Mammalian target of rapamycin as a target in hematological malignancies

Biphasic Rapamycin Effects in Lymphoma and Carcinoma Treatment

mTOR drives tumor growth but also supports T-cell function, rendering the applications of mTOR inhibitors complex especially in T-cell malignancies. Here, we studied the effects of the mTOR inhibitor rapamycin in mouse EL4 T-cell lymphoma. Typical pharmacologic rapamycin (1-8 mg/kg) significantly reduced tumor burden via direct suppression of tumor cell proliferation and improved survival in EL4 challenge independent of antitumor immunity. Denileukin diftitox (DD)-mediated depletion of regulatory T cells significantly slowed EL4 growth in vivo in a T-cell-dependent fashion. However, typical rapamycin inhibited T-cell activation and tumor infiltration in vivo and failed to boost DD treatment effects. Low-dose (LD) rapamycin (75 μg/kg) increased potentially beneficial CD44hiCD62L⁺ CD8⁺ central memory T cells in EL4 challenge, but without clinical benefit. LD rapamycin significantly enhanced DD treatment efficacy, but DD plus LD rapamycin treatment effects were independent of antitumor immunity. Instead, rapamycin upregulated EL4 IL2 receptor in vitro and in vivo, facilitating direct DD tumor cell killing. LD rapamycin augmented DD efficacy against B16 melanoma and a human B-cell lymphoma, but not against human Jurkat T-cell lymphoma or ID8agg ovarian cancer cells. Treatment effects correlated with IL2R expression, but mechanisms in some tumors were not fully defined. Overall, our data define a distinct, biphasic mechanisms of action of mTOR inhibition at doses that are clinically exploitable, including in T-cell lymphomas. Cancer Res; 77(2); 520-31. ©2016 AACR.

A critical role for non-coding RNA GAS5 in growth arrest and rapamycin inhibition in human T-lymphocytes

Non-coding RNA GAS5 (growth arrest-specific transcript 5) is a 5'-TOP (5'-terminal oligopyrimidine tract) RNA, whose translation, and consequently also stability, is controlled by the mTOR (mammalian target of rapamycin) pathway. GAS5 was identified by functional expression cloning and is necessary and sufficient for normal growth arrest in both leukaemic and untransformed human T-lymphocytes. GAS5 is also required for the inhibitory effects of rapamycin and its analogues on T-cells. The striking functional effects of GAS5 may be mediated through the snoRNAs (small nucleolar RNAs) encoded in its introns and/or through the unusual folding of the mRNA itself, which sequesters, and therefore inhibits, the glucocorticoid receptor.

Inhibition of human T-cell proliferation by mammalian target of rapamycin (mTOR) antagonists requires noncoding RNA growth-arrest-specific transcript 5 (GAS5)

The central importance of the serine/threonine protein kinase mTOR (mammalian Target of Rapamycin) in the control of cell growth and proliferation is well established. However, our knowledge both of the upstream pathways controlling mTOR activity and of the downstream events mediating these effects is still seriously incomplete. We report a previously unsuspected role for the nonprotein-coding RNA GAS5 in the inhibition of T-cell proliferation produced by mTOR antagonists such as rapamycin. GAS5 transcripts are up-regulated during growth arrest and after rapamycin treatment, and GAS5 has recently been shown to be necessary and sufficient for normal T-cell growth arrest. Down-regulation of GAS5 using RNA interference protects both leukemic and primary human T cells from the inhibition of proliferation produced by mTOR antagonists. The GAS5 transcript is a member of the 5' terminal oligopyrimidine class of RNAs, which is specifically controlled at the level of translation by the mTOR pathway, and the effects of GAS5 on the cell cycle provide a novel and important link to the control of proliferation. These observations point to a significant advance in our understanding of the mechanism of action of mTOR inhibitors, which is likely to lead to improvements in immunosuppressive and cancer therapy.

Advances in the management and understanding of autoimmune lymphoproliferative syndrome (ALPS)

Autoimmune lymphoproliferative syndrome (ALPS) is a disorder of T cell dysregulation caused by defective Fas-mediated apoptosis. Patients with ALPS can develop a myriad of clinical manifestations including lymphadenopathy, hepatosplenomegaly, autoimmunity and increased rates of malignancy. ALPS may be more common that originally thought, and testing for ALPS should be considered in patients with unexplained lymphadenopathy, hepatosplenomegaly, and/or autoimmunity. As the pathophysiology of ALPS is better characterized, a number of targeted therapies are in preclinical development and clinical trials with promising early results. This review describes the clinical and laboratory manifestations found in ALPS patients, as well as the molecular basis for the disease and new advances in treatment.

The effect of rapamycin on bone growth in rabbits

mTOR is a nutrient-sensing protein kinase that regulates numerous cellular processes. Our prior studies using the mTOR inhibitor, rapamycin, indicate an important role for mTOR in chondrogenesis. We extended our observations to a physiological, in vivo model of bone growth, direct infusion of rapamycin into the proximal tibial growth plates of rabbits. Rapamycin or DMSO vehicle was infused directly into growth plates by an osmotic minipump for 8 weeks. Tibial growth was followed radiographically. At the end of the experiment, growth plates were recovered for histological analysis. Six animals were studied. No untoward effects of rapamycin infusion were found. Bone growth of limbs exposed to rapamycin was slower than control limbs, particularly during the period of most rapid growth. Histological analysis revealed that growth plate height in the rapamycin-infused limbs was reduced. Both the hypertrophic and proliferative zones were significantly smaller in the rapamycin-infused limbs. Direct infusion of rapamycin into proximal tibial growth plates decreased the size of the growth plate and inhibited overall long bone growth. Rapamycin appears to affect both the proliferative and hypertrophic zones of the tibial growth plate. Our results indicate that nutrients may exert a direct effect on long bone growth via mTOR-mediated modulation of chondrogenesis at the growth plate. and suggest that the possible inhibitory effects of rapamycin on skeletal growth warrant further attention before its use in children.

Treatment with sirolimus results in complete responses in patients with autoimmune lymphoproliferative syndrome

We hypothesized that sirolimus, an mTOR inhibitor, may be effective in patients with autoimmune lymphoproliferative syndrome (ALPS) and treated patients who were intolerant to or failed other therapies. Four patients were treated for autoimmune cytopenias; all had a rapid complete or near complete response. Two patients were treated for autoimmune arthritis and colitis, demonstrating marked improvement. Three patients had complete resolution of lymphadenopathy and splenomegaly and all patients had a reduction in double negative T cells, a population hallmark of the disease. Based on these significant responses, we recommend that sirolimus be considered as second-line therapy for patients with steroid-refractory disease.