Blockade of mTOR signaling potentiates the ability of histone deacetylase inhibitor to induce growth arrest and differentiation of acute myelogenous leukemia cells

Inhibition of mTOR signaling by oleanolic acid contributes to its anti-tumor activity in osteosarcoma cells

Oleanolic acid (OA), a pentacyclic triterpenoid exhibits potent anti-tumor activity against many tumor cell lines. But the mechanisms through which OA inhibits osteosarcoma cells are not known. The mammalian target of rapamycin (mTOR) serves as a central regulator of cell growth, proliferation, survival, and metabolism by integrating intracellular and extracellular signals. In this study, we examined effects of OA on proliferation, cell cycle progression, apoptosis in osteosarcoma cells, and involvement of mTOR signaling in this process. OA inhibited cell proliferation and colony formation, induced G1 arrest in osteosarcoma MG63 and Saos-2 cells dose and time dependently. The protein level of cyclin D1, which plays critical role in G1 to S phase transition and servers as a downstream target of mTOR complex 1 (mTORC1) was down-regulated by OA. Phosphorylation of p70 ribosomal S6 kinase 1 (p70 S6K1) (T389) and S6 (S235/236), mediators of mTORC1 signaling in controlling protein translation and cell growth, was also inhibited by OA. Furthermore, OA inhibited phosphorylation of Akt, a pro-survival factor and substrate for mTORC2. Inactivation of Akt correlated with pro-apoptotic role of OA in osteosarcoma cells, as manifested by an increase in annexin V-FITC binding, cleavage of poly (ADP-ribose) polymerase (PARP) and activation of caspases 3. Our results suggest that OA is a promising agent for treatment of osteosarcoma and mTOR signaling may contribute to its anti-tumor effects on osteosarcoma cells.

Potential of mTOR inhibitors as therapeutic agents in hematological malignancies

Despite significant advances in the treatment of hematological malignancies over the last decade, morbidity and mortality from these disorders remain high. New discoveries in the pathogenesis of these malignancies have led to better understanding of these diseases and new thinking in drug development. mTOR is a downstream effector of the PI3K/Akt (protein kinase B) signaling pathway that mediates cell survival and proliferation and is known to be deregulated in many cancers. Preclinical activity of mTOR inhibitors has been very promising in various hematological malignancies. Rapamycin analogs with relatively favorable pharmaceutical properties, including temsirolimus (CCI-779), everolimus (RAD001) and deforolimus (AP23573), are under clinical evaluations in patients with hematologic malignancies. They have shown encouraging results thus far and a favorable toxicity profile. Their utility, mainly as cytostatic agents, needs to be further explored in combination with pre-existing chemotherapeutic agents for various hematological malignancies.

Autophagy regulates myeloid cell differentiation by p62/SQSTM1-mediated degradation of PML-RARα oncoprotein

PML-RARα oncoprotein is a fusion protein of promyelocytic leukemia (PML) and the retinoic acid receptor-α (RARα) and causes acute promyelocytic leukemias (APL). A hallmark of all-trans retinoic acid (ATRA) responses in APL is PML-RARα degradation which promotes cell differentiation. Here, we demonstrated that autophagy is a crucial regulator of PML-RARα degradation. Inhibition of autophagy by short hairpin (sh) RNA that target essential autophagy genes such as Atg1, Atg5 and PI3KC3 and by autophagy inhibitors (e.g. 3-methyladenine), blocked PML-RARα degradation and subsequently granulocytic differentiation of human myeloid leukemic cells. In contrast, rapamycin, the mTOR kinase inhibitor, enhanced autophagy and promoted ATRA-induced PML-RARα degradation and myeloid cell differentiation. Moreover, PML-RARα co-immunoprecipitated with ubiquitin-binding adaptor protein p62/SQSTM1, which is degraded through autophagy. Furthermore, knockdown of p62/SQSTM1 inhibited ATRA-induced PML-RARα degradation and myeloid cell differentiation. The identification of PML-RARα as a target of autophagy provides new insight into the mechanism of action of ATRA and its specificity for APL.

