Distinct effects of mitogens and the actin cytoskeleton on CREB and pocket protein phosphorylation control the extent and timing of cyclin A promoter activity.

Soluble mitogens and adhesion-dependent organization of the actin cytoskeleton are required for cells to enter S phase in fibroblasts. The induction of cyclin A is also required for S-phase entry, and we now report that distinct effects of mitogens and the actin cytoskeleton on the phosphorylation of CREB and pocket proteins regulate the extent and timing of cyclin A promoter activity, respectively. First, we show that CREB phosphorylation and binding to the cyclic AMP response element (CRE) determines the extent, but not the timing, of cyclin A promoter activity. Second, we show that pocket protein inactivation regulates the timing, but not the extent, of cyclin A promoter activity. CREB phosphorylation and CRE occupancy are regulated by soluble mitogens alone, while the phosphorylation of pocket proteins requires both mitogens and the organized actin cytoskeleton. Mechanistically, cytoskeletal integrity controls pocket protein phosphorylation by allowing for sustained ERK signaling and, thereby, the expression of cyclin D1. Our results lead to a model of cyclin A gene regulation in which mitogens play a permissive role by stimulating early G(1)-phase phosphorylation of CREB and a distinct regulatory role by cooperating with the organized actin cytoskeleton to regulate the duration of ERK signaling, the expression of cyclin D1, and the timing of pocket protein phosphorylation.

miR-433 is aberrantly expressed in myeloproliferative neoplasms and suppresses hematopoietic cell growth and differentiation.

BCR-ABL-negative myeloproliferative neoplasms (MPNs) are most frequently characterized by the JAK2V617F gain-of-function mutation, but several studies showed that JAK2V617F may not be the initiating event in MPN development, and recent publications indicate that additional alterations such as chromatin modification and microRNA (miRNA) deregulation may have an important role in MPN pathogenesis. Here we report that 61 miRNAs were significantly deregulated in CD34+ cells from MPN patients compared with controls (P<0.01). Global miRNA analysis also revealed that polycythemia vera (JAKV617F) and essential thrombocythemia (JAK2 wild type) patients have significantly different miRNA expression profiles from each other. Among the deregulated miRNAs, expression of miR-134, -214 and -433 was not affected by changes in JAK2 activity, suggesting that additional signaling pathways are responsible for the deregulation of these miRNAs in MPN. Despite its upregulation in MPN CD34+ and during normal erythropoiesis, both overexpression and knockdown studies suggest that miR-433 negatively regulates CD34+ proliferation and differentiation ex vivo. Its novel target GBP2 is downregulated during normal erythropoiesis and regulates proliferation and erythroid differentiation in TF-1 cells, indicating that miR-433 negatively regulates hematopoietic cell proliferation and erythropoiesis by directly targeting GBP2.

Analysis of genomic aberrations and gene expression profiling identifies novel lesions and pathways in myeloproliferative neoplasms.

Polycythemia vera (PV), essential thrombocythemia and primary myelofibrosis, are myeloproliferative neoplasms (MPNs) with distinct clinical features and are associated with the JAK2V617F mutation. To identify genomic anomalies involved in the pathogenesis of these disorders, we profiled 87 MPN patients using Affymetrix 250K single-nucleotide polymorphism (SNP) arrays. Aberrations affecting chr9 were the most frequently observed and included 9pLOH (n=16), trisomy 9 (n=6) and amplifications of 9p13.3-23.3 (n=1), 9q33.1-34.13 (n=1) and 9q34.13 (n=6). Patients with trisomy 9 were associated with elevated JAK2V617F mutant allele burden, suggesting that gain of chr9 represents an alternative mechanism for increasing JAK2V617F dosage. Gene expression profiling of patients with and without chr9 abnormalities (+9, 9pLOH), identified genes potentially involved in disease pathogenesis including JAK2, STAT5B and MAPK14. We also observed recurrent gains of 1p36.31-36.33 (n=6), 17q21.2-q21.31 (n=5) and 17q25.1-25.3 (n=5) and deletions affecting 18p11.31-11.32 (n=8). Combined SNP and gene expression analysis identified aberrations affecting components of a non-canonical PRC2 complex (EZH1, SUZ12 and JARID2) and genes comprising a 'HSC signature' (MLLT3, SMARCA2 and PBX1). We show that NFIB, which is amplified in 7/87 MPN patients and upregulated in PV CD34+ cells, protects cells from apoptosis induced by cytokine withdrawal.

How do cancer cells acquire the fuel needed to support cell growth?

In this paper we consider whether the dependency of metazoan cells on extracellular signals to maintain cell survival results in an important barrier that must be overcome during carcinogenesis. It is now generally accepted that a major barrier to cancer comes from the inability of cells to enter and progress through the cell cycle in a cell-autonomous fashion. Most of the oncogenes studied over the last two decades contribute to the ability of the cancer cell to enter and progress through the cell cycle in the absence of the instructional signals normally imparted by extracellular growth factors. Over the last two decades, it has begun to be appreciated that there is a second potential barrier to transformation. It appears that all cells in multicellular organisms need extracellular signals not only to initiate proliferation, but also to maintain cell survival. Every cell in our body expresses the proteins necessary to execute its own death by apoptosis. A cell will activate this apoptotic program by default unless it receives signals from the extracellular environment that allow the cell to suppress the apoptotic machinery it expresses. It now appears that the molecular basis of this suppression lies in the signaling pathways that regulate cellular nutrient uptake and direct the metabolic fate of those nutrients.

Reduced expression of (alpha)5(beta)1 integrin prevents spreading-dependent cell proliferation.

Cell adhesion to substratum results in the initiation of integrin signaling and an integrin-dependent organization of the cytoskeleton (cell spreading). To address the potential relationships between these events and cell proliferation, we transfected NRK fibroblasts with an antisense cDNA encoding a 1.3 kb ATG-spanning portion of (alpha)5 integrin subunit and obtained stable clones in which the surface expression of (alpha)5(beta)1 integrin was selectively reduced. (alpha)5-antisense NRK cells are less spread than the control transfectants, have poorly defined stress fibers, and an increased amount of cortical actin. The antisense clones remained anchorage-dependent, but they proliferated very slowly. Serum dose-response curves showed that they have an impaired response to mitogens. Importantly, cell spreading and stress fiber formation could be completely restored by plating the antisense cells on collagen, but cell spreading failed to rescue proliferation. These data indicate that cell spreading can be uncoupled from cell proliferation and that cytoskeletal organization is important for NRK cell proliferation because it enforces the proliferative effect of (alpha)5(beta)1 integrin. Our results also indicate that reduced surface expression of (alpha)5(beta)1 integrin is not sufficient to confer the anchorage-independent phenotype to nontransformed cells.

cDNA characterization and chromosome mapping of the human GAS2 gene.

Murine Gas2 is a microfilament-associated protein whose expression is increased at growth arrest in mammalian cells. During apoptosis, Gas2 is specifically cleaved at its C-terminus by a still unknown ICE-like protease, and the processed protein induces dramatic rearrangements in the cytoskeleton when overexpressed in several cell types. Here we report the characterization of a cDNA encoding the human homologue of Gas2, showing high conservation with the murine counterpart at the protein level. Fluorescence in situ hybridization analysis and radiation hybrid mapping localized the GAS2 gene on human chromosome 11p14.3-p15.2, in a region homologous to the gas2 region on mouse chromosome 7.