OR01-6 IGF-1 Receptor Contributes to GH Signaling and Influences GH-induced GHR Downregulation in LNCaP Human Prostate Cancer Cells

Program: Abstracts - Orals, Featured Poster Presentations, and Posters
Session: OR01-Cell Specific GH & IGF-1 Signaling
Saturday, June 15, 2013: 11:30 AM-1:00 PM
Presentation Start Time: 12:45 PM
Room 134 (Moscone Center)
Ashiya Buckels*1, Yue Zhang1, Yujun Gan1, Yao Huang2, Jing Jiang1 and Stuart Jonathan Frank1
1University of Alabama at Birmingham, Birmingham, AL, 2St. Joseph's Hospital and Medical Center, Phoenix, AZ
Growth hormone (GH) triggers intracellular signaling by binding GH receptor (GHR) and activating the GHR-associated cytoplasmic tyrosine kinase, JAK2. We previously found (1) that GH-induced GHR tyrosine phosphorylation by JAK2 is required for downstream activation of the STAT5 transcription factor, but not for activation of JAK2 itself; however, a GHR rendered resistant to tyrosine phosphorylation by mutation of its intracellular tyrosine residues was also resistant to GH-induced downregulation. These and other data (2) suggest that the pace and degree of GH-induced GHR downregulation are correlated with the degree of GH-induced JAK2 activation and GHR tyrosine phosphorylation. We have also observed that the ability of GH to induce acute downstream signaling (activation of STAT5) is reduced by Cre-lox-mediated silencing of IGF-1 receptor (IGF-1R) expression in murine osteoblasts (3). We now investigate the relationship between IGF-1R levels, acute GH signaling, and GH-induced GHR downregulation in a separate system – human LNCaP prostate cancer cells. We compared LNCaP cells that express an shRNA targeting the IGF-1R (shIGF-1R cells) to control LNCaP cells in which an empty vector is expressed (vector cells). We verified substantial IGF-1R protein reduction in shIGF-1R cell extracts and found that acute GH-induced GHR, JAK2, and STAT5 tyrosine phosphorylation were reduced in the shIGF-1R cells over a 2-30 min exposure at various GH concentrations. Our previous conclusions about the relationship between GH-induced GHR tyrosine phosphorylation and GH-induced GHR downregulation predicted that the pace and degree of GHR loss in GH-treated shIGF-1R cells would be less than in vector cells. In fact, we observed the opposite result. GH induced more rapid and complete loss of GHR protein, as assessed by specific immunoblotting, in the cells with reduced IGF-1R. These data suggest that the presence of IGF-1R (even unliganded) contributes very proximally to GH signaling by augmenting GH-induced JAK2 activation and GHR phosphorylation, but may also stabilize the activated GHR, perhaps prolonging its signaling. The mechanism of this novel impact of IGF-1R on the fate of the GH-activated GHR is yet unknown, but could relate to effects on GHR endocytosis or post-endocytic GHR trafficking.

(1) Deng, L., et al., Endocrinol 2012; 153:2311 (2) Deng, L., et al., Mol Endocrinol 2007; 21:1537 (3) Gan, Y., et al., Mol Endocrinol 2010; 24:644

Nothing to Disclose: AB, YZ, YG, YH, JJ, SJF

*Please take note of The Endocrine Society's News Embargo Policy at http://www.endo-society.org/endo2013/media.cfm

Sources of Research Support: NIH R01 DK 46395 (to SJF)
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