| 1. |
- Bahram, Fuad, et al.
(författare)
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Posttranslational regulation of Myc function in response to phorbol ester/interferon-gamma-induced differentiation of v-Myc-transformed U-937 monoblasts
- 1999
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Ingår i: Blood. - 0006-4971 .- 1528-0020. ; 93:11, s. 3900-3912
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Tidskriftsartikel (refereegranskat)abstract
- The transcription factors of the Myc/Max/Mad network are important regulators of cell growth, differentiation, and apoptosis and are frequently involved in tumor development. Constitutive expression of v-Myc blocks phorbol ester (TPA)-induced differentiation of human U-937 monoblasts. However, costimulation with interferon-gamma (IFN-gamma) and TPA restores terminal differentiation and G1 cell-cycle arrest despite continuous expression of v-Myc. The mechanism by which TPA + IFN-gamma counteract v-Myc activity has not been unravelled. Our results show that TPA + IFN-gamma treatment led to an inhibition of v-Myc- and c-Myc-dependent transcription, and a specific reduction of v-Myc:Max complexes and associated DNA-binding activity, whereas the steady state level of the v-Myc protein was only marginally affected. In contrast, TPA + IFN-gamma costimulation neither increased the expression of Mad1 or other mad/mnt family genes nor altered heterodimerization or DNA-binding activity of Mad1. The reduced amount of v-Myc:Max heterodimers in response to treatment was accompanied by partial dephosphorylation of v-Myc and c-Myc. Phosphatase treatment of Myc:Max complexes lead to their dissociation, thus mimicking the effect of TPA + IFN-gamma. In addition to modulation of the expression of Myc/Max/Mad network proteins, posttranslational negative regulation of Myc by external signals may, therefore, be an alternative biologically important level of control with potential therapeutic relevance for hematopoietic and other tumors with deregulated Myc expression.
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| 2. |
- Dimberg, Anna, et al.
(författare)
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Retinoic acid-induced cell cycle arrest of human myeloid cell lines is associated with sequential down-regulation of c-Myc and cyclin E and post-transcriptional upregulation of p27Kip1
- 2002
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Ingår i: Blood. - 0006-4971 .- 1528-0020. ; 99:6, s. 2199-2206
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Tidskriftsartikel (refereegranskat)abstract
- All-trans retinoic acid (ATRA) is a potential therapeutic agent for the treatment of hematopoietic malignancies, because of its function as an inducer of terminal differentiation of leukemic blasts. Although the efficacy of ATRA as an anticancer drug has been demonstrated by the successful treatment of acute promyelocytic leukemia (APL), the molecular mechanisms of ATRA-induced cell cycle arrest of myeloid cells have not been fully investigated. In this study, we show that the onset of ATRA-induced G0/G1 arrest of human monoblastic U-937 cells is linked to a sharp down-regulation of c-Myc and cyclin E levels and an increase in p21WAF1/CIP1 expression. This is followed by an increase in p27Kip1 protein expression due to enhanced protein stability. The importance of an early decrease in Myc expression for these events was demonstrated by the failure of a U-937 subline with constitutive exogenous expression of v-Myc to cell cycle arrest and regulate cyclin E and p27Kip1 in response to ATRA. Preceding the initiation of G1 arrest, a transient rise in retinoblastoma protein (pRb), p107, and cyclin A levels was detected. Later, a rapid fall in the levels of cyclins A and B and a coordinate dephosphorylation of pRb at Ser780, Ser795, and Ser807/811 coincided with the accumulation of cells in G1. These results thus identify a decrease in c-Myc and cyclin E levels and a posttranscriptional up-regulation of p27Kip1 as important early changes, and position them in the complex chain of events regulating ATRA-induced cell cycle arrest of myeloid cells.
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| 3. |
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| 4. |
- Wu, Siqin, et al.
(författare)
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TGF-beta enforces senescence in Myc-transformed hematopoietic tumor cells through induction of Mad1 and repression of Myc activity
- 2009
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Ingår i: Experimental Cell Research. - : Elsevier BV. - 0014-4827 .- 1090-2422. ; 315:18, s. 3099-3111
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Tidskriftsartikel (refereegranskat)abstract
- Inhibition of tumor growth factor (TGF)-beta-mediated cell cycle exit is considered an important tumorigenic function of Myc oncoproteins. Here we found that TGF-beta1 enforced G(1) cell cycle arrest and cellular senescence in human U-937 myeloid tumor cells ectopically expressing v-Myc, which contains a stabilizing mutation frequently found in lymphomas. This correlated with induced expression of the Myc antagonist Mad1, resulting in replacement of Myc for Mad1 at target promoters, reduced histone acetylation and strong repression of Myc-driven transcription. The latter was partially reversed by histone deacetylase (HDAC) inhibitors, consistent with involvement of Mad1. Importantly, knockdown of MAD1 expression prevented TGF-beta1-induced senescence, underscoring that Mad1 is a crucial component of this process. Enforced Mad1 expression sensitized U-937-myc cells to TGF-beta and restored phorbol ester-induced cell cycle exit, but could not alone induce G(1) arrest, suggesting that Mad1 is required but not sufficient for cellular senescence. Our results thus demonstrate that TGF-beta can override Myc activity despite a stabilizing cancer mutation and induce senescence in myeloid tumor cells, at least in part by induction of Mad1. TGF-beta-induced senescence, or signals mimicking this pathway, could therefore potentially be explored as a therapeutic principle for treating hematopoietic and other tumors with deregulated MYC expression.
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