Are found in fruits, vegetables, olive oil, tea and red wine [22]. Plants produce flavonoids as secondary metabolites PubMed ID:https://www.ncbi.nlm.nih.gov/pubmed/28300835 for protection against micro organisms, U. V.light, spread of disease and gives colour to flowers. Chrysin is 5,7-dihydroxy flavone that was found to be cytotoxic with EC50 value of 100 M in wide range of cell lines such as breast (MCF-7, MDA-MB-231 cells), colon (Lovo, DLD-1) and prostate cancer cells [23,24]. Emerging evidences have shown that Histone deacetylase inhibitors (HDACi) such as Trichostatin A (TSA), NBM-HD-1, 3, 3′ Diindolyl methane (DIM) were found to be not only inhibit histone deacetylase activity but also decrease the Akt activity that eventually lead to growth purchase Tyrphostin AG 490 inhibition as well as apoptosis [25-28]. Recent studies have shown the Akt inhibitory activity and apoptotic inducing nature of chrysin [29,30]. But the exact molecular mechanism of action of chrysin was not studied. In the present study we have identified that chrysin functions as HDAC-8 inhibitor and how chrysin controls the cell cycle and cause G1 cell cycle arrest by regulating various cell cycle proteins and histone modifications (H3acK14, H4acK12, H4acK16 and H3K9 me2) at p21 promoter. Here we establish the role of STAT response element (-684/-692) in the transcriptional activity of p21.ResultsIsolation, purification and characterization of novel flavonoidsChrysin (C15H10O4) and its two derivatives, oroxylin-A and methoxy-chrysin (Additional file 1: Figure S1), were extracted from the dried stem bark of the Oroxylum indicum plant using petroleum ether extraction and from the soluble fractions of the same extract using acetone (Additional file 1). The identities and structures were established by NMR (Additional file 1: Figure S2, Figure S3) and ESIMS analyses (Additional file 1: Table S1, Table S3). ThePal-Bhadra et al. BMC Cancer 2012, 12:180 http://www.biomedcentral.com/1471-2407/12/Page 3 ofidentities were verified by comparing the spectroscopic results as described earlier [31]. The compounds were purified further by HPLC. The HPLC fractions that provide greater than 97?9 level of purity of the compounds were considered further (Additional file 1: Figure S1).The base structure of all three compounds is flavonoid. In oroxylin-A, one methoxy group (Meo) at 6th position of chrysin, while in methoxy chrysin, the methoxy group substitution at 7th position of chrysin (Additional file 1: Figure S1). The presence of the voluminous hydrophobic (OH) substitute at the R6 in the chrysin causes an inhibitory effect on DNA cross-linking.Role of Chrysin in cell cycle progressionThe effect of the chrysin and its analogues (Figure 1A) on the cell viability and cell cycle was determined by MTT assay and FACS analysis in human neoplastic A375 cells. The incubation in 40 M chrysin or its derivatives showed marked inhibition on cell proliferation. Chrysin having 2 hydroxyl groups cause 50 of cell death at 40 Mconcentration. At lower concentration (10 and 20 M) a trace level of cytotoxic effect was noticed (data not shown). A375 cells treated with analogues of chrysin, oroxylin A and methoxy chrysin show less cytotoxicity (Figure 1B). But cytotoxicity was not increased proportionate to the higher concentration (120 M) in A375 cell line over 48 h of incubation. We also compare the effect of chrysin with known HDAC inhibitor Trichostatin A to understand the role of chrysin on cell viability relative to standard HDAC inhibitor TSA (Figure 1C). In order to u.