Adenoviral delivery of Tousled kinase for the protection of salivary glands against ionizing radiation damage.

Oral complications of salivary hypofunction often afflict cancer patients undergoing radiotherapy for head and neck cancers. Dry mouth or xerostomia is an undesirable consequence of radiotherapy that compromises normal oral functions in addition to causing odynophagia and increasing the patient's risk of oral infections and dental caries. Radiation-induced xerostomia is irreversible, and palliative measures to provide symptomatic relief remain the mainstay of treatment. Previously, we identified a splice variant of a cellular kinase, Tousled-like kinase 1B (TLK1B), which when overexpressed protects normal epithelial cells against ionizing radiation (IR)-induced cell death. To address the need to protect salivary glands in patients undergoing regional radiotherapy, we investigated whether preemptive expression of TLK1B in salivary glands protects against IR. In stably-derived salivary cell lines in vitro, TLK1B expression increased cell survival after IR. Cells expressing exogenous TLK1B were less radiosensitive (A5-TLK1B, α/ß=0.67 Gy; ParC5-TLK1B, α/ß=4.3 Gy) compared to control cells (A5-BK, α/ß=1.7 Gy; ParC5-BK, α/ß=32.7 Gy). Using a recombinant adenovirus serotype 5 viral vector for TLK1B gene transfer into rat submandibular salivary glands in vivo, we demonstrated that TLK1B protects the saliva-secreting acinar cells and better preserves salivary gland function against IR relative to control glands. After a single fraction of 16 Gy, the decline in salivary function at 8 weeks was less pronounced in TLK1B-treated animals (40%) as compared to saline-treated controls (67%). Histopathological analysis demonstrated increase in acinar atrophy, decrease in acinar cell number, and increase in inflammatory infiltrate and fibrosis in irradiated control tissues relative to TLK1B-treated glands. These results show the radioprotective benefits of TLK1B and implicate its usefulness in the management of regional radiotherapy-induced xerostomia.

Expression of antisense RNA against initiation factor eIF-4E mRNA in HeLa cells results in lengthened cell division times, diminished translation rates, and reduced levels of both eIF-4E and the p220 component of eIF-4F.

HeLa cells were transformed to express antisense RNA against initiation factor eIF-4E mRNA from an inducible promoter. In the absence of inducer, these cells (AS cells) were morphologically similar to control cells but grew four- to sevenfold more slowly. Induction of antisense RNA production was lethal. Both eIF-4E mRNA and protein levels were reduced in proportion to the degree of antisense RNA expression, as were the rates of protein synthesis in vivo and in vitro. Polysomes were disaggregated with a concomitant increase in ribosomal subunits. Translation in vitro was restored by addition of the initiation factor complex eIF-4F but not by eIF-4E alone. Immunological analysis revealed that the p220 component of eIF-4F was decreased in extracts of AS cells and undetectable in AS cells treated with inducer, suggesting that p220 and eIF-4E levels are coordinately regulated. eIF-4A, another component of eIF-4F, was unaltered.

Targeting and killing of prostate cancer cells using lentiviral constructs containing a sequence recognized by translation factor eIF4E and a prostate-specific promoter.

To develop a gene therapy that would selectively kill prostate cancer cells while sparing normal cells, we have constructed lentiviral vectors that contain a therapeutic gene with a short DNA sequence in the 5'-untranslated region (UTR) that is recognized by the translation initiation factor, eIF4E, which is often overexpressed in malignant cells. Infection of cancer (LNCaP, PC-3M, DU145, and MCF-7 cells) and noncancer cell lines (BPH-1, 267-B1, Plat-E, and Huvec-c cells) with lentivirus having a CMV-promoter and EGFP reporter resulted in high levels of EGFP expression in all cells, whereas, inclusion of the eIF4E UTR recognition sequence restricted high expression to cancer cells and Plat-E cells, which also express substantial levels of eIF4E. Infection of the cells with lentiviral vectors having this UTR in front of the HSV thymidine kinase suicide gene resulted in differential sensitivity to the killing effects of ganciclovir, with at least 100-fold more drug required to kill noncancer cells than cancer cells. Furthermore, in experiments where the CMV promoter was replaced by the prostate-specific ARR(2)PB promoter, the killing effects of ganciclovir were restricted to prostate cancer cells and not seen in nonprostate cancer cells. Our results indicate that combined translational regulation, by incorporation of an eIF4E-UTR recognition sequence into a therapeutic gene, together with transcriptional regulation with a prostate-specific promoter, may provide a means to selectively destroy prostate cancer cells while sparing normal prostate cells.

A translationally regulated Tousled kinase phosphorylates histone H3 and confers radioresistance when overexpressed.

