Brk/Protein tyrosine kinase 6 phosphorylates p27KIP1, regulating the activity of cyclin D-cyclin-dependent kinase 4.
Cyclin D and cyclin-dependent kinase 4 (cdk4) are overexpressed in a variety of tumors, but their levels are not accurate indicators of oncogenic activity because an accessory factor such as p27(Kip1) is required to assemble this unstable dimer. Additionally, tyrosine (Y) phosphorylation of p27 (pY88) is required to activate cdk4, acting as an "on/off switch." We identified two SH3 recruitment domains within p27 that modulate pY88, thereby modulating cdk4 activity. Via an SH3-PXXP interaction screen, we identified Brk (breast tumor-related kinase) as a high-affinity p27 kinase. Modulation of Brk in breast cancer cells modulates pY88 and increases resistance to the cdk4 inhibitor PD 0332991. An alternatively spliced form of Brk (Alt Brk) which contains its SH3 domain blocks pY88 and acts as an endogenous cdk4 inhibitor, identifying a potentially targetable regulatory region within p27. Brk is overexpressed in 60% of breast carcinomas, suggesting that this facilitates cell cycle progression by modulating cdk4 through p27 Y phosphorylation. p27 has been considered a tumor suppressor, but our data strengthen the idea that it should also be considered an oncoprotein, responsible for cyclin D-cdk4 activity.
School of Graduate Studies, SUNY Downstate Medical Center, Brooklyn, New York, USA.
Priyank
P
Patel
Institute of Medical Microbiology and Hygiene, University of Regensburg, Regensburg, Germany.
Benedikt
B
Asbach
School of Graduate Studies, SUNY Downstate Medical Center, Brooklyn, New York, USA.
Elina
E
Shteyn
Departments of Pediatrics and Cell Biology, SUNY Downstate Medical Center, Brooklyn, New York, USA.
Cindy
C
Gomez
College of Medicine, SUNY Downstate Medical Center, Brooklyn, New York, USA.
Alexander
A
Coltoff
Departments of Pediatrics and Cell Biology, SUNY Downstate Medical Center, Brooklyn, New York, USA.
Sadia
S
Bhuyan
Department of Biochemistry and Molecular Genetics, University of Illinois at Chicago, Chicago, Illinois, USA.
Angela L
AL
Tyner
Institute of Medical Microbiology and Hygiene, University of Regensburg, Regensburg, Germany.
Ralf
R
Wagner
Departments of Pediatrics and Cell Biology, SUNY Downstate Medical Center, Brooklyn, New York, USA stacy.blain@downstate.edu.
Stacy W
SW
Blain
D001940:Breast / Q000201:enzymology; D001943:Breast Neoplasms / Q000201:enzymology*; D045744:Cell Line, Tumor; D056741:Cyclin D / Q000378:metabolism*; D051358:Cyclin-Dependent Kinase 4 / Q000378:metabolism*; D050760:Cyclin-Dependent Kinase Inhibitor p27 / Q000378:metabolism*; D005260:Female; D006801:Humans; D009363:Neoplasm Proteins / Q000737:chemistry / Q000378:metabolism*; D010766:Phosphorylation; D060066:Protein Interaction Maps; D011505:Protein-Tyrosine Kinases / Q000737:chemistry / Q000378:metabolism*; D018909:src Homology Domains
D056741:Cyclin D; D009363:Neoplasm Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D011505:Protein-Tyrosine Kinases; C088375:PTK6 protein, human; D051358:Cyclin-Dependent Kinase 4