Protein N␣ -terminal acetylation is one of the most common protein modifications in eukaryotic cells. In yeast, three major complexes, NatA, NatB, and NatC, catalyze nearly all N-terminal acetylation, acetylating specific subsets of protein N termini. In human cells, only the NatA and NatB complexes have been described. We here identify and characterize the human NatC (hNatC) complex, containing the catalytic subunit hMak3 and the auxiliary subunits hMak10 and hMak31. This complex associates with ribosomes, and hMak3 acetylates Met-Leu protein N termini in vitro, suggesting a model in which the human NatC complex functions in cotranslational N-terminal acetylation. Small interfering RNA-mediated knockdown of NatC subunits results in p53-dependent cell death and reduced growth of human cell lines. As a consequence of hMAK3 knockdown, p53 is stabilized and phosphorylated and there is a significant transcriptional activation of proapoptotic genes downstream of p53. Knockdown of hMAK3 alters the subcellular localization of the Arf-like GTPase hArl8b, supporting that hArl8b is a hMak3 substrate in vivo. Taken together, hNatC-mediated N-terminal acetylation is important for maintenance of protein function and cell viability in human cells.

Background and ObjectivesChemokines are soluble mediators involved in angiogenesis, cellular growth control and immunomodulation. In the present study we investigated the effects of various chemokines on proliferation of acute myelogenous leukemia (AML) cells and constitutive chemokine release by primary AML cells. Design and MethodsNative human AML cells derived from 68 consecutive patients were cultured in vitro. We investigated AML cell proliferation ( 3 H-thymidine incorporation, colony formation), chemokine receptor expression, constitutive chemokine release and chemotaxis of normal peripheral blood mononuclear cells. ResultsExogenous chemokines usually did not have any effect on AML blast proliferation in the absence of hematopoietic growth factors, but when investigating growth factordependent (interleukin 3 + granulocyte-macrophage colony-stimulating factor + stem cell factor) proliferation in suspension cultures the following patient subsets were identified: (i) patients whose cells showed chemokine-induced growth enhancement (8 patients); (ii) divergent effects on proliferation (15 patients); and (iii) no effect (most patients). These patient subsets did not differ in chemokine receptor expression, but, compared to CD34 -AML cells, CD34 + cells showed higher expression of several receptors. Chemokines also increased the proliferation of clonogenic AML cells from the first subset of patients. Furthermore, a broad constitutive chemokine release profile was detected for most patients, and the following chemokine clusters could be identified: CCL2-4/CXCL1/8, CCL5/CXCL9-11 (possibly also CCL23) and CCL13/17/22/24/CXCL5 (possibly also CXCL6). Only the CCL2-4/CXCL1/8 cluster showed significant correlations between corresponding mRNA levels and NFκB levels/activation. The chemotaxis of normal immunocompetent cells for patients without constitutive chemokine release was observed to be decreased. Interpretation and ConclusionsDifferences in chemokine responsiveness as well as chemokine release contribute to patient heterogeneity in AML. Patients with AML can be classified into distinct subsets according to their chemokine responsiveness and chemokine release profile. key words: acute myelogenous, leukemia, chemokine, NFKB, CXCR2 Manuscript received April 11, 2006. Manuscript accepted February 5, 2007 Chemokines are a family of soluble proteins that are involved in a wide range of biological processes with relevance for hematologic malignancies, including cell trafficking, regulation of cell proliferation and apoptosis, immunoregulation, normal hematopoiesis and angiogenesis.1-5 The chemokines are grouped into the two major subclasses, CCL and CXCL chemokines, which interact with CCR and CXCR membrane receptors, respectively. Another classification of chemokines is: (i) homeostatic (also called constitutive) chemokines that bind to single receptors; and (ii) inflammatory (also called inducible) chemokines that bind to several receptors, and each of these receptors can usually bind several chemokines. 1-5Acute...

