Role of sphingosine 1-phosphate in trafficking and mobilization of hematopoietic stem cells.

PURPOSE OF REVIEW: The 'mobilization' of hematopoietic stem cells (HSCs) out of the bone marrow and into the peripheral blood is used clinically to obtain HSCs for transplantation. Although generally successful, mobilization protocols remain imperfect and the mechanisms involved are not fully understood. This review discusses the latest findings in respect to the mechanisms involved in the egress of HSCs from the bone marrow into the circulation and the potential for these recent developments to improve mobilization procedures. RECENT FINDINGS: It has recently become apparent that the bioactive lipid sphingosine 1-phosphate (S1P) plays an active role in attracting HSCs into the peripheral blood. S1P is the first factor identified that provides a chemoattractant gradient promoting the movement of HSCs into the peripheral blood. Drugs that mimic S1P are available with others in development, raising the possibility of increasing the strength of the egress signal and thereby improving the efficacy of mobilization procedures. SUMMARY: S1P is the first egress factor described for HSCs, but the details of the underlying biology are only just emerging. Although manipulating the S1P axis to enhance mobilization protocols is an exciting possibility, much needs to be learned before improvements in mobilization strategies can be realized.

Regulation of the bone marrow microenvironment by G-CSF: Effects of G-CSF on acute lymphoblastic leukaemia.

It has been suggested that disruption of the lymphoid niche by G-CSF may be of therapeutic benefit to patients with acute lymphoblastic leukaemia. We used a xenograft model to determine the effect of G-CSF on ALL progression in a minimal residual disease setting. Consistent with the effects on normal murine B cell progenitors, G-CSF slowed disease in the majority of ALL xenografts tested, suggesting that G-CSF may provide benefits beyond neutrophil recovery for ALL patients. However, two of eight xenografts demonstrated accelerated disease progression. G-CSF could be detrimental for these patients due to expansion of the malignant clone.

A Phase I/II Study of the mTOR Inhibitor Everolimus in Combination with HyperCVAD Chemotherapy in Patients with Relapsed/Refractory Acute Lymphoblastic Leukemia.

PURPOSE: Previous studies suggest a potential therapeutic role for mTOR inhibition in lymphoid malignancies. This single-center phase I/II study was designed to test the safety and efficacy of the mTOR inhibitor everolimus in combination with HyperCVAD chemotherapy in relapsed/refractory acute lymphoblastic leukemia (ALL). EXPERIMENTAL DESIGN: Twenty-four patients were treated; 15 received everolimus 5 mg/day and 9 received 10 mg/day with HyperCVAD. RESULTS: The median age of patients was 25 years (range, 11-64) and median number of prior treatments was 2 (range, 1-7). Grade 3 mucositis was the dose-limiting toxicity and the maximum tolerated everolimus dose was 5 mg/day. Responses included complete remission (CR) in 6 patients (25%), CR without platelet recovery (CRp) in 1 (4%), and CR without recovery of counts (CRi) in 1 (4%), for an overall response rate of 33%. In addition, partial response (PR) was noted in 2 patients (8%). Seven of 11 patients treated in first salvage achieved CR/CRp (64%). The median OS was 29 weeks for patients in first salvage versus 15 weeks for patients in second salvage and beyond (P ≤ 0.001). A response was noted in 5 of 10 (50%) heavily pretreated T-ALL patients (median of 4 prior salvage regimens). Everolimus significantly inhibited phosphorylation of S6RP, but this did not correlate with response. No significant decreases in p4EBP1 and pAkt levels were noted. Responders had higher everolimus dose-adjusted area under the curve (P = 0.025) and lower clearance (P = 0.025) than nonresponders. CONCLUSIONS: The combination of HyperCVAD and everolimus is well tolerated and moderately effective in relapsed ALL, specifically T-ALL.

Silencer of death domains controls cell death through tumour necrosis factor-receptor 1 and caspase-10 in acute lymphoblastic leukemia.

