Retinoic acid-induced differentiation of retrovirus-infected HL-60 cells is associated with enhanced transcription from the viral long terminal repeat

All-trans retinoic acid increases transgene expression in MSCV-transduced cells, via a mechanism that is retinoid receptor dependent but independent of cellular differentiation

Treatment of MSCV-GFP-transduced HL60 promyelocytic cells with all-trans retinoic acid (ATRA) resulted in a significant increase in GFP expression. The increased GFP expression was observed by 16 hr and was dependent on de novo protein production. This effect was specific to ATRA and unrelated to cell differentiation because it was not induced by dimethyl sulfoxide. Furthermore, a similar increase in GFP expression was observed in MSCV-GFP-transfected K562 cells, which do not differentiate when exposed to ATRA. Significantly increased GFP expression was seen at doses as low as 0.5 nM ATRA and was abrogated by AGN193109, an antagonist of retinoid signaling. We therefore conclude that this increase in gene expression is mediated by retinoic acid receptors. The long terminal repeat (LTR) region of MSCV contains candidate retinoic acid response elements and response elements for the ATRA-inducible transcription factor C/EBPalpha. We suggest that the increase in GFP expression is driven by the action of ATRA-activated host cell transcription factors. These findings offer a method to increase the expression of retroviral transgenes either in vitro or in vivo by treatment with low doses of retinoic acid that are clinically achievable and well tolerated. This use of inducible host cell transcription factors offers an alternative to engineering novel LTR regulatory sequences in order to increase transgene expression.

Exploring the functional complexity of cellular proteins by protein knockout

Comprehensive dissection of protein functions entails more complicated manipulations than simply eliminating the protein of interest. Established knockdown technologies, such as RNA interference, antisense oligodeoxynucleotides, or ribozymes, are limited for specific applications such as modulating protein levels or specific targeting of a posttranslationally modified subpopulation. Here we show that the engineered Skp1, Cullin 1, and F-box-containing betaTrCP substrate receptor ubiquitin-proteolytic system, designated protein knockout, could achieve not only total elimination but also rapid and systematic reduction of a given cellular protein. Stable expression of a single engineered betaTrCP demonstrated simultaneous and sustained degradation of the entire retinoblastoma family proteins. Furthermore, the engineered betaTrCP was capable of selecting hypo- but not hyperphosphorylated forms of retinoblastoma for degradation. The engineered betaTrCP has been extensively modified to increase its specificity in substrate selection. This optimized protein-knockout system offers a powerful and versatile proteomic tool to dissect diverse functional properties of cellular proteins in somatic cells.

Polyoma middle T antigen in HL-60 cells accelerates hematopoietic myeloid and monocytic cell differentiation

Expression of the polyoma virus middle T antigen in HL-60 cells accelerates their differentiation in response to both monocytic and granulocytic differentiation-inducing agents. Middle T-expressing cells treated with the granulocytic inducer retinoic acid or the monocytic inducer 1,25-dihydroxy vitamin D3 differentiated 24 h earlier than parental, mock-electroporated, or vector control cell lines. The rapid onset of differentiation correlated with an increase in the cellular level of the middle T protein as well as two known retinoic-acid-inducible markers in HL-60 cells: the paxillin and transglutaminase gene products. The accelerated functional differentiation response and expression of retinoic-acid-inducible markers indicate that middle T played a causal role in differentiation. Thus, expression of the polyoma middle T antigen in HL-60 cells enhanced a variety of molecular changes associated with cellular differentiation.

Ectopic TAL-1/SCL expression in phenotypically normal or leukemic myeloid precursors: proliferative and antiapoptotic effects coupled with a differentiation blockade

