Targeting p90 ribosomal S6 kinase eliminates tumor-initiating cells by inactivating Y-box binding protein-1 in triple-negative breast cancers.
Y-box binding protein-1 (YB-1) is the first reported oncogenic transcription factor to induce the tumor-initiating cell (TIC) surface marker CD44 in triple-negative breast cancer (TNBC) cells. In order for CD44 to be induced, YB-1 must be phosphorylated at S102 by p90 ribosomal S6 kinase (RSK). We therefore questioned whether RSK might be a tractable molecular target to eliminate TICs. In support of this idea, injection of MDA-MB-231 cells expressing Flag-YB-1 into mice increased tumor growth as well as enhanced CD44 expression. Despite enrichment for TICs, these cells were sensitive to RSK inhibition when treated ex vivo with BI-D1870. Targeting RSK2 with small interfering RNA (siRNA) or small molecule RSK kinase inhibitors (SL0101 and BI-D1870) blocked TNBC monolayer cell growth by ∼100%. In a diverse panel of breast tumor cell line models RSK2 siRNA predominantly targeted models of TNBC. RSK2 inhibition decreased CD44 promoter activity, CD44 mRNA, protein expression, and mammosphere formation. CD44(+) cells had higher P-RSK(S221/227) , P-YB-1(S102) , and mitotic activity relative to CD44(-) cells. Importantly, RSK2 inhibition specifically suppressed the growth of TICs and triggered cell death. Moreover, silencing RSK2 delayed tumor initiation in mice. In patients, RSK2 mRNA was associated with poor disease-free survival in a cohort of 244 women with breast cancer that had not received adjuvant treatment, and its expression was highest in the basal-like breast cancer subtype. Taking this further, we report that P-RSK(S221/227) is present in primary TNBCs and correlates with P-YB-1(S102) as well as CD44. In conclusion, RSK2 inhibition provides a novel therapeutic avenue for TNBC and holds the promise of eliminating TICs.
Department of Pediatrics, University of British Columbia, Vancouver, BC, Canada.
Anna L
AL
Stratford
Kristen
K
Reipas
Kaiji
K
Hu
Abbas
A
Fotovati
Rachel
R
Brough
Jessica
J
Frankum
Mandeep
M
Takhar
Peter
P
Watson
Alan
A
Ashworth
Christopher J
CJ
Lord
Annette
A
Lasham
Cristin G
CG
Print
Sandra E
SE
Dunn
D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000235:genetics; D001578:Benzopyrans / Q000494:pharmacology; D015153:Blotting, Western; D001943:Breast Neoplasms / Q000188:drug therapy / Q000378:metabolism* / Q000628:therapy; D045744:Cell Line, Tumor; D005260:Female; D005434:Flow Cytometry; D005455:Fluorescent Antibody Technique; D006801:Humans; D018960:Hyaluronan Receptors / Q000235:genetics / Q000378:metabolism; D051379:Mice; D016688:Mice, Inbred NOD; D016513:Mice, SCID; D009005:Monosaccharides / Q000494:pharmacology; D011401:Promoter Regions, Genetic / Q000235:genetics; D011621:Pteridines / Q000494:pharmacology; D060888:Real-Time Polymerase Chain Reaction; D038744:Ribosomal Protein S6 Kinases, 90-kDa / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D051840:Y-Box-Binding Protein 1 / Q000235:genetics / Q000378:metabolism*
C516541:BI D1870; D001578:Benzopyrans; D018960:Hyaluronan Receptors; D009005:Monosaccharides; D011621:Pteridines; C497449:SL0101; D051840:Y-Box-Binding Protein 1; D038744:Ribosomal Protein S6 Kinases, 90-kDa