Effect of histone deacetylase inhibitor valproic acid on progenitor cells of acute myeloid leukemia

Histone deacetylase inhibitor valproic acid (VPA) was recently shown to enhance proliferation and self-renewal of normal hematopoietic stem cells, raising the possibility that VPA may also support growth of leukemic progenitor cells (LPC). Here, VPA maintains a significantly higher proportion of CD34+ LPC and colony forming units compared to control cultures in six AML samples, but selectively reduces leukemic cell numbers in another AML sample with expression of AML1/ETO. Our data suggest a differential effect of VPA on the small population of AML progenitor cells and the bulk of aberrantly differentiated blasts in the majority of AML samples tested.

Clinical trial of valproic acid and all-trans retinoic acid in patients with poor-risk acute myeloid leukemia

BACKGROUND

Valproic acid (VPA), a histone deacetylase (HDAC) inhibitor, induced in vitro differentiation of primary acute myeloid leukemia (AML) blasts, an effect enhanced by all-trans retinoic acid (ATRA). Clinical responses to VPA were recently observed in patients with myelodysplastic syndrome (MDS). Herein, the authors have described results of a clinical trial with VPA plus ATRA in 26 patients with poor-risk AML.

METHODS

VPA (5-10 mg/kg starting dose) and ATRA (45 mg/m(2)) were administered orally. Low-dose AraC or hydroxyurea were permitted to control leukocytosis. Biologic activity of VPA was confirmed by serial analysis of HDAC2 protein levels in peripheral blood (PB) mononuclear cells.

RESULTS

Nineteen of 26 patients completed at least 4 weeks of VPA/ATRA treatment; 7 patients were withdrawn prematurely because of rapidly progressive disease (n = 3) or unacceptable neurologic and cardiovascular toxicity (n = 4). Additional cytoreductive treatment was required in 58% of patients enrolled. Median treatment duration was 3 months. No patient achieved complete remission, one with de novo AML had a minor response, and two patients with secondary AML arising from myeloproliferative disorder (MPD) achieved a partial remission and clearance of PB blasts, respectively. The latter responses were accompanied by profound granulocytosis and erythrocytosis in both patients, reminiscent of the response pattern known from ATRA treatment of acute promyelocytic leukemia. However, cytogenetic analysis of isolated CD34(+) cells and granulocytes did not reveal terminal differentiation of leukemic blasts.

CONCLUSIONS

Treatment with VPA/ATRA results in transient disease control in a subset of patients with AML that has evolved from a myeloproliferative disorder but not in patients with a primary or MDS-related AML.

Mechanisms of resistance to natural killer cell-mediated cytotoxicity in acute lymphoblastic leukemia

OBJECTIVE

Natural killer (NK) cell-mediated cytotoxicity contributes to the innate immune response against numerous malignancies, including leukemias. Acute lymphoblastic leukemias (ALL) often display a high degree of resistance, the mechanisms of which have not been elucidated.

METHODS

We used the well-characterized NK cell line NK-92 as a model to investigate whether mechanisms commonly implicated in tumor escape from NK cell killing are relevant for ALL.

RESULTS

We demonstrate selective resistance of B-precursor ALL to NK-92 cytotoxicity even in the absence of inhibitory killer cell immunoglobulin-like receptors (KIR), except for KIR2DL4. We also show that human leukocyte antigen-G, a ligand of KIR2DL4, expressed on a subset of ALL, does not mediate resistance of NK-cell mediated lysis. Similarly, intracellular adhesion molecule/lymphocyte function-associated antigen-1 interaction did not contribute significantly to resistance. In contrast the NK-sensitive T-ALL (MOLT-4) expressed moderate amounts of MHC class I chain-related gene AB (MICA/B) a ligand for the NK cell activating receptor NKG2D, while expression of MICA/B was absent in resistant B-ALL cell lines.

CONCLUSIONS

The NK cell-resistance of B-lineage ALLs does not appear to involve inhibitory mechanisms, but suggests deficient NK cell activation. Thus, immunostrategies designed to enhance ALL sensitivity toward NK cell-mediated cytotoxicity should focus on mechanisms of NK cell activation.

Valproic Acid Stimulates Proliferation and Self-renewal of Hematopoietic Stem Cells

Histone deacetylase inhibitors have attracted considerable attention because of their ability to overcome the differentiation block in leukemic blasts, an effect achieved either alone or in combination with differentiating agents, such as all-trans retinoic acid. We have previously reported favorable effects of the potent histone deacetylase inhibitor valproic acid in combination with all-trans retinoic acid in patients with advanced acute myeloid leukemia leading to blast cell reduction and improvement of hemoglobin. These effects were accompanied by hypergranulocytosis most likely due to an enhancement of nonleukemic myelopoiesis and the suppression of malignant hematopoiesis rather than enforced differentiation of the leukemic cells. These data prompted us to investigate the effect of valproic acid on normal hematopoietic stem cells (HSC). Here we show that valproic acid increases both proliferation and self-renewal of HSC. It accelerates cell cycle progression of HSC accompanied by a down-regulation of p21(cip-1/waf-1). Furthermore, valproic acid inhibits GSK3beta by phosphorylation on Ser9 accompanied by an activation of the Wnt signaling pathway as well as by an up-regulation of HoxB4, a target gene of Wnt signaling. Both are known to directly stimulate the proliferation of HSC and to expand the HSC pool. In summary, we here show that valproic acid, known to induce differentiation or apoptosis in leukemic blasts, stimulates the proliferation of normal HSC, an effect with a potential effect on its future role in the treatment of acute myeloid leukemia.

Rho family small GTPases control migration of hematopoietic progenitor cells into multicellular spheroids of bone marrow stroma cells

Seeding of hematopoietic progenitor cells (HPC) into the bone marrow requires a complex interaction between cell membrane and adhesion systems and cell signaling pathways. We established a multicellular, spheroid coculture model to study HPC migration in a three-dimensional stromal environment. Here, entry of primary CD34(+) cells into stroma cell spheroids was independent of the integrins very late antigen (VLA)-4, VLA-5, lymphocyte function-associated antigen-1, and the chemokine receptor CXCR4. Experiments using a panel of bacterial toxins selectively targeting key regulators of cellular locomotion, the Rho family small GTPases Rho, Rac, and Cdc42, revealed a considerable reduction or even abrogation of TF-1 cell migration without an increase of apoptosis or impairment of proliferation. Pertussis toxin, an inhibitor of Galpha(i) proteins, showed a similar effect. In some in vitro invasion assays, phosphatidylinositol-3 kinase (PI-3K) was shown to mediate Rac- and Cdc42-induced cell motility and invasion. However, inhibition of the PI-3K pathway by LY294002 did not impair TF-1 cell migration in our three-dimensional model system.