Thorsten Heinzel

Valproic acid defines a novel class of HDAC inhibitors inducing differentiation of transformed cells

Histone deacetylases (HDACs) play important roles in transcriptional regulation and pathogenesis of cancer. Thus, HDAC inhibitors are candidate drugs for differentiation therapy of cancer. Here, we show that the well‐tolerated antiepileptic drug valproic acid is a powerful HDAC inhibitor. Valproic acid relieves HDAC‐dependent transcriptional repression and causes hyperacetylation of histones in cultured cells and in vivo. Valproic acid inhibits HDAC activity in vitro, most probably by binding to the catalytic center of HDACs. Most importantly, valproic acid induces differentiation of carcinoma cells, transformed hematopoietic progenitor cells and leukemic blasts from acute myeloid leukemia patients. More over, tumor growth and metastasis formation are significantly reduced in animal experiments. Therefore, valproic acid might serve as an effective drug for cancer therapy.

Acetylation of non-histone proteins modulates cellular signalling at multiple levels

This review focuses on the posttranslational acetylation of non-histone proteins, which determines vital regulatory processes. The recruitment of histone acetyltransferases and histone deacetylases to the transcriptional machinery is a key element in the dynamic regulation of genes controlling cellular proliferation and differentiation. A steadily growing number of identified acetylated non-histone proteins demonstrate that reversible lysine acetylation affects mRNA stability, and the localisation, interaction, degradation and function of proteins. Interestingly, most non-histone proteins targeted by acetylation are relevant for tumourigenesis, cancer cell proliferation and immune functions. Therefore inhibitors of histone deacetylases are considered as candidate drugs for cancer therapy. Histone deacetylase inhibitors alter histone acetylation and chromatin structure, which modulates gene expression, as well as promoting the acetylation of non-histone proteins. Here, we summarise the complex effects of dynamic alterations in the cellular acetylome on physiologically relevant pathways.

The histone deacetylase inhibitor valproic acid selectively induces proteasomal degradation of HDAC2

Histone‐modifying enzymes play essential roles in physiological and aberrant gene regulation. Since histone deacetylases (HDACs) are promising targets of cancer therapy, it is important to understand the mechanisms of HDAC regulation. Selective modulators of HDAC isoenzymes could serve as efficient and well‐tolerated drugs. We show that HDAC2 undergoes basal turnover by the ubiquitin–proteasome pathway. Valproic acid (VPA), in addition to selectively inhibiting the catalytic activity of class I HDACs, induces proteasomal degradation of HDAC2, in contrast to other inhibitors such as trichostatin A (TSA). Basal and VPA‐induced HDAC2 turnover critically depend on the E2 ubiquitin conjugase Ubc8 and the E3 ubiquitin ligase RLIM. Ubc8 gene expression is induced by both VPA and TSA, whereas only TSA simultaneously reduces RLIM protein levels and therefore fails to induce HDAC2 degradation. Thus, poly‐ubiquitination and proteasomal degradation provide an isoenzyme‐selective mechanism for downregulation of HDAC2.

Histone deacetylase as a therapeutic target

The maintenance of health depends on the coordinated and tightly regulated expression of genetic information. Certain forms of leukemia have become paradigms for the pathogenic role of aberrant repression of differentiation genes. In these acute leukemias, fusion proteins generated by chromosomal translocations no longer function as transcriptional activators, but instead repress target genes by recruiting histone deacetylases (HDACs). The potential benefit of HDAC inhibition has been established by the use of enzyme inhibitors in vitro and in a single reported case of experimental therapy. Because recently identified HDAC inhibitors appear to overcome many drawbacks of early inhibitory compounds in clinical use, the stage is set to test the therapeutic value of HDAC inhibition in leukemias and in other diseases, including solid tumors and aberrant hormonal signaling. This review summarizes the range of diseases expected to respond to HDAC inhibition.

A phosphorylation-acetylation switch regulates STAT1 signaling

Cytokines such as interferons (IFNs) activate signal transducers and activators of transcription (STATs) via phosphorylation. Histone deacetylases (HDACs) and the histone acetyltransferase (HAT) CBP dynamically regulate STAT1 acetylation. Here we show that acetylation of STAT1 counteracts IFN-induced STAT1 phosphorylation, nuclear translocation, DNA binding, and target gene expression. Biochemical and genetic experiments altering the HAT/HDAC activity ratio and STAT1 mutants reveal that a phospho-acetyl switch regulates STAT1 signaling via CBP, HDAC3, and the T-cell protein tyrosine phosphatase (TCP45). Strikingly, inhibition of STAT1 signaling via CBP-mediated acetylation is distinct from the functions of this HAT in transcriptional activation. STAT1 acetylation induces binding of TCP45, which catalyzes dephosphorylation and latency of STAT1. Our results provide a deeper understanding of the modulation of STAT1 activity. These findings reveal a new layer of physiologically relevant STAT1 regulation and suggest that a previously unidentified balance between phosphorylation and acetylation affects cytokine signaling.

Clinical trial of valproic acid and all-trans retinoic acid in patients with poor-risk acute myeloid leukemia

Valproic acid (VPA), a histone deacetylase (HDAC) inhibitor, induced in vitro differentiation of primary acute myeloid leukemia (AML) blasts, an effect enhanced by all‐trans retinoic acid (ATRA). Clinical responses to VPA were recently observed in patients with myelodysplastic syndrome (MDS). Herein, the authors have described results of a clinical trial with VPA plus ATRA in 26 patients with poor‐risk AML.

