Bortezomib Inhibits mTOR Pathway in Multiple Myeloma Cell Lines Via Induced Expression of REDD1.

Pharmacogenomic profiles of genes involved in bortezomib - dexamethasone response may help to understand resistance and could provide new therapeutic targets as well as contributing to novel prognostic markers in multiple myeloma. We have used gene expression profiling to analyze the complex signaling pathways regulating the response to bortezomib - dexamethasone. Gene expression profiles were established in 9 cell lines, derived from 9 myeloma patients, incubated or not with a combination of bortezomib 10 nM and dexamethasone 1 μM. These concentrations correspond to the ones used for patients in the IFM 2005-01. Cells were collected after 6 hours of treatment. We focused our interest in early response genes, making the hypothesis that the comprehension of early effects would help to better understand the mechanisms of resistance that take place in at least two third of myeloma patients. Supervised analysis with permutations identified significantly up regulated genes involved in stress responses (heat shocks proteins, RTP801/dig2/REDD1/DDIT4), endoplasmic reticulum stress (HERP/HERPUD1, gadd145/CHOP/DDIT3), ubiquitin/proteasome pathway (proteasome 26S subunits PSMB7, PSMC4, PSMD3 and PSMD13), unfolded protein response (such as SQSTM1, ATF4) or redox equilibrium (PLRX, PRDX1). We assumed that these genes might represent a molecular signature of response to bortezomib and provide important insight into the complex mechanisms of action of these drugs. We focused on REDD1 a gene cloned in 2002 that is known to be rapidly induced by a wide variety of stress conditions (arsenic, hypoxia, dexamethasone, thapsigargin, tunimycin and heat shock) and DNA damages (ionizing radiation, ultraviolet radiation, DNA alkylant). We found that both REDD1 gene and protein expression were early and highly induced after bortezomib exposure alone or in combinaison with dexamethasone. This effect was dependent upon cell line: REDD1 was overexpressed within two hours in resistant cell lines in association with a cell size decrease while in sensitive cell lines, neither REDD1 induction nor morphological changes occured. REDD1 induction was associated with the dephosphorylation of S6K1, a key substrat of mTOR, a protein kinase which controls cell growth and cell size in response to various signals. SiRNA studies confirmed that bortezomib lead to a negative regulation of mRTor activity mediated by REDD1: disruption of REDD1 abrogates both S6K1 phosphorylation and early transitory cell size reduction. Our results are in accordance with data obtained in mouse showing an early regulation of mTOR pathway and cellular proliferation induced by REDD1 expression in response to stress. Our study suggests that mTOR regulation could be a resistance mechanism mediated by REDD1 expression. As we found that REDD1 was differentially induced in primary plasma cells from patients, this gene expression could help to predict response to bortezomib. Our objective is now to clarify the pathway that links bortezomib to REDD1 in multiple myeloma and to investigate REDD1 expression in patients enrolled in IFM 2005-01 clinical trial.