ABSTRACT
Medulloblastoma is the most common malignant brain tumor in children. Although aggressive surgery, radiation, and chemotherapy have improved outcomes, survivors suffer severe long-term side effects, and many patients still succumb to their disease. For patients whose tumors are driven by mutations in the sonic hedgehog (SHH) pathway, SHH antagonists offer some hope. However, many SHH-associated medulloblastomas do not respond to these drugs, and those that do may develop resistance. Therefore, more effective treatment strategies are needed for both SHH and non-SHH-associated medulloblastoma. One such strategy involves targeting the cells that are critical for maintaining tumor growth, known as tumor-propagating cells (TPC). We previously identified a population of TPCs in tumors from patched mutant mice, a model for SHH-dependent medulloblastoma. These cells express the surface antigen CD15/SSEA-1 and have elevated levels of genes associated with the G2-M phases of the cell cycle. Here, we show that CD15(+) cells progress more rapidly through the cell cycle than CD15(-) cells and contain an increased proportion of cells in G2-M, suggesting that they might be vulnerable to inhibitors of this phase. Indeed, exposure of tumor cells to inhibitors of Aurora kinase (Aurk) and Polo-like kinases (Plk), key regulators of G2-M, induces cell-cycle arrest, apoptosis, and enhanced sensitivity to conventional chemotherapy. Moreover, treatment of tumor-bearing mice with these agents significantly inhibits tumor progression. Importantly, cells from human patient-derived medulloblastoma xenografts are also sensitive to Aurk and Plk inhibitors. Our findings suggest that targeting G2-M regulators may represent a novel approach for treatment of human medulloblastoma.
Subject(s)
Aurora Kinase A/antagonists & inhibitors , Brain Neoplasms/genetics , Cell Cycle Proteins/antagonists & inhibitors , Hedgehog Proteins/genetics , Medulloblastoma/drug therapy , Medulloblastoma/enzymology , Protein Kinase Inhibitors/pharmacology , Protein Serine-Threonine Kinases/antagonists & inhibitors , Proto-Oncogene Proteins/antagonists & inhibitors , Animals , Aurora Kinase A/genetics , Aurora Kinase A/metabolism , Brain Neoplasms/metabolism , Brain Neoplasms/pathology , Cell Cycle Proteins/genetics , Cell Cycle Proteins/metabolism , Cell Proliferation , Hedgehog Proteins/metabolism , Humans , Medulloblastoma/genetics , Medulloblastoma/pathology , Mice , Mice, Inbred C57BL , Mice, Mutant Strains , Molecular Targeted Therapy , Protein Serine-Threonine Kinases/genetics , Protein Serine-Threonine Kinases/metabolism , Proto-Oncogene Proteins/genetics , Proto-Oncogene Proteins/metabolism , Signal Transduction , Polo-Like Kinase 1ABSTRACT
In mice, Quaking (Qk) is required for myelin formation; in humans, it has been associated with psychiatric disease. QK regulates the stability, subcellular localization, and alternative splicing of several myelin-related transcripts, yet little is known about how QK governs these activities. Here, we show that QK enhances Hnrnpa1 mRNA stability by binding a conserved 3' UTR sequence with high affinity and specificity. A single nucleotide mutation in the binding site eliminates QK-dependent regulation, as does reduction of QK by RNAi. Analysis of exon expression across the transcriptome reveals that QK and hnRNP A1 regulate an overlapping subset of transcripts. Thus, a simple interpretation is that QK regulates a large set of oligodendrocyte precursor genes indirectly by increasing the intracellular concentration of hnRNP A1. Together, the data show that hnRNP A1 is an important QK target that contributes to its control of myelin gene expression.
Subject(s)
3' Untranslated Regions , Gene Expression Regulation , Heterogeneous-Nuclear Ribonucleoprotein Group A-B/metabolism , Oligodendroglia/metabolism , RNA-Binding Proteins/genetics , Alternative Splicing , Animals , Base Sequence , Cell Differentiation , Cell Line , Conserved Sequence , Exons , Heterogeneous Nuclear Ribonucleoprotein A1 , Heterogeneous-Nuclear Ribonucleoprotein Group A-B/genetics , Humans , Mice , Myelin-Associated Glycoprotein/genetics , Oligodendroglia/cytology , Oligonucleotide Array Sequence Analysis , RNA Stability , RNA, Small Interfering/genetics , Rats , Sequence AlignmentABSTRACT
Embryonic development requires maternal proteins and RNA. In Caenorhabditis elegans, a gradient of CCCH tandem zinc finger (TZF) proteins coordinates axis polarization and germline differentiation. These proteins govern expression from maternal mRNAs by an unknown mechanism. Here we show that the TZF protein MEX-5, a primary anterior determinant, is an RNA-binding protein that recognizes linear RNA sequences with high affinity but low specificity. The minimal binding site is a tract of six or more uridines within a 9-13-nucleotide window. This sequence is remarkably abundant in the 3'-untranslated region of C. elegans transcripts, demonstrating that MEX-5 alone cannot specify mRNA target selection. In contrast, human TZF homologs tristetraprolin and ERF-2 bind with high specificity to UUAUUUAUU elements. We show that mutation of a single amino acid in each MEX-5 zinc finger confers tristetraprolin-like specificity to this protein. We propose that divergence of this discriminator residue modulates the RNA-binding specificity in this protein class. This residue is variable in nematode TZF proteins, but is invariant in other metazoans. Therefore, the divergence of TZF proteins and their critical role in early development is likely a nematode-specific adaptation.
