Table 1 Functional or molecular targeting effects of valproic acid (VPA) on acute myeloid leukemia (AML) cells

Intracellular signaling and trafficking

Expression of the CXCR-4 receptor is decreased in CD34- AML cells, whereas increased expression is observed in CD34+ leukemic cells [25]. In vivo treatment with VPA in combination with ATRA alters the phosphorylation status and the phosphoresponsiveness of several intracellular signaling pathways, but the effects differ between patients [35]. VPA increases p21 but downregulates c-Myc expression at a transcriptional level [18]. Modulation of CBP activity and interaction with PML nuclear bodies may contribute to the effects of VPA [38]. The nucleolar morphology and function is altered [36]. VPA also alters the overall expression pattern of the various HDACs [16].

AML cell proliferation, differentiation and apoptosis

VPA has an antiproliferative effect that is dose-dependent. The effects differ between patients and at lower concentrations even enhancement of proliferation is seen for a subset of patients [37]. VPA reprograms the differentiation program in AML cells, especially in cells with a myelomonocytic phenotype [19]. Animal studies of APL suggest that terminal granulocytic differentiation can also be seen [30]. Differentiation is especially seen in t(8;21) AML cells [31].

Effects on leukemic stem cells

In animal models of APL, VPA causes rapid disease regression in induction of granulocytic differentiation, but discontinuation is associated with immediate disease relapse, suggesting that leukemia-initiating cell activity is not affected by VPA [30]. Studies in human cells also suggest that VPA spare or increase immature AML cells during in vitro culture [17]. Direct associations between epigenetic modifications and reprogramming of normal as well as cancer stem cells are now emerging for other malignancies [33].

Effects on t(8;21) AML

In contrast to other AML subsets, VPA inhibits not only the mature AML cells but also the immature progenitors in AML1/ETO [17]. The drug targets the AML1/ETO-HDAC complex, and thereby alters gene expression and induces differentiation [31]. VPA has specific effects in this AML subset. The drug induces differentiation followed by apoptosis and accompanied by increased expression of repressed AML1 target genes [31].

Effects on antileukemic immune reactivity

In combination with 5-AZA, VPA causes induction of specific T cell responses against cancer-associated antigens [24]. The drug also increases the susceptibility to NK cell-mediated lysis through upregulation of NK cell ligands on the leukemic cells [32, 34]; the NK cells then target leukemic stem cells [29]. This effect is also seen for ATRA [34]. AML cells are also sensitized to TRAIL/Apo2L-induced apoptosis by VPA [27]. Spontaneous in vitro apoptosis is associated with immunogenic apoptosis with HSP release and calreticulin exposure; VPA does not interfere with this expression during stress-induced apoptosis [20].

Chemosensitivity and chemoresistance

A recent experimental study suggested that VPA induces a broad chemoresistance phenotype in AML cells [26]. However, clinical data does not support this observation since VPA can be combined with cytarabine, hydroxyurea and 6-mercaptopurin in the treatment of AML patients [22, 23]. One marker of sensitivity may be UTX (KDM6A), which has a functional relationship between protein acetylation and lysine-specific methylation [21]. Resistance programs have also been identified that compensate for the HDAC inhibitor-induced global hyperacetylation, and these programs include MAPKAPK2, HSP90AA1, HSP90AB1 and ACTB [21]. One study also suggested serum HSP90 as a possible marker of sensitivity to VPA [22]. Cellular high expression of FOSB may be another sensitivity marker [28].