Histone deacetylases and cancer: causes and therapies.

Together, histone acetyltransferases and histone deacetylases (HDACs) determine the acetylation status of histones. This acetylation affects the regulation of gene expression, and inhibitors of HDACs have been found to cause growth arrest, differentiation and/or apoptosis of many tumours cells by altering the transcription of a small number of genes. HDAC inhibitors are proving to be an exciting therapeutic approach to cancer, but how do they exert this effect?

An ultrastructural study of early morphogenetic events during the establishment of fetal hepatic erythropoiesis.

Morphogenetic events are described which characterize early stages of the interaction between mesenchyme and expanding epithelial cell cords derived from the hepatic endodermal diverticulum in the C57BL/6J mouse. This interaction culminates in the differentiation of hepatic epithelial and hematopoietic tissues. No basement membrane separates the presumptive hepatic epithelial cells from the adjacent mesenchyme, while intercellular attachments, both adherent junctions and desmosomes, are established transiently between heterologous cell types across this epithelio-mesenchymal interface. Yolk sac-derived erythroblasts found in the primitive liver are distinguished morphologically from endogenous hepatic erythroid cells; they are confined to the vascular compartment and are not, apparently, precursors for hepatic erythropoiesis. The earliest recognizable endogenous hepatic hematopoietic cells appear, extravascularly, among those mesenchymal cells in intimate contact with the endodermal epithelium between the 10(1/4) and 10(1/2) gestational day. Definitive erythropoiesis commences between the 10(1/2) and 11th fetal days. The ultrastructure of these primitive hepatic erythroid cells (proerythroblasts) and their transition to more mature forms (basophilic and polychromatophilic erythroblasts) are described.

Erythropoietin effects on fetal mouse erythroid cells. I. Cell population and hemoglobin synthesis.

The effect of the hormone, erythropoietin, on cultures of erythroblasts derived from the livers of fetal C57BL/6J mice was examined. An increase both in the content and in the rate of synthesis of normal adult mouse globin chains was detected in hormone-treated cultures. The rate of protein synthesis by individual erythroblasts does not increase in response to the hormone, whereas the absolute number of hemoglobin-synthesizing cells does increase and accounts for the observed stimulation of hemoglobin synthesis. The principal effect of erythropoietin appears to be upon the population of immature erythroid precursor cells which persists in the presence of the hormone, the cells maintaining their ability to replicate, and their capacity to differentiate into hemoglobinizing erythroblasts. In the absence of hormone, already committed erythroblasts continue their development, but erythropoiesis is not sustained.

Protein synthesis: its control in erythropoiesis.

Erythropoiesis in the fetal mouse provides a model to study several important aspects of the regulation of cell differentiation and differentiated protein synthesis. Changes in the patterns of hemoglobins formed during fetal and postfetal development are shown to be associated with the substitution of the liver erythroid cell line. In the course of differentiation of yolk sac erythroid cells there are at least two classes of proteins distinguishable with respect to dependence on continued RNA formatoin. The bulk of nuclear proteins, "nondifferentiated" proteins, appear to be dependent on relatively short-lived messenger RNA while synthesis of differentiated proteins, the hemoglobins, proceeds on relatively stable molecules of messenger RNA. Hemoglobin formation occurs in those cells which are actively synthesizing DNA and dividing. On the average, two to three cell divisions may occur after the formation and stabilization of the messenger RNA for globin. Yolk sac erythropoiesis, at least from day 10 of gestation, is unresponsive to erythropoietin. By comparison, in fetal liver erythropoiesis, the hormone, erythropoietin, acts selectively on the most immature erythroid cell precursor to induce differentiation, cell replication, and hemoglobin formation. The erythropoietin responsive cell in the liver is apparently differentiated from the progenitor, pluripotential stem cell and committed to erythroblast formation and hemoglobin synthesis on exposure to the hormone. The initial effects of erythropoietin on macromolecular synthesis are to stimulate RNA synthesis, which temporally is followed by cell replication and the increase in hemoglobin formation. During liver erythropoiesis, there appears to be a transition from hemoglobin synthesis dependent on RNA formation to hemoglobin synthesis directed by relatively stable messenger RNA.

