Inhibition of cell proliferation by CD44: Akt is inactivated and EGR-1 is down-regulated.
OBJECTIVE
CD44 is a transmembrane glycoprotein and can facilitate signal transduction by serving as a platform for molecular recruitment and assembly. A number of studies have suggested that CD44 can either positively or negatively regulate cell proliferation. The purpose of this study was to investigate how CD44 can inhibit cell proliferation.


METHODS
We engineered E6.1 Jurkat cells to express CD44. Importantly, these cells lack endogenous CD44 expression. Molecular pathways involved with cell proliferation were studied using RT(2)-PCR array, siRNA, Western blotting and by employing pharmacological inhibitors of ERK1/2, p38 and the PI3K/Akt pathways.


RESULTS
We found that CD44 expression significantly inhibited cell proliferation and down-regulated EGR-1 expression and EGR-1 targets cyclin D1 and cyclin D2. Transfection of control E6.1 Jurkat cells with EGR-1 siRNA also inhibited cell proliferation, confirming its role. Disruption of the PI3K/Akt pathway with pharmacological inhibitors reduced both EGR-1 expression and cell proliferation, recapitulating the properties of CD44 expressing cells. Akt was hypophosphorylated in cells expressing CD44 showing its potential role in negatively regulating Akt activation. Strikingly, constitutively active Akt rescued the proliferation defect showing requirement for active Akt, in our system.


CONCLUSIONS
Our results suggest a novel pathway by which CD44 inactivates Akt, down-regulates EGR-1 expression and inhibits cell proliferation.
Zhang
L-S
LS
Department of Psychiatry and Behavioral Health, University of North Texas Health Science Center, Fort Worth, TX 76107, USA.
Ma
H-W
HW
Greyner
H J
HJ
Zuo
W
W
Mummert
M E
ME
D015153:Blotting, Western; D049109:Cell Proliferation / Q000187:drug effects; D019938:Cyclin D1 / Q000235:genetics / Q000378:metabolism*; D056742:Cyclin D2; D015536:Down-Regulation; D006801:Humans; D051057:Proto-Oncogene Proteins c-akt / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D034741:RNA, Small Interfering / Q000235:genetics; D015398:Signal Transduction / Q000235:genetics / Q000502:physiology; D014162:Transfection
C080336:CCND2 protein, human; D056742:Cyclin D2; D034741:RNA, Small Interfering; D019938:Cyclin D1; D051057:Proto-Oncogene Proteins c-akt