Combined inhibition of PI3K and mTOR exerts synergistic antiproliferative effect, but diminishes differentiative properties of rapamycin in acute myeloid leukemia cells

A novel strategy has been suggested to enhance rapamycin-based cancer therapy through combining mammalian target of rapamycin (mTOR)-inhibitors with an inhibitor of the phosphatydilinositol 3-kinase PI3K/Akt or mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway. However, recent study demonstrated the potentiating effect of rapamycin on all-trans-retinoic acid (ATRA)-mediated differentiation of acute myelogenous leukemia (AML) cells, prompting us to investigate the effects of longitudinal inhibition of PI3K/Akt/mTOR signaling pathway on both proliferation and differentiative capacity of AML. In NB4, HL-60, U937 and K562 cell lines, rapamycin exerted minimal antiproliferative effects, and combining PI3K inhibitor LY 294002 and rapamycin inhibited proliferation more than LY 294002 alone. Rapamycin potentiated differentiation of ATRA-treated NB4 cells, but the combination of rapamycin and LY 294002 inhibited the expression of CD11b in both ATRA- and phorbol myristate acetate (PMA)-stimulated cells more than PI3K inhibitor alone. These results demonstrate that, although the combination of PI3K inhibitor and rapamycin is more effective in inhibiting proliferation of AML, the concomitant inhibition of PI3K and mTOR by LY 294002 and rapamycin has more inhibitory effects on ATRA-mediated differentiation than the presence of PI3K-inhibitor alone, and diminishes positive effects of rapamycin on leukemia cell differentiation.

The role of HDACs inhibitors in childhood and adolescence acute leukemias

Acute leukemia is the most common type of childhood and adolescence cancer, characterized by clonal proliferation of variably differentiated myeloid or lymphoid precursors. Recent insights into the molecular pathogenesis of leukemia have shown that epigenetic modifications, such as deacetylation of histones and DNA methylation, play crucial roles in leukemogenesis, by transcriptional silencing of critical genes. Histone deacetylases (HDACs) are potential targets in the treatment of leukaemia, and, as a consequence, inhibitors of HDACs (HDIs) are being studied for therapeutic purposes. HDIs promote or enhance several different anticancer mechanisms, such as apoptosis, cell cycle arrest, and cellular differentiation and, therefore, are in evidence as promising treatment for children and adolescents with acute leukemia, in monotherapy or in association with other anticancer drugs. Here we review the main preclinical and clinical studies regarding the use of HDIs in treating childhood and adolescence leukemia.

The DAC system and associations with acute leukemias and myelodysplastic syndromes

Imbalances of histone acetyltransferase (HAT) and deacetylase activity (DAC) that result in deregulated gene expression are commonly observed in leukemias. These alterations provide the basis for novel therapeutic approaches that target the epigenetic mechanisms implicated in leukemogenesis. As the acetylation status of histones has been linked to transcriptional regulation of genes involved particularly in differentiation and apoptosis, DAC inhibitors (DACi) have attracted considerable attention for treatment of hematologic malignancies. DACi encompass a structurally diverse family of compounds that are being explored as single agents as well as in combination with chemotherapeutic drugs, small molecule inhibitors of signaling pathways and hypomethylating agents. While DACi have shown clear evidence of activity in acute myeloid leukemia, myelodysplastic syndromes and lymphoid malignancies, their precise role in treatment of these different entities remain to be elucidated. Successful development of these compounds as elements of novel targeted treatment strategies for leukemia will require that clinical studies be performed in conjunction with translational research including efforts to identify predictive biomarkers.