VDR-Alien: a novel, DNA-selective vitamin D3 receptor-corepressor partnership

The vitamin D receptor (VDR) is a transcription factor that transmits incoming 1,25‐dihy‐droxyvitamin D3 (1α,25(OH)2D3) signaling via combined contact with coactivator proteins and specific DNA binding sites (VDREs), which ultimately results in activation of transcription. In contrast, the mechanisms of transcriptional repression via the VDR are less well understood. This study documents VDR‐dependent transcriptional repression largely via histone deacety‐lase (HDAC) activity. Direct, ligand‐sensitive protein‐protein interaction of the VDR with the nuclear receptor corepressor (NCoR) and a novel corepressor, called Alien, was demonstrated to be comparable but independent of the VDR AF‐2 trans‐activation domain. Functional assays indicated that Alien, but not NCoR, displays selectivity for different VDRE structures for transferring these repressive effects into gene regulatory activities. Moreover, superrepression via Alien was found to be affected only in part by HDAC inhibitors such as trichostatin A. Finally, for a dissociation of VDR‐Alien complexes in vitro and in vivo, higher ligand concentrations were needed than for a dissociation of VDR‐NCoR complexes. This suggests that Alien and NCoR are using different interfaces for interaction with the VDR and different pathways for mediating superrepression, which in turn characterizes Alien as a representative of a new class of corepressors. Taken together, association of the VDR with corepressor proteins provides a further level of transcriptional regulation, which is emerging as a complex network of protein‐protein interaction‐mediated control.—Polly, P., Herdick, M., Moehren, U., Baniahmad, A., Heinzel, T., Carlberg, C. VDR‐Alien: a novel, VDR‐Alien: a novel, DNA‐selective vitamin D3 receptor‐corepressor partnership. FASEBJ. 14, 1455–1463 (2000)

Valproic acid (VPA) in patients with refractory advanced cancer: a dose escalating phase I clinical trial

Altered histone deacetylase (HDAC) activity has been identified in several types of cancer. This study was designed to determine the safety and maximum tolerated dose (MTD) of valproic acid (VPA) as an HDAC inhibitor in cancer patients. Twenty-six pre-treated patients with progressing solid tumours were enrolled in dose-escalating three-patient cohorts, starting at a dose of VPA 30 mg kg−1 day−1. VPA was administered as an 1-h infusion daily for 5 consecutive days in a 21-day cycle. Neurocognitive impairment dominated the toxicity profile, with grade 3 or 4 neurological side effects occurring in 8 out of 26 patients. No grade 3 or 4 haematological toxicity was observed. The MTD of infusional VPA was 60 mg kg−1 day−1. Biomonitoring of peripheral blood lymphocytes demonstrated the induction of histone hyperacetylation in the majority of patients and downmodulation of HDAC2. Pharmacokinetic studies showed increased mean and maximum serum VPA concentrations >120 and >250 mg l−1, respectively, in the 90 and 120 mg kg−1 cohorts, correlating well with the incidence of dose-limiting toxicity (DLT). Neurotoxicity was the main DLT of infusional VPA, doses up to 60 mg kg−1 day−1 for 5 consecutive days are well tolerated and show detectable biological activity. Further investigations are warranted to evaluate the effectivity of VPA alone and in combination with other cytotoxic drugs.

HDACi--targets beyond chromatin.

Histone deacetylases (HDACs) play an important role in gene regulation. Inhibitors of HDACs (HDACi) are novel anti-cancer drugs, which induce histone (hyper-) acetylation and counteract aberrant gene repression. On the other hand, HDACi treatment can also result in decreased gene expression, and targeting HDACs affects more than chromatin. Recently, HDACi were shown to evoke non-histone protein acetylation, which can alter signaling networks relevant for tumorgenesis. Furthermore, HDACi can promote the degradation of (proto-) oncoproteins. Here, we summarize these findings and discuss how these substances could be beneficial for the treatment and prevention of human ailments, such as cancer and unbalanced immune functions.

Nuclear export is essential for the tumor-promoting activity of survivin

Survivin appears to function as an apoptosis inhibitor and a regulator of cell division during development and tumorigenesis. Here we report the molecular characterization of the nucleocytoplasmic transport of survivin and its potential implications for tumorigenesis. We identified an evolutionary conserved Crm1‐dependent nuclear export signal (NES) in survivin. In dividing cells, the NES is essential for tethering survivin and the survivin/Aurora‐B kinase complex to the mitotic machinery, which in turn appears to be essential for proper cell division. In addition, export seems to be required for the cytoprotective activity of survivin, as export‐deficient survivin fails to protect tumor cells against chemo‐and radiotherapy‐induced apoptosis. These findings appear to be clinically relevant since preferential nuclear localization of survivin correlated with enhanced survival in colorectal cancer patients. Targeting survivins nuclear export by the application of NES‐specific antibodies promoted its nuclear accumulation and inhibited its cytoprotective function. We demonstrate that nuclear export is essential for the biological activity of survivin and promote the identification of molecular decoys to specifically interfere with survivins nuclear export as potential anticancer therapeutics. Knauer, S. K., Krämer, O. H., Knösel, T., Engels, K., Rödel, F., Kovács, A. F., Dietmaier, W., Klein‐Hitpass, L., Habtemichael, N., Schweitzer, A., Brieger, J., Rödel, C., Mann, W., Petersen, I., Heinzel, T., Stauber, R. H. Nuclear export is essential for the tumor‐promoting activity of survivin. FASEB J. 21, 207–216 (2007)