Dual Activation of NRF2 in Multiple Myeloma and Bone Marrow Mesenchymal Stromal Cells Regulates Chemotherapy Resistance

Background

The cornerstone treatments of multiple myeloma (MM) are proteasome inhibitors bortezomib (BZ) and carfilzomib (CFZ). However, MM still remains incurable for that MM cells rapidly develop resistance to chemotherapy. Nuclear factor (erythroid-derived 2)-like 2 (NRF2) pathways have been shown to contribute to the malignant phenotypes of several cancers through effects on proliferation and drug sensitivity. NRF2 functions to rapidly change the sensitivity of the cells environment to oxidants and electrophiles by stimulating the transcriptional activation of drug metabolism and antioxidant genes. NRF2 is negatively regulated by proteasome degradation through its inhibitor KEAP1. The aim of this study was to determine if proteasome inhibitor induced NRF2 signalling orchestrates survival of MM in the bone marrow (BM) microenvironment.

Methods

To investigate the role of NRF2 in the MM microenvironment primary human MM and BM mesenchymal stromal cells (MSC) were obtained under UK ethical approval (LREC ref 07/H0310/146). NRF2 activity in MM and BM-MSC was measured by NRF2 protein expression, target genes expression and using promoter assays. Lentiviral mediated shRNA knockdown of NRF2 in the MM and BM-MSC. The NRF2 inhibitor, brusatol was used to verify the knockdown experiments.

Results

Results show that primary MM and MM cell lines have increased NRF2 activity in response to the proteasome inhibitors BZ and CFZ as measured by increased nuclear NRF2, increased NRF2 regulated genes and increased ARE activity in the promoter of heme oxygenase-1. Expression of basal NRF2 was high in the majority of primary MM cells and cell lines tested. Pharmacological inhibition and shRNA mediated knock-down of NRF2 showed a significant reduction in survival of MM cells, when treated alone and in combination with BZ or CFZ. Investigations also revealed that BM-MSC had increased NRF2 activity in response to BZ and CFZ. Moreover, knockdown of NRF2 in BM-MSC or pharmacological inhibition of NRF2 in BM-MSC/MM co-cultures reverses the protection conferred to MM by BM-MSC in response to BZ and CFZ.

Conclusion:

Here we show the first description of NRF2 driven cytoprotective responses in MM. We show that NRF2 in MM is activated by both BZ and CFZ which subsequently activates pro-survival mechanisms in response to proteasome inhibition. Furthermore, NRF2 is also activated in the BM microenvironment by BZ and CFZ, which also confers protection to MM. This highlights the importance of NRF2 in regulating MM drug resistance within the BM microenvironment through independent actions in both the tumour and the non-malignant BM-MSC which support it.