Histone deacetylase inhibitors induce remission in transgenic models of therapy-resistant acute promyelocytic leukemia.

Acute promyelocytic leukemia (APL) is associated with chromosomal translocations, invariably involving the retinoic acid receptor alpha (RAR alpha) gene fused to one of several distinct loci, including the PML or PLZF genes, involved in t(15;17) or t(11;17), respectively. Patients with t(15;17) APL respond well to retinoic acid (RA) and other treatments, whereas those with t(11;17) APL do not. The PML-RAR alpha and PLZF-RAR alpha fusion oncoproteins function as aberrant transcriptional repressors, in part by recruiting nuclear receptor-transcriptional corepressors and histone deacetylases (HDACs). Transgenic mice harboring the RAR alpha fusion genes develop forms of leukemia that faithfully recapitulate both the clinical features and the response to RA observed in humans with the corresponding translocations. Here, we investigated the effects of HDAC inhibitors (HDACIs) in vitro and in these animal models. In cells from PLZF-RAR alpha/RAR alpha-PLZF transgenic mice and cells harboring t(15;17), HDACIs induced apoptosis and dramatic growth inhibition, effects that could be potentiated by RA. HDACIs also increased RA-induced differentiation. HDACIs, but not RA, induced accumulation of acetylated histones. Using microarray analysis, we identified genes induced by RA, HDACIs, or both together. In combination with RA, all HDACIs tested overcame the transcriptional repression exerted by the RAR alpha fusion oncoproteins. In vivo, HDACIs induced accumulation of acetylated histones in target organs. Strikingly, this combination of agents induced leukemia remission and prolonged survival, without apparent toxic side effects.

Transcriptional and post-transcriptional regulation of globin gene accumulation in murine erythroleukemia cells.

The mechanism responsible for the accumulation of newly synthesized alpha- and beta-globin mRNA in the cytoplasm of induced murine erythroleukemia cells was examined by nuclear mRNA nascent chain elongation (run-off transcription). Hexamethylenebisacetimide, a potent inducer of murine erythroleukemia cell differention, induced high levels of both alpha- and beta-globin gene transcription within 48 to 72 h in culture. Butyric acid, a modest inducer of murine erythroleukemia cells, induced a somewhat lower level of globin gene transcription. With both inducers, alpha-globin transcriptional rates exceeded those of beta-globin. Hemin, on the other hand, showed no detectable increase over the basal rate observed in uninduced cells, even at a time (48 h) when newly synthesized globin mRNA was accumulating in the cytoplasm. These results suggest that there are at least two mechanisms responsible for regulating alpha- and beta-globin structural gene expression in induced murine erythroleukemia cells and that the mechanisms involved are inducer dependent. Hexamethylenebisacetimide and butyric acid increase the rate at which globin genes are transcribed, but hemin appears to allow constitutive levels of transcripts to accumulate.

Suppression of cyclin-dependent kinase 4 during induced differentiation of erythroleukemia cells.

Differentiation of murine erythroleukemia cells induced by hexamethylene bisacetamide (HMBA) is associated with accumulation of underphosphorylated retinoblastoma protein (pRB) and an increase in retinoblastoma (RB) gene expression. Here we show that HMBA causes a rapid decrease in the level of cyclin-dependent kinase 4 (cdk4) protein. This decrease results from decreased stability of the protein, while the rate of synthesis of the protein is not affected by HMBA. The decrease in the level of cdk4 protein is followed by suppression of the pRB kinase activity associated with cdk4. Cyclin D3, which can bind and activated cdk4, is increased in HMBA-induced cells and is found in complex with pRB and the transcription factor E2F. In uninduced cells cyclin D3 complexes with pRB and E2F are barely detected. At the later stages of differentiation, MEL cells become arrested in G1 and cdk2 kinase activity is suppressed; this is accompanied by a decrease in the level of cyclin A and cdk2 proteins. Cells transfected with cdk4, which continue to overexpress cdk4 protein during culture with HMBA, are resistant to HMBA-induced differentiation. In contrast, overexpression of cdk2 protein does not inhibit induced differentiation. These findings suggest that suppression of cdk4 is a critical event in the pathway leading to terminal differentiation of erythroleukemia cells.

