All-trans retinoic acid and hematopoietic growth factors regulating the growth and differentiation of blast progenitors in acute promyelocytic leukemia

Variant Acute Promyelocytic Leukemia Presenting Without Auer Rods Highlights the Need for Correlation with Cytogenetic Data in Leukemia Diagnosis

Variant acute promyelocytic leukemia (vAPL) is a rare leukemia characterized by rearrangement between RARα and a non-PML partner gene. This type of leukemia can be difficult to recognize by histomorphologic evaluation, particularly in patients with few or no Auer rods, and by flow cytometry, but it can be identified by distinct cytogenetic features. Herein, we report on a patient with vAPL with t(11;17)(q23;q21) who presented an initial diagnostic challenge. Detailed flow cytometry findings are presented for this rare entity. Our case study also presents novel treatment (chemotherapy in combination with venetoclax) chosen based on mechanistic data from preclinical studies.

ATRA inhibits the proliferation of DU145 prostate cancer cells through reducing the methylation level of HOXB13 gene

All-trans retinoic acid (ATRA) has been widely investigated for treatments of many cancers including prostate cancer. HOXB13, silenced in androgen receptor-negative (AR(-)) prostate cancer cells, plays a role in AR(-) prostate cancer cell growth arrest. In this study we intended to elucidate the mechanisms that are involved in the proliferation inhibition of AR(-) prostate cancer cells triggered by ATRA. We discovered that ATRA was able to induce the growth arrest and to increase HOXB13 expression in AR(-) prostate cancer cells. Both EZH2 and DNMT3b participated in the repression of HOXB13 expression through an epigenetic mechanism involving DNA and histone methylation modifications. Specifically, EZH2 recruited DNMT3b to HOXB13 promoter to form a repression complex. Moreover, ATRA could upregulate HOXB13 through decreasing EZH2 and DNMT3b expressions and reducing their interactions with the HOXB13 promoter. Concurrently, the methylation level of the HOXB13 promoter was reduced upon the treatment of ATRA. Results from this study implicated a novel effect of ATRA in inhibition of the growth of AR(-) resistant human prostate cancer cells through alteration of HOXB13 expression as a result of epigenetic modifications.

Myeloid-derived suppressor cells: general characteristics and relevance to clinical management of pancreatic cancer

Recent studies describe a heterogeneous population of cells of the myeloid lineage, termed myeloid derived suppressor cells (MDSC), which are observed with increased prevalence in the peripheral blood and tumor microenvironment of cancer patients, including pancreatic cancer. Accumulation of MDSC in the peripheral circulation has been related to extent of disease, and correlates with stage. MDSC have primarily been implicated in promoting tumor growth by suppressing antitumor immunity. There is also compelling evidence MDSC are also involved in angiogenesis and metastatic spread. Two main subsets of MDSC have been identified in cancer patients: a monocytic subset, characterized by expression of CD14, and a granulocytic subset characterized by expression of CD15. Both subsets of MDSC actively suppress host immunity through a variety of mechanisms including production of reactive oxygen species and arginase. Just as in humans, accumulation of monocytic and granulocytic MDSC has been noted in the bone marrow, spleen, peripheral circulation, and tumors of tumor bearing mice. Successful targeting of MDSC in mice is associated with improved immune responses, delayed tumor growth, improved survival, and increased efficacy of vaccine therapy. By further elucidating mechanisms of MDSC recruitment and maintenance in the tumor environment, strategies could be developed to reverse immune tolerance to tumor. We discuss here what is currently known about MDSC as well as some potential strategies targeting MDSC in the context of our work on pancreatic cancer and recent literature. Due to the number of new reports on MDSC, the most pertinent ones have been selected.

