c-fos gene expression in cell revertants from a transformed to a pseudonormal phenotype

Therapeutic potential and mechanism of action of oligonucleotides and ribozymes

Specific inactivation of gene expression is an attractive approach for rational drug design to combat degenerative diseases and infectious agents. Oligonucleotide-directed triple-helix formation at cis-acting elements of gene promoters, short oligonucleotides containing base sequences that are complementary to the messenger RNA (antisense oligos), and RNA enzymes (ribozymes) that specifically cleave messenger RNA molecules are currently being used both as experimental tools and as therapeutic agents. Mechanisms of action of various oligonucleotide-based drugs, recent developments in the drug-delivery approaches, and future potentials are discussed in this review.

Specific inhibition of c-fos proto-oncogene expression by triple-helix-forming oligonucleotides

The promoter region of the c-fos oncogene 5' flanking sequence contains enhancer elements crucial for binding nuclear factors that regulate transcription following cell proliferation and differentiation. Single-stranded deoxyoligonucleotides were chosen for modulation of c-fos protooncogene expression because of their high-affinity binding to specific nucleotide sequences. We designed two oligonucleotides that form a triple-helix complex on the retinoblastoma gene product-responsible element of the c-fos oncogene. Modification of the DNA triplex with dimethyl sulfate and affinity cleaving assays demonstrate that the predicted oligonucleotides form a DNA triplex structure with the c-fos promoter in a sequence-specific manner. Tumorigenic and non-tumorigenic fibroblasts were transiently transfected with fos-CAT plasmid modified with alkylating triplex-forming oligonucleotide reagents. A dramatic depression of CAT activity was found when the cross-linked triple helix complex at the retinoblastoma gene product-related site of the c-fos promoter was used. These experiments suggest that transcription of individual genes can be selectively modulated in cell culture by sequence specific triplex formation in regulatory enhancer sequences.

The reversion of highly tumorigenic cell lines to non-tumorigenic phenotype is associated with c-jun down-expression

Using model spontaneously reverting cell lines, c-jun, junB, junD and c-fos oncogene expression was investigated. c-jun, but not junB, junD or c-fos, was overexpressed in highly tumorigenic clones. The reversion of cells to the non-tumorigenic phenotype resulted in a dramatic decrease in c-jun expression. CAT assays revealed that c-jun overexpression in tumorigenic cells was associated with higher transcription activity. No correlation between c-jun oncogene expression and AP-1 transcription factor activity in tumorigenic and non-tumorigenic clones was found.

Identification of binding sites for transcription factors NF-kappa B and AP-2 in the promoter region of the human heme oxygenase 1 gene

Heme oxygenase (HO) is the rate-limiting enzyme in heme catabolism and its activity is induced by many agents, including its substrate heme, heavy metals, UV radiation, and other injurious oxidant conditions. We examined the presence of several regulatory elements in the promoter region of the human HO-1 gene which could possibly account for its induction in response to diverse agents or influences. Heme treatment increased both HO activity and HO-1 mRNA in the human erythroleukemic cell line K562. Electrophoretic mobility-shift assays of nuclear protein extracts from heme-treated and control cells with specific oligonucleotide probes containing binding sites for known transcription factors, including AP-1, AP-2, Sp1, NF-kappa B, CTF/NF1, TFIID, OKT1, and CREB, and oligonucleotides containing serum-, metal-, and glucocorticoid-responsive elements demonstrated a specific and marked increase in the NF-kappa B and AP-2 transcription factors and, to a lesser extent, an increase in AP-1. No significant increase in other transcription factors over the control, untreated cells was observed. DNase I footprint assays using purified transcription factors revealed the presence of NF-kappa B and AP-2 binding sites in the proximal part of the promoter region of the human HO-1 gene. Moreover, nucleotide sequence analysis of the HO-1 promoter region showed that the protected regions encompassed NF-kappa B and AP-2 consensus binding sites. The presence of regulatory sequences for the binding of transcription factors such as NF-kappa B and AP-2, whose activation is associated with the immediate response of the cell to an injury, may be an indication of the important role which HO-1 may play in defense mechanisms against tissue injury.

Characterization of a 142-bp fragment of the murine c-fos oncogene promoter upstream of the SIF-binding element

We previously reported that in transformed mouse sarcoma cells of spontaneous origin and in revertants transfected with a fos-cat fusion, the 600-bp c-fos promoter region provides chloramphenicol acetyltransferase activity. In the present study, we investigated the binding of transcriptional factor protein(s) to a region (-503 to -361) upstream of the sis (platelet-derived growth factor)-inducible factor (SIF)-binding element. Gel electrophoresis retardation (GER) assay clearly demonstrated the appearance of strong binding activity to a newly described fragment in the 142-bp region studied. Further analysis using synthetic oligodeoxyribonucleotides and GER defined a binding region of 30 bp (AvaI-AvaII) from -503 to -472 that partially overlaps with a region known to bind fos promoter binding site 2 (FBS2). DNase I footprint analysis discovered a novel sequence in the upstream region of the c-fos promoter to which protein(s) in nuclear extracts from various mouse and human cells bind. This factor(s) is not identical to most known transcriptional factors present in the promoter region of nuclear oncogenes. A proximal part of this fragment is very conservative and contains several AP-2-like-binding sites.