Pharicin B stabilizes retinoic acid receptor-α and presents synergistic differentiation induction with ATRA in myeloid leukemic cells

All-trans retinoic acid (ATRA), a natural ligand for the retinoic acid receptors (RARs), induces clinical remission in most acute promyelocytic leukemia (APL) patients through the induction of differentiation and/or eradication of leukemia-initiating cells. Here, we identify a novel natural ent-kaurene diterpenoid derived from Isodon pharicus leaves, called pharicin B, that can rapidly stabilize RAR-α protein in various acute myeloid leukemic (AML) cell lines and primary leukemic cells from AML patients, even in the presence of ATRA, which is known to induce the loss of RAR-α protein. Pharicin B also enhances ATRA-dependent the transcriptional activity of RAR-α protein in the promyelocytic leukemia-RARα-positive APL cell line NB4 cells. We also showed that pharicin B presents a synergistic or additive differentiation-enhancing effect when used in combination with ATRA in several AML cell lines and, especially, some primary leukemic cells from APL patients. In addition, pharicin B can overcome retinoid resistance in 2 of 3 NB4-derived ATRA-resistant subclones. These findings provide a good example for chemical biology-based investigations of pathophysiological and therapeutic significances of RAR-α and PML-RAR-α proteins. The effectiveness of the ATRA/pharicin B combination warrants further investigation on their use as a therapeutic strategy for AML patients.

Perspectives on inhibiting mTOR as a future treatment strategy for hematological malignancies

Mammalian target of rapamycin (mTOR) is a protein kinase implicated in the regulation of various cellular processes, including those required for tumor development, such as the initiation of mRNA translation, cell-cycle progression and cellular proliferation. In a wide range of hematological malignancies, the mTORC1 signaling pathway has been found to be deregulated and has been designed as a major target for tumor therapy. Given that pre-clinical studies have clearly established the therapeutic value of mTORC1 inhibition, numerous clinical trials of rapamycin and its derivates (rapalogs) are ongoing for treatment of these diseases. At this time, although disease stabilization and tumor regression have been observed, objective responses in some tumor types have been modest. Nevertheless, some of the mechanisms underlying cancer-cell resistance to rapamycin have now been described, thereby leading to the development of new strategy to efficiently target mTOR signaling in these diseases. In this review, we discuss the rationale for using mTOR inhibitors as novel therapies for a variety of hematological, malignancies with a focus on promising new perspectives for these approaches.

(Arene)Ru(II) complexes of epidermal growth factor receptor inhibiting tyrphostins with enhanced selectivity and cytotoxicity in cancer cells

Ru(eta6-arene) complexes of epidermal growth factor receptor (EGFR) inhibiting tyrphostins 1a and 1b were prepared, characterized and tested for DNA interaction and bioactivity in four human tumor cell lines. The intrinsic cytotoxicity and cell line selectivity of o-hydroxyanisol 1a was greatly enhanced in its Ru(eta6-p-cymene) complex 2a and in its Ru(eta6-toluene) complex 3a. Complex 2a was particularly efficacious against multi-drug resistant EGFR(+) MCF-7/Topo breast carcinoma cells and also against mTOR-dependent EGFR(-) HL-60 leukemia cells. Complex 3a showed enhanced activity only against 518A2 melanoma cells and HL-60 cells, which are both known to express the mTOR protein. DNA was strongly metallated (ca. 1.7-2%) by all new Ru complexes without undergoing topological changes. Apparently, by complexation to Ru fragments tyrphostin derivatives can address additional biological targets in a manner instrumental to antitumoral strategies.

Vorinostat enhances the activity of temsirolimus in renal cell carcinoma through suppression of survivin levels

PURPOSE

The mammalian target of rapamycin (mTOR) inhibitor temsirolimus has exhibited promising anticancer activity for the treatment of renal cell cancers (RCC). Survivin expression has been implicated in drug resistance and reducing its levels with the histone deacetylase (HDAC) inhibitor vorinostat may enhance the anticancer activity of temsirolimus.

EXPERIMENTAL DESIGN

The sensitivity of RCC cell lines to the combination of temsirolimus and vorinostat was determined by measuring cell viability, clonogenic survival, and apoptosis. The effects of this combination on survivin levels were determined in vitro and in vivo. Survivin expression was silenced using small interfering RNA to evaluate its role in determining sensitivity to temsirolimus and vorinostat. The effect of the combination on angiogenesis was also determined in RCC xenograft models.