Genetic Analyses Permit the Differentiation Between Reactive Malfunctions (‘Promyelocyte Arrest’) and Arising Promyelocyte Leukemia in a Pregnant Patient With a History of a Medulloblastoma

We report the case of a 24-year-old woman with a history of radiotherapy for a cerebellar medulloblastoma 2 years prior to detection of a lymph node metastasis of the former disease and a pancytopenia in the peripheral blood. On bone marrow (BM) examination promyelocyte leukemia vs. a reactive 'promyelocyte arrest' were discussed. The translocation t(15;17) was found in some nuclei and there was a PML-RARalpha gene rearrangement detectable by RT-PCR. Furthermore, there was BM infiltration by the primary cancer. All these results led to the diagnosis of a relapse of the medulloblastoma and of a beginning promyelocyte leukemia. As the patient was pregnant, she had to be parted with the baby to facilitate intensive chemotherapy. She did not respond to a therapeutic regimen specific for promyelocytic leukemia but achieved complete remission of the medulloblastoma as well as the leukemia after the administration of polychemotherapy specific for medulloblastoma. One year later, she suffered from a relapse of her leukemia. Now nearly all cells showed a t(15;17) aberration. Immunophenotype analyses showed a shift to a more undifferentiated blast phenotype that was, however, still HLA-DR negative. The patient again received chemotherapy for leukemia but developed a sepsis 3 months later and died of pancytopenia ensuing her leukemia. There was no clinical evidence for recurrence of the medulloblastoma.

Accumulation of immunosuppressive CD11b+ myeloid cells correlates with the failure to prevent tumor growth in the anterior chamber of the eye

The purpose of these studies is to determine why an immunogenic tumor grows unchecked in the anterior chamber (a.c.) of the eye. The OVA-expressing EL4 tumor, E.G7-OVA, was injected into the a.c. or skin of immunocompetent and immunodeficient mice. Tumor growth and tumor-specific immune responses were monitored. Ocular tumor-infiltrating leukocytes were characterized phenotypically and functionally. Growth of E.G7-OVA was inhibited when limiting numbers of cells were injected in the skin but not in the a.c. of C57BL/6 mice, although both routes primed OVA-specific immune responses, which prevented the growth of a subsequent injection with E.G7-OVA in the skin or opposite eye. Tumor regression was OVA-specific because growth of the parental EL-4 tumor was not inhibited in primed mice. E.G7-OVA growth in the skin was not inhibited in immunodeficient Rag(-/-) or CD8 T cell-deficient mice, suggesting that CD8(+) CTLs mediate tumor elimination. CD8(+) T cell numbers were significantly increased in eyes of mice primed with E.G7-OVA, but few were detected in primary ocular tumors. Nevertheless, growth of E.G7-OVA was retarded in the a.c. of TCR-transgenic OT-I mice, and CD8(+) T cell numbers were increased within eyes, suggesting that tumor-specific CD8(+) CTLs migrated into and controlled primary ocular tumor growth. E.G7-OVA did not lose antigenicity or become immunosuppressive after 13 days of growth in the eye. However, CD11b(+) cells accumulated in primary ocular tumors and contained potent immunosuppressive activity when assayed in vitro. Thus, CD11b(+) cells that accumulate within the eye as tumors develop in the a.c. may contribute to immune evasion by primary ocular tumors by inhibiting CTLs within the eye.

Identification of B-cell translocation gene 1 as a biomarker for monitoring the remission of acute myeloid leukemia

Acute myeloid leukemia (AML) is a biologically heterogeneous disease of the hematopoietic system characterized by a clonal accumulation of immature blast cells in bone marrow. We used a proteomic approach based on two-dimensional electrophoresis and mass spectrometry to search for biomarkers related to the complete remission (CR) state of AML patients. We detected one AML-related protein, which was identified as the B-cell translocation gene 1 (BTG1) protein that belongs to anti-proliferative protein family. In the CR state of AML-M2 and M3 patients (by French-American-British subtype classification), the BTG1 protein was upregulated in bone marrow mononuclear cells. It was also expressed robustly in normal bone marrow mononuclear cells. In addition, the BTG1 levels in AML-M2 patients in a non-remission state after therapy did not increase as they did before therapy. Overexpression of BTG1 mRNA was also observed in the CR state of all-trans-retinoic acid (ATRA)-treated AML-M3 patients and ATRA-treated HL-60 cells. Taken together, these results suggest that BTG1 may play a role in the differentiation process of myeloid cells and can therefore be used as a potential treatment-related biomarker for monitoring the remission status of AML-M2 and M3 patients.