The TAL-1 gene specifies a basic helix-loop-helix domain (bHLH) transcription factor, which heterodimerizes with E2A gene family proteins. tal-1 protein is abnormally expressed in the majority of T-cell acute lymphoblastic leukemias (T-ALLs). tal-1 is expressed and plays a significant role in normal erythropoietic differentiation and maturation, while its expression in early myeloid differentiation is abruptly shut off at the level of late progenitors/early differentiated precursors (G. L. Condorelli, L. Vitelli, M. Valtieri, I. Marta, E. Montesoro, V. Lulli, R. Baer, and C. Peschle, Blood 86:164-175, 1995). We show that in late myeloid progenitors (the phenotypically normal murine 32D cell line) and early leukemic precursors (the human HL-60 promyelocytic leukemia cell line) ectopic tal-1 expression induces (i) a proliferative effect under suboptimal culture conditions (i.e., low growth factor and serum concentrations respectively), via an antiapoptotic effect in 32D cells or increased DNA synthesis in HL-60 cells, and (ii) a total or marked inhibitory effect on differentiation, respectively, on granulocyte colony-stimulating factor-induced granulopoiesis in 32D cells or retinoic acid- and vitamin D3-induced granulo- and monocytopoiesis in HL-60 cells. Furthermore, experiments with 32D temperature-sensitive p53 cells indicate that aberrant tal-1 expression at the permissive temperature does not exert a proliferative effect but causes p53-mediated apoptosis, i.e., the tal-1 proliferative effect depends on the integrity of the cell cycle checkpoints of the host cell, as observed for c-myc and other oncogenes. tal-1 mutant experiments indicate that ectopic tal-1 effects are mediated by both the DNA-binding and the heterodimerization domains, while the N-terminally truncated tal-1 variant (M3) expressed in T-ALL malignant cells mimics the effects of the wild-type protein. Altogether, our results (i) indicate proliferative and antidifferentiative effects of ectopic tal-1 expression, (ii) shed light on the underlying mechanisms (i.e., requirement for the integrity of the tal-1 bHLH domain and cell cycle checkpoints in the host cell, particularly p53), and (iii) provide new experimental models to further investigate these mechanisms.

Vitamin A levels and human immunodeficiency virus load in injection drug users

Although low plasma vitamin A levels are associated with increased mortality and higher vertical transmission during human immunodeficiency virus (HIV) infection, it is unknown whether plasma low vitamin A levels are a marker for circulating HIV load. We conducted a cross-sectional study within a prospective cohort study of injection drug users in order to evaluate the relationship between plasma vitamin A levels and HIV viral load. Plasma vitamin A level was measured by high-performance liquid chromatography. Infectious viral load was measured by quantitative microculture of serial fivefold dilutions of 10(6) peripheral blood mononuclear cells. A total of 284 HIV-infected adults (79 women, 205 men) were studied. Plasma vitamin A levels consistent with deficiency were found in 28.9% of adults. A total of 38.0% of women and 25.3% of men had vitamin A deficiency (P < 0.04). The median infectious viral load for the entire study population was 8 infectious units per million cells. No significant relationship between plasma vitamin A levels and infectious viral load was observed in these injection drug users. This study suggests that there is no correlation between HIV viral load and plasma vitamin A levels in injection drug users, and these variables may represent independent risk factors during HIV infection. HIV-infected adult women appear to be at higher risk of developing vitamin A deficiency.

High-level expression from a cytomegalovirus promoter in macrophage cells

To identify vectors that provide optimal gene expression in human hematopoietic cells, we investigated retroviruses containing the adenosine deaminase (ADA) gene under the transcriptional control of the promoters/enhancers of Moloney murine leukemia virus and the human cytomegalovirus (CMV) immediate-early gene. ADA expression was monitored in transduced human multipotential promyelocytic leukemic HL-60 cells and human monocytic leukemic THP-1 cells. HL-60 cells can be induced by phorbol ester to differentiate into macrophage lineage cells and by retinoic acid into granulocytic cells. THP-1 cells undergo phorbol ester-induced differentiation to macrophage cells. In LNCA-transduced HL-60 derived macrophage cells, ADA controlled by the CMV promoter was expressed at 100.0 mumol/hr.mg, in contrast to 1.2 mumol/hr.mg from LN-transduced control cells. LNCA-transduced THP-1 macrophage cells showed a similar increase in ADA activity over control cells. These elevated enzyme activities were confirmed by Northern blots, which showed substantial increases in ADA mRNA derived from the CMV promoter. This suggests use of the CMV promoter for gene therapy targeted at macrophages, as, for example, in the treatment of lysosomal storage disorders such as Gaucher disease. These inducible cell lines have allowed the evaluation of transduced gene expression in proliferating and differentiating hematopoietic cells that may serve as an in vitro model of bone marrow-targeted gene therapy.

