Because PBMCs isolated from healthy human volunteers are considered ‘normal’ rather than transformed, a key question concerns the biological significance of histone modifications observed following intake of foods such as broccoli sprouts. What benefit might be derived from the rapid and transient reversal of histone ‘marks’ in normal cells, in terms of the genes silenced and unsilenced? We have proposed recently [8
] that epigenetic changes induced by weak ligands might prime normal cells to respond effectively to exogenous insults (toxins, oxidative stress, etc.), activating genes such as P21
to facilitate cell cycle arrest and/or apoptosis, thereby safeguarding against progression to neoplasia (). Rather than the current view of HDAC inhibitors as agents for cancer therapy, dietary HDAC inhibitors might be important for cancer chemoprevention, due to a lifetime of subtle modifications to the histone code.
If this view is indeed correct, how widespread might such HDAC inhibitors be in the human diet, and could they ameliorate other chronic conditions such as cardiovascular disease and neurodegeneration? This is an important question, because ‘epigenetics’ is now known to impact multiple areas, and the underlying mechanisms are central to basic stem cell biology, loss of pleuripotency during differentiation and cell fate determination, and developmental patterning [1
Given such widespread implications, it is interesting to speculate further about SFN and other dietary HDAC inhibitors and their impact on development and chronic disease susceptibility. In addition to SFN, there are many other known or putative diet-derived HDAC inhibitors. Butyrate is the smallest known HDAC inhibitor (reviewed in Ref. [6
]), and contains a simple three-carbon ‘spacer’ attached to a carboxylic acid group (). This compound is derived from the fermentation of dietary fiber and represents the primary metabolic fuel for the colonocytes, where it is present at millimolar concentrations in the large bowel. A second dietary agent reported to inhibit HDAC activity in vitro is the garlic compound DADS [23
], which through metabolism can generate S
-allylmercaptocysteine () and related intermediates containing a spacer ending with a carboxylic acid functional group. As discussed elsewhere [8
], deacetylation of SFN-NAC generates SFN-cysteine (SFN-Cys), a metabolite of SFN that fits well in the HDAC active site (, inset). Molecular modeling studies with other dietary compounds, such as biotin, α-lipoic acid, and metabolites of vitamin E and conjugated linoleic acids, also provided support for their role as putative HDAC inhibitors (). Sulforaphene, erucin, and phenylbutyl isothiocyanate, which contain a similar spacer length as SFN, each had comparable HDAC inhibitory activities [8
], consistent with the Cys moiety occupying the active site and the isothiocyanate ‘cap’ group influencing accessibility to the binding pocket. Similar findings have been reported for structural analogs of TSA, in which the spacer and hydroxamic acid group were retained while substituting the cap group (reviewed in Ref. [8
]). It is interesting to note that retinoic acid also has a cap group, spacer, and carboxylic acid functional group, but drug resistant cases of promyelocytic leukemia respond to retinoids only when coupled with potent HDAC inhibitors [24
]. This might be due to poor fit of retinoids with HDACs that associate with the oncogenic RAR-PLZF fusion protein (reviewed in Ref. [8
Fig. 5 Dietary HDAC inhibitors. HDAC inhibition has been reported in vitro and/or in vivo for butyrate, garlic organosulfur compounds, and metabolites of SFN, such as SFN-NAC and SFN-Cys, whereas other compounds shown are hypothetical HDAC inhibitors (see text). (more ...)
HDAC inhibitors alone can de-repress epigenetically silenced genes in certain cancers, but there is growing interest in combining such compounds with agents that alter DNA methylation, thereby optimizing therapeutic efficacy through enhanced epigenetic gene activation. In theory, dietary HDAC inhibitors might cooperate with other food components known to inhibit DNA methyltransferases (DNMTs), such as soy isoflavones or tea catechins [25
]. The tea polyphenol epigallocatechin-3-gallate (EGCG) was reported to inhibit DNMT in vitro [26
], but a pilot study with SFN in combination with EGCG revealed no significant protection in Apcmin
mice, even though each compound alone suppressed the growth of intestinal polyps [27
]. Further studies are needed to explain this discrepancy, including the possible involvement of confounding pharmacokinetics and metabolism in vivo.
Finally, the mechanisms discussed herein are pertinent to class I and class II HDACs, but certain dietary agents might alter HDAC activities through other mechanisms, as reported for theophylline in alveolar macrophages from patients with chronic obstructive pulmonary disease [28
], and for resveratrol in the activation of human SIRT1 [29
]. The latter enzyme belongs to the NAD+
-dependent SIR2 family, designated as class III HDACs, which do not typically respond to TSA. For more on this topic, the reader is directed to a discussion of sirtuin-activating compounds and their possible role in aging and neurodegenerative diseases [30
In summary, interest in epigenetic mechanisms continues unabated and is impacting on treatment options in the clinic, such as with vorinostat (SAHA) in patients with cutaneous T-cell lymphoma. Potent HDAC inhibitors are seen as promising adjuncts to currently used chemotherapy, through epigenetic mechanisms that activate apoptosis and enhance the debulking of tumors and their subsequent regression. However, with the realization that HDAC inhibitors also exist in the diet, we must begin to expand our horizons and question what role epigenetic modifications might play in normal, non-transformed cells. We have hypothesized that the dietary agents such as SFN, DADS and butyrate prime normal cells epigenetically so that they respond most effectively to external insults. However, more work is needed to confirm or refute this idea, given the transient and reversible nature of the epigenetic changes detected (e.g. with broccoli sprouts in human volunteers). There also is a need to better define the precise mechanisms involved, such as the specific HDAC targets and the downstream pathways affected. These mechanisms could be cell-type specific, due to the unique epigenetic marks laid down in each tissue; thus, protection theoretically might be achieved with the same dietary agent against cancer development in the colon, or motor neuron loss in neurodegenerative disorders, or aberrant vascular changes leading to stroke. This is an optimistic view, but promising results obtained with HDAC inhibitors in Huntington’s disease, epilepsy, and bipolar disorder [32
] suggest that ‘epigenetics’ will likely impact upon multiple disease areas, not simply cancer therapeutics.