Effects of all-trans retinoic acid (ATRA) on human myeloma cells

All-trans retinoic acid (ATRA) is a natural oxidative metabolite of Vitamin A (retinol) and is known to be a regulator of cell proliferation differentiation, especially in various malignant cells. The cyto-differentiating action of ATRA has led to its usage in the treatment of several malignancies, particularly acute promyelocytic leukemia (APL). There have been many reports regarding the cell biological effects of ATRA on human myeloma cells and a few clinical trials. Most of these reports have revealed growth inhibition by ATRA mediated by down-regulation of the IL-6/IL-6R auto/paracrine loop, and upregulation of p21/Cip1. Here, we review previous reports and introduce experimental results obtained using various myeloma cell lines established in our laboratory.

Proposition of treatment of cancer cells by nuclear protein mixtures from embryonic cells

Several kinds of cancer cells are shown to revert to normal state by action of chemical or biochemical differentiation agents. The analogy between cancer and embryonic cells, with regard to the expression of oncogenes, and the presence, in young embryos, of regulating factors, lead to the proposition of treatment of cancer cells by extracts of nuclei from embryo cells. Preliminary experiments with these extracts, on hepatoma cells in culture, have shown growth and DNA synthesis inhibitions, without cell toxicity, and a prolongation of survival of rats injected with the treated cancer cells.

Signaling revisited in acute promyelocytic leukemia

Although transcription factors are still the main focus to understanding leukemogenesis, recent results strongly suggest that alteration of a receptor and/or subsequent signaling plays a critical and co-operative role in the pathogenesis of acute myeloid leukemia (AML). The t(15;17) translocation, found in 95% of APL, encodes a PML-RARalpha fusion protein. A main model proposed for acute promyelocytic leukemia (APL) is that PML-RARalpha exerts its oncogenic effects by repressing retinoic acid-inducible genes critical to myeloid differentiation. Dysregulation of these genes may result in abnormal signaling, thereby freeing pre-leukemic cells from controls which normally induce the onset of differentiation. It is also likely that treatment of APL cells by retinoic acid induces de novo up-regulation of the same genes which are dominantly repressed by PML-RARalpha and whose expression is required for reactivation of the differentiation program. Identification of such genes together with the signaling pathways interrupted at the early stages of leukemia transformation and reactivated during retinoic acid-induced differentiation in APL cells will contribute to the development of new molecular targets for treatment of leukemia.

Mechanism of immune dysfunction in cancer mediated by immature Gr-1+ myeloid cells

The mechanism of tumor-associated T cell dysfunction remains an unresolved problem of tumor immunology. Development of T cell defects in tumor-bearing hosts are often associated with increased production of immature myeloid cells. In tumor-bearing mice, these immature myeloid cells are represented by a population of Gr-1(+) cells. In this study we investigated an effect of these cells on T cell function. Gr-1(+) cells were isolated from MethA sarcoma or C3 tumor-bearing mice using cell sorting. These Gr-1(+) cells expressed myeloid cell marker CD11b and MHC class I molecules, but they lacked expression of MHC class II molecules. Tumor-induced Gr-1(+) cells did not affect T cell responses to Con A and to a peptide presented by MHC class II. In sharp contrast, Gr-1(+) cells completely blocked T cell response to a peptide presented by MHC class I in vitro and in vivo. Block of the specific MHC class I molecules on the surface of Gr-1(+) cells completely abrogated the observed effects of these cells. Thus, immature myeloid cells specifically inhibited CD8-mediated Ag-specific T cell response, but not CD4-mediated T cell response. Differentiation of Gr-1(+) cells in the presence of growth factors and all-trans retinoic acid completely eliminated inhibitory potential of these cells. This may suggest a new approach to cancer treatment.

Increased production of immature myeloid cells in cancer patients: a mechanism of immunosuppression in cancer

Defective dendritic cell (DC) function caused by abnormal differentiation of these cells is an important mechanism of tumor escape from immune system control. Previously, we have demonstrated that the number and function of DC were dramatically reduced in cancer patients. This effect was closely associated with accumulation of immature cells (ImC) in peripheral blood. In this study, we investigated the nature and functional role of those ImC. Using flow cytometry, electron microscopy, colony formation assays, and cell differentiation in the presence of different cell growth factors, we have determined that the population of ImC is composed of a small percentage (<2%) of hemopoietic progenitor cells, with all other cells being represented by MHC class I-positive myeloid cells. About one-third of ImC were immature macrophages and DC, and the remaining cells were immature myeloid cells at earlier stages of differentiation. These cells were differentiated into mature DC in the presence of 1 microM all-trans-retinoic acid. Removal of ImC from DC fractions completely restored the ability of the DC to stimulate allogeneic T cells. In two different experimental systems ImC inhibited Ag-specific T cell responses. Thus, immature myeloid cells generated in large numbers in cancer patients are able to directly inhibit Ag-specific T cell responses. This may represent a new mechanism of immune suppression in cancer and may suggest a new approach to cancer treatment.

