Induction in human osteoblastic cells (SaOS2) of the early response genesfos, jun, andmyc by the amino terminal fragment (ATF) of urokinase

Abstract

Previous studies have demonstrated that overexpression of urinary plasminogen activator (uPA) in rat prostate cancer cells results in increased skeletal metastases, which are primarily of the osteoblastic variety. The osseous activation induced by the metastases appears to be mediated through the amino terminal fragment (ATF) of uPA, which lacks the catalytic domain and can act as a growth factor for osteoblasts. To explore further the mechanism of action of uPA in bone cells, we evaluated the effects of ATF on modulating the expression of various proto-oncogenes. Human-osteoblast-derived osteosarcoma cells, SaOS2, were treated with graded doses of ATF for 10-120 min, and effects on early response proto-oncogenes were monitored. ATF increased c-myc, c-jun, and c-fos gene expression in a time-dependent manner for up to 60 min, after which mRNA levels fell. The maximum induction was seen in c-fos gene expression, which was found to be dose dependent. This effect of ATF was localized to its growth-factorlike domain. Examination of the half life of these transcripts in the presence of the transcriptional inhibitor actinomycin D demonstrated that ATF does not alter the stability of c-fos mRNA in these bone cells. Nuclear run-off assays indicated that ATF effects were due to stimulation of c-fos gene transcription. An increase in c-fos protein levels was correlated with the augmentation of its mRNA in ATF-treated SaOS2 cells. Pretreatment of SaOS2 cells with the protein tyrosine kinase inhibitor herbimycin and recombinant soluble uPA receptor (uPAR) caused a significant reduction in the ability of ATF to induce c-fos expression. These results demonstrate a novel role for uPA in activating early response proto-oncogenes, in particular c-fos, which plays an important role in bone cell growth and differentiation and may be a key factor in the signal transduction pathway of ATF.

siRNA-mediated simultaneous downregulation of uPA and its receptor inhibits angiogenesis and invasiveness triggering apoptosis in breast cancer cells

“…The binding of uPA to uPAR in some cell types causes the transmission of signaling over the cell membrane leading to increased motility (40). This ligand engagement of uPAR has been reported to cause the transient activation of a variety of signaling proteins including src-family of members (37,41,42), ERK-1 and ERK-2 (36,43), FAK (44,45), fos, jun and myc (46), and rac (47). In fact, inhibitors of these pathways block the effects of uPA (40).…”

Molecular basis of the spectrum of skeletal complications of neoplasia

“…25 The ATF can signal to the osteoblast through this receptor and activate protooncogenes, such as c-fos, that may contribute to osteoblastic stimulation. 26 The residue of the uPA molecule, now termed low-molecular-weight uPA, can continue to exert proteolytic activity and may activate latent growth factors, such as transforming growth factor ␤. Therefore, this enzyme-growth factor system may contribute to the development of focal osteoblastic metastases.…”

Nuclear translocation of urokinase-type plasminogen activator

“…9 Signal transduction by this scuPA fragment may be mediated in part by LRP 10 and certain integrins. 11 However, there is limited information as to the mechanism by which uPA modifies gene transcription, [12][13][14][15] and our previous studies have provided reason to hypothesize that cells express additional uPA-binding proteins that possess distinct signal-transducing activities involved in cell contractility, migration, and differentiation. 9,10,16 We previously reported that uPAR forms a complex on the surface of human SMCs with nucleolin, 17 a protein that regulates the organization of nucleolar chromatin, packaging of pre-RNA, rDNA transcription, and ribosome assembly (reviewed in Tuteja and Tuteja 18 ).…”