A cross-talk between stromal cell-derived factor-1 and transforming growth factor-beta controls the quiescence/cycling switch of CD34(+) progenitors through FoxO3 and mammalian target of rapamycin.

Cell cycle regulation plays a fundamental role in stem cell biology. A balance between quiescence and proliferation of hematopoietic stem cells in interaction with the microenvironment is critical for sustaining long-term hematopoiesis and for protection against stress. We analyzed the molecular mechanisms by which stromal cell-derived factor-1 (SDF-1) exhibited a cell cycle-promoting effect and interacted with transforming growth factor-beta (TGF-beta), which has negative effects on cell cycle orchestration of human hematopoietic CD34(+) progenitor cells. We demonstrated that a low concentration of SDF-1 modulated the expression of key cell cycle regulators such as cyclins, cyclin-dependent kinase inhibitors, and TGF-beta target genes, confirming its cell cycle-promoting effect. We showed that a cross-talk between SDF-1- and TGF-beta-related signaling pathways involving phosphatidylinositol 3-kinase (PI3K)/Akt phosphorylation participated in the control of CD34(+) cell cycling. We demonstrated a pivotal role of two downstream effectors of the PI3K/Akt pathway, FoxO3a and mammalian target of rapamycin, as connectors in the SDF-1-/TGF-beta-induced control of the cycling/quiescence switch and proposed a model integrating a dialogue between the two molecules in cell cycle progression. Our data shed new light on the signaling pathways involved in SDF-1 cell cycle-promoting activity and suggest that the balance between SDF-1- and TGF-beta-activated pathways is critical for the regulation of hematopoietic progenitor cell cycle status.

Nuclear retention of the tumor suppressor cPML by the homeodomain protein TGIF restricts TGF-beta signaling.

The homeodomain protein TGIF has been implicated in the negative regulation of TGF-beta signaling. In this study, we report an unexpected role of TGIF in the inhibition of Smad2 phosphorylation, which occurs by a mechanism independent of its association with Smad2. This inhibitory function of TGIF is executed in concert with c-Jun, which facilitates the interaction of TGIF with cPML, resulting in the nuclear sequestration of cPML and the disruption of the cPML-SARA complex. Notably, knockdown of TGIF by siRNA caused increased association of cPML with SARA and cytoplasmic accumulation of cPML. Furthermore, c-Jun(-/-) fibroblasts exhibit enhanced association of cPML with SARA. c-Jun(-/-) fibroblasts also lose their sensitivity to TGIF-mediated disruption of the cPML-SARA complex and of nuclear sequestration of cPML. We suggest that the interaction of TGIF with cPML through c-Jun may negatively regulate TGF-beta signaling through controlling the localization of cPML and, consequently, the assembly of the cPML-SARA complex.

A novel biological assay to detect the active form of TGF-beta in urine to monitor renal allograft rejection.

BACKGROUND: Transforming growth factor-beta (TGF-beta) plays an important role in renal fibrosis. Measurement of the concentration of the active form of TGF-beta particularly in urine may help our understanding of the mechanism of chronic allograft nephropathy and could be used as a diagnostic tool. However, TGF-beta release and activation are complex and, consequently, there is currently no accurate way to measure TGF-beta activity. METHODS: TGF-beta-sensitive BL41 cells were stably transfected with a reporter plasmid harboring a synthetic TGF-beta-inducible DNA sequence upstream from the luciferase gene. Cells were incubated with urine samples from normal donors or transplanted recipients with or without patent nephropathy, and the active form of TGF-beta was determined as luciferase activity. RESULTS: We have established a cell line which expresses luciferase activity in response to active TGF-beta in a dose-dependent manner. Moreover, the use of a histone deacetylase inhibitor greatly increased sensitivity to TGF-beta and also stabilized luciferase inductibility. This test is highly specific to active TGF-beta. Detectable levels of TGF-beta were found in urine from patients with renal dysfunction due to acute or chronic renal allograft rejection (P < 0.001), but not in that from patients with stable, correctly functional kidneys. CONCLUSION: We describe a highly sensitive and specific assay for active TGF-beta. We also show that, in cases of renal allograft, TGF-beta expression is highly and significantly correlated with acute or chronic rejections.

Bcr-Abl activates the AKT/Fox O3 signalling pathway to restrict transforming growth factor-beta-mediated cytostatic signals.

The fusion of Abl with either Bcr or Tel in human leukaemia leads to the constitutive activation of Abl tyrosine kinase, which in turn induces growth-factor-independent proliferation and cell survival. However, the mechanism by which Bcr-Abl induces cellular transformation has not yet been well characterized. Here, we show that Bcr-Abl-expressing cells are resistant to growth inhibition and apoptosis mediated by transforming growth factor-beta (TGF-beta). Interestingly, we observed that the suppressive effects of Bcr-Abl on TGF-beta responses were not mediated by an impairment of Smad signalling, which is believed to act as the principal mediator of TGF-beta responses. In contrast, we found that Bcr-Abl can target the protein kinase AKT and the transcription factor Fox O3 to interfere with growth inhibition and apoptosis in response to TGF-beta. Our results show a novel mechanism of cellular transformation by the oncogenic fusion protein Bcr-Abl through suppression of the cytostatic actions of TGF-beta.

Evidence for a role of MSK1 in transforming growth factor-beta-mediated responses through p38alpha and Smad signaling pathways.

Smad proteins are central mediators of the transforming growth factor-beta (TGF-beta) superfamily signaling. The mitogen-activated protein kinase (MAPK) p38 is also one of the downstream targets required for TGF-beta-mediated responses. Although the interplay between the p38 and Smad signaling pathways might allow cells to display diverse patterns of responses to TGF-beta, the mechanism of this cross-talk is not well established. We report here that inhibition of the p38alpha isoform suppressed the ability of Smad3 to mediate TGF-beta-induced transcriptional responses. The inhibition of p38 activity blocked TGF-beta-mediated phosphorylation of the MSK1 kinase, a substrate of p38 that plays an important role in the remodeling of chromatin. Moreover, we observed that expression of dominant-interfering mutants of MSK1 blocked the binding of Smad3 to the coactivator p300 in response to TGF-beta signaling. These data reveal a new mechanism whereby the Smad signaling pathway and the p38 cascade are integrated in the nucleus to activate gene expression.