Reversible acetylation modulates p54nrb/NONO-mediated expression of the interleukin 8 gene.

The non-POU domain-containing octamer-binding protein (NONO, also referred to as p54nrb) is a multifunctional nuclear protein engaging in transcriptional regulation, mRNA splicing, nuclear retention of defective RNA, and DNA repair. Emerging evidence has demonstrated that p54nrb is subjected to various posttranslational modifications, including phosphorylation and methylation, which may be important regulators of its multifunction. However, among these modifications, direct evidence of p54nrb acetylation and its underlying mechanism remains unclear. In this study, we reported that lysine 371 of p54nrb was reversibly acetylated by the acetyltransferase general control non-depressible 5 (GCN5) and deacetylase sirtuin 1 (SIRT1), which was crucial for activity of p54nrb to inhibit interleukin-8 (IL-8) expression. Mechanistically, GCN5-mediated acetylation attenuates the recruitment of p54nrb on its core binding motif within the IL-8 gene promoter, preferentially increasing the expression of the IL-8 gene. In contrast, deacetylation by SIRT1 reverses this process. Altogether, our data suggest that reversible acetylation is an important switch for the multiple nuclear functions of p54nrb/NONO.

Phospholipase D2 is a positive regulator of sirtuin 1 and modulates p53-mediated apoptosis via sirtuin 1.

Sirtuin 1 (SIRT1) is a nicotinamide adenine dinucleotide-dependent histone deacetylase that plays diverse physiological roles. However, little is known about the regulation of SIRT1 activity. Here, we show that phospholipase D2 (PLD2), but not PLD1, selectively interacts with SIRT1 and increases the deacetylase activity of SIRT1. PLD2 does not interact with the other isozymes of SIRT (SIRT2-7). Two leucine residues in the LXXLL motif (L173 and L174) in the phox domain of PLD2 interact with the region essential for SIRT1 activity. PLD2 stimulates the SIRT1-mediated deacetylation of p53 independent of its lipase activity. In our study, mutagenesis of the LXXLL motif suppressed the interaction of PLD2 with SIRT1 and inhibited SIRT1-mediated p53 deacetylation and p53-induced transactivation of proapoptotic genes. Ultimately, overexpression of wild-type PLD2 but not that of LXXLL-mutant PLD2 protected cells against etoposide-induced apoptosis. Moreover, PLD2 did not protect against apoptosis induced by SIRT1 depletion under genotoxic stress. Collectively, our results suggest that PLD2 is a positive regulator of SIRT1 and modulates p53-mediated apoptosis via SIRT1.

Intracellular NAD+ Depletion Confers a Priming Signal for NLRP3 Inflammasome Activation.

Nicotinamide adenine dinucleotide (NAD+) is an important cofactor in many redox and non-redox NAD+-consuming enzyme reactions. Intracellular NAD+ level steadily declines with age, but its role in the innate immune potential of myeloid cells remains elusive. In this study, we explored whether NAD+ depletion by FK866, a highly specific inhibitor of the NAD salvage pathway, can affect pattern recognition receptor-mediated responses in macrophages. NAD+-depleted mouse bone marrow-derived macrophages (BMDMs) exhibited similar levels of proinflammatory cytokine production in response to LPS or poly (I:C) stimulation compared with untreated cells. Instead, FK866 facilitated robust caspase-1 activation in BMDMs in the presence of NLRP3-activating signals such as ATP and nigericin, a potassium ionophore. However, this FK866-mediated caspase-1 activation was completely abolished in Nlrp3-deficient macrophages. FK866 plus nigericin stimulation caused an NLRP3-dependent assembly of inflammasome complex. In contrast, restoration of NAD+ level by supplementation with nicotinamide mononucleotide abrogated the FK866-mediated caspase-1 cleavage. FK866 did not induce or increase the expression levels of NLRP3 and interleukin (IL)-1ß but drove mitochondrial retrograde transport into the perinuclear region. FK866-nigericin-induced mitochondrial transport is critical for caspase-1 cleavage in macrophages. Consistent with the in vitro experiments, intradermal coinjection of FK866 and ATP resulted in robust IL-1ß expression and caspase-1 activation in the skin of wild-type, but not Nlrp3-deficient mice. Collectively, our data suggest that NAD+ depletion provides a non-transcriptional priming signal for NLRP3 activation via mitochondrial perinuclear clustering, and aging-associated NAD+ decline can trigger NLRP3 inflammasome activation in ATP-rich environments.

Naa20, the catalytic subunit of NatB complex, contributes to hepatocellular carcinoma by regulating the LKB1-AMPK-mTOR axis.

N-α-acetyltransferase 20 (Naa20), which is a catalytic subunit of the N-terminal acetyltransferase B (NatB) complex, has recently been reported to be implicated in hepatocellular carcinoma (HCC) progression and autophagy, but the underlying mechanism remains unclear. Here, we report that based on bioinformatic analysis of Gene Expression Omnibus and The Cancer Genome Atlas data sets, Naa20 expression is much higher in HCC tumors than in normal tissues, promoting oncogenic properties in HCC cells. Mechanistically, Naa20 inhibits the activity of AMP-activated protein kinase (AMPK) to promote the mammalian target of rapamycin signaling pathway, which contributes to cell proliferation, as well as autophagy, through its N-terminal acetyltransferase (NAT) activity. We further show that liver kinase B1 (LKB1), a major regulator of AMPK activity, can be N-terminally acetylated by NatB in vitro, but also probably by NatB and/or other members of the NAT family in vivo, which may have a negative effect on AMPK activity through downregulation of LKB1 phosphorylation at S428. Indeed, p-LKB1 (S428) and p-AMPK levels are enhanced in Naa20-deficient cells, as well as in cells expressing the nonacetylated LKB1-MPE mutant; moreover, importantly, LKB1 deficiency reverses the molecular and cellular events driven by Naa20 knockdown. Taken together, our findings suggest that N-terminal acetylation of LKB1 by Naa20 may inhibit the LKB1-AMPK signaling pathway, which contributes to tumorigenesis and autophagy in HCC.

Deacetylation by SIRT1 promotes the tumor-suppressive activity of HINT1 by enhancing its binding capacity for ß-catenin or MITF in colon cancer and melanoma cells.

Histidine triad nucleotide-binding protein 1 (HINT1), which belongs to the evolutionarily conserved HIT superfamily, has been shown to possess a tumor-suppressive function by binding to and inhibiting several oncogenic transcription factors, such as ß-catenin and microphthalmia transcription factor (MITF), in various types of cancer cells. However, the regulatory mechanism that mediates the binding capacity of HINT1 for partner transcription factors remains elusive. Here, we report that HINT1 is acetylated by CBP at K21 and K30 and deacetylated by SIRT1. Deacetylation of HINT1 by SIRT1 increases the capacity of HINT1 to bind to ß-catenin or MITF. As a result, the tumor-suppressive function of HINT1 is increased. In support of this, the deacetylation mimetic HINT1 mutant HINT1 2KR was found to significantly reduce cellular proliferation in colon cancer and melanoma cells and tumorigenesis in xenograft assays. Thus, this study reveals an acetylation-dependent regulatory mechanism that governs the tumor-suppressive function of HINT1.