Histone deacetylase 3 (HDAC3) is one of four members of the human class I HDACs that regulates gene expression by deacetylation of histones and nonhistone proteins. Early studies have suggested that HDAC3 activity is regulated by association with the corepressors N-CoR and SMRT. Here we demonstrate that, in addition to protein-protein interactions with NCoR/SMRT, the activity of HDAC3 is regulated by both phosphorylation and dephosphorylation. A protein kinase CK2 phosphoacceptor site in the HDAC3 protein was identified at position Ser 424 , which is a nonconserved residue among the class I HDACs. Mutation of this residue was found to reduce deacetylase activity. Interestingly, unlike other class I HDACs, HDAC3 uniquely copurifies with the catalytic and regulatory subunits of the protein serine/threonine phosphatase 4 complex (PP4 c /PP4 R1 ). Furthermore, HDAC3 complexes displayed protein phosphatase activity and a series of subsequent mutational analyses revealed that the N terminus of HDAC3 (residues 1-122) was both necessary and sufficient for HDAC3-PP4 c interactions. Significantly, both overexpression and siRNA knock-down approaches, and analysis of cells devoid of PP4 c , unequivocally show that HDAC3 activity is inversely proportional to the cellular abundance of PP4 c . These findings therefore further highlight the importance of protein-protein interactions and extend the significance of dephosphorylation in the regulation of HDAC activity, as well as present a novel alternative pathway by which HDAC3 activity is regulated.

Estrogen receptors are phosphoproteins which can be activated by ligands, kinase activators, or phosphatase inhibitors. Our previous study showed that p38 mitogen-activated protein kinase was involved in estrogen receptor activation by estrogens and MEKK1. Here, we report estrogen receptor-dependent p38 activation by estrogens in endometrial adenocarcinoma cells and in vitro and in vivo phosphorylation of the estrogen receptor ␣ mediated through p38. The phosphorylation site was identified as threonine-311 (Thr 311 ), located in helix 1 of the hormone-binding domain. The mutation of threonine-311 to alanine did not affect estrogen binding of the receptor but compromised its interaction with coactivators. Suppression of p38 activity or mutation of the site inhibited the estrogen-induced receptor nuclear localization as well as its transcriptional activation by estrogens and MEKK1. The inhibition of the p38 signal pathway by a specific chemical inhibitor blocked the biological activities of estrogens in regulating endogenous gene expression as well as endometrial cancer cell growth. Our studies demonstrate the role of estrogen receptor phosphorylation induced by the natural ligand in estrogen receptor's cellular distribution and its significant contribution to the growthstimulating activity of estrogens in endometrial cancer cells.Estrogens are female sex steroid hormones that control development, maintenance, and regulation of the female reproductive phenotype and behavior. They also stimulate the growth of normal and transformed epithelial cells of the female reproductive systems. The effect of estrogens is mediated through both estrogen receptors ␣ and ␤ (ER␣ and ER␤), which belong to the nuclear hormone receptor superfamily, a group of ligand-regulated, zinc finger-containing transcription factors (11,40). The superfamily includes not only receptors for classical steroids such as estrogens, androgens, progesterones, and glucocorticoids, but also receptors for steroid analogues and nonsteroid ligands such as vitamin D, thyroid, and retinoic acids, as well as orphan receptors for which the ligand is unknown.Unlike the thyroid and vitamin D receptors, which reside in the nucleus in the absence of ligands, receptors for classical steroids such as ER␣ are targeted to the nucleus after binding with estrogens or selective estrogen receptor modulators such as tamoxifen. In contrast, the pure ER␣ antagonist ICI 182,780 directs the ER␣ to the cytoplasm (6). Of importance in this respect are three clusters of basic amino acids, similar to the nuclear localization signals found in simian virus 40 large T antigen, which were identified in the DNA binding domain and the hinge region of ER␣ (44). The nuclear localization signals are constitutively active and do not seem to explain the estrogen effect on the ER␣ nuclear localization (44). It is therefore possible that estrogen-induced targeting of ER␣ to the nucleus is mediated through other mechanisms.Studies in recent years have provided increasing evidence that nuclear loc...

