Substantial data support major roles for bone-derived TGF-␤ 1 and tumor-derived parathyroid hormone-related protein (PTHrP) in the vicious cycle of local bone destruction that characterizes osteolytic metastases. Tumor-produced PTHrP stimulates osteoclastic bone resorption to result in the bone destruction associated with breast cancer metastases (1, 2). Neutralizing antibodies to PTHrP not only decreased osteoclastic bone resorption but also inhibited the development of metastases to bone by the human breast cancer cell line, MDA-MB-231 (3). TGF-␤, stored in bone matrix (4) and released locally in active form during osteoclastic resorption (5), stimulates PTHrP production by tumor cells (6 -8). A dominantnegative TGF-␤ type II receptor (T␤RII⌬cyt) stably expressed in the MDA-MB-231 breast cancer line rendered the cells unresponsive to TGF-␤ and inhibited TGF-␤-induced PTHrP secretion and the development of bone metastases in a mouse model. This dominant-negative type II blockade was reversed by a constitutively active TGF-␤ type I receptor (T␤RI(T204D)). Furthermore, transfection of the cDNA for PTHrP into the dominant-negative MDA-MB-231 line also increased PTHrP production and accelerated bone metastases (9). These published data establish that TGF-␤ in bone can promote osteolysis by increasing PTHrP secretion from breast cancer cells. They do not, however, exclude contributions from other TGF-␤-responsive tumor factors. Here we demonstrate that PTHrP is the central mediator of TGF-␤-induced osteolytic metastasis. We also show that TGF-␤ increases PTHrP secretion from MDA-MB-231 cells by signaling through both Smad and p38 MAP kinase pathways.

Background: RNA-binding proteins (RBPs) function as master regulators of gene expression. Alterations in RBP expression and function are often observed in cancer and influence critical pathways implicated in tumor initiation and growth. Identification and characterization of oncogenic RBPs and their regulatory networks provide new opportunities for targeted therapy. Results: We identify the RNA-binding protein SERBP1 as a novel regulator of glioblastoma (GBM) development. High SERBP1 expression is prevalent in GBMs and correlates with poor patient survival and poor response to chemo-and radiotherapy. SERBP1 knockdown causes delay in tumor growth and impacts cancer-relevant phenotypes in GBM and glioma stem cell lines. RNAcompete identifies a GC-rich region as SERBP1-binding motif; subsequent genomic and functional analyses establish SERBP1 regulation role in metabolic routes preferentially used by cancer cells. An important consequence of these functions is SERBP1 impact on methionine production. SERBP1 knockdown decreases methionine levels causing a subsequent reduction in histone methylation as shown for H3K27me3 and upregulation of genes associated with neurogenesis, neuronal differentiation, and function. Further analysis demonstrates that several of these genes are downregulated in GBM, potentially through epigenetic silencing as indicated by the presence of H3K27me3 sites. Conclusions: SERBP1 is the first example of an RNA-binding protein functioning as a central regulator of cancer metabolism and indirect modulator of epigenetic regulation in GBM. By bridging these two processes, SERBP1 enhances glioma stem cell phenotypes and contributes to GBM poorly differentiated state.

RNA-binding proteins (RBPs) function as master regulators of gene expression. Alterations in their levels are often observed in tumors with numerous oncogenic RBPs identified in recent years. Musashi1 (Msi1) is an RBP and stem cell gene that controls the balance between self-renewal and differentiation. High Msi1 levels have been observed in multiple tumors including glioblastoma and are often associated with poor patient outcomes and tumor growth. A comprehensive genomic analysis identified a network of cell cycle/division and DNA replication genes and established these processes as Msi1’s core regulatory functions in glioblastoma. Msi1 controls this gene network via two mechanisms: direct interaction and indirect regulation mediated by the transcription factors E2F2 and E2F8. Moreover, glioblastoma lines with Msi1 knockout (KO) displayed increased sensitivity to cell cycle and DNA replication inhibitors. Our results suggest that a drug combination strategy (Msi1 + cell cycle/DNA replication inhibitors) could be a viable route to treat glioblastoma.

◥ 13-Cis-retinoic acid (RA) is typically used in postremission maintenance therapy in patients with neuroblastoma. However, side effects and recurrence are often observed. We investigated the use of miRNAs as a strategy to replace RA as promoters of differentiation. miR-124 was identified as the top candidate in a functional screen. Genomic target analysis indicated that repression of a network of transcription factors (TF) could be mediating most of miR-124's effect in driving differentiation. To advance miR-124 mimic use in therapy and better define its mechanism of action, a high-throughput siRNA morphologic screen focusing on its TF targets was conducted and ELF4 was identified as a leading candidate for miR-124 repression. By altering its expression levels, we showed that ELF4 maintains neuroblastoma in an undifferentiated state and promotes proliferation. Moreover, ELF4 transgenic expression was able to counteract the neurogenic effect of miR-124 in neuroblastoma cells. With RNA sequencing, we established the main role of ELF4 to be regulation of cell-cycle progression, specifically through the DREAM complex. Interestingly, several cell-cycle genes activated by ELF4 are repressed by miR-124, suggesting that they might form a TF-miRNA regulatory loop. Finally, we showed that high ELF4 expression is often observed in neuroblastomas and is associated with poor survival.Implications: miR-124 induces neuroblastoma differentiation partially through the downregulation of TF ELF4, which drives neuroblastoma proliferation and its undifferentiated phenotype.