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Molecular and Cellular Biology, December 2003, p. 9389-9404, Vol. 23, No. 24
0270-7306/03/$08.00+0     DOI: 10.1128/MCB.23.24.9389-9404.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Caveolin-1 Maintains Activated Akt in Prostate Cancer Cells through Scaffolding Domain Binding Site Interactions with and Inhibition of Serine/Threonine Protein Phosphatases PP1 and PP2A

Likun Li,1 Cheng Hui Ren,1 Salahaldin A. Tahir,1 Chengzhen Ren,1 and Timothy C. Thompson1,2,3*

Scott Department of Urology,1 Department of Molecular and Cellular Biology,2 Department of Radiology, Baylor College of Medicine, Houston, Texas 770303

Received 17 April 2003/ Returned for modification 4 June 2003/ Accepted 9 September 2003

Previously it has been reported that caveolin-1 (cav-1) has antiapoptotic activities in prostate cancer cells and functions downstream of androgenic stimulation. In this study, we demonstrate that cav-1 overexpression significantly reduced thapsigargin (Tg)-stimulated apoptosis. Examination of the phosphatidylinositol 3-kinase (PI3-K)/Akt signaling cascade revealed higher activities of PDK1 and Akt but not PI3-K in cav-1-stimulated cells compared to control cells. We subsequently found that cav-1 interacts with and inhibits serine/threonine protein phosphatases PP1 and PP2A through scaffolding domain binding site interactions. Deletion of the cav-1 scaffolding domain significantly reduces phosphorylated Akt and cell viability compared with wild-type cav-1. Analysis of potential substrates for PP1 and PP2A revealed that cav-1-mediated inhibition of PP1 and PP2A leads to increased PDK1, Akt, and ERK1/2 activities. We demonstrate that increased Akt activities are largely responsible for cav-1-mediated cell survival using dominant-negative Akt mutants and specific inhibitors to MEK1/MEK and show that cav-1 increases the half-life of phosphorylated PDK1 and Akt after inhibition of PI3-K by LY294002. We further demonstrate that cav-1-stimulated Akt activities lead to increased phosphorylation of multiple Akt substrates, including GSK3, FKHR, and MDM2. In addition, overexpression of cav-1 significantly increases translocation of phosphorylated androgen receptor to nucleus. Our studies therefore reveal a novel mechanism of Akt activation in prostate cancer and potentially other malignancies.

* Corresponding author. Mailing address: Scott Department of Urology, Baylor College of Medicine, 6560 Fannin, Suite 2100, Houston, TX 77030. Phone: (713) 799-8718. Fax: (713) 794-7983. E-mail: timothyt{at}www.urol.bcm.tmc.edu.

Molecular and Cellular Biology, December 2003, p. 9389-9404, Vol. 23, No. 24
0022-538X/03/$08.00+0     DOI: 10.1128/MCB.23.24.9389-9404.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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