Histone deacetylases (HDACs) are enzymes that catalyze the removal of acetyl groups from lysine residues of histone and nonhistone proteins. Recent studies suggest that they are key regulators of many cellular events, including cell proliferation and cancer development. Human class I HDACs possess homology to the yeast RPD3 protein and include HDAC1, HDAC2, HDAC3, and HDAC8. While HDAC1, HDAC2, and HDAC3 have been characterized extensively, almost nothing is known about HDAC8. Here we report that HDAC8 is phosphorylated by cyclic AMP-dependent protein kinase A (PKA) in vitro and in vivo. The PKA phosphoacceptor site of HDAC8 is Ser 39 , a nonconserved residue among class I HDACs. Mutation of Ser 39 to Ala enhances the deacetylase activity of HDAC8. In contrast, mutation of Ser 39 to Glu or induction of HDAC8 phosphorylation by forskolin, a potent activator of adenyl cyclase, decreases HDAC8's enzymatic activity. Remarkably, inhibition of HDAC8 activity by hyperphosphorylation leads to hyperacetylation of histones H3 and H4, suggesting that PKA-mediated phosphorylation of HDAC8 plays a central role in the overall acetylation status of histones.In eukaryotes, genomic DNA is wrapped tightly around core histones to form nucleosomes, the fundamental building blocks of chromatin. Nucleosomes, once regarded as inert structural particles, are now considered integral and dynamic components of the machineries responsible for gene regulation. Many different enzymes and protein complexes are known to bring about changes in the state of chromatin by numerous mechanisms, with resultant effects on gene expression. One class of complexes alters DNA packaging (remodels chromatin) in an ATP-dependent manner (4, 29). Another class of chromatinaltering factors acts by covalently modifying histone proteins (5). These modifications include acetylation, phosphorylation, methylation, ubiquitination, and ADP-ribosylation. The bestcharacterized posttranslational histone modification is acetylation, which is catalyzed by histone acetyltransferase (HAT) enzymes. Histone acetylation is a reversible process that is regulated by the opposing activities of HATs and histone deacetylases (HDACs). Generally, hyperacetylation of histones results in transcriptional activation whereas deacetylation correlates with transcriptional silencing. Consistent with this generalization, transcriptional activators are often associated with HAT activity whereas HDACs frequently form complexes with transcriptional repressors (24). Therefore, these two regulatory processes work in harmony to achieve appropriate levels of gene expression. Several oncogenes and tumor suppressors (pRb, BRCA-1, BRCA-2, PML-RAR, and a zinc finger protein mutated in leukemia) have been shown to be associated with HATs or HDACs (41).HDAC proteins are vital regulators of fundamental cellular events, including cell cycle progression, differentiation, and tumorigenesis (37,45). A small-molecule inhibitor of HDAC, trichostatin A (TSA), arrests mammalian cells in both G 1 and G 2 (31, 44), w...

Androgens are the male steroid hormone responsible for the development, maintenance, and regulation of the male phenotype and reproductive physiology. The activity of androgens is mediated through the androgen receptor (7,12,31,49), which belongs to the steroid/thyroid nuclear receptor superfamily, a group of ligand-regulated, zinc finger-containing transcription factors (11,50).Similar to other steroid receptors, the androgen receptor, both structurally and functionally, is modular in nature and composed of an N-terminal A/B region (NT) containing the major activation function (AF-1), a DNA binding domain containing two zinc fingers, a short hinge region, and a C-terminal ligand binding domain which contains a weaker activation function (AF-2) (21, 22). A third activation function, AF-5, has also been described (21).Although modular in nature, the different domains of androgen receptor act in a highly coordinated fashion during the receptor activation. In the absence of androgens, the ligand binding domain represses the activation functions until androgens bind to it and relieve the repression by inducing the formation of a conformation suitable for interaction with a complex of transcriptional coactivators, including common coactivators (1, 34), steroid receptor coactivators (3,16,18,40), as well as possible androgen receptor-selective coactivators (51). The coactivators often are histone acetyltransferases themselves or associated with the activity. They convey the effect of androgen receptor on androgen response elementmediated transcription by directly remodeling the chromatin structure through the acetylation of core histones.In addition to androgen response element-mediated gene regulation, androgens have also been reported to regulate the expression of many genes in which no well-defined androgen response elements have been identified. Kallio et al. (23) have shown that, in a proper context, androgen receptor is capable of eliciting both trans-repression and trans-activation through protein-protein interactions with other transcription factors without interacting directly with specific DNA elements.Besides their established physiological functions, androgens are implicated in multiple pathological processes, including benign prostatic hyperplasia and prostate cancer. Physiological levels of androgens are chronically required for the normal growth and development of the prostate gland. In addition to the stimulation of cell proliferation, inhibition of prostatic cell death plays an important role in the maintenance of the total prostatic cell number by androgens in adulthood (19). Similar to normal prostatic glandular epithelial cells, most prostate cancer cells are androgen receptor positive in situ and remain

