Intracellular antibodies (intrabodies) for gene therapy of infectious diseases

Intracellular antibodies (intrabodies) represent a new class of neutralizing molecules with a potential use in gene therapy. Intrabodies are engineered single-chain antibodies in which the variable domain of the heavy chain is joined to the variable domain of the light chain through a peptide linker, preserving the affinity of the parent antibody. Intrabodies are expressed inside cells and directed to different subcellular compartments where they can exert their function more effectively. The effects of intrabodies have been investigated using structural, regulatory, and enzymatic proteins of the human immunodeficiency virus (HIV-1) as targets. These intrabodies have demonstrated their versatility by controlling early as well as late events of the viral life cycle. In this article, we review studies of the use of intrabodies as research tools and therapeutic agents against HIV-1.

Post-transcriptional regulation of erythropoietin mRNA stability by erythropoietin mRNA-binding protein

We have previously identified a sequence in the 3'-untranslated region (3'-UTR) of erythropoietin (Epo) mRNA which binds a protein(s), erythropoietin mRNA-binding protein (ERBP). A mutant lacking the ERBP binding site (EpoM) was generated. Hep3B cells were stably transfected with a wild-type Epo (EpoWT) cDNA or EpoM cDNA construct located downstream of a promoter of cytomegalovirus. Following inhibition of transcription, the half-lives of EpoWT and EpoM mRNAs were 7 h and 2.5 h in normoxia, respectively. The EpoM mRNA half-life remained unchanged in hypoxia. EpoWT mRNA half-life increased approximately 40% in response to a 6-h hypoxic pre-exposure and an additional approximately 50% when pre-exposed to 12 h hypoxia. The steady-state level of EpoWT mRNA was 4-fold that of EpoM mRNA reflecting the difference in mRNA decay rates in normoxia. The Epo protein level expressed from exogenous EpoM was unchanged in both normoxia and hypoxia. In contrast, the Epo protein level expressed from exogenous EpoWT increased 50% in hypoxia when compared with normoxia. These observations were further supported by chimeric chloramphenicol acetyltransferase and Epo-3'-UTR constructs. We have demonstrated that Epo mRNA stability was modulated in normoxia and further by hypoxia, therefore, providing evidence that Epo is regulated at the post-transcriptional level through ERBP complex formation.

Interaction of erythropoietin RNA binding protein with erythropoietin RNA requires an association with heat shock protein 70

Synthesis of erythropoietin (Epo), the glycoprotein hormone that regulates red blood cell formation, is induced in response to low oxygen stress (hypoxia), and is regulated at both transcriptional and post-transcriptional levels. We have previously described an Epo RNA binding protein (ERBP) which specifically binds to the 3'-untranslated region of Epo mRNA and is likely involved in the regulation of Epo mRNA stability. Since heat shock proteins (hsps) are induced in response to a variety of stresses, including hypoxia, we tested the possibility that hsps are involved in ERBP-Epo RNA complex formation. When human anti-hsp70 antibody was added to ERBP-containing human hepatoma cell (Hep3B) lysates, the ERBP-Epo RNA complex was inhibited in an electrophoretic mobility band shift assay. In addition, the anti-hsp70 antibody-inhibited complex could be rescued if lysates were pretreated with purified inducible hsp70, but not with bovine serum albumin (BSA). In vivo studies using quercetin to inhibit hsp70 induction support the notion that hsp70 is involved in ERBP-Epo RNA complex formation. Taken together, these findings suggest involvement of hsp70 in ERBP-Epo mRNA complex formation, and our model suggests a novel role for hsps in the regulation of EPO mRNA stability.

Changes in redox affect the activity of erythropoietin RNA binding protein

We have previously identified a cytosolic protein, erythropoietin RNA binding protein (ERBP), which is up-regulated in certain tissues in response to hypoxia. To further characterize the interaction of ERBP and erythropoietin (EPO) mRNA, we have examined the role of reduction-oxidation in the EPO mRNA binding mechanism of ERBP isolated from human hepatoma cells (Hep3B). Reducing agents dithiothreitol (DTT) and 2-mercaptoethanol (2-ME) increased ERBP binding activity in a concentration-dependent manner, whereas the oxidizing agent, diamide, abolished ERBP binding activity. In addition, treatment of Hep3B cell lysates with the irreversible sulfhydryl alkylating agent N-ethylmaleimide resulted in inhibition of the EPO mRNA-ERBP complex. Taken together, these findings suggest that sulfhydryl groups may play a role in vivo in the regulation of EPO production through the modulation of ERBP binding activity.

Hypoxia up-regulates the activity of a novel erythropoietin mRNA binding protein

The mechanisms which control the production of erythropoietin (Epo) remain enigmatic. Recent data suggest that the half-time of Epo messenger RNA (mRNA) is increased by hypoxia in Hep 3B cells, a human hepatoma line. The post-transcriptional regulation of other rapidly degraded mRNAs is mediated by sequence-specific mRNA binding proteins. In order to determine if Epo mRNA specific binding proteins exist, we probed cytosolic lysates from Hep 3B cells and mouse tissues with radiolabeled Epo RNA. A cytosolic protein that binds specifically to Epo RNA was identified in the Epo-producing, hepatoblastoma Hep 3B cell line by gel mobility shift assay. This protein was identified in both normoxic and hypoxic cells and bound specifically to a 120-base fragment of the 3'-untranslated region (3'-UTR) of Epo mRNA. Binding was completed with unlabeled Epo RNA, but not with granulocyte-macrophage colony-stimulating factor RNA. Ultraviolet light cross-linked Epo RNA-protein complexes migrated as two bands of 70 and 135-140 kD on sodium dodecyl sulfate-polyacrylamide gels. Binding activity was markedly increased in brain and spleen lysates from mice subjected to 24 h of hypoxia. Therefore, the post-transcriptional regulation of Epo expression in response to hypoxia may in part be due to the interaction of Epo RNA with its specific binding protein.

Association of the atrial natriuretic factor receptor with guanylate cyclase in solubilized rat glomerular membranes

The elution profile of solubilized rat glomerular membranes from a gel filtration column showed two peaks of 125I-ANF (atrial natriuretic factor) binding (367 +/- 21, 156 +/- 12 KDa). Over 85% of the total binding for the extract was in the 367 KDa peak. Guanylate cyclase activity was correlated with 125I-ANF specific binding. ANF activation of guanylate cyclase was also observed. As observed previously with particulate membrane, Scatchard-analysis of ANF binding data with the solubilized extract was consistent with a two-site model. Both affinities (Kd's), 4 pM and 1 nM, are within the range of blood concentrations reported for ANF. These observations suggest that most rat glomerular ANF receptors are large molecular complexes coupled with guanylate cyclase in the 300-350 KDa size range.