Overexpression of TCL1 activates the endoplasmic reticulum stress response: a novel mechanism of leukemic progression in mice

Abstract

IntroductionChronic lymphocytic leukemia (CLL) represents 30% of adult leukemia and is an incurable B-cell malignancy. Malignant CLL cells use a limited repertoire of immunoglobulin heavy and light chain genes to manufacture their BCRs, [1][2][3] and are very responsive to in vitro anti-IgM stimulation. 4,5 Thus, Ag stimulation has been proposed to drive malignant progression of CLL.The functions of the endoplasmic reticulum (ER) and its associated molecules in CLL have not attracted extensive investigative efforts because CLL cells do not exhibit a readily prominent ER structure like professional secretory cells. Although no protein Ag has been shown to drive malignant progression of CLL in vivo, exposure to Toll-like receptor ligands activates CLL cells, allowing rapid proliferation, 6,7 a cellular process accompanied by robust production and folding of membrane receptors and secretory proteins in the ER. We hypothesize that the ER may play an important role in malignant progression of CLL. First, electron microscopy examinations of human CLL cells showed clear ER expansions and immunoglobulin staining in the ER. [8][9][10] Second, treatments that target ER-Golgi protein transport or inhibit BiP (HSP70 in the ER) and GRP94 (HSP90 in the ER) can sensitize CLL cells to drug-induced apoptosis. 9,11,12 The IRE-1/XBP-1 pathway is activated in response to stress conditions like proteotoxicity or hypoxia in the ER, but it also plays important roles in maintaining basal cellular functions. 13,14 IRE-1 is an ER-resident transmembrane protein that contains a stress sensor domain in the lumen of the ER, and a serine/threonine kinase domain linked to an RNase domain in the cytoplasm. On stress conditions, IRE-1 oligomerizes via its luminal domains in the ER, bringing together the cytoplasmic kinase domains which can undergo autophosphorylation and up-regulate IRE-1's RNase activity. The IRE-1 RNase then splices 26 nucleotides from the mature XBP-1 mRNA, allowing the spliced XBP-1 mRNA to encode the functional 54-kDa transcription factor XBP-1. [15][16][17] XBP-1 regulates a panel of important genes 18 and can crosstalk with other B-cell transcription factors, such as IRF4 and Blimp-1. 19 Overexpression of XBP-1 in B cells has been shown to cause monoclonal gammopathy of undetermined significance, a precursor condition for multiple myeloma. 20 Here, we investigate the roles of the ER stress response in the E-TCL1 CLL mouse model, in which the TCL1 gene is under the control of the immunoglobulin heavy chain promoter/enhancer driving TCL1 overexpression in B cells. 21 TCL1 is expressed in ϳ 90% human CLL patients, 22 and its overexpression is associated with strong BCR signaling, 23,24 allowing malignant CLL cells to undergo high-rate proliferation. E-TCL1 mice initially develop a preleukemic state with clear CD5 ϩ IgM ϩ B-cell characteristics in the blood, spleen, lymph nodes, and bone marrow, and slowly progress to the full-blown monoclonal CLL stage with all clinical features of aggressive human CLL. 21,25 Similar to huma...

Mechanistic rationale for targeting the unfolded protein response in pre-B acute lymphoblastic leukemia

“…2, S6) on XBP1 expression and activity. A previous study showed that overexpression of TCL1 results in increased ER stress (37). In addition, results by us and others demonstrate that oncogenic tyrosine kinase activity results in ectopic expression of Prdm1 (23) and XBP1 (this study; Fig.…”

“…STF-083010 and A106 were previously described (15,37) and obtained from Axon Medchem and VWR. AZD5363 and PD325901 were obtained from Axon Medchem and dissolved in DMSO and stored at −20°C for further experiments.…”

“…S8 B and C). Removal of the A107 moiety results in a significantly smaller molecule and hence increases inhibitory specificity of the remaining A106 compound (37).…”

“…S8A). A recent study showed that spontaneous hydrolysis of STF-083010 results in two fragments, termed A106 and A107 (37) (Fig. 6B), and the A106 moiety retained full inhibitory activity on ERN1 endoribonuclease activity (37).…”

IRE1α RNase–dependent lipid homeostasis promotes survival in Myc-transformed cancers

“…Raji, Daudi, Ramos, EB-2, SK-N-AS, and BE2C cells were obtained from ATCC in 2016, and the Kelly cell line was obtained from Sigma-Aldrich. MEC1, MEC2, and WaC3 cells were described previously (58). The 8498 cells were obtained from Alexander L. Kovalchuk and Herbert C. Morse III (both from the National Institute of Allergy and Infectious Disease, NIH, Rockville, Maryland, USA).…”