Repression of breast cancer cell growth by proteasome inhibitorsin vitro: impact of mitogen-activated protein kinase phosphatase 1

Abstract

Mitogen activated protein kinase phosphatase-1 (MKP-1) has emerged as an important protein mediating breast cancer oncogenesis and chemoresistance to cancer chemotherapies, especially proteasome inhibitors. In this in vitro study, we utilized the breast cancer epithelial cell lines MCF-7 and MDA-MB-231, in comparison to MCF-10A control cells, to examine the impact of MKP-1 on breast cancer cell growth and repression by proteasome inhibitors. We confirm that proteasome inhibitors MG-132 and bortezomib induce MKP-1 protein upregulation and we show that one of the ways in which bortezomib increases MKP-1 in breast cancer cells, in addition to inhibition of ubiquitin-proteasome system, is via upregulation of MKP-1 mRNA expression in p38 MAPK-mediated manner. Notably, these effects are specific to cancer cells, as bortezomib activated p38 MAPK and induced MKP-1 in MCF-7 and MDA-MB-231 breast cancer cells, but not in control cells (MCF-10A). We took a dual approach toward targeting MKP-1 to show that bortezomib-induced effects are enhanced. Firstly, treatment with the non-specific MKP-1 inhibitor triptolide reduces breast cancer cell growth and augments proteasome inhibitor-induced effects. Secondly, specific knock-down of MKP-1 with siRNA significantly repressed cell viability by reduced cyclin D1 expression, and enhanced repression of cancer cell growth by proteasome inhibitors. Taken together, these results indicate that removing the unwanted (MKP-1-inducing) effects of bortezomib significantly improves the efficacy of proteasome inhibition in breast cancer cells. Thus, future development of drugs targeting MKP-1 offer promise of combination therapies with reduced toxicity and enhanced cell death in breast cancer.

Bortezomib inhibits lung fibrosis and fibroblast activation without proteasome inhibition

“…A number of reports of BTZ actions in Fibs employed doses of 10 nM-1 μM along with continuous exposure for prolonged time intervals of 24-48 h (16, 17, 25). To define conditions in Fibs (or MyoFibs established by 48 h pretreatment with TGF-β) in which BTZ does and does not inhibit the proteasome, we examined a range of doses and also took advantage of the reversibility of its known proteasome inhibitory actions.…”

“…This effect could be explained by inhibition of kinase activation and/or enhancement of phosphatase-mediated dephosphorylation. There is precedent for BTZ increasing de novo expression and/or activity of phosphatases (25, 32). To evaluate the importance of gene induction, we pretreated Fibs with 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole (DRB), a reversible inhibitor of transcription, and then removed it and changed the medium prior to assessing the ability of BTZ to inhibit MyoFib differentiation ( Figure 8C ).…”

Novel strategies to improve the endocrine therapy of breast cancer

“…Estrogen-dependent cells, cultured long term in estrogen-depleted medium, rely on mTOR signaling for growth and are excessively sensitive to its inhibition. 25 In addition, mTOR inhibition restores sensitivity of endocrine-resistant breast cancer cells to endocrine therapy. 26 , 27 …”

c-Jun N-Terminal Kinase Inactivation by Mitogen-Activated Protein Kinase Phosphatase 1 Determines Resistance to Taxanes and Anthracyclines in Breast Cancer

“…For instance, in breast cancer cells, doxorubicin is able to activate JNK pathway to achieve its antitumoral effect (14,15). On the contrary, MKP-1 mediates different tumor responses to anticancer therapy, depending on its activity or inactivity: MKP-1 activates antiapoptotic pathways in response to proteasome inhibitors (16,17) as well as antiproliferative activity to PR in breast cancer cells (18). Of relevance, MKP-1 inhibition by small molecules enhanced the antitumoral effect of paclitaxel (19), and transient expression modulation of MKP-1 defined breast cancer cells' ability to survive after exposure to different cytotoxic agents (13).…”