Complex post-transcriptional regulation of EGF-receptor expression by EGF and TGF-α in human prostate cancer cells

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Abstract
The epidermal growth factor receptor (EGFR) plays an important role in the development and progression of prostate cancer and its overexpression is associated with decreased survival. With progression, prostate cancer cells switch from epidermal growth factor (EGF) to transforming growth factor α (TGF-α) synthesis, which contributes to autocrine growth and unrestrained proliferation. To define the molecular mechanisms involved in the regulation of EGFR expression by EGF and TGF-α we studied three human prostate cancer cell lines, androgen-responsive (LNCaP) and -unresponsive (DU145 and PC3). Here we show that TGF-α stabilized EGFR mRNA two- to threefold in all three cell lines, whilst EGF stabilized EGFR mRNA ~ twofold in LNCaP and DU145 cells, but not in PC3 cells. Both ligands increased EGFR transcription in LNCaP and DU145 cells, with less effect in PC3 cells. In all three cell lines EGF reduced total EGFR protein levels more than TGF-α, but this was associated with a greater increase in de novo protein synthesis with EGF compared to TGF-α. Only EGF, however, shortened EGFR protein stability (half-life decreased from 5 h to 120 min), resulting in rapid disappearance of newly synthesized EGFR protein. Both ligands increased total LNCaP and DU145 cell numbers. These studies demonstrate that the EGF- and TGF-α-induced upregulation of EGFR mRNA and protein in human prostate cancer cell lines is complex and occurs at multiple, transcriptional and post-transcriptional levels. Taken together, these data provide novel insight into the molecular mechanisms by which TGF-α would preferentially maintain an autocrine loop in human prostate cancer cells. Furthermore, this work suggests that in human prostate cancer cells ligand-specific differential intracellular trafficking of the EGFR plays a major role in regulating its expression.