The human NatA protein N ␣ -terminal-acetyltransferase complex is responsible for cotranslational Nterminal acetylation of proteins with Ser, Ala, Thr, Gly, and Val N termini. The NatA complex is composed of the catalytic subunit hNaa10p (hArd1) and the auxiliary subunit hNaa15p (hNat1/NATH). Using immunoprecipitation coupled with mass spectrometry, we identified endogenous HYPK, a Huntingtin (Htt)-interacting protein, as a novel stable interactor of NatA. HYPK has chaperone-like properties preventing Htt aggregation. HYPK, hNaa10p, and hNaa15p were associated with polysome fractions, indicating a function of HYPK associated with the NatA complex during protein translation. Knockdown of both hNAA10 and hNAA15 decreased HYPK protein levels, possibly indicating that NatA is required for the stability of HYPK. The biological importance of HYPK was evident from HYPK-knockdown HeLa cells displaying apoptosis and cell cycle arrest in the G 0 /G 1 phase. Knockdown of HYPK or hNAA10 resulted in increased aggregation of an Htt-enhanced green fluorescent protein (Htt-EGFP) fusion with expanded polyglutamine stretches, suggesting that both HYPK and NatA prevent Htt aggregation. Furthermore, we demonstrated that HYPK is required for N-terminal acetylation of the known in vivo NatA substrate protein PCNP. Taken together, the data indicate that the physical interaction between HYPK and NatA seems to be of functional importance both for Htt aggregation and for N-terminal acetylation.

Protein N(alpha)-terminal acetylation is a conserved and widespread protein modification in eukaryotes. Several studies have linked it to normal cell function and cancer development, but nevertheless, little is known about its biological function. In yeast, protein N(alpha)-terminal acetylation is performed by the N-acetyltransferase complexes NatA, NatB and NatC. In humans, only the NatA complex has been identified and characterized. In the present study we present the components of hNatB (human NatB complex). It consists of the Nat3p homologue hNAT3 (human N-acetyltransferase 3) and the Mdm20p homologue hMDM20 (human mitochondrial distribution and morphology 20). They form a stable complex and in vitro display sequence-specific N(alpha)-acetyltransferase activity on a peptide with the N-terminus Met-Asp-. hNAT3 and hMDM20 co-sediment with ribosomal pellets, thus supporting a model where hNatB acts co-translationally on nascent polypeptides. Specific knockdown of hNAT3 and hMDM20 disrupts normal cell-cycle progression, and induces growth inhibition in HeLa cells and the thyroid cancer cell line CAL-62. hNAT3 knockdown results in an increase in G(0)/G(1)-phase cells, whereas hMDM20 knockdown decreased the fraction of cells in G(0)/G(1)-phase and increased the fraction of cells in the sub-G(0)/G(1)-phase. In summary, we show for the first time a vertebrate NatB protein N(alpha)-acetyltransferase complex essential for normal cell proliferation.

Characterization of epigenetic events in carcinogenesis has led to the discovery of a new class of oncogenes and thereby a new class of therapeutic targets. Among the new therapeutic approaches are modulation of protein lysine acetylation through inhibition of histone deacetylases (HDACs). HDACs deacetylate histones as well as transcription factors and can modulate gene expression through both these mechanisms in normal and malignant cells. Furthermore, acetylation is an important posttranslational modulation of several proteins involved in the regulation of cell proliferation, differentiation and apoptosis in normal as well as cancer cells. Even though several HDAC inhibitors have been characterized in vitro, only a limited number of these agents are in clinical trials. Various HDAC inhibitors differ in their toxicity profile when comparing the side effects described in the available clinical studies of HDAC inhibition in the treatment of cancer. These drugs may also affect normal hematopoiesis; hematologic toxicity is common to many drugs but stimulation of hematopoiesis seems to occur for others. HDAC inhibitors usually affect <10% of the genes in cancer cells. Divergent effects of HDAC inhibition on the global gene expression profiles have been described when testing various cancer cells, and this is further complicated by altered HDAC expression induced by HDAC inhibitors. However, increased p21 expression seems to be a common characteristic for most studies, suggesting an important role of this molecule during HDAC inhibitory treatment. Even though the initial studies are encouraging, additional in vitro and in vivo pharmacological characterization is definitely needed.