Histone deacetylases (HDACs) are enzymes that regulate the functions of histones as well as nonhistones by catalyzing the removal of acetyl groups from lysine residues. HDACs regulate many biological processes, including the cell division cycle and tumorigenesis. Although recent studies have implicated HDAC8 in tumor cell proliferation, the molecular mechanisms linking HDAC8 to cell growth remain unknown. Here, we report that the human ortholog of the yeast ever-shorter telomeres 1B (EST1B) binds HDAC8. This interaction is regulated by protein kinase A-mediated HDAC8 phosphorylation and protects human EST1B (hEST1B) from ubiquitin-mediated degradation. Phosphorylated HDAC8 preferentially recruits Hsp70 to a complex that inhibits the CHIP (C-terminal heat shock protein interacting protein) E3 ligase-mediated degradation of hEST1B. Importantly, HDAC8 regulation of hEST1B protein stability modulates total telomerase enzymatic activity. Our findings reveal a novel mechanism by which HDAC8 contributes to tumorigenesis by regulating telomerase activity.Histone acetylation and deacetylation are dynamic posttranslational modifications mediated by the opposing enzymatic activities of histone acetyltransferases (HATs) and histone deacetylases (HDACs), respectively. These reversible modifications regulate diverse biological functions ranging from cell cycle control to tumor formation. Increasing evidence suggests the need for a balance between HAT and HDAC activities for appropriate gene expression. Many studies have demonstrated the adverse consequences of abnormal HAT and HDAC expressions. For example, the inappropriate recruitment of HDACs has been implicated in both developmental defects and human diseases such as promyelocytic leukemia and lymphoma (34). HDAC inhibitors display antitumor activity by promoting either tumor cell apoptosis or normal cell differentiation (37).Based on similarity to yeast proteins, human HDACs can be divided into three different groups. Class I HDACs (HDAC1, 2, 3, and 8) share extensive amino acid sequence homology with the yeast RPD3 protein. Class II HDACs (HDAC4,5,6,7,9, and 10) are homologous to the yeast HDA1 protein.Class III HDACs (SIRT1, 2, 3, 4, 5, 6, and 7) share significant homology with the yeast SIR2 protein and require NAD ϩ for enzymatic activity. A new human HDAC, HDAC11, was reported to exhibit properties of both class I and II HDACs (16, 18).The least studied and understood class I HDAC is HDAC8. The HDAC8 cDNA was identified initially by three independent groups using sequence homology database searches for class I HDACs (3, 23, 54). The HDAC8 gene encodes a 377-amino-acid protein with a predicted molecular mass of 45 kDa.HDAC8 mRNA is expressed in multiple human tissues, including liver, heart, brain, lung, pancreas, placenta, prostate, and kidney. While HDAC8 can deacetylate all core histones in vitro, it preferentially deacetylates histones H3 and H4. Consistent with the presence of a putative nuclear localization signal, HDAC8 is localized predominantly in the nucleu...

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Estrogens are mitogens that stimulate the growth of both normal and transformed epithelial cells of the female reproductive system. The effect of estrogens is mediated through the estrogen receptors, which are ligand-regulated transcription factors. Tamoxifen, a selective estrogen receptor modulator, functions as an estrogen receptor antagonist in breast but an agonist in uterus. In the current study, we show that coexpression of a constitutively active MEKK1, but not RAF or MEKK2, significantly increases the transcriptional activity of the receptor in endometrial and ovarian cancer cells. The expression of wild-type MEKK1 and an active Rac1, which functions upstream of MEKK1, also increased the activity of the receptor while coexpression of dominant negative MEKK1 blocked the Rac1 induction, indicating that endogenous MEKK1 is capable of activating the receptor. Additional experiments demonstrated that the MEKK1-induced activation was mediated through both Jun N-terminal kinases and p38/Hog1 and was independent of the known phosphorylation sites on the receptor. p38, but not Jun N-terminal kinases, efficiently phosphorylated the receptor in immunocomplex kinase assays, suggesting a differential involvement of the two kinases in the receptor activation. More importantly, the expression of the constitutively active MEKK1 increased the agonistic activity of 4-hydroxytamoxifen to a level comparable to that of 17beta-estradiol and fully blocked its antagonistic activity. These findings suggest that the uterine-specific agonistic activity of the tamoxifen compound may be determined by the status of kinases acting downstream of MEKK1.