A novel mechanism of Ha-ras oncogene action: regulation of fibronectin mRNA levels by a nuclear posttranscriptional event.

Abstract

Although loss of cell surface fibronectin (FN) is a hallmark of many oncogenically transformed cells, the mechanisms responsible for this phenomenon remain poorly understood. The present study utilized the nontumorigenic human osteosarcoma cell line TE-85 to investigate the effects of induced Ha-ras oncogene expression on FN biosynthesis. TE-85 cells were stably transfected with metallothionein-Ha-ras fusion genes, and the effects of metal-induced ras expression on FN biosynthesis were determined. Induction of the ras oncogene, but not proto-oncogene, was accompanied by a decrease in total FN mRNA and protein levels. Transfection experiments indicated that these oncogene effects were not due to reduced FN promoter activity, suggesting that a posttranscriptional mechanism was involved. The most common mechanism of posttranscriptional regulation affects cytoplasmic mRNA stability. However, in this study the down-regulation of FN was identified as a nuclear event. A component of the ras effect was due to a mechanism affecting accumulation of processed nuclear FN RNA. Mechanisms that would generate such an effect include altered RNA processing and altered stability of the processed message in the nucleus. There was no effect of ras on FN mRNA poly(A) tail length or site of polyadenylation. There was also no evidence for altered splicing at the ED-B domain of FN mRNA. This demonstration of nuclear posttranscriptional down-regulation of FN by the Ha-ras oncogene identifies a new level at which ras oncoproteins can regulate gene expression and thus contribute to development of the malignant phenotype.Fibronectins (FNs) are large glycoproteins implicated in a wide variety of cellular properties including cell adhesion, morphology, migration, differentiation, and transformation (21). FNs exist in extracellular matrices (cellular FN) and in soluble form in body fluids (plasma FN) and are composed of two similar but nonidentical subunits held together by disulfide bonds. Alternative splicing of the primary transcript of a single gene located on human chromosome 2 generates numerous distinct FN transcripts which account for the individual subunits and for the plasma and cellular isoforms. Loss of cell surface FN is a hallmark of many oncogenically transformed cells and has been correlated with the acquisition of tumorigenic and metastatic potentials (21). This effect has been observed in cells transformed with a variety of oncogenes; however, there is still little known about the mechanisms by which oncogenes affect FN biosynthesis. In two cases, transformation of chick embryo fibroblasts by src and growth stimulation of rat embryo fibroblasts by ElA, the primary defects in FN production are reported to be at the level of transcription (31, 45).The ras family of genes encode related 21-kDa plasma membrane-associated proteins that bind guanine nucleotides and are involved in signal transduction and cell growth (2). ras oncogenes are mutated in many types of human tumors. As a result of mutation, there is a loss of control of...

Urokinase Receptor and Fibronectin Regulate the ERKMAPK to p38MAPK Activity Ratios That Determine Carcinoma Cell Proliferation or Dormancy In Vivo

“…Interestingly, one report suggested that p38 activity in fibroblasts is negatively regulated by the presence of surface-associated fibrillar FN (Bourdoulous et al, 1998). We entertained this possibility in spite of the fact that in a majority of published reports oncogenic transformation was shown to be linked to a reduced cell surface FN level (Chandler et al, 1994;Gu and Oliver 1995;Akamatsu et al, 1996;Werbajh et al, 1998). We postulated that, in epithelial tumor cells, uPAR-dependent activation of ␣5␤1 will lead to assembly of FN fibrils, which will block p38 MAPK activity and inhibit this growth-arresting pathway.…”

Downregulation of fibronectin transcription in highly metastatic adenocarcinoma cells

“…The observed decrease in transcription activity observed in FN-CAT transfections accounts for the 40-fold reduction in steady state levels of FN mRNA, leaving little room for posttranscriptional mechanisms as those caused by Ha-Ras in human osteosarcoma cells [31].…”

Ras Induces Elevation of E2F-1 mRNA Levels

“…3D). The effects of Ras on mRNA stability have been documented for other mRNA molecules, such as vascular endothelial growth factor, fibronectin, nuclear factor 1, and ornithine decarboxylase (43)(44)(45)(46).…”