Conversion of differentiation inducer resistance to differentiation inducer sensitivity in erythroleukemia cells.

Hexamethylene bisacetamide (HMBA) is a potent inducer of differentiation of murine erythroleukemia cells (MELC). Commitment, the irreversible initiation of the program of terminal-cell differentiation, is first detected in HMBA-sensitive DS19-SC9 MELC in culture after 10 to 12 h of exposure to HMBA. Vincristine (VC)-resistant MELC derived from the DS19-SC9 MELC line display increased sensitivity to HMBA and become committed with little or no latent period. In the present study, we showed that the MELC line R1, which is resistant to HMBA-mediated differentiation, became sensitive to inducer if selected for a low level of VC resistance (less than 10 ng of VC per ml). Four independently derived VC-resistant cell lines from HMBA-resistant R1 cells, designated R1[VCR]a to R1[VCR]d, acquired sensitivity to HMBA and the accelerated kinetics of commitment that are characteristic of VC-resistant MELC derived from the parental DS19-SC9 cells. The calcium channel blocker verapamil suppresses the VC resistance of R1[VCR] cells but does not alter the accelerated response to HMBA. In R1[VCR] cells there was no detectable increase in the level of the 140-kilodalton P-glycoprotein. Transient inhibition of protein synthesis during the latent period delays inducer-mediated commitment of VC-sensitive DS19-SC9 MELC but does not alter the accelerated commitment kinetics of R1[VCR]a cells. Previously, we have reported evidence that protein kinase C beta (PKC beta) plays a role in HMBA-induced MELC differentiation and that compared with DS19-SC9 cells, R1 cells have a relatively low level and R1[VCR]a cells have a high level of PKC beta. These findings suggest that (i) acquisition of VC resistance overcomes the block acquired by R1 cells to HMBA-mediated differentiation; (ii) the accelerated kinetics of HMBA-induced commitment of VC-resistant MELC is not dependent on the verapamil-sensitive transport channel that is responsible, at least in part, for resistance to VC; (iii) in VC-resistant MELC, there is constitutive expression or accumulation of a protein required for HMBA-induced differentiation; and (iv) an elevated level of PKC beta activity may play a role in the altered response of R1[VCR] and other VC-resistant MELC to HMBA.

Histone deacetylase inhibitors: inducers of differentiation or apoptosis of transformed cells.

Histone deacetylase (HDAC) inhibitors have been shown to be potent inducers of growth arrest, differentiation, and/or apoptotic cell death of transformed cells in vitro and in vivo. One class of HDAC inhibitors, hydroxamic acid-based hybrid polar compounds (HPCs), induce differentiation at micromolar or lower concentrations. Studies (x-ray crystallographic) showed that the catalytic site of HDAC has a tubular structure with a zinc atom at its base and that these HDAC inhibitors, such as suberoylanilide hydroxamic acid and trichostatin A, fit into this structure with the hydroxamic moiety of the inhibitor binding to the zinc. HDAC inhibitors cause acetylated histones to accumulate in both tumor and normal tissues, and this accumulation can be used as a marker of the biologic activity of the HDAC inhibitors. Hydroxamic acid-based HPCs act selectively to inhibit tumor cell growth at levels that have little or no toxicity for normal cells. These compounds also act selectively on gene expression, altering the expression of only about 2% of the genes expressed in cultured tumor cells. In general, chromatin fractions enriched in actively transcribed genes are also enriched in highly acetylated core histones, whereas silent genes are associated with nucleosomes with a low level of acetylation. However, HDACs can also acetylate proteins other than histones in nucleosomes. The role that these other targets play in the induction of cell growth arrest, differentiation, and/or apoptotic cell death has not been determined. Our working hypothesis is that inhibition of HDAC activity leads to the modulation of expression of a specific set of genes that, in turn, result in growth arrest, differentiation, and/or apoptotic cell death. The hydroxamic acid-based HPCs are potentially effective agents for cancer therapy and, possibly, cancer chemoprevention.

