The Effect of a Histone Deacetylase Inhibitor – Valproic Acid – on Nucleoli in Human Leukaemic Myeloblasts (nucleoli / histone deacetylase inhibitor / leukaemic myeloblasts)

The present study was undertaken to provide more information on nucleolar changes induced by a histone deacetylase inhibitor such as valproic acid in leukaemic myeloblasts at the single-cell level. For this study, RNA in nucleoli was visualized by a simple but sensitive cytochemical procedure in unfixed cytospins of short-term bone marrow cultures from patients suffering from acute myeloid leukaemia. Valproic acid in leukaemic myeloblasts markedly reduced the nucleolar size and also produced significant transformation of “active” to “resting” and “inactive” nucleoli that reflected the alteration of the nucleolar transcription in sensitive myeloblasts. On this occasion it should be added that valproic acid significantly increased the incidence of altered myeloblasts that changed to apoptotic cells or apoptotic bodies and cell ghosts. In contrast to the above-mentioned decreased nucleolar size, the nucleolar RNA concentration, expressed by computerassisted RNA image densitometry in valproic acidtreated myeloblasts, was not significantly changed. The results of the present study clearly indicated that the nucleolar size and transformation of “active” to “sleeping” or “inactive” nucleoli are convenient markers of the sensitivity and alteration of leukaemic myeloblasts produced by a histone deacetylase inhibitor, valproic acid, at the single-cell level. Introduction Previous studies clearly demonstrated that histone deacetylase inhibitors (HDACIs), including valproic acid or sodium valproate (VPA), alter gene expression and act as cytostatic agents that inhibit cell growth, influence cell differentiation and induce the apoptotic process. On the other hand, the mechanisms of HDACI action are less understood although they apparently act on nuclear chromatin structure and gene transcription (Blaheta et al., 2005; Han et al., 2006; Santini et al., 2007; Smetana et al., 2007, 2008a; Emanuele et al., 2008; Eot-Houllier, 2009; Hrebackova et al., 2010; Papi et al., 2010; Tan et al., 2010). The anti-tumour effects of VPA were noted on a variety of malignant tumours including established cell cultures of malignant myeloblasts (Han et al., 2006; Santini et al., 2007; Smetana et al., 2007; Duenas-Gonzalez et al., 2008; Hrebackova et al., 2010; Papi et al., 2010; Tan et al., 2010). However, the direct effects of VPA on the cell nucleolus of malignant cells were less studied (Smetana et al., 2007), although these nuclear components are very convenient markers of cell states at the single-cell level (Smetana, 2002, 2005). The present study was undertaken to provide more information on the effect of VPA on nucleolar bodies of malignant cells and short-term cultures of myeloblasts from selected patients suffering from acute myeloid leukaemia. They appeared to be a very convenient model for such studies because they possessed a satisfactory number of these cells for such study. On this occasion it should be briefly mentioned that nucleoli are gene-rich multifunctional cell organelles that participate in cell proliferation, differentiation, maturation, aging and cell death (Busch and Smetana, 1970; Pederson, 1998; Olson et al, 2000; Smetana, 2005; Boisvert et al., 2007). They are also sites of the ribosomal RNA transcription and assembly of ribosomal particles that migrate to the cytoplasm and represent a substantial part of the cell proteosynthetic machinery (Busch and Smetana, 1970; Fakan and Puvion, 1980; Wachtler and Stahl, 1993; Hozák, 1995; Tschochner and Hurt, 2003; Raška et al., 2006). Received May 24, 2010. Accepted June 17, 2010. The present study was supported in part by the Research Development Project VZ 0002373601 of the Ministry of Health of the Czech Republic and Research Development Project VZ 0021620813 of the Ministry of Education, Youth and Sports of the Czech Republic. Corresponding author: Karel Smetana, Institute of Haematology and Blood Transfusion, U nemocnice 1, Prague 2, 128 20, Czech Republic. Phone: (+420) 221 977 271; Fax: (+420) 221 977 249; e-mail: [email protected] Abbreviations: HDACI – histone deacetylase inhibitor; VPA – valproic acid. Folia Biologica (Praha) 56, 201-205 (2010)