Lovastatin induces a pronounced differentiation response in acute myeloid leukemias

We recently identified HMG-CoA reductase, the rate-limiting enzyme of the mevalonate pathway, as a potential therapeutic target of various retinoic acid responsive cancers. Lovastatin, a competitive inhibitor of HMG-CoA reductase, induced a retinoic acid-like differentiation response followed by extensive apoptosis in neuroblastoma cell lines at relatively low concentrations (<20 microM) of this agent. More recently, we demonstrated that acute myeloid leukemias but not acute lymphocytic leukemias also displayed increased sensitivity to lovastatin-induced apoptosis. In this study, we examined the ability of lovastatin to induce differentiation of acute myeloid leukemic cells and to evaluate the role differentiation may hold in the anti-leukemic properties of this agent. Increased expression of the leukocyte integrins CD11b and CD18 as well as down-regulation of the anti-apoptotic gene bcl-2 are associated with late stage differentiation of the myeloid lineage and retinoic acid induced maturation of acute myeloid leukemic cells. Lovastatin exposure induced increased expression of CD11b and CD18 markers similar to retinoic acid treatment. Following 24 hrs exposure to 20 microM lovastatin, all 7 acute myeloid leukemia cell lines tested showed a decrease in bcl-2 mRNA expression while only 1/5 acute lymphocytic leukemia cell lines showed a similar response. A role for bcl-2 in the apoptotic response of acute myeloid leukemia cells to lovastatin was demonstrated as exogenous constitutive expression of bcl-2 in the AML-5 cell line inhibited apoptosis in a time and dose dependent manner. Thus, lovastatin exposure of acute myeloid leukemia cells induced a differentiation response that may contribute to the therapeutic potential of this agent in the treatment of this disease.

Combination of granulocyte colony-stimulating factor and low-dose cytosine arabinoside further enhances myeloid differentiation in leukemia cells in vitro

We examined the differentiation-inducing effect on freshly isolated myeloid leukemia cells in liquid suspension culture by combined treatment with granulocyte colony-stimulating factor (G-CSF) plus low-dose cytosine arabinoside (Ara-C; 5-10 ng/ml) in 25 patients with acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) in leukemic transformation. Culture with G-CSF alone showed leukemic cell growth stimulation in 15 out of the 25 cases (60%) and induction of cell differentiation in 19 out of the 25 cases (76%), respectively. In 23 cases (92%), either growth stimulation and/or differentiation induction of leukemia cells was observed in response to G-CSF. This suggests that most myeloid leukemia cells are able to respond to G-CSF stimulation. In addition, treatment of cells with low-dose Ara-C alone resulted in the enhancement of myeloid specific antigens expression in 16 cases (64%). Treatment of leukemia cells with higher concentrations of Ara-C (over 50 ng/ml) alone resulted in cytocidal effects but not in the induction of differentiation. Furthermore, 15 cases (60%) showed pronounced myeloid differentiation of leukemia cells after combined exposure to G-CSF plus low-dose Ara-C as compared with cells treated with either G-CSF or Ara-C alone. The enhanced effect of differentiation induction by combining G-CSF plus low-dose Ara-C was also observed in a murine myeloid leukemia cell line WEHI-3B in vitro. These data suggest that treatment with G-CSF plus low-dose Ara-C is capable of inducing differentiation of leukemic cells in vitro, and also appears to be useful for the differentiation-based therapy of patients with AML and MDS.

Induction of tumor cell differentiation

It is proposed that cell proliferation with reduced individual cell growth (total protein accumulation) is necessary, but not sufficient, for cell differentiation. These conditions may facilitate transcription and accumulation of histones H1 and/or H1o relative to the core histones. This may have a critical role in cell differentiation.