Resistance to apoptosis remains a significant problem in drug resistance and treatment failure in malignant disease. NO-aspirin is a novel drug that has efficacy against a number of solid tumours, and can inhibit Wnt signaling, and although we have shown Wnt signaling to be important for acute lymphoblastic leukemia (ALL) cell proliferation and survival inhibition of Wnt signaling does not appear to be involved in the induction of ALL cell death. Treatment of B lineage ALL cell lines and patient ALL cells with NO-aspirin induced rapid apoptotic cell death mediated via the extrinsic death pathway. Apoptosis was dependent on caspase-10 in association with the formation of the death-inducing signaling complex (DISC) incorporating pro-caspase-10 and tumor necrosis factor receptor 1 (TNF-R1). There was no measurable increase in TNF-R1 or TNF-α in response to NO-aspirin, suggesting that the process was ligand-independent. Consistent with this, expression of silencer of death domain (SODD) was reduced following NO-aspirin exposure and lentiviral mediated shRNA knockdown of SODD suppressed expansion of transduced cells confirming the importance of SODD for ALL cell survival. Considering that SODD and caspase-10 are frequently over-expressed in ALL, interfering with these proteins may provide a new strategy for the treatment of this and potentially other cancers.

mTOR inhibition by everolimus in childhood acute lymphoblastic leukemia induces caspase-independent cell death.

Increasingly, anti-cancer medications are being reported to induce cell death mechanisms other than apoptosis. Activating alternate death mechanisms introduces the potential to kill cells that have defects in their apoptotic machinery, as is commonly observed in cancer cells, including in hematological malignancies. We, and others, have previously reported that the mTOR inhibitor everolimus has pre-clinical efficacy and induces caspase-independent cell death in acute lymphoblastic leukemia cells. Furthermore, everolimus is currently in clinical trial for acute lymphoblastic leukemia. Here we characterize the death mechanism activated by everolimus in acute lymphoblastic leukemia cells. We find that cell death is caspase-independent and lacks the morphology associated with apoptosis. Although mitochondrial depolarization is an early event, permeabilization of the outer mitochondrial membrane only occurs after cell death has occurred. While morphological and biochemical evidence shows that autophagy is clearly present it is not responsible for the observed cell death. There are a number of features consistent with paraptosis including morphology, caspase-independence, and the requirement for new protein synthesis. However in contrast to some reports of paraptosis, the activation of JNK signaling was not required for everolimus-induced cell death. Overall in acute lymphoblastic leukemia cells everolimus induces a cell death that resembles paraptosis.

Plerixafor (AMD3100) induces prolonged mobilization of acute lymphoblastic leukemia cells and increases the proportion of cycling cells in the blood in mice.

The CXCR4 antagonist Plerixafor (AMD3100) induces the rapid mobilization of hematopoietic stem and progenitor cells into the blood in mice and humans. AMD3100 similarly induces the mobilization of human acute lymphoblastic leukemia (ALL) cells into the blood in mice. In this study, the temporal response of pre-B ALL cells to AMD3100 was compared with that of normal hematopoietic progenitor cells (HPC) using an NOD/SCID xenograft model of ALL and BALB/c mice, respectively. ALL cells remained in the circulation up to 6 hours after AMD3100 administration, by which time normal HPCs were no longer detectable. AMD3100 also increased the proportion of actively cycling ALL cells in the peripheral blood. Together, these data suggest that ALL cells are more sensitive to the effects of bone marrow disruption than normal progenitors. Using the NOD/SCID xenograft model, we demonstrated that AMD3100 increased the efficacy of the cell cycle specific drug vincristine, resulting in reduced disease levels in the blood and spleens of animals over 3 weeks and extended the survival of NOD/SCID mice with ALL. These data demonstrate that mobilizing agents can increase the therapeutic effect of cell cycle dependent chemotherapeutic agents.