Inhibition of RSK with the novel small-molecule inhibitor LJI308 overcomes chemoresistance by eliminating cancer stem cells

ABSTRACT The triple-negative breast cancer (TNBC) subtype is enriched in cancer stem cells (CSCs) and clinically correlated with the highest rate of recurrence. Several studies implicate the RSK pathway as being pivotal for the growth and proliferation of CSCs, which are postulated to drive tumor relapse. We now address the potential for the newly developed RSK inhibitor LJI308 to target the CSC population and repress TNBC growth and dissemination. Overexpression of the Y-box binding protein-1 (YB-1) oncogene in human mammary epithelial cells (HMECs) drove TNBC tumor formation characterized by a multi-drug resistance phenotype, yet these cells were sensitive to LJI308 in addition to the classic RSK inhibitors BI-D1870 and luteolin. Notably, LJI308 specifically targeted transformed cells as it had little effect on the non-tumorigenic parental HMECs. Loss of cell growth, both in 2D and 3D culture, was attributed to LJI308-induced apoptosis. We discovered CD44+/CD49f+ TNBC cells to be less sensitive to chemotherapy compared to the isogenic CD44-/CD49f-cells. However, inhibition of RSK using LJI308, BI-D1870, or luteolin was sufficient to eradicate the CSC population. We conclude that targeting RSK using specific and potent inhibitors, such as LJI308, delivers the promise of inhibiting the growth of TNBC

Inhibiting p90 ribosomal S6 kinase (RSK)/Y-box binding protein-1 (YB-1) signaling is a novel targeted therapeutic strategy with the ability to overcome drug resistance in triple-negative breast cancer

Despite advances in treating breast cancer, disease recurrence rates remain high and secondary tumors are often refractory to chemotherapy. Currently, the treatment for triple-negative breast cancer (TNBC) relies upon conventional chemotherapeutics as no targeted therapies are available. Although these tumors initially respond well, they paradoxically have the highest relapse rates. Y-box binding protein-1 (YB-1) is an oncogenic transcription/translation factor abundantly expressed in TNBC (~70% of patients) and associated with disease relapse. It is activated predominantly by phosphorylation via p90 ribosomal S6 kinase (RSK). Once activated YB-1 up-regulates the tumor-initiating cell (TIC) marker, CD44 and promotes drug resistance. These data suggest that blocking YB-1’s activation via RSK inhibition may suppress growth and attenuate the development of chemoresistance in TNBC. Through an unbiased, functional viability screen comparing breast cancer subtypes, we identified RSK2 as a novel target for TNBC. Pharmacological or siRNA inhibition of RSK2 blocks activation of YB-1, which subsequently decreases growth in TNBC cell lines and delays tumor initiation in immunocompromised mice. Contrary to most conventional chemotherapies, inhibiting RSK/YB-1 signaling eliminates the CD44⁺/CD24‾ cell fraction rather than enriching for it. In an effort to identify novel RSK inhibitors, we screened “off-patent” compounds and identified the flavonoid, luteolin, as a RSK inhibitor. We validated that luteolin inhibits RSK in cell-free assays and further demonstrated it blocks the RSK/YB-1/Notch4 signaling pathway. Luteolin phenotypically mirrored the effects of established RSK inhibitor, BI-D1870, and suppressed growth in TNBC (including CD44⁺/CD24‾-sorted cells) providing further support for the use of RSK inhibitors to treat this subtype. Finally, we demonstrate that cells that survive standard-of-care chemotherapeutics (paclitaxel and epirubicin) exhibit elevated RSK/YB-1 signaling. Inhibiting this pathway sensitizes TNBC to chemotherapy and reduces the residual cell burden. Importantly, RSK inhibition also demonstrates efficacy against a multidrug resistant cell line and primary, drug-refractory TNBC. When taken together, our data identify RSK as a promising target for the treatment of TNBC. RSK inhibition has the unique ability to eliminate CD44⁺/CD24‾cells and overcome broad-spectrum chemoresistance by blocking activation of YB-1 and as such holds potential to reduce relapse in this aggressive subtype.Medicine, Faculty ofMedicine, Department ofExperimental Medicine, Division ofGraduat

YB-1 enhances ABCG2 expression in breast cancer to elicit chemoresistance that can be circumvented using RSK inhibitors

Despite advances in the treatment and management of breast cancer, recurrence remains a significant problem. Current dogma dictates that populations of drug resistant cells within a heterogeneous tumour are responsible for relapse. In this study, we demonstrate that the oncogenic transcription factor Y-box binding protein-1 (YB-1) regulates the expression of ABCG2, an ATP-binding transporter involved in multidrug resistance. Introducing YB-1 into human mammary epithelial cells (HMECs) was associated with increased ABCG2 promoter acetylation and gene expression. Accordingly, these cells acquired resistance to chemotherapeutic substrates of ABCG2, notably 5-fluoruracil, doxorubicin, and gefitinib. The acetylation-mediated relaxation of the ABCG2 promoter allowed for direct YB-1 binding and transcriptional regulation. As such, repressing YB-1 activation by RSK using the small molecule inhibitors BI-D1870 and luteolin led to a decrease in ABCG2 expression and resensitization of cells to chemotherapy. We found that treatment with luteolin or NVP-LJI308, a next-generation RSK inhibitor, was sufficient to induce apoptosis in triple-negative breast cancer cells. Moreover, RSK inhibition repressed soft agar colony growth and mammosphere formation. Relapse is believed to initiate from a drug resistant stem-like subpopulation within a heterogeneous tumour. Sorting for CD44+/CD49f+ cells revealed them to have an over 2-fold increase in ABCG2 expression relative to bulk tumour cells. As a result, they were refractory to traditional chemotherapy, but sensitive to the RSK inhibitors BI-D1870, NVP-LJI308, and luteolin. Therefore, targeting the RSK/YB-1 pathway inhibits ABCG2 and could represent a potential avenue to overcome multidrug resistance