| The neuregulins (NRGs) are a group of alternatively-spliced polypeptide ligands, the receptors for which are ErbB3 and ErbB4, members of the epidermal growth factor receptor (EGFR)/ErbB1 family of receptor tyrosine kinases (RTKs). ErbB RTKs, upon binding ligand, dimerize and become activated by crossphosphorylation on tyrosine residues. In this report, the signaling and cell biology of NRGs were studied in LNCaP, a human prostate carcinoma (PCA) cell line. As LNCaP expresses ErbB1, ErbB2, and Erb133, but not ErbB4, NRG signals must necessarily be transmitted through ErbB3. NRG stimulation of LNCaP was found to induce tyrosine phosphorylation of ErbB3 and ErbB2, but not ErbB I. The downstream signaling pathways of P13-K and ERK were strongly activated, while those of JNK/SAPK and mHOG/p38 MAP kinase were turned on to a lesser extent. PLC-gamma and STATs 1, 3, and 5 were activated by TGFalpha, a ligand of ErbB1, but were not turned on by NRG. The cell biological effects of NRG differed from TGFalpha, also. While NRG induced detachment and eventual apoptosis of LNCaP, TGFalpha, stimulated modest proliferation. An Erb132-deficient LNCaP subline (R1-LNCaP) and pharmacological kinase inhibitors were used to investigate the NRG signals controlling its physiological effects. NRG activation of ERK and P13-K were significantly reduced in R1-LNCaP, and the cell line did not detach in response to NRG treatment. In contrast, TGFalpha-induced activation of the ERK and P13-K pathways was relatively unaltered in R1-LNCaP, demonstrating specificity of ErbB2 for NRG signaling. This is probably due to the reliance of the catalytically-impaired ErbB3 on ErbB2 for heterodimization and activation, while ErbB1 is capable of activation by homodimerization. The specific P13-K inhibitor LY294002 completely blocked NRG-induced detachment and the striking cytoskeletal rearrangements which accompany it. PD98059, an inhibitor of MEK (ERK kinase), was not able to block NRG-stimulated detachment. This work identifies cell detachment as a novel function of NRG. In the LNCaP human PCA line, this cell biological effect requires the participation of ErbB2 and P13-K. These findings have significant implications for neuregulin signal transduction and cell biology, and for prostate cancer progression and metastasis. |
