Activation of Stat3 through a Phosphomimetic Serine727 Promotes Prostate Tumorigenesis Independent of Tyrosine705 phosphorylation

• Published in: Cancer research (Chicago, Ill.) Vol. 68; no. 19; pp. 7736 - 7741
• Main Authors: Qin, Haiyan R, Kim, Han-Jong, Kim, Joon-Young, Hurt, Elaine M, Klarmann, George J, Kawasaki, Brian T, Serrat, Maria A Duhagon, Farrar, William L
• Format: Journal Article
• Language: English
• Published: 01-10-2008
• ISSN: 0008-5472; 1538-7445
• DOI: 10.1158/0008-5472.CAN-08-1125

Aberrantly activated Stat3 is implicated in the development of various human cancers. Y705 phosphorylation is conventionally thought to be required for Stat3 signal-dependent activation and appears to play an essential role in some malignancies. Recently, it was shown that Stat3 is activated through novel and non-canonical mechanisms, including phosphorylation at S727. Here, we investigate S727 phosphorylation activation of Stat3 and subsequent effects in prostate cancer development, independent of Y705 phosphorylation, using mutated Stat3 in the human prostate cancer cell line LNCaP. We demonstrate mutation of S727 to the phosphomimetic residue Glu and inactivation of Y705 (Y705F/S727E) resulted in a remarkable growth advantage in low-serum, enhanced anchorage-independent growth in soft agar and increased tumorgenicity in NOD/SCID mice, possibly by direct activation of downstream proto-oncogenes
c-myc, mcl-1
and
survivin
. Y705F/S727E mutant cells were more invasive than Y705F/S727A (inactivation of Y705 and S727) mutant cells, and more Y705F/S727E mutant Stat3 was localized in the nuclei relative to Y705F/S727A mutant Stat3 at the steady-state. Furthermore, the Y705F/S727E but not the Y705F/S727A mutant induced anchorage-independent growth of noncancerous prostate epithelial cells (RWPE-1). We further show that Stat3 is phosphorylated at S727 in 65% of malignant prostate tissues (n=20) relative to 25% of normal prostate tissues (n=4). Moreover, there is a positive correlation between phosphoS727-Stat3 expression and Gleason score in these prostate cancer tissues (
P
= 0.05). Our data suggests for the first time that S727 phosphorylation is sufficient to activate Stat3, thereby driving prostate tumorigenesis independent of Y705 phosphorylation.