Expression from leukocyte integrin promoters in retroviral vectors

Human gene therapy for diseases involving leukocytes would be facilitated by the identification of specific promoter/enhancer sequences capable of directing high levels of tissue and stage-specific expression of the requisite cDNA when used in a retroviral vector. We tested the promoter sequences from the leukocyte integrin CD11a (LFA-1), CD11b (Mac-1), and CD18 subunits in retroviral vectors to express a reporter gene, adenosine deaminase, in the human leukocyte cell lines K562 and HL-60. The leukocyte integrins are expressed in leukocytes, and they are inducible in HL-60 cells, a model system for myeloid differentiation. Although the leukocyte integrin promoter/enhancer sequences direct the expression of reporter genes in myeloid lineage cell lines in transient transfection assays, in these studies, the leukocyte integrin promoters direct low levels of reporter gene expression following retroviral-mediated transduction in K562 and HL-60 cells and selection of stable integrants. Treatment of HL-60 cells transduced with retroviral vectors containing the leukocyte integrin promoters with retinoic acid or phorbol myristate acetate results in less than a two-fold increase in reporter gene expression. These studies indicate that: (i) expression from the leukocyte integrin promoters from stable integrants in retroviral vectors does not parallel the results observed in transient transfection assays, and (ii) additional promoter/enhancer sequences will likely be required for these promoters to direct high levels of tissue and stage-specific expression in retroviral vectors.

Induced expression from the Moloney murine leukemia virus long terminal repeat during differentiation of human myeloid cells is mediated through its transcriptional enhancer

Transcription from the Moloney murine leukemia virus (Mo-MuLV) long terminal repeat (LTR) is inhibited in murine stem cells and induced during maturation of these cells. We have investigated whether alterations in the activity of this viral regulatory element also occur during differentiation of human myeloid leukemia cells. The Mo-MuLV LTR and the simian virus 40 (SV40) early promoter were introduced into HL-60 promyelocytes on Epstein-Barr virus-derived chloramphenicol acetyltransferase expression vectors. When these cells were induced to terminally differentiate, transcription from the Mo-MuLV LTR was induced approximately 10-fold. Expression from the SV40 promoter remained constant during differentiation of these cells. Replacing the SV40 transcriptional enhancer with the Mo-MuLV LTR transcriptional enhancer rendered the SV40 promoter inducible during differentiation. We conclude that sequences within the transcriptional enhancer of the Mo-MuLV LTR contain cis-acting elements responsible for induction of gene expression during differentiation of human myeloid cells.

Efficient retroviral transfer of a mouse c-myc construct into HL60

We introduced an LTR-driven mouse c-myc second and third exon, Tn5Neo gene construct into the inducible human leukemia line HL60 using an amphotropic retroviral vector system. Over 90% of the cells became neo-resistant and the transfected myc gene was transcribed in several neomycin resistant clones. Making use of the simultaneous presence of the different myc genes in the same cell, we compared expression of the corresponding mRNAs after differentiation and their decay mechanisms.

Induced expression from the Moloney murine leukemia virus long terminal repeat during differentiation of human myeloid cells is mediated through its transcriptional enhancer

Transcription from the Moloney murine leukemia virus (Mo-MuLV) long terminal repeat (LTR) is inhibited in murine stem cells and induced during maturation of these cells. We have investigated whether alterations in the activity of this viral regulatory element also occur during differentiation of human myeloid leukemia cells. The Mo-MuLV LTR and the simian virus 40 (SV40) early promoter were introduced into HL-60 promyelocytes on Epstein-Barr virus-derived chloramphenicol acetyltransferase expression vectors. When these cells were induced to terminally differentiate, transcription from the Mo-MuLV LTR was induced approximately 10-fold. Expression from the SV40 promoter remained constant during differentiation of these cells. Replacing the SV40 transcriptional enhancer with the Mo-MuLV LTR transcriptional enhancer rendered the SV40 promoter inducible during differentiation. We conclude that sequences within the transcriptional enhancer of the Mo-MuLV LTR contain cis-acting elements responsible for induction of gene expression during differentiation of human myeloid cells.