Valproic acid (2-propylvaleric acid, 2-propylpentanoic acid or n-dipropylacetic acid) (see ), derived from valeric acid () (naturally produced by valerian, Valeriana officinali
s) (see ), was first synthesized in 1882 by Burton [1
]. It is a branched short-chain fatty acid, forming a clear liquid at room temperature, and whose half-life is 9 to 16 hours. For nearly a century, this molecule was used as a “physiologically inert” solvent for organic compounds. It was in 1963, during a study focused on molecules with potential anti-convulsive activity, in which VPA was used as a molecular carrier, that the pharmacological activity of VPA was demonstrated: VPA prevented pentylenetetrazol-induced convulsions in rodents [2
In the human brain, VPA alters the activity of the neurotransmitter Gamma Amino Butyrate (GABA) by potentialising the inhibitory activity of GABA through several mechanisms, including inhibition of GABA degradation, inhibition of GABA Transaminobutyratre (ABAT), increased GABA synthesis, and decreased turnover [5
]. Moreover, VPA attenuates N-Methyl-D-Aspartate-mediated excitation [6
] and blocks Na+
channels (voltage-dependent L type CACNA1 type C, D, N, and F), and voltage-gated K+
channels (SCN) [8
Besides its clinical use as an anticonvulsant and mood-stabilizing drug [9
], VPA presents beneficial effects in clinical depression [10
], absence seizures [11
], tonic-clonic seizures, complex partial seizures [13
], juvenile myoclonic epilepsy [14
], seizures associated with Lennox-Gastaut syndrome [15
], migraine headaches, and schizophrenia. VPA as a therapeutic agent is commercially available as Depakote, Depakote ER, Depakene, Depacon, Stavzor, Mylproin, Ergenyl, Dipropylacetic acid, Myproic Acid, Dipropylacetate, and Convulex.
More recently VPA has been described as an HDAC inhibitor, resulting in an increased interest for its use in cancer therapy. Chromatin is formed of DNA packaged in nucleosome structures, constituted by 146 base-pair DNA sequence winding around an octamere of histones (two copies of each histone: H2A, H2B, H3, and H4) held in place by histone H1. The condensed form of chromatin (heterochromatin) is inactive in terms of transcription whereas the decondensed form (euchromatin) corresponds to an active form. The transition between euchromatin and heterochromatin is dependent upon two families of proteins: histone acetyl transferases (HATs), and histone deacetylases (HDACs). It has been established that histone acetylation leads to relaxation of the nucleosome structure, releasing the DNA and allowing transcription. Inhibition of HDAC promotes decondensed chromatin formation, thereby promoting the expression of genes.
VPA, as well as other HDAC inhibitors (HDACi), is able to alter expression of many genes. Corresponding proteins were described to play important roles in cellular activity and could influence several important pathways such as cell cycle control, differentiation, DNA repair, and apoptosis [16
VPA specifically targets 2 of the 4 classes of HDACs: class I, subclasses Ia and Ib, and class II, subclass IIa. Within subclass IIa, HDAC9 is an exception to this modulation, being activated by VPA, which is also true for HDAC11 [20
]. HDAC 6, 8, and 10 are not modulated. It is interesting to mention that HDAC classes I and II have been reported to be strongly implicated in neuronal function, which could partially explain the action of VPA in neural pathologies.
DNA methylation also contributes to the regulation of gene expression. Hypermethylation of the promoter, usually corresponding to inhibition of gene expression, is controlled by DNA methytransferase (DNMT). Demethylation of nucleic acid has been commonly associated with passive processes corresponding to inhibition of maintenance methylation during S-Phase of the cell cycle. The existence of DNA demethylase was shown a decade ago, resulting in a demethylated active DNA form [21
]. HDACi have been associated with demethylation of DNA, and since 2001, were associated with the active demethylated form. The exact mechanism is not yet known, but it seems that VPA does not directly enhance the enzymatic activity of DNA demethylase. However, through HDACi activity, VPA enables methylated DNA to be more accessible, which is confirmed by the observation that inhibition of HAT diminishes the demethylation effect triggered by VPA [23
]. In addition, it has been shown that valproic acid downregulates expression of proteins essential for chromatin maintenance: SMCs 1-6 (Structural Maintenance of chromatin 1 to 6), DNMT1 (DNA methyl transferase-1), and HP1 (Heterochromatin Protein-1) [25
]. The effects upon transcription are observed after less than 24 hours, while 48 hours are needed to see the effects upon protein levels, which correlates with DNA decondensation (shown in breast cancer cell lines).
Recently, it has been shown that VPA is also able to induce mono-, di-, or tri-methylation of histone 3, particularly at lysine 9 (H3K4) [20
]. Methylation of histones at this lysine is associated with increased transcriptional activity. However, this phenomenon and its purpose are not currently clear, considering the specific site of methylation, and the fact that it only occurs on already hyperacetylated histones, and near-demethylated genes. It is assumed that this modification could serve to stabilize the transiently released form of chromatin, mediated by histone acetylation [28
Among many drugs named as “molecular therapies,” epigenetic drugs are between the most encouraging, because in contrast to other drugs that target the expression of a molecule or a family of molecules, they target chromatin through associated proteins (HDAC, DNMT, HP1, and SMCs). Thus, epigenetic drugs affect the expression of many proteins and therefore may be applicable to a wide range of pathologies, especially cancer, where multiple antioncogens are repressed during carcinogenesis. Epigenetic drugs could particularly target these repressed tumor suppressor genes.
Moreover, given that the balance of acetylation and deacetylation, under the control of HAT and HDAC, is not restricted to histones [29
], it can be hypothesized that VPA, like other HDACi, could modulate molecular activity in addition to transcription. Targeted genes could be Ku (releasing BAX), STAT3, HSP90, p53, and various transcription factors. Candidates have already been mentioned in a preliminary study in 2005 [30
For all the signaling pathways modulated, it has not been established if VPA acts through epigenetic regulation, inhibition of acetylation of molecules other than histones, or by other molecular mechanisms.