Anti-invasive and antimetastatic activities of ribosomal protein S6 kinase 4 in breast cancer cells.
OBJECTIVE
We have previously shown that p90 ribosomal protein S6 kinase 4 (RSK4), an X-linked gene, is highly up-regulated in mammary tumors of MMTV-c-Myc transgenic mice. In this study, we further investigated whether RSK4 inhibits or promotes breast tumor growth and progression.


METHODS
Stable overexpression or small interfering RNA-mediated knockdown of RSK4 was done in the MDA-MB-231 cell line. Stable clones were tested for cell proliferation, anchorage-independent growth in soft agar, invasive and metastatic ability of these clones in vitro and tumorigenesis, invasive and metastatic ability in vivo in severe combined immunodeficient mice.


RESULTS
Here, we show that exogenous expression of RSK4 resulted in decreased cell proliferation and increased accumulation of cells in G(0)-G(1) phase, which paralleled with enhanced expression of tumor suppressor genes: retinoblastoma protein, retinobl astoma-associated 46 kDa protein, and p21 protein. Overexpression of RSK4 resulted in reduced colony formation in soft agar and suppressed invasive and migratory activities of MDA-MB-231 cells both in vitro and in vivo. Importantly, RSK4-overexpressing cells showed up-regulation of claudin-2 and down-regulation of CXCR4, both of these play roles in invasion and chemotaxis.


CONCLUSIONS
These results indicate that RSK4 expression may limit the oncogenic, invasive, and metastatic potential of breast cancer cells. Anti-invasive and antimetastatic activities of RSK4 may be, in part, due to its regulation of claudin-2. Increased expression of RSK4 in c-Myc-overexpressing cells and a dose-dependent induction of luciferase reporter gene activity suggest that c-Myc may regulate RSK4 expression.
Thakur
Archana
A
Department of Pathology, Barbara Ann Karmanos Cancer Institute, Wayne State University School of Medicine, Detroit, Michigan, USA.
Sun
Yuan
Y
Bollig
Aliccia
A
Wu
Jack
J
Biliran
Hector
H
Banerjee
Sanjeev
S
Sarkar
Fazlul H
FH
Liao
D Joshua
DJ
D000818:Animals; D015153:Blotting, Western; D001943:Breast Neoplasms / Q000201:enzymology* / Q000473:pathology; D045744:Cell Line, Tumor; D002465:Cell Movement / Q000502:physiology; D049109:Cell Proliferation; D057167:Claudins; D005260:Female; D015972:Gene Expression Regulation, Neoplastic / Q000502:physiology*; D006801:Humans; D007150:Immunohistochemistry; D008565:Membrane Proteins / Q000096:biosynthesis; D051379:Mice; D016513:Mice, SCID; D009361:Neoplasm Invasiveness; D019718:Receptors, CXCR4 / Q000096:biosynthesis; D020133:Reverse Transcriptase Polymerase Chain Reaction; D038744:Ribosomal Protein S6 Kinases, 90-kDa / Q000378:metabolism*
C491875:CLDN2 protein, human; C527276:CXCR4 protein, human; D057167:Claudins; D008565:Membrane Proteins; D019718:Receptors, CXCR4; C501305:RPS6KA6 protein, human; D038744:Ribosomal Protein S6 Kinases, 90-kDa