Raf-MEK-ERK pathway and cellular transformation.

Reciprocal regulation by TLR4 and TGF-β in tumor-initiating stem-like cells

Tumor-initiating stem-like cells (TICs) are resistant to chemotherapy and associated with hepatocellular carcinoma (HCC) caused by HCV and/or alcohol-related chronic liver injury. Using HCV Tg mouse models and patients with HCC, we isolated CD133(+) TICs and identified the pluripotency marker NANOG as a direct target of TLR4, which drives the tumor-initiating activity of TICs. These TLR4/NANOG-dependent TICs were defective in the TGF-β tumor suppressor pathway. Functional oncogene screening of a TIC cDNA library identified Yap1 and Igf2bp3 as NANOG-dependent genes that inactivate TGF-β signaling. Mechanistically, we determined that YAP1 mediates cytoplasmic retention of phosphorylated SMAD3 and suppresses SMAD3 phosphorylation/activation by the IGF2BP3/AKT/mTOR pathway. Silencing of both YAP1 and IGF2BP3 restored TGF-β signaling, inhibited pluripotency genes and tumorigenesis, and abrogated chemoresistance of TICs. Mice with defective TGF-β signaling (Spnb2(+/-) mice) exhibited enhanced liver TLR4 expression and developed HCC in a TLR4-dependent manner. Taken together, these results suggest that the activated TLR4/NANOG oncogenic pathway is linked to suppression of cytostatic TGF-β signaling and could potentially serve as a therapeutic target for HCV-related HCC.

c-Myc regulates RNA splicing of the A-Raf kinase and its activation of the ERK pathway

A-Raf kinase can inhibit apoptosis by binding to the proapoptotic mammalian sterile 20-like kinase (MST2). This function relies on expression of hnRNP H, which ensures the correct splicing of a-raf mRNA needed to produce full-length A-Raf protein. Here, we showed that expression of hnRNP H and production of full-length A-Raf is positively controlled by c-Myc. Low c-Myc reduces hnRNP H expression and switches a-raf splicing to produce A-Raf(short), a truncated protein. Importantly, A-Raf(short) fails to regulate MST2 but retains the Ras-binding domain such that it functions as a dominant negative mutant suppressing Ras activation and transformation. Human colon and head and neck cancers exhibit high hnRNP H and high c-Myc levels resulting in enhanced A-Raf expression and reduced expression of A-Raf(short). Conversely, in normal cells and tissues in which c-Myc and hnRNP H are low, A-Raf(short) suppresses extracellular signal regulated kinase activation such that it may act as a safeguard against oncogenic transformation. Our findings offered a new paradigm to understand how c-Myc coordinates diverse cell functions by directly affecting alternate splicing of key signaling components.

Transforming activity of purinergic receptor P2Y, G protein coupled, 8 revealed by retroviral expression screening

Biphenotypic acute leukemia (BAL) is a relatively rare subtype of acute leukemia characterized by the presence of both myeloid and lymphoid cell surface antigens. We have now screened for transforming genes in BAL blasts with the use of the focus formation assay with a retroviral cDNA expression library constructed from malignant blasts isolated from a BAL patient. Some of the retroviral inserts recovered from transformed foci were found to encode wild-type purinergic receptor P2Y, G protein coupled, 8 (P2RY8). The oncogenic potential of P2RY8 was confirmed with the in vitro focus formation assay as well as with an in vivo tumorigenicity assay in nude mice. A variety of luciferase-based reporter assays revealed that P2RY8 increased both the trans-activation activities of CREB and Elk-1 as well as the transcriptional activities of the serum response element and enhancer-promoter fragments of the c-Fos and c-Myc genes. Quantitation of P2RY8 mRNA in CD34(+) cells of bone marrow showed that P2RY8 expression is frequently increased in leukemia patients, especially in those with refractory disease. Our data thus reveal an abundant expression of P2RY8 in leukemic cells and its unexpected role in the pathogenesis of acute leukemia.

Non-solid oncogenes in solid tumors: EML4-ALK fusion genes in lung cancer

It is generally accepted that recurrent chromosome translocations play a major role in the molecular pathogenesis of hematological malignancies but not of solid tumors. However, chromosome translocations involving the e26 transformation-specific sequence transcription factor loci have been demonstrated recently in many prostate cancer cases. Furthermore, through a functional screening with retroviral cDNA expression libraries, we have discovered the fusion-type protein tyrosine kinase echinoderm microtubule-associated protein like-4 (EML4)-anaplastic lymphoma kinase (ALK) in non-small cell lung cancer (NSCLC) specimens. A recurrent chromosome translocation, inv(2)(p21p23), in NSCLC generates fused mRNA encoding the amino-terminal half of EML4 ligated to the intracellular region of the receptor-type protein tyrosine kinase ALK. EML4-ALK oligomerizes constitutively in cells through the coiled coil domain within the EML4 region, and becomes activated to exert a marked oncogenicity both in vitro and in vivo. Break and fusion points within the EML4 locus may diverge in NSCLC cells to generate various isoforms of EML4-ALK, which may constitute approximately 5% of NSCLC cases, at least in the Asian ethnic group. In the present review I summarize how detection of EML4-ALK cDNA may become a sensitive diagnostic means for NSCLC cases that are positive for the fusion gene, and discuss whether suppression of ALK enzymatic activity could be an effective treatment strategy against this intractable disorder.