Changes in DNA associated with induction of erythroid differentiation by dimethyl sulfoxide in murine erythroleukemia cells.

The Friend virus-infected murine erythroleukemia cell can be induced to differentiate along erythroid cells in culture with various compounds, including dimethyl sulfoxide. DNA from murine erythroleukemia cells cultured with dimethyl sulfoxide shows a decrease in sedimentation rate in alkaline sucrose gradients after alkali lysis of the cells. These changes can be detected as early as 27 hr after the beginning of culture. Similar results are observed with DNA of the cells cultured with other inducers, butyric acid and dimethylacetamide, but not with DNA from a variant cell line resistant to induction with dimethyl sulfoxide. Ultraviolet irradiation, which is known to cause similar changes in the sedimentation rate of DNA in alkaline sucrose gradients, induces differentiation of the murine erythroleukemia cells. These studies suggest that alterations in DNA may be related to events involved in the induction of differentiation of murine erythroleukemia cells by dimethyl sulfoxide.

Effect of hexamethylene bisacetamide on the commitment to differentiation of murine erythroleukemia cells.

Friend virus-transformed murine erythroleukemia cells express the program of erythropoietic differentiation under the influence of the previously described, potent inducing agent, hexamethylene bisacetamide. Commitment to differentiation, defined as the ability to continue the processes of differentiation in the absence of inducer, has been examined at the single-cell level, with a combination of suspension and cell-cloning techniques. Recruitment of committed cells is shown to occur prior to the detectable accumulation of hemoglobin or the appearance of morphological changes characteristic or erythroid maturation. The stability of the commitment of murine erythroleukemia cells to differentiate is found to be dependent upon both the concentration of hexamethylene bisacetamide and the duration of exposure to the inducing agent. Under conditions less than optimal for induction, a single cell can give rise to a colony containing both differentiated and undifferentiated cells. On the basis of these findings, it is suggested that fully stabilized differentiation, in addition to the previously demonstrated requirement for the inducing agent to be present during a cell-cycle S phase, involves subsequent stabilizing event(s) caused by a direct or indirect action of the inducing agent.

Two cytodifferentiation agent-induced pathways, differentiation and apoptosis, are distinguished by the expression of human papillomavirus 16 E7 in human bladder carcinoma cells.

Many transformed cells have been found to lose the capacity to proliferate and undergo differentiation following exposure to hybrid polar agents. This study investigates the mechanism by which hexamethylene bisacetamide (HMBA) suppresses the proliferation of the human bladder carcinoma line, T24. We found that following a 24-h exposure to HMBA, T24 proliferation was inhibited, and cells arrested in G1 phase and underwent morphological maturation. HMBA-induced cessation of proliferation was mediated, in part, by effects on cell cycle regulatory proteins. In T24 cells cultured without HMBA, E2F complexes predominantly with p107. In culture with inducer, p107 protein decreased, pRB and p130 were converted to underphosphorylated forms, and E2F was shifted into complexes with pRB and p130. To determine whether the formation of pRB:E2F and p130:E2F complexes was required for the HMBA-induced G1 arrest, the ability of the pocket proteins to bind E2F was blocked by enforced expression of human papillomavirus 16 E7. Following culture with HMBA, the T24 clones expressing E7 died, whereas vector-alone T24 clones arrested in G1 phase. T24/E7-1 cells did not form pRB:E2F or p130:E2F complexes upon culture with HMBA; rather, E2F was present in its free form. T24/E7-1 cells cultured with HMBA initially accumulate in G1. By day 2, they have entered into S phase, and by day 3, over 80% of the cells became apoptotic. Taken together, these studies enlarge the repertoire of demonstrated developmental pathways that may be triggered in transformed cells, depending upon their molecular status, and may provide potential therapeutic opportunities for cancer.

