Transcriptional activation of the metallothionein I gene by electric pulses in vivo: basis for the development of a new gene switch system

A novel estrogen receptor intramolecular folding-based titratable transgene expression system

The use of regulated gene expression systems is important for successful gene therapy applications. In this study, ligand-induced structural change in the estrogen receptor (ER) was used to develop a novel ER intramolecular folding-based transcriptional activation system. The system was studied using ER-variants of different lengths, flanked on either side by the GAL4-DNA-binding domain and the VP16-transactivation domain (GAL4(DBD)-ER-VP16). The ER ligands of different types showed efficient ligand-regulated transactivation. We also characterized a bidirectional transactivation system based on the ER and demonstrated its utility in titrating both reporter and therapeutic gene expression. The ligand-regulated transactivation system developed by using a mutant form of the ER (G521T, lacking affinity for the endogenous ligand 17beta-estradiol, whereas maintaining affinity for other ligands) showed efficient activation by the ligand raloxifene in living mice without significant interference from the circulating endogenous ligand. The ligand-regulated transactivation system was used to test the therapeutic efficiency of the tumor suppressor protein p53 in HepG2 (p53(+/+)) and SKBr3 (p53(-/-)/mutant-p53(+/+)) cells in culture and tumor xenografts in living mice. The multifunctional capabilities of this system should be useful for gene therapy applications, to study ER biology, to evaluate gene regulation, ER ligand screening, and ER ligand biocharacterization in cells and living animals.

Differential gene expression in skeletal muscle cells after membrane depolarization

Skeletal muscle is a highly plastic tissue with a remarkable capacity to adapt itself to challenges imposed by contractile activity. Adaptive response, that include hypertrophy and activation of oxidative mechanisms have been associated with transient changes in transcriptional activity of specific genes. To define the set of genes regulated by a depolarizing stimulus, we used 22 K mouse oligonucleotide microarrays. Total RNA from C2C12 myotubes was obtained at 2, 4, 18, and 24 h after high K+ stimulation. cDNA from control and depolarized samples was labeled with cyanine 3 or 5 dyes prior to microarray hybridization. Loess normalization followed by statistical analysis resulted in 423 differentially expressed genes using an unadjusted P-value < or = 0.01 as cut off. Depolarization affects transcriptional activity of a limited number of genes, mainly associated with metabolism, cell communication and response to stress. A number of genes related to Ca2+ signaling pathways are induced at 4 h, reinforcing the potential role of Ca2+ in early steps of signal transduction that leads to gene expression. Significant changes in the expression of molecules involved in muscle cell structure were observed; K+-depolarization increased Tnni1 and Acta1 mRNA levels in both differentiated C2C12 and rat skeletal muscle cells in primary culture. Of these two, depolarization induced slow Ca2+ transients appear to have a role only in the regulation of Tnni1 transcriptional activity. We suggest that depolarization induced expression of a small set of genes may underlie Ca2+ dependent plasticity of skeletal muscle cells.

Positive in vivo heterologous gene regulation by electric pulses delivery with metallothionein I gene promoter

BACKGROUND

In vivo electrotransfer is a physical method of gene delivery in various tissues and organs. It is a promising strategy for the systemic secretion of therapeutic proteins and for DNA vaccination. Nevertheless, for the success of gene therapy in clinics, it is essential to develop gene regulation systems. The existing systems described in the literature all rely on the creation of an artificial transcription factor and/or an inducer drug. New strategies based on endogenous regulatable elements are being developed. We have previously identified the murine metallothionein promoter as an endogenous promoter inducible by controlled electric stimuli applied for electrotransfer experiments. We report here a regulation strategy based on this murine metallothionein promoter in a plasmid context using electric pulses delivery as an inducer.

METHODS

Plasmids containing different constructions of the murine metallothionein I (mMT-I) promoter were transfected in mice tibialis-cranalis muscles using the simple skeletal muscle electrotransfer method. The regulation system was studied with the murine secreted alkaline phosphatase (MUSEAP) reporter gene.

RESULTS

The mMT-I promoter can be transiently induced in vivo by application of electric fields. Its inducibility was analyzed in a plasmid context. We demonstrated that the mechanism of this transcriptional induction is not mediated by the cellular entry of metal ions. The ARE (antioxidant-responsive element) sequence was identified as the element responsive to the electric field stimulation.