RESULTS

Vorinostat synergistically improved the anticancer activity of temsirolimus in a panel of RCC cell lines in vitro and in two xenograft models in vivo. While each single agent led to a modest decrease in survivin levels, the combination dramatically reduced its expression, which correlated with an induction of apoptosis. Silencing survivin levels induced apoptosis and significantly improved the efficacy of temsirolimus and vorinostat. In addition, the temsirolimus/vorinostat combination led to a strong reduction in angiogenesis.

CONCLUSIONS

Vorinostat augmented the anticancer activity of temsirolimus in both in vitro and in vivo models of RCC. The effectiveness of the combination was due to a decrease in survivin levels and corresponding induction of apoptosis, and enhanced inhibition of angiogenesis. Targeting survivin may be a promising therapeutic strategy to improve RCC therapy.

p53 is critical for the Aurora B kinase inhibitor-mediated apoptosis in acute myelogenous leukemia cells

We previously showed that AZD1152-HQPA, the inhibitor of Aurora B kinase potently induced growth arrest and apoptosis of various types of human leukemia cells including MV4-11 acute myelogenous leukemia (AML) cells, although the molecular mechanisms by which this class of kinase inhibitors induces apoptosis remain to be fully elucidated. We have recently established the MV4-11 subline, designated as MV4-11 TP53 R248W, which possesses transcriptionally inactive R248W mutation in the TP53 gene. MV4-11 TP53 R248W cells were relatively resistant to AZD1152-HQPA-mediated growth arrest, as measured by MTT and clonogenic assays. AZD1152-HQPA (10-100 nM, 48 h) strikingly induced apoptosis of MV4-11 cells, as assessed by Annexin V binding, loss of mitochondrial outer membrane potential, and activation of caspase cascade, in parallel with up-regulation of p53 and its target molecules Bax and Noxa. Notably, AZD1152-HQPA (10-100 nM, 48 h) induced polyploidy rather than apoptosis in MV4-11 TP53 R248W cells. The polyploid cells were eventually eliminated via apoptosis at later time period (72-120 h) in association with up-regulation of p73. Taken together, p53 plays an important role in AZD1152-HQPA-induced growth arrest and early onset of apoptosis in AML cells. P73 may mediate the late onset of apoptosis to eliminate the polyploid cells caused by the inhibitor of Aurora B kinase.

Evaluation of in vivo antineoplastic effects of rapamycin in patients with chemotherapy-refractory AML

BACKGROUND

The mammalian target of rapamycin (mTOR) has recently been identified as a potential target in acute myeloid leukemia (AML).

METHODS

We treated 5 patients with chemotherapy-refractory AML with the mTOR-inhibitor rapamycin at 2mg per os daily for 14 days, with dose adjustment allowed to reach a target serum rapamycin concentration of 10-20 ng/mL. Four of five patients received additional hydroxyurea at constant dose during treatment with rapamycin.

RESULTS

Two patients achieved a leukocyte response, in one of them, a prolonged response was seen. In the other patients, blast counts remained stable or increased during rapamycin therapy. We did not observe severe hematologic or non-hematologic side effects of rapamycin.

CONCLUSION

Rapamycin at 2mg per day acts mildly cytoreductive in a subgroup of patients with refractory AML. Higher doses and drug combinations may be required to obtain long lasting anti-leukemic effects in these patients.

Histone deacetylase inhibitors: a new perspective for the treatment of leukemia

Histone deacetylase inhibitors (HDIs) promote or enhance several different anticancer mechanisms and therefore are in evidence as potential antileukemia agents. Studies on leukemia have provided examples for their functional implications in cancer development and progression, as well as their relevance for therapeutic targeting. A number of HDIs have been tested in clinical trials and have been proven safe with significant clinical activity. The use of HDIs in association with other molecules, such as classical chemotherapeutic drugs and DNA demethylating agents, has been implied as a promising treatment alternative for leukemia patients in the future. Here we describe the histone deacetylase inhibitors that have been tested in clinical trials for the treatment of leukemia and lymphoma. We conclude that further clinical trials involving a broader number of HDIs used either alone or in combination with other agents are needed to consolidate the use of these epigenetic modulators on leukemia therapy.

