Death domain-associated protein (Daxx) is a multifunctional protein that modulates both cell death and transcription. Several recent studies have indicated that Daxx is a mediator of lymphocyte death and/or growth suppression, although the detailed mechanism is unclear. Previously, we reported that Daxx suppresses IL-6 family cytokine-induced gene expression by interacting with STAT3. STAT3 is important for the growth and survival of lymphocytes; therefore, we here examined the role of Daxx in the gp130/STAT3-dependent cell growth/survival signals. We found that Daxx suppresses the gp130/STAT3-dependent cell growth and that Daxx endogenously interacts with STAT3 and inhibits the DNA-binding activity of STAT3. Moreover, small-interfering RNA-mediated knockdown of Daxx enhanced the expression of STAT3-target genes and accelerated the STAT3-mediated cell cycle progression. In addition, knockdown of Daxx-attenuated lactate dehydrogenase leakage from cells, indicating that Daxx positively regulates cell death during gp130/STAT3-mediated cell proliferation. Notably, Daxx specifically suppressed the levels of Bcl2 mRNA and protein, even in cytokine-unstimulated cells, indicating that Daxx regulates Bcl2 expression independently of activated STAT3. These results suggest that Daxx suppresses gp130-mediated cell growth and survival by two independent mechanisms: inhibition of STAT3-induced transcription and down-regulation of Bcl2 expression.Key words: Cell death . Daxx . gp130 . IL-6 . STAT3
IntroductionDeath domain-associated protein (Daxx) is a multifunctional protein that modulates both cell death and transcription [1]. Daxx is present in most cell types and is mainly located in the nucleus. Because previous studies have demonstrated that Daxx can bear both proand anti-cell-death activities depending on the stimulus and the cell type [1,2], the precise role of Daxx, in particular its ability to promote or hinder cell death, remains controversial. Nevertheless, in lymphocytes, the following observations indicate that Daxx has procell death and/or growth-suppressing functions. First, Daxx is induced by type I interferon in pro-B cells and is required for interferon-induced apoptosis [3]. Second, in Con A-stimulated splenocytes, Daxx up-regulation and interaction with PML correlate with the induction of B-cell apoptosis [4]. Third, CD40-induced proliferation is profoundly reduced in transgenic B cells over-expressing Daxx [5]. Fourth, analysis of T-cell-specific transgenic mice expressing a dominant negative form of Daxx (Daxx-DN) [18][19][20], and the chromatin-modifying protein a-thalassemia syndrome protein [21,22], it seems possible that Daxx regulates cellular processes by regulating transcription of specific genes under different conditions. We have previously shown that Daxx suppresses STAT3-mediated transcriptional activity and has a role in regulating IL-6 family cytokine signaling and gene induction in several cancer cell lines [16].The IL-6 family of cytokines utilizes the membrane glycoprotein gp130 as a cri...