Heterogeneity of murine erythroleukemia cells with respect to tumor promoter-mediated inhibition of cell differentiation.

Spontaneous and induced differentiation of murine erythroleukemia cells (strain 745A DS19 ) is reversibly inhibited by 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent promoter of mouse skin carcinogenesis, and by other tumor-promoting macrocyclic plant diterpenes, but it is not by nonpromoting diterpenes. Twelve clones randomly isolated from this strain vary in their response to TPA. All clones are induced to differentiate by several compounds, the most potent of which is hexamethylene bisacetamide. In six clones TPA (100 ng/ml) caused greater than 90% inhibition of differentiation, as measured by the appearance of benzidine-reactive cells. In two clones cell differentiation was not inhibited by TPA even at concentrations as high as 1 microgram/ml. In four clones, differentiation was only partially inhibited (16 to 47%) by TPA. Clones resistant to TPA inhibition of differentiation were also resistant to structurally related tumor-promoting agents. The isolation of variant cell lines, sensitive and resistant to TPA, provides a tool for elucidating the mechanism of tumor promoter-mediated inhibition of cell differentiation.

Antagonistic effect of butyrate on hexamethylene bisacetamide induced differentiation of murine erythroleukemia cells.

Butyrate, at concentrations greater than 0.75 mM, induces hemoglobin accumulation in murine erythroleukemia cells (MELC). At concentrations below 0.75 mM, butyrate inhibits hemoglobin accumulation induced by hexamethylene bisacetamide (HMBA) as well as HMBA induced commitment to terminal cell division. The blocking of HMBA induced differentiation does not result from growth inhibition. When cells were exposed to HMBA and butyrate for 4 days and then both inducers were removed, the cells did not terminally divide. On the other hand, cells exposed to HMBA for 4 days, with subsequent removal of HMBA, did go on to terminally divide. Thus, butyrate blocks the ability of HMBA to accumulate the intracellular signals for terminal cell division. A 48-h pretreatment of cells with butyrate did not inhibit the ability of subsequent HMBA treatment, after butyrate removal, to induce terminal cell division. These results might suggest that cells do not generate a memory of exposure to HMBA in the presence of butyrate or a memory of exposure to butyrate when used as a pretreatment under these conditions. HMBA, at concentrations below 1.0 mM, does not induce MELC differentiation, but such concentrations actually enhance dimethyl sulfoxide induced differentiation of MELC. Equimolar concentrations of short chain fatty acids (1 to 7 carbons) were tested for their ability to block HMBA induced differentiation of MELC. Butyrate and valerate (4 and 5 carbons, respectively) had blocking activities similar to each other, whereas the other fatty acids exhibited little or no blocking of HMBA induced differentiation.

Phase I trial and clinical pharmacological evaluation of hexamethylene bisacetamide administration by ten-day continuous intravenous infusion at twenty-eight-day intervals.