Inhibition of mammalian target of rapamycin signaling potentiates the effects of all-trans retinoic acid to induce growth arrest and differentiation of human acute myelogenous leukemia cells

Our study explored the drug interaction of all-trans retinoic acid (ATRA) and RAD001 (everolimus), the inhibitor of mammalian target of rapamycin complex 1 (mTORC1), in acute myelogenous leukemia (AML) NB4 and HL60 cells. RAD001 (10 nM) significantly enhanced the ATRA-induced growth arrest and differentiation of these cells, as measured by colony-forming assay and cell cycle analysis, and expression of CD11b cell surface antigen and nitroblue tetrazolium reduction, respectively. ATRA (0.1-1 microM) upregulated levels of RTP801, a negative regulator of mTORC1, and inhibited mTORC1 signaling as assessed by measurement of the levels of p-p70S6K and p-4E-BP1 in HL60 and NB4 cells. ATRA (0.1-1 microM) in combination with RAD001 (10 nM) strikingly downregulated the levels of p-70S6K and p-4E-BP1 without affecting the total amount of these proteins. Notably, RAD001 (10 nM) significantly augmented ATRA-induced expression of CCAAT/enhancer-binding protein epsilon (C/EBPepsilon) and p27(kip1) and downregulated levels of c-Myc in these cells. Furthermore, RAD001 (5 mg/kg) enhanced the ability of ATRA (10 mg/kg) to inhibit the proliferation of HL60 cells growing as tumor xenografts in immune-deficient nude mice. Taken together, concomitant blockade of the RA and mTORC1 signaling may be a promising treatment strategy for individuals with AML.

Mammalian target of rapamycin inhibitors and their potential role in therapy in leukaemia and other haematological malignancies

The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that functions as a key regulator of cell growth, protein synthesis, and cell-cycle progression through interactions with a number of signalling pathways, including PI3K/AKT, ras, TCL1, and BCR/ABL. Many haematological malignancies have aberrant activation of the mTOR and related signalling pathways. Accordingly, mTOR inhibitors, a class of signal transduction inhibitors that were originally developed as immunosuppressive agents, are being investigated in preclinical models and clinical trials for a number of haematological malignancies. Sirolimus and second-generation mTOR inhibitors, such as temsirolimus and everolimus, are safe and relatively well-tolerated, making them potentially attractive as single agents or in combination with conventional cytotoxics and other targeted therapies. Promising early clinical data suggests activity of mTOR inhibitors in a number of haematological diseases, including acute lymphoblastic leukaemia, chronic myeloid leukaemia, mantle cell lymphoma, anaplastic large cell lymphoma, and lymphoproliferative disorders. This review describes the rationale for using mTOR inhibitors in a variety of haematological diseases with a focus on their use in leukaemia.

Translational research in complex etiopathogenesis and therapy of hematological malignancies: the specific role of tyrosine kinases signaling and inhibition

During the recent genomics and proteomics era, high-resolution, genome-wide approaches have revealed numerous promising new drug targets and disease biomarkers, accelerating and emphasizing the need for targeted molecular therapy compounds. Significant progress has been made in understanding the pathogenesis of hematological malignancies there by, revealing new drug targets. Introduction of multiple new technologies in cancer research have significantly improved the drug discovery process, leading to key success in targeted cancer therapeutics, including tyrosine kinase inhibitors. The studies of receptor tyrosine kinases and their role in malignant transformation are already translated from the preclinical level (cell-based and animal models) to clinical studies, enabling the more complete understanding of tumor cell biology and improvement of tumor therapy.