Many mechanisms have been reported to be involved in HDAC inhibition-induced cytotoxicity 
. HDAC inhibitors not only inhibit cell proliferation but also induce cell death once the inhibition is strong enough.
It is well known that p21cip1
is a cyclin-dependent kinase inhibitor that directly inhibits the activity of cyclin E/CDK2 and cyclin D/CDK4/6 complexes, thus inhibiting Rb phosphorylation. p21cip1
functions as a regulator of cell cycle progression at S phase. Highly expressed p21cip1
would inhibit cell proliferation. What is interesting is that even though the expression of p27kip1
gene and mRNA remained unchanged after LC treatment, p27kip1
protein level dose- and time-dependently increased with LC treatment. p27kip1
protein, like p21cip1
, increased at a relatively low dose and at an early time point, which implied that p27 protein accumulation is possibly regulated at a post-translational level. Previous studies have reported that protein modification by phosphorylation or acetylation would affect the stability of modified proteins 
. It is a general mechanism by which protein acetylation or sumoylation modulates ubiquitination-dependent proteasome proteolysis 
. For example, both acetylation and ubiquitination can modify the same lysine residues at the C terminus of p53, implicating a role of acetylation in the regulation of p53 stability 
. Since LC could induce the acetylation of multiple proteins, protein acetylation by interfering protein ubiquitination could be one of the mechanisms to affect p27 protein degradation, thus inducing p27 accumulation, which need to be confirmed in the future study. As shown, LC could dose-dependently decreased Rb phosphorylation, a downstream target of both p21 and p27 protein. Therefore, p27 protein accumulation together with p21cip1
high expression would contribute to LC-mediated cytotoxicity.
However, we have also noticed that after LC treatment increased not only histone protein acetylation but also other protein acetylation as well (), which suggests that other mechanisms are possibly involved in LC-mediated cytotoxicity in cancer cells. In recent years, protein acetylation has emerged as a major posttranslational modification for proteins 
. The regulatory scope of lysine acetylation is broad and comparable with that of other major posttranslational modifications including protein phosphorylation, sumoylation, ubiquitination and methylation 
. Lysine acetylation is a reversible posttranslational modification of proteins and plays a key role in regulating gene expression. 3600 lysine acetylation sites on 1750 proteins and quantified acetylation changes in response to the deacetylase inhibitors have been identified 
. It is found that lysine acetylation preferentially targets large macromolecular complexes involved in diverse cellular processes, such as chromatin remodeling, cell cycle, nuclear transport, splicing, and actin nucleation 
. Other study further confirmed that virtually every enzyme in glycolysis, gluconeogenesis, the tricarboxylic acid cycle, the urea cycle, fatty acid metabolism, and glycogen metabolism was found to be acetylated in human liver tissue, suggesting that lysine acetylation plays a major role in cell metabolisim and cell viability 
We have confirmed that LC is a HDAC inhibitor. It was found that LC treatment mainly arrested cancer cell proliferation and only slightly induced cell death, which is possibly due to the following two reasons: on one hand, LC is not a strong HDAC inhibitor compared to classical inhibitors including TSA but LC and Buty exerted their inhibiting effects at a similar mM level; on the other hand, relative low level of LC could efficiently induced high expression of p21cip1
which has been reported to block HDAC inhibition-induced apoptosis 
We found that normal thymocytes are more resistant to LC treatment-induced cytotoxicity than cancer cells (). In cancer cells, LC treatment dose-dependently decreased cell viability but in normal thymocytes, LC exerted very weak effect on cell viability. These results are consistent to previous reports by using HDAC inhibitor TSA 
. TSA more selectively induced cancer cytotoxicity than normal cells. This difference has been verified in the in vivo
experiment (). Both in vitro
and in cultured cells, it was found that LC treatment not only inhibited HDAC activities and induced histone acetylation in cancer cells but also in normal cells, but the cytotoxicity induced in normal and cancer cells are different. Therefore, even though the mechanism is unclear, the active states of HDAC in the cells are possibly responsible for the difference 
. As regard to the different effects of LC on normal and tumor tissues, besides the sensitivity to HDAC inhibition, other mechanisms are possibly involved in this difference. Since normal lymphocytes were sensitive while cancer cells were resistant to oligomycin treatment for ATP generation, this implies that in cancer cells, the oxidative phosphorylation system worked in normal cells but not in cancer cells consistent to previous reports 
. In normal thymocytes, LC treatment efficiently induced ATP generation, indicating that LC could be used in normal cells, while in cancer cells LC treatment failed to generate ATP, indicating that LC could not be used for ATP generation. Therefore, reasonably LC would have more potential to affect other targets like HDAC in cancer cells than in normal cells. Even though in cultured cancer cells LC could slightly induced cell death, no cell death in tumor tissues was found like in other normal tissues (data not shown), this is possibly due to that the LC concentration in vivo
is not as high as in vitro
LC has been recognized to play an important role in cellular energy metabolism. The current study has found that HDAC is a new molecular target of LC.
Many studies have been done to determine the effectiveness of LC for fat burning. Also it has been clinically used in cancer patients with fatigue and carnitine deficiency 
. It has been reported that a deficiency of LC is a risk factor for liver cancer. Furthermore, it was found that long-term LC supplementation may prevent the development of liver cancer 
. Even though it has been used under many clinical conditions, the mechanism is still unclear. HDAC inhibitor has been developed as anti-cancer drugs 
. It has been reported previously that histone acetylation mediated by HDAC inhibition could block cell proliferation and induce cell death 
. These data verified that LC mediated histone acetylation via
inhibiting HDAC which at least partially contributed to its cytotoxicity. Therefore, LC would be promising in cancer therapeutics.
LC is not as strong as other HDAC inhibitors like TSA, therefore, LC alone may not be a more potent anti-cancer agent than other HDAC inhibitors, but the importance relies on that LC is an intracellular molecule well known to transport acyl CoA for ATP production under physiological conditions. It has been reported that intracellular LC concentration is at a low mM level 
, and this dose of LC is at a level to at least partially inhibit HDAC activities in most of the cells. We have predicted that LC has the potential to interact with HDAC, therefore, it is possible that the cellular LC and HDAC are in a binding state under physiological conditions, which is worth to be further investigated in the future work.