Tamoxifen-induced cytotoxicity in breast cancer cells is mediated by glucose-regulated protein 78 (GRP78) via AKT (Thr308) regulation.
Glucose regulated protein 78 (GRP78) has recently been suggested to be associated with drug resistance in breast cancer patients. However, the precise role of GRP78 in drug resistance and the involved signaling pathways are not clearly understood. In the present study, we show that among a panel of drugs, namely Paclitaxel (TAX), Doxorubicin (DOX), 5-fluorouracil (5-FU), UCN-01 and Tamoxifen (TAM) used, TAM alone up-regulated the expression of GRP78 significantly and induced apoptosis in MCF-7 and MDA-MB-231 cells. Interestingly, inhibition of GRP78 by a specific pharmacological inhibitor, VER-155008 augmented TAM-induced apoptosis, and overexpression of GRP78 rendered the cells resistant to TAM-induced cell death suggesting a role for GRP78 in TAM-induced cytotoxicity. Mechanistically, the expression of phosphorylated AKT as determined by Western blot analyses revealed that TAM selectively upregulated phosphorylation of AKT at Thr308 but not at Ser473, and siRNA silencing of GRP78 resulted in inhibition of AKT phosphorylation at Thr308 but not at Ser473. Further, a GRP78 inhibitor, VER155008 inhibited TAM-induced phosphorylation of GSK3β, a downstream substrate of AKT. These results, thus suggests a role for GRP78 in TAM-induced AKT activation. Additionally, co-localization studies by immunofluorescence, and immunoprecipitation experiments demonstrated a complex formation of AKT and GRP78. Furthermore, in glucose-free medium, the cells were sensitized to TAM-induced cell death that was associated with reduced AKT phosphorylation at Thr308, thus strengthening the association of AKT regulation with drug response. Collectively, our findings identify a role of GRP78 in AKT regulation in response to TAM in breast cancer cells.
Dept. of Biotechnology, Savitribai Phule Pune University, Pune, 411007, Maharashtra, India.
Radha
R
Pujari
Dept. of Biotechnology, Savitribai Phule Pune University, Pune, 411007, Maharashtra, India.
Jemy
J
Jose
Dept. of Biotechnology, Savitribai Phule Pune University, Pune, 411007, Maharashtra, India.
Varsha
V
Bhavnani
National Centre for Cell Science, Pune University Campus, Pune, 411007, Maharashtra, India.
Natesh
N
Kumar
National Centre for Cell Science, Pune University Campus, Pune, 411007, Maharashtra, India. Electronic address: padma@nccs.res.in.
Padma
P
Shastry
Dept. of Biotechnology, Savitribai Phule Pune University, Pune, 411007, Maharashtra, India. Electronic address: jkpal@unipune.ac.in.
Jayanta K
JK
Pal
D000970:Antineoplastic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects; D001943:Breast Neoplasms / Q000473:pathology*; D045744:Cell Line, Tumor; D004347:Drug Interactions; D019008:Drug Resistance, Neoplasm / Q000187:drug effects; D000091342:Endoplasmic Reticulum Chaperone BiP; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D005947:Glucose / Q000494:pharmacology; D006360:Heat-Shock Proteins / Q000378:metabolism*; D006801:Humans; D007328:Insulin / Q000494:pharmacology; D010766:Phosphorylation / Q000187:drug effects; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D013629:Tamoxifen / Q000494:pharmacology*
D000970:Antineoplastic Agents; D000091342:Endoplasmic Reticulum Chaperone BiP; C000717247:HSPA5 protein, human; D006360:Heat-Shock Proteins; D007328:Insulin; D013629:Tamoxifen; D051057:Proto-Oncogene Proteins c-akt; D005947:Glucose
AKT signaling; Breast cancer; Chemoresistance; Endoplasmic reticulum stress; Tamoxifen