We have treated 33 patients with different types of advanced cancer by 10-day continuous i.v. infusion courses of hexamethylene bisacetamide (HMBA), a drug that produces differentiation of a variety of transformed cell lines on prolonged exposure in vitro to drug concentrations of 3 to 5 mM. In this dose-finding and pharmacokinetic study, five dosage levels were explored from 12 to 28 g/m2/day. Patients who had not shown progression of disease were given repeat courses of therapy at 28-day intervals. Seventy-two courses of therapy were administered; 17 patients received one course; eight patients received two; six patients received three; and one patient each received four and 17+ courses, respectively. The maximal tolerated dose was 28 g/m2/day for 10 days; the dose-limiting toxic effects were thrombocytopenia with hemorrhage and central nervous system dysfunction manifesting as disorientation and confusion. Based on these studies the recommended dosage for Phase II studies by the 10-day schedule is 24 g/m2/day. Pharmacokinetic studies demonstrated rapid clearance of HMBA from plasma; the decay phase data fit a one compartment model with a mean plasma half-life of 2.5 h and a range from 0.6 to 5.8 h. Mean plasma steady-state levels in our patients were 0.37, 0.58, 0.86, 0.88, and 1.42 mM, at the 12-, 16-, 20-, 24-, and 28-g/m2/day dosage levels, respectively. The data indicate that plasma HMBA concentrations of 1 mM can be maintained for 10 days with acceptable patient tolerance, but that HMBA concentrations in excess of 1.4 mM for 10 days are associated with substantial hematological and central nervous system toxicity. Objective antitumor effects were observed in five patients; one woman with non-small cell lung cancer, who has received 17+ courses over a period of 28+ mo, achieved a partial remission that continues at 28+ mo on therapy. Transient regression of cutaneous metastases was observed in three patients with breast carcinoma and one patient with colorectal carcinoma.

The histone deacetylase inhibitor, CBHA, inhibits growth of human neuroblastoma xenografts in vivo, alone and synergistically with all-trans retinoic acid.

Histone deacetylase inhibitors (HDACIs) inhibit the growth of a variety of transformed cells in culture. We demonstrated previously that the hybrid-polar HDACI m-carboxycinnamic acid bis-hydroxamide (CBHA) induces apoptosis of human neuroblastoma in vitro and is effective in lower doses when combined with retinoids. The current study investigates the effect of CBHA on the growth of human neuroblastoma in vivo, both alone and in combination with all-trans retinoic acid (atRA), using a severe combined immunodeficiency-mouse xenograft model. CBHA (50, 100, and 200 mg/kg/day) inhibited growth of SMS-KCN-69n tumor xenografts in a dose-dependent fashion, with 200 mg/kg CBHA resulting in a complete suppression of tumor growth. The efficacy of 50 and 100 mg/kg CBHA was enhanced by the addition of 2.5 mg/kg atRA. This dose of atRA was ineffective when administered alone. Treatment was accompanied by mild weight loss in all groups except the lowest dose of CBHA. Our results suggest HDACIs alone or combined with retinoids may have therapeutic utility for neuroblastoma.

Differential modulation of protein kinase C isoforms in erythroleukemia during induced differentiation.

Induction of erythroid differentiation of murine erythroleukemia cells (MELC) by exposure to hexamethylene bisacetamide (HMBA) involves the modulation of protein kinase C (PKC) activity. Using immuno- and Northern blot techniques, we have demonstrated that MELC express a pattern of PKC isoforms which includes PKC alpha, PKC delta, PKC epsilon, PKC zeta, and PKC eta. We show that MELC resistant to induction by HMBA express significantly less of the nPKC isoform, PKC delta, and slightly less PKC epsilon. Recovery of HMBA sensitivity is associated with reexpression of PKC delta protein. Upon exposure to HMBA, there is a fall in cytosolic PKC delta and PKC epsilon accompanied by a transient increase in membrane-associated forms of these PKC isoforms. HMBA-resistant MELC fail to display this isoform-specific translocation of PKC. Induction of differentiation is accompanied, over the next 24 h of exposure to HMBA, by a progressive fall in cellular PKC activity, associated with a progressive fall in the cellular content of PKC delta, PKC epsilon, and PKC zeta. These studies suggest that PKC delta, and possibly PKC epsilon and PKC zeta as well, play a role in the pathway of HMBA-mediated terminal cell differentiation of MELC.

Hybrid polar histone deacetylase inhibitor induces apoptosis and CD95/CD95 ligand expression in human neuroblastoma.

