Utilization of multiple polyadenylation signals in the human RHOA protooncogene

Abstract

Little is known regarding the regulation of expression of the RHOA protooncogene, a member of the family of genes encoding Ras-related GTP-binding proteins. We have previously reported that the 3′ untranslated region ( UTR) of RHOA was contained within a genomic sequence which flanked the 5′ end of the human glutathione peroxidase 1-encoding gene [J.A. Moscow et al. J. Biol. Chem. 267 (1992) 5949–5958]. Our previous studies revealed the presence of multiple (1.8 and 1.5kb) RHOA mRNA species in breast cancer cell lines and of three putative polyadenylation signals in the RHOA 3′ UTR. In this report, we have isolated several RHOA cDNAs from a multidrug-resistant MCF-7 human breast cancer cell line. Sequence analyses of these RHOA cDNA clones indicate that multiple polyadenylation signals are used to terminate RHOA transcripts. RNase-protection analysis demonstrated that all three polyadenylation signals are utilized in breast cancer cell lines and RNA stability studies demonstrated that RHOA RNA species with different 3′ ends have equivalent stability. Since little is known about the RNA expression of RHOA in human tumors, and since both activated and non-activated RHOA genes possess transformation potential, we analyzed RHOA mRNA in lung and colon tumors by Northern blot and RNase-protection analyses. In all eight lung tumors examined, RHOA RNA levels were decreased relative to the level in normal surrounding tissue, whereas RHOA expression was decreased in only two of six colon tumors. We also found that lovastatin-induced cell cycle arrest resulted in increased RHOA RNA expression in breast cancer cell lines. RNase-protection analysis of RHOA RNA from these tumor and cell line specimens demonstrated that the relative abundance of RNA transcripts utilizing these three polyadenylation signals did not vary with total RHOA mRNA levels.