CONCLUSIONS

This time-control of the expression of a therapeutic gene by physical stimuli could be of value in the context of gene regulation for gene therapy.

Metallothionein-I/II double knockout mice are no more sensitive to the carcinogenic effects of nickel subsulfide than wild-type mice

Metallothionein (MT) is a high-affinity metal-binding protein thought to mitigate the toxicity of various metals. MT may limit the toxicity of a metal by direct binding or through action as an antioxidant for metals that generate reactive oxygen species. Nickel compounds have carcinogenic potential in humans and animals, possibly by production of oxidative stress. The impact of MT deficiency on the carcinogenic effects of nickel is unknown. Thus, groups (n=25) of male MT-I/II double knockout (MT-null) or MT wild-type (WT) mice were exposed to a single treatment of nickel (0.5 or 1.0 mg Ni3S2/site, intramuscularly, [i.m.], into both hind legs), or left untreated (control) and observed over the next 104 weeks. There were no differences in the incidence of spontaneous tumors in MT-null and WT mice. Nickel induced injection site fibrosarcomas in a dose-related fashion to a similar extent in both WT and MT-null mice. Nickel-treatment had no effect on total lung tumor incidence, although some phenotypic-specific differences occurred in the proportion of benign and malignant pulmonary tumors. Overall, MT-null mice appear no more sensitive to the carcinogenic effects of nickel than WT mice. Thus, poor MT production does not appear to be a predisposing factor for nickel carcinogenesis.

Inducible production of erythropoietin using intramuscular injection of block copolymer/DNA formulation

BACKGROUND

We have previously shown that intramuscular injection of plasmid DNA formulated with a non-ionic amphiphile synthetic vector [poly(ethylene oxide)(13)-poly(propylene oxide)(30)-poly(ethylene oxide)(13) block copolymer; PE6400] increases reporter gene expression compared with naked DNA. We have now investigated this simple non-viral formulation for production of secreted proteins from the mouse skeletal muscle.

METHODS

Plasmids encoding either constitutive human secreted alkaline phosphatase or murine erythropoietin inducible via a Tet-on system were formulated with PE6400 and intramuscularly injected into the mouse tibial anterior muscle.

RESULTS

PE6400/DNA formulation led to an increased amount of recombinant alkaline phosphatase secreted from skeletal muscle as compared with naked DNA. In the presence of doxycycline, a single injection of 10 microg plasmid encoding inducible murine erythropoietin formulated with PE6400 significantly increased the hematocrit, whereas the same amount of DNA in the absence of PE6400 had no effect. The increase in the hematocrit was stable for 42 days. The tetracycline-inducible promoter permitted pharmacological control of hematocrit level after DNA intramuscular injection. However, 4 months post-injection the hematocrit returned to its pre-injection value, even in the presence of doxycycline. This phenomenon was likely caused by an immune response against the tetracycline-activated transcription factor.

CONCLUSIONS

Intramuscular injection of plasmid DNA formulated with PE6400 provides an efficient and simple method for secretion and production of non-muscle proteins.

Minimal influence of metallothionein over-expression on nickel carcinogenesis in mice

Metallothionein (MT) is a metal-binding protein associated with tolerance to metals and oxidative stress. Nickel is a metal carcinogen potentially acting through oxidative attack on critical biomolecules. We investigated the role of MT in nickel carcinogenesis using MT-transgenic mice that constitutively over-express MT-I in all tissues tested. Groups of 25 male MT-transgenic and wild type (C57BL/6; WT) mice received intramuscular injections of nickel subsulfide (Ni3S2) in both thighs at doses of 0 (control), 0.5, or 1.0 mg/site at 12 weeks of age and were observed for 104 weeks. Injection site tumors (ISTs; primarily fibrosarcomas) started occurring 45 weeks after nickel injection and IST incidence was similar in the WT (control - 0%, 0.5 mg/site - 20%, 1.0 mg/site - 40%) and MT-transgenic mice (control - 0%, 0.5mg/site - 28%, 1.0mg/site - 29%.). At the 0.5 mg/site dose the average time to IST in MT-transgenic mice was approximately 13 weeks shorter than in WT mice. Spontaneous lung tumors developed in 25% of control WT mice but none developed in control MT-transgenic mice. A nickel dose-related trend for increased lung tumors occurred in MT-transgenic mice but not in WT mice. Thus, the over-expression of MT did not significantly mitigate the carcinogenic response to nickel.