Calreticulin, a Molecular Chaperone in the Endoplasmic Reticulum, Modulates Radiosensitivity of Human Glioblastoma U251MG Cells

Abstract

Radiotherapy is the primary and most important adjuvant therapy for malignant gliomas. Although the mechanism of radiation resistance in gliomas has been studied for decades, it is still not clear how the resistance is related with functions of molecular chaperones in the endoplasmic reticulum. Calreticulin (CRT) is a Ca 2+ -binding molecular chaperone in the endoplasmic reticulum. Recently, it was reported that changes in intracellular Ca 2+ homeostasis play a role in the modulation of apoptosis. In the present study, we found that the level of CRT was higher in neuroglioma H4 cells than in glioblastoma cells (U251MG and T98G), and was well correlated with the sensitivity to ;-irradiation. To examine the role of CRT in the radiosensitivity of malignant gliomas, the CRT gene was introduced into U251MG cells, which express low levels of CRT, and the effect of overexpression of CRT on the radiosensitivity was examined. The cells transfected with the CRT gene exhibited enhanced radiationinduced apoptosis compared with untransfected control cells. In CRT-overexpressing cells, cell survival signaling via Akt was markedly suppressed. Furthermore, the gene expression of protein phosphatase 2AcA (PP2AcA), which is responsible for the dephosphorylation and inactivation of Akt, was up-regulated in CRT-overexpressing cells, and the regulation was dependent on Ca 2+. Thus, overexpression of CRT modulates radiation-induced apoptosis by suppressing Akt signaling through the up-regulation of PP2AcA expression via altered Ca 2+ homeostasis. These results show the novel mechanism by which CRT is involved in the regulation of radiosensitivity and radiation-induced apoptosis in malignant glioma cells.

Calreticulin promotes cell motility and enhances resistance to anoikis through STAT3–CTTN–Akt pathway in esophageal squamous cell carcinoma

“…It has recently been reported that CRT-LRP1 receptor complex has a critical role in thrombospondin 1 signaling to increase cell survival of fibroblasts in anoikis by downregulating apoptotic signaling and stimulating Akt activity (Pallero et al, 2008). However, in human glioblastoma U251MG cells, overexpression of CRT-enhanced radiation-induced apoptosis by suppressing Akt signaling (Okunaga et al, 2006). Elevation of CRT promoted apoptosis during cardiac differentiation by negatively regulating Akt activation by upregulating the serine/threonine phosphatase PP2A (Kageyama et al, 2002).…”

“…Recent evidence has shown that CRT protects fibroblasts from anoikis through binding to LRP1 and TSP-1 (Pallero et al, 2008). Akt activation or inactivation has been reported to play an important role in CRT-associated apoptosis (Kageyama et al, 2002;Okunaga et al, 2006;Lim et al, 2008). CRT-deficient mice show embryonic lethal characteristics due to changes in cell adhesiveness and impairment of cardiac development and function (Coppolino et al, 1997;Rauch et al, 2000;Mesaeli and Phillipson, 2004).…”

Endoplasmic reticulum proteostasis in glioblastoma—From molecular mechanisms to therapeutic perspectives

“…The presence of a single-nucleotide polymorphism (rs12435998) in suppressor of Lin-12-like protein 1 (SEL1L), an ERAD E3 ubiquitin ligase adaptor subunit, predicts better response of patients to radio-chemotherapy and is associated with longer survival (214). Similarly, the ER-resident lectinlike chaperone calreticulin modulates radiosensitivity of the U251MG cell line (215). Collectively, these results indicate that ER proteostasis might be an important factor in GBM cells' sensitivity to irradiation.…”

Calreticulin expression in the clonal plasma cells of patients with systemic light-chain (AL-) amyloidosis is associated with response to high-dose melphalan

“…14 Among its functions, calreticulin has been shown in various cell types to be a chaperone involved in N-linked glycosylation, a regulator of calcium homeostasis, a ligand for integrins, a modulator of steroid hormone nuclear receptors, an inhibitor of angiogenesis (vasostatin), a self-antigen, a vehicle for antigen delivery, and a component of the phagocytic synapse. [54][55][56][57][58][59] Calreticulin with its links to endoplasmic calcium stores has also been studied in relationship to apoptosis. 58,60,61 HeLa cells inducible for calreticulin are more sensitive to drug-dependent apoptosis than wild type, and the sensitivity of glioblastoma cells to radiation directly correlates with the calreticulin level.…”