t-DARPP regulates phosphatidylinositol-3-kinase-dependent cell growth in breast cancer.
BACKGROUND
Recent reports have shown that t-DARPP (truncated isoform of DARPP-32) can mediate trastuzumab resistance in breast cancer cell models. In this study, we evaluated expression of t-DARPP in human primary breast tumors, and investigated the role of t-DARPP in regulating growth and proliferation in breast cancer cells.


RESULTS
Quantitative real time RT-PCR analysis using primers specific for t-DARPP demonstrated overexpression of t-DARPP in 36% of breast cancers (13/36) as opposed to absent to very low t-DARPP expression in normal breast tissue (p < 0.05). The mRNA overexpression of t-DARPP was overwhelmingly observed in ductal carcinomas, including invasive ductal carcinomas and intraductal carcinomas, rather than other types of breast cancers. The immunohistochemistry analysis of DARPP-32/t-DARPP protein(s) expression in breast cancer tissue microarray that contained 59 tumors and matched normal tissues when available indicated overexpression in 35.5% of primary breast tumors that were more frequent in invasive ductal carcinomas (43.7%; 21/48). In vitro studies showed that stable overexpression of t-DARPP in MCF-7 cells positively regulated proliferation and anchorage-dependent and -independent growth. Furthermore, this effect was concomitant with induction of phosphorylation of AKT(ser473) and its downstream target phospho(ser9) GSK3β, and increased Cyclin D1 and C-Myc protein levels. The knockdown of endogenous t-DARPP in HCC1569 cells led to a marked decrease in phosphorylation of AKTs(ser473) and GSK3β(ser9). The use of PI3K inhibitor LY294002 or Akt siRNA abrogated the t-DARPP-mediated phosphorylation of AKT(ser473) and led to a significant reduction in cell growth.


CONCLUSIONS
Our findings underscore the potential role of t-DARPP in regulating cell growth and proliferation through PI3 kinase-dependent mechanism.
Vangamudi
Bhavatarini
B
Department of Surgery, Vanderbilt University Medical Center, Nashville, Tennessee, USA.
Peng
Dun-Fa
DF
Cai
Qiuyin
Q
El-Rifai
Wael
W
Zheng
Wei
W
Belkhiri
Abbes
A
D001943:Breast Neoplasms / Q000235:genetics / Q000378:metabolism*; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects / Q000235:genetics; D002867:Chromones / Q000494:pharmacology; D019938:Cyclin D1 / Q000378:metabolism; D051937:Dopamine and cAMP-Regulated Phosphoprotein 32 / Q000235:genetics / Q000378:metabolism*; D005260:Female; D006801:Humans; D015151:Immunoblotting; D007150:Immunohistochemistry; D009025:Morpholines / Q000494:pharmacology; D019869:Phosphatidylinositol 3-Kinases / Q000235:genetics / Q000378:metabolism*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010766:Phosphorylation / Q000187:drug effects; D016133:Polymerase Chain Reaction; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism; D034741:RNA, Small Interfering
D002867:Chromones; D019938:Cyclin D1; D051937:Dopamine and cAMP-Regulated Phosphoprotein 32; D009025:Morpholines; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D051057:Proto-Oncogene Proteins c-akt; D016271:Proto-Oncogene Proteins c-myc; D034741:RNA, Small Interfering; C489427:MYC protein, human; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one