Reduction of Cripto-1 expression by a hammerhead-shaped RNA molecule results from inhibition of translation rather than mRNA cleavage

Cripto: a tumor growth factor and more

Cripto, a growth factor with an EGF-like domain, and the first member of the EGF-CFC family of genes to be sequenced and characterized, contributes to deregulated growth of cancer cells. A role for Cripto in tumor development has been described in the human and the mouse. Members of the EGF-CFC family are found only in vertebrates: CFC proteins in zebrafish, Xenopus, chick, mouse and human have been characterized and indicate some common general functions in development. Cripto expression was first found in human and mouse embryonal carcinoma cells and male teratocarcinomas, and was demonstrated to be over-expressed in breast, cervical, ovarian, gastric, lung, colon, and pancreatic carcinomas in contrast to normal tissues where Cripto expression was invariably low or absent. Cripto may play a role in mammary tumorigenesis, since in vitro, Cripto induces mammary cell proliferation, reduces apoptosis, increases cell migration, and inhibits milk protein expression. This prediction is strengthened by observations of Cripto expression in 80% of human and mouse mammary tumors. At least three important roles for Cripto in development have created considerable interest, and each activity may be distinct in its mechanism of receptor signaling. One role is in the patterning of the anterior-posterior axis of the early embryo, a second is a crucial role in the development of the heart, and a third is in potentiating branching morphogenesis and modulating differentiation in the developing mammary gland. Whether these properties are functions of different forms of Cripto, different Cripto receptors or the distinct domains within this 15-38 kDa glycoprotein are examined here, but much remains to be revealed about this evolutionarily conserved gene product. Since all Cripto receptors have not yet been determined with certainty, future possible uses as therapeutic targets remain to be developed. Cripto is released or shed from expressing cells and may serve as an accessible marker gene in the early to mid-progressive stages of breast and other cancers. Meanwhile some speculations on possible receptor complexes for Cripto signaling in mammary cells are offered here as a spur to further discoveries.

Translational suppression by hammerhead ribozymes and inactive variants in S. cerevisiae

The activity of hammerhead ribozymes in S. cerevisiae was assessed using two ribozymes that were designed to intramolecularly attack the hepatitis B viral X mRNA. The ribozymes effectively suppressed the expression of the X-lacZ fusion gene, when they were inserted at the 5' end of the X mRNA. The ribozymes cleaved the target RNA efficiently at the targeted phosphodiester bond, but the inactive mutants carrying G5-to-A substitution in the core did not, as the total RNA preparations of yeast extracts was assayed by primer extension. These G5A mutants, however, exerted the suppression as effectively as the wild-type ribozymes. The results, with several mutations introduced to a ribozyme, suggested that either mere formation of hammerhead-like structures with the three stems, or the formation of any two stems, could inhibit translation. Thus, the hammerhead-like structures, leading to cleavage or not, could effectively suppress translation, especially when formed around the initiation codon. The G5-to-A and U7-to-G mutations and replacement of the stem-II hairpin tetraloop did not appear to affect the formation of the inhibitory structure(s). The inhibition that was observed when stems I and III were directly connected without a loop or with a stem II hairpin was completely relieved when they were connected with only the loop of stem II (not containing the stem portion).

Inhibition of Fas-mediated apoptosis in mouse insulinoma betaTC-3 cells via an anti-Fas ribozyme

In this study we have designed and constructed an anti-Fas ribozyme and show that it can specifically cleave the Fas mRNA in vitro. Moreover, to test its efficacy ex vivo, we transfected the anti-Fas ribozyme into betaTC-3 insulinoma cells, using a RNA polymerase III promoter to drive its expression. Like pancreatic beta cells, betaTC-3 cells do not constitutively express Fas, but Fas expression can be induced with IL-1 and IFN-gamma. Transfected cells expressed an average of 5000 copies of anti-Fas ribozyme transcript per cell as assessed by reverse transcriptase-real-time PCR. After IL-1/IFN-gamma treatment, betaTC-3 cells transfected with the anti-Fas ribozyme expressed 80% less Fas compared with mock-transfected cells. In addition, the anti-Fas ribozyme also inhibited Fas expression in NIT-1 insulinoma cells and in primary cultures of dispersed pancreatic islet cells. Inhibition of de novo Fas expression in betaTC-3 cells expressing the anti-Fas ribozyme correlated with resistance to Fas-mediated apoptosis as determined by the number of cells exhibiting caspase 3 proteolytic activity. Hence, we have engineered a ribozyme capable of preventing Fas expression in the betaTC-3 pancreatic insulinoma cell line and conferring resistance to Fas-mediated apoptosis. We suggest that ribozymes may be potentially useful to engineer resistance to apoptosis in transplantable beta cells, a feature that may significantly improve the survival of islet cell grafts.

RNA hairpin loops repress protein synthesis more strongly than hammerhead ribozymes

A general study has been carried out to determine how well hammerhead ribozymes might reduce levels of specific protein synthesis in living cells, compared with RNA hairpin loops as stable but noncleaving controls. Four different experiments are described. First, a wide variety of hammerhead ribozymes, as well as hairpin loops, was cloned into a gene-expression cassette for beta-galactosidase, upstream of the coding sequences for that reporter gene, and expressed from plasmids in several strains of Escherichia coli. The results show that ribozymes, when acting intramolecularly in E. coli, do not significantly reduce the amount of protein synthesized from any construct. As a control, long RNA hairpin loops do greatly reduce the amount of protein made. Secondly, we studied the transcription-translation of these same plasmids in a cell extract from E. coli. Once again, hammerhead ribozymes show no effect on levels of beta-galactosidase, whereas long RNA hairpin loops produce a strong reduction, by apparent attentuation at the level of translation. Thirdly, we added an SV40 promoter to each plasmid, in order to study the effects of these gene-regulators on protein synthesis in Chinese hamster ovary cells. Here active intramolecular ribozymes produce a slight reduction in beta-galactosidase, whereas long RNA hairpin loops produce an even stronger reduction than before. Those hairpin loops apparently induce degradation of their own mRNA in Chinese hamster ovary cells, by a mechanism not seen in E. coli. Finally, analyses of total RNA by S1-trimming show that hammerhead ribozymes will self-cleave a mRNA by a total of no more than 45-50% in E. coli, compared with 70-80% in vitro. Other analyses using Northern blotting were unable to detect any ribozyme cleavage in E. coli or Chinese hamster ovary cells. In summary, the ability of hammerhead ribozymes to reduce protein synthesis appears weak or nonexistent in all the cellular systems tested. By comparison, long RNA hairpin loops reduce protein synthesis strongly: by an apparent attentuation mechanism in E. coli or by a novel degradation of their own mRNA in Chinese hamster ovary cells.