Inhibitors of histone deacetylase (HDAC) have been shown to have both apoptotic and differentiating effects on various tumor cells. M-carboxycinnamic acid bishydroxamide (CBHA) is a recently developed hybrid polar compound structurally related to hexamethylene bisacetamide. CBHA is a potent inhibitor of HDAC activity. CBHA induces cellular growth arrest and differentiation in model tumor systems. We undertook an investigation of the effects of CBHA on human neuroblastoma cell lines in vitro. When added to cultures of a panel of neuroblastoma cell lines, CBHA induced the accumulation of acetylated histones H3 and H4, consistent with the inhibition of HDAC. Concentrations of CBHA between 0.5 microM and 4 microM led to apoptosis in nine of nine neuroblastoma cell lines. Apoptosis was assessed by DNA fragmentation analysis and the appearance of a sub-G1 (<2N ploidy) population by flow cytometric analysis. The addition of a caspase inhibitor (benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone) completely abrogated CBHA-induced apoptosis in three of three cell lines. The addition of cycloheximide greatly reduced CBHA-induced apoptosis, suggesting that apoptotic induction was dependent on de novo protein synthesis. In addition, CBHA induced the expression of both CD95 (APO-1/Fas) and CD95 ligand within 12 h. The effect of CBHA on human neuroblastoma cells suggests that this agent and structurally related synthetic hybrid polar compounds have therapeutic potential for the treatment of this malignancy.

Suberoylanilide hydroxamic acid, an inhibitor of histone deacetylase, suppresses the growth of prostate cancer cells in vitro and in vivo.

Suberoylanilide hydroxamic acid (SAHA) is the prototype of a family of hybrid polar compounds that induce growth arrest in transformed cells and show promise for the treatment of cancer. SAHA induces differentiation and/or apoptosis in certain transformed cells in culture and is a potent inhibitor of histone deacetylases. In this study, we examined the effects of SAHA on the growth of human prostate cancer cells in culture and on the growth of the CWR22 human prostate xenograft in nude mice. SAHA suppressed the growth of the LNCaP, PC-3, and TSU-Pr1 cell lines at micromolar concentrations (2.5-7.5 microM). SAHA induced dose-dependent cell death in the LNCaP cells. In mice with transplanted CWR222 human prostate tumors, SAHA (25, 50, and 100 mg/kg/day) caused significant suppression of tumor growth compared with mice receiving vehicle alone; treatment with 50 mg/kg/day resulted in a 97% reduction in the mean final tumor volume compared with controls. At this dose, there was no detectable toxicity as evaluated by weight gain and necropsy examination. Increased accumulation of acetylated core histones was detected in the CWR22 tumors within 6 h of SAHA administration. SAHA induced prostate-specific antigen mRNA expression in CWR22 prostate cancer cells, resulting in higher levels of serum prostate-specific antigen than predicted from tumor volume alone. The results suggest that hydroxamic acid-based hybrid polar compounds inhibit prostate cancer cell growth and may be useful, relatively nontoxic agents for the treatment of prostate carcinoma.

Modulation of the c-myb, c-myc and p53 mRNA and protein levels during induced murine erythroleukemia cell differentiation.

The induction of murine erythroleukemia cells (MELC) to terminal differentiation by hexamethylene bisacetamide (HMBA) is accompanied by changes in the levels of c-myb and c-myc mRNA, and in p53 protein levels. We simultaneously examined the effects of HMBA on modulation of c-myb, c-myc and p53 mRNA and protein levels, and examined the relationship between these changes and commitment to terminal cell division. In MELC cultured with HMBA, c-myb protein levels paralleled c-myb mRNA levels except at 24h, when the protein level was equivalent to the level in control cultures, whereas the mRNA had decreased. The c-myc protein paralleled c-myc mRNA throughout induction. The p53 mRNA and protein behaved in a discordant fashion. The p53 protein decreased to very low levels between 4 and 8 h and remained low, while the mRNA, which initially decreased, reaccumulated by 24 and 48 h. Transfer of MELC after 12 to 48 h of culture with HMBA to medium without inducer resulted in rapid (less than 3 h) reaccumulation of the c-myb mRNA, c-myb protein, and p53 protein, and cessation of recruitment of cells to commitment. Cells already induced to commit to terminal differentiation continued to express the differentiated phenotype.