ABSTRACT
Glucocorticoids (GCs) are used for treatment of various hematopoietic malignancies owing to their ability to induce apoptosis. A major obstacle in leukemia therapy is the emergence of GC-resistant cells. Hence, combinatory treatment protocols should be developed that convert GC-resistant leukemia cells into sensitive ones. Here we demonstrate that the broad-acting kinase inhibitor staurosporine (STS) confers GC-sensitivity on GC-resistant T lymphoma cells expressing elevated levels of either Bcl-2 or Bcl-XL, but not on GC-resistant myelogenic leukemia cells expressing Mcl-1 in addition to Bcl-2 and/or Bcl-XL. In T lymphoma cells, STS induces the expression of the pro-apoptotic orphan receptor Nur77 that overcomes the anti-apoptotic effect of Bcl-2, thus enabling GCinduced apoptosis. However, in the myelogenic leukemia cells, STS does not upregulate Nur77. In these cells, the glucocorticoid receptor (GR) is rapidly downregulated by GC and the anti-apoptotic Mcl-1 protein is upregulated by STS, thereby leading to an even more resistant phenotype. Altogether, our data provide a molecular basis for the differential apoptotic response of T lymphoma versus myelogenic leukemia cells to STS and GC. The former being sensitized to GC-induced apoptosis by STS, whereas in the latter, STS intensifies GC resistance. The cell type specific responses should be taken into consideration when combinatory therapy is used for treating hematopoietic malignancies.
Subject(s)
Cell Survival/drug effects , DNA-Binding Proteins/physiology , Glucocorticoids/physiology , Phosphotransferases/antagonists & inhibitors , Proto-Oncogene Proteins c-bcl-2/physiology , Receptors, Cytoplasmic and Nuclear/physiology , Receptors, Steroid/physiology , Staurosporine/pharmacology , Transcription Factors/physiology , Apoptosis/drug effects , Apoptosis/physiology , Cell Line, Tumor , Cell Survival/physiology , Drug Resistance, Neoplasm , Glucocorticoids/pharmacology , Humans , Leukemia, Myeloid , Lymphoma, T-Cell , Myeloid Cell Leukemia Sequence 1 Protein , Neoplasm Proteins/metabolism , Nuclear Receptor Subfamily 4, Group A, Member 1 , Proto-Oncogene Proteins c-bcl-2/metabolism , Receptors, Glucocorticoid/metabolism , bcl-X Protein/physiologyABSTRACT
Recent data cast new light on the mechanisms by which glucocorticoids (GCs) elicit apoptosis of thymocytes and leukemia cells. Here we attempt to integrate recent studies by others and us, which provide a novel insight to this apoptotic process. In the last few years it was made clear that there is a tight cooperation between genomic and non-genomic effects exerted by GC receptors (GRs). GC invokes major alterations in the gene expression profile through GR-mediated transactivation and transrepression, which ultimately tip the balance between pro-survival and pro-apoptotic proteins. Although essential in shaping the cell's proteome, these genomic effects are insufficient to elicit apoptotic death and additional signals are required for activating the pro-apoptotic proteins. Several non-genomic effects have been described that occur immediately following exposure to GC, which are imperative for the induction of apoptosis. We have recently observed that GC induces instant GR translocation to the mitochondria in GC-sensitive, but not in GC-resistant, T lymphoid cells. This response contrasts the nuclear translocation of GR occurring in both cell types. We propose that the sustained elevation of GR in the mitochondria following GC exposure is crucial for triggering apoptosis.
Subject(s)
Apoptosis , Glucocorticoids/pharmacology , Receptors, Glucocorticoid/agonists , Animals , Apoptosis Regulatory Proteins/genetics , Apoptosis Regulatory Proteins/metabolism , Cell Line, Tumor , Gene Expression Regulation/drug effects , Humans , Mitochondria/metabolism , Protein Kinases/metabolism , Protein Transport , Receptors, Glucocorticoid/genetics , Receptors, Glucocorticoid/metabolism , Signal TransductionABSTRACT
"Negative selection" and "death by neglect" are governed by apoptotic processes occurring in the thymus that shape the repertoire of maturing T cells. We have previously developed an in vitro model that recapitulates "death by neglect": Co-cultivation of double positive (DP) thymocytes or thymic lymphoma cells (PD1.6) with thymic epithelial cells (TEC) caused TcR-independent apoptosis of the former. We further demonstrated that this apoptosis could be attenuated by aminoglutethimide, an inhibitor of steroid synthesis, suggesting a role of TEC-derived glucocorticoids (GC) in this death process. We have now substantiated the role of the GC-glucocorticoid receptor (GR) axis by using a GC-resistant subline (PD1.6Dex(-)) obtained from the GC-sensitive PD1.6 cells by repeated exposures to increasing doses of dexamethasone (Dex). The PD1.6Dex(-) cells barely express GR and are much less sensitive to TEC-induced apoptosis. Re-expression of GR in PD1.6Dex(-) cells restored their sensitivity to both Dex and TEC, highlighting the central role of GR in these apoptotic processes. Likewise, repeated exposures of PD1.6 cells to TEC led to the selection of TEC-resistant cells (PD1.6TEC(-)) that are insensitive to corticosterone and less sensitive to Dex, though their GR level was only moderately reduced. This is in line with the low levels of corticosterone secreted by TEC. Altogether, our data show that TEC eliminates DP thymic lymphoma cells in a GR-dependent manner and modulates the GC sensitivity of the surviving cells.
