PTEN/AKT pathway involved in histone deacetylases inhibitor induced cell growth inhibition and apoptosis of oral squamous cell carcinoma cells
Introduction
Histone deacetylases (HDACs) are enzymes that regulate chromatin structure and function by catalyzing the hydrolysis of the acetyl group from the N-terminal lysine residues of the nucleosomal core histones.1 In active chromatin, the core histones are highly hyperacetylated, whereas in inactive chromatin, the core histones are deacetylated and often cytosine methylated.1 Transcriptional activators usually bind and recruit histone acetyl transferases (HATs) to activate genes expression. In contrast, transcriptional repressors and co-repressors can interact with HDACs to inactivate genes expression.1
Inhibition of HDACs can induce silenced genes re-expression and induce cell growth arrest and apoptosis in a wide variety of tumor cells, including breast cancer cells, prostate cancer cells, hepatoma cells, pancreatic adenocarcinoma cells, lymphoma cells, lung cancer cells, and leukemic cells.2, 3, 4, 5, 6, 7, 8 Therefore, HDACs inhibitors are recognized as potential anticancer agents. One of the HDACs inhibitors, suberoylanilide hydroxamic acid (SAHA), has been in clinical trials for patients with both hematologic and solid tumors.9, 10 Moreover, SAHA has also been the first FDA-approved HDACs inhibitor for treatment of cutaneous T-cell lymphoma.11 The proapoptotic effects of HDACs inhibitors have been mainly correlated with the accumulation of acetylated histones and consequent changes in tumor suppressor genes expression.1 However, alteration of the functions of non-histone substrates also plays important roles in HDACs inhibitors induced cell growth arrest and apoptosis.12 Although several different signal pathways are demonstrated to be involved in the proapoptotic effects of HDACs inhibitors,4, 5, 6, 7, 8 the underlying molecular mechanisms are still not fully elucidated.
The PTEN (Phosphatase and tensin homolog deleted on chromosome 10) gene is a tumor suppressor that reverses the action of phosphoinositide 3-kinase (PI3K) by catalyzing the removal of the 3′ phosphate of phosphatidylinositol 3,4,5-trisphosphate (PIP3), which is necessary for AKT activation.13, 14, 15 PTEN has a single catalytic domain possessing both lipid phospho-inositol and protein phosphatase activities. The lipid phospho-inositol phosphatase activity is essential for PTEN to antagonizes the activity of growth factor-stimulated PI3K/AKT signaling pathway that activates many downstream cellular processes, including cell growth, apoptosis, and cell motility.13, 14, 15, 16 Activation of PTEN blocks cell cycle in the G1 phase and thereby to suppress tumor formation and progression.17 Besides p53 and Erg-1,18, 19 few other agents are known to activate PTEN so far. Considering AKT is involved in proapoptotic effects of HDACs inhibitors,12 it was theoretically and practically important to examine whether or not PTEN was activated by HDACs inhibitors.
Oral squamous cell carcinoma (OSCC) is the most common type of oral cancer and does not respond well to chemotherapy along. However, chemotherapy still plays important roles in the treatment of OSCC. It is usually used in combination with surgery or radiation therapy, or to treat advanced unresectable OSCC in the palliative setting.20 Although the combinational approaches are effective for OSCC, the currently used agents still have some serious side effects. Therefore, it is still clinically needed to develop novel anticancer agents to fight oral cancer. Although HDACs inhibitors are potential anticancer drugs, the effects of HDACs inhibitors on OSCC cells remain unknown.
The purpose of the present study was to evaluate the effects of trichostatin A (TSA), one of the HDACs inhibitors, on cell proliferation and apoptosis of OSCC cells, and to examine whether or not PTEN/AKT pathway was involved in TSA induced cell growth inhibition and apoptosis of the cells.
Section snippets
Cell culture
TCa83 cells, previously established from human tongue OSCC in Peking University School of Stomatology and already been used in many studies,21, 22 were cultured in DMEM (Gibco/Invitrogen) with 10% fetal bovine serum at 37 °C with 5% CO2 in a humidified atmosphere. For cell treatment, TSA (Sigma) dissolved in Me2SO was added in culture medium, while for corresponding controls, the same volume of solvent was added in culture medium.
MTT assay
Cell proliferation was assessed with MTT
Induction of cell growth inhibition and apoptosis of OSCC cells by TSA treatment
To examine the effects of TSA on OSCC cells, TCa83 cells were treated with TSA at concentrations of 0, 0.5, 1.0, and 2.0 μg/ml for different time periods. As shown in Fig. 1, MTT assay showed that the proliferation of TCa83 cells was inhibited by TSA in a dose-dependent manner after treatment for 24 h (Fig. 1A and B). Continued treatment with TSA for 36 h induced TCa83 cells apoptosis, also in a dose-dependent manner (Fig. 1B and C) and differentiation as indicated by neurite extension was seen (
Discussion
In the present study, we showed that PTEN/AKT signal pathway was involved in HDACs inhibitor TSA induced inhibition of cell growth and apoptosis of OSCC cells. Previous studies have demonstrated that rare cell lines are able to escape from the proapoptotic effects of HDACs inhibitors. Consistently, the present study showed that HDACs inhibitor TSA had similar proapoptotic effects on OSCC cells (Fig. 1). The results further supported that HDACs inhibitors could be a broad-spectrum anticancer
Conflict of Interest Statement
None declared.
Acknowledgements
This work was supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (No. 2006-331).
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