Regulation of c-myb expression in ML-1 human myeloblastic leukemia cells by c-ets-1 protein

Thrombopoietin regulates c-Myb expression by modulating micro RNA 150 expression

OBJECTIVE

Mice harboring c-Myb hypomorphic mutations display enhanced thrombopoiesis because of increased numbers of megakaryocytes and their progenitors. Thrombopoietin induces these same effects, which lead us to hypothesize that the hormone acts through modulation of c-Myb expression, as c-Myb levels falls during thrombopoietin-induced megakaryocyte (MK) maturation. Micro RNAs (miRs) downregulate gene expression by binding to the 3' untranslated region (UTR) of specific messenger RNAs (mRNAs); we noted that the 3'UTR of c-Myb contains four miR-150 binding sites.

MATERIALS AND METHODS

We used quantitative reverse transcriptase polymerase chain reaction, Western blotting, and reporter gene analyses to assess the response of c-Myb to thrombopoietin stimulation and to gain of and loss of miR-150 expression.

RESULTS

We found that thrombopoietin reduced c-Myb mRNA and protein levels within 7 hours in megakaryocytes and UT7/thrombopoietin (TPO) cells. Using a reporter gene containing the c-Myb 3'UTR region, including its four miR150 binding sites, we found that expression of miR150 reduced luciferase expression to 50% of baseline at 24 hours and to 25% at 48 hours in UT7/TPO cells. Quantitative polymerase chain reaction and Western blotting also revealed that miR-150 reduced endogenous c-Myb mRNA and protein to 50% in UT7/TPO cells, and to 65% in mature megakaryocytes. Converse experiments utilizing anti-miR150 increased luciferase activity twofold over control anti-miR. Finally, TPO increased miR150 expression 1.8-fold within 24 hours and 3.4-fold within 48 hours.

CONCLUSIONS

These findings establish that miR150 downmodulates c-Myb levels, and because TPO affects miR150 expression, our results indicate that, in addition to affecting MK progenitor cell growth, TPO downmodulates c-Myb expression through induction of miR-150.

B-Myb regulates the A(2B) adenosine receptor in vascular smooth muscle cells

The A(2B) adenosine receptor (A(2B)AR) has been described to control various vascular functions, including inhibition of smooth muscle cell proliferation. Here, we sought to understand the regulation of A(2B)AR gene expression in aortic vascular smooth muscle cells (VSMCs), with a focus on the proliferation phase. Assays with A(2B)AR-beta-gal aortic VSMCs, in which the endogenous A(2B)AR gene promoter drives the expression of prokaryotic beta-galactosidase (beta-gal) instead of the endogenous A(2B)AR gene, show that beta-gal expression is upregulated when the cells are induced to exit from cell cycle arrest. Similarly, the level of A(2B)AR mRNA is upregulated in proliferating primary aortic VSMCs. In search of related mechanisms, it was noted that the A(2B)AR gene promoter contains several putative binding sites for the proliferation-inducing transcription factor, B-Myb. Using a clone of the 5' region upstream of the mouse A(2B)AR gene linked to a reporter gene, B-Myb site deletion mutants were generated. It was determined that B-Myb upregulates the A(2B)AR gene promoter, and specific promoter binding sites were identified as functional. In accordance, B-Myb also elevates endogenous A(2B)AR mRNA and receptor activity, and this activity decreases cell proliferation. Our data are novel in that they show that this proliferation-inhibiting A(2B)AR is itself an inducible receptor regulated by B-Myb.

Transforming growth factor-beta1 causes transcriptional activation of CD34 and preserves haematopoietic stem/progenitor cell activity

Stem/progenitor cells endowed with in vitro and in vivo haematopoietic activity express the surface protein CD34. Transforming growth factor beta1 (TGF-beta1) is one of the soluble molecules that regulate cell cycle and differentiation of haematopoietic cells, but has pleiotropic activities depending on the state of responsiveness of the target cells. It has previously been shown that TGF-beta1 maintains human CD34+ haematopoietic progenitors in an undifferentiated state, independently of any cell cycle effect. Here, we have shown that TGF-beta1 upregulates the human CD34, an effect that was evident in primary stem/progenitor cells (CD34+lin-) both at the transcriptional and protein levels, and was not associated with any relevant effect on cell growth. The presence of TGF-beta1 influenced differentiation, maintaining primary CD34+/Lin- in an undifferentiated state. This effect was associated with Smad activation and with a dramatic decrease in p38 phosphorylation. Moreover, blocking p38 phosphorylation by the SB202190 inhibitor increased CD34 RNA levels but did not enhance CD34 protein expression in CD34+/Lin- cells, suggesting that modulation of multiple signalling pathways is necessary to reproduce TGF-beta1 effects. These data establish the role that TGF-beta1 has in the modulation of the CD34 stem/progenitor protein and stem/progenitor functions, providing important clues for understanding haematopoietic development and a potential tool for the modulation of human haematopoiesis.

Transforming growth factor-beta1 transcriptionally activates CD34 and prevents induced differentiation of TF-1 cells in the absence of any cell-cycle effects

A number of cytokines modulate self-renewal and differentiation of hematopoietic elements. Among these is transforming growth factor beta1 (TGF-beta1), which regulates cell cycle and differentiation of hematopoietic cells, but has pleiotropic activities depending on the state of responsiveness of the target cells. It has been previously shown by us and other authors that TGF-beta1 maintains human CD34(+) hematopoietic progenitors in an undifferentiated state, independently of any cell cycle effects, and that depletion of TGF-beta1 triggers differentiation accompanied by a decrease in CD34 antigen expression. In the present work, we show that exogenous TGF-beta1 upregulates the human CD34 antigen in the CD34(+) cell lines TF-1 and KG-1a, but not in the more differentiated CD34(-) cell lines HL-60 and K-562. We further studied this effect in the pluripotent erythroleukemia cell line TF-1. Here, TGF-beta1 did not effect cell growth, but induced transcriptional activation of full-length CD34 and prevented differentiation induced by differentiating agents. This effect was associated with nuclear translocation of Smad-2, activation of TAK-1, and with a dramatic decrease in p38 phosphorylation. In other systems TGF-beta1 has been shown to activate a TGF-beta-activated kinase 1 (TAK1), which in turn, activates p38. The specific inhibitor of p38 phosphorylation, SB202190, also increased CD34 RNA expression, indicating the existence of a link between p-38 inhibition by TGF-beta1 and CD34 overexpression. Our data demonstrate that TGF-beta1 transcriptionally activates CD34 and prevents differentiation of TF-1 cells by acting independently through the Smad, TAK1 and p38 pathways, and thus provide important clues for the understanding of hematopoietic development and a potential tool to modify response of hematopoietic cells to mitogens or differentiating agents.

In vitro effects of insulin-like growth factor-1 (IGF-1) on proliferation and constitutive cytokine secretion by acute myelogenous leukemia blasts

The effects of insulin-like growth factor 1 (IGF-1) on autonomous proliferation, cytokine-dependent proliferation and constitutive cytokine secretion by acute myelogenous leukemia (AML) blasts were investigated using serum-free in vitro conditions. IGF-1 enhanced AML blast proliferation independent of the presence of other exogenous cytokines only for 2 of 21 patients, but for 10 additional patients IGF-1 altered blast proliferation in the presence of certain exogenous cytokines or cytokine combinations. IGF-1 had minor effects on AML blast cytokine secretion only for a subset of the patients (decreased levels for 1 patient, increased levels for 7 patients). Our in vitro observations indicate that IGF-1 can modulate AML blast proliferation and/or cytokine secretion for a subset of patients.