The gene Tousled of Arabidopsis Thaliana encodes a protein kinase which, when mutated, results in abnormal flower development. From a library of mRNAs that are translationally upregulated by overexpression of the translation initiation factor 4E, we identified a mammalian Tousled Like kinase (TLK1B). The human TLK1B mRNA contains a 5'UTR 1088-nt-long with two upstream AUG codons, and was found to be very inhibitory for translation. The TLK1B protein localizes almost exclusively to the nuclei. TLK1B overexpression in mammalian cells rendered them more resistant to ionizing radiation (IR). Purified TLK1B phosphorylated histone H3 at S(10) with high specificity both in a mix of core histones and in isolated chromatin, suggesting that histone H3 is a physiological substrate for TLK1B. Moreover, overexpression of TLK1B in transfected cells resulted in a higher degree of H3 phosphorylation. Expression of TLK1B in a yeast strain that harbors a temperature-sensitive mutation of the major H3 kinase, Ipl1, complemented the growth defect; restored normal levels of histone H3 phosphorylation; and increased their resistance to IR. Phosphorylation of H3 has been linked to the activation of the immediate-early genes upon mitogenic stimulation, and to chromatin condensation during mitotic/meiotic events. A possible role for TLK1B in radioprotection is discussed.

Differential expression of Myc1 and Myc2 isoforms in cells transformed by eIF4E: evidence for internal ribosome repositioning in the human c-myc 5'UTR.

eIF4E is essential for translation initiation, but its overexpression causes malignant transformation. Recent work demonstrated that eIF4E/F participates in exposing and locating alternate translation start codons during scanning. Translation initiation of several important protooncogenes and growth-regulators, such as Myc and FGF-2, can start at CUG start codon(s) upstream of the normal open reading frame (ORF). The resulting amino-terminal extension alters the properties of these proteins and their intracellular distribution. In cells overexpressing eIF4E, c-myc is overexpressed and particularly the larger, CUG-initiated form (Myc1). Recent reports suggest that synthesis of Myc2, the normally expressed AUG-initiated form, is mediated by an IRES. To determine what role eIF4E might play in c-myc expression, the c-myc 5' untranslated region (UTR) was fused in-frame to CAT reporters, and several more derivative constructs were made. In vitro translation experiments (with and without eIF4E/F); expression in CHO cells transformed with eIF4E; and deletion/mutation analysis demonstrated that Myc1 is translated by a scanning mechanism, while Myc2 is translated by Internal Ribosome Repositioning. Moreover, the existence of a true IRES in the 5'UTR was contradicted by its failure to direct translation of a circular transcript, in contrast to hsp70. The c-myc 5'UTR also failed to engage in translation in the absence of functional eIF4F, after cleavage of the eIF4G component with CVB4 protease-2A. The Internal Repositioning Element (IRPE) in c-myc 5'UTR was delimited to nucleotides (nt) 394-440 from the P1 transcription start site.

Detection of the proto-oncogene eIF4E in surgical margins may predict recurrence in head and neck cancer.

Head and neck squamous cell cancers (HNSCC) have a high local recurrence rate due to incomplete tumor resection. The use of molecular markers to establish surgical margins may decrease local recurrence. Surgical margins are determined by histopathologic analysis on frozen sections. We postulate that genetic and molecular changes precede gross histologic alterations. Tumor markers may improve the reliability of pathology examination, but those evaluated to date lack the sensitivity needed for routine clinical use. Western blot analysis showed elevated eIF4E in all 26 HNSCC in contrast to its low expression in benign lesions. Surgical margins were analysed for eIF4E in 23 patients. Twelve patients showed elevated eIF4E in histologically negative margins. Cancer has recurred in 5 of the 12 patients as opposed to none of the 11 patients with eIF4E negative margins (P= 0.02, Log rank test). This is the first report of eIF4E in HNSCC, as a sensitive and specific marker for HNSCC, with potential for defining clear resection margins. The correlation between elevated levels of eIF4E at the margins and recurrence highlights its ability to detect malignant cells prior to clear-cut alterations in morphology. The accuracy and simplicity of these assays underscore the usefulness of eIF4E in managing HNSCC.

Elevated expression of eIF4E and FGF-2 isoforms during vascularization of breast carcinomas.

The translation initiation factor eIF4E is a novel protooncogene found over expressed in most breast carcinomas (Kerekatte et al., 1995), but the pathology where this elevation is initially manifested and its possible role in cancer progression are unknown. We report that eIF4E is markedly increased in vascularized malignant ductules of invasive carcinomas, whereas necrotic and avascular ductal carcinomas in situ display significantly lower levels. eIF4E facilitates the synthesis of FGF-2, a powerful tumor angiogenic factor. Conversely, reducing eIF4E with antisense RNA in MDA-435 cells suppresses their tumorigenic and angiogenic properties, consistent with loss of FGF-2 synthesis. These findings suggest a causal role for eIF4E in tumor vascularization.

Analysis of surgical margins with the molecular marker eIF4E: a prognostic factor in patients with head and neck cancer.

PURPOSE: Complete excision of cancer is guided by histologic assessment of surgical margins. Molecular markers may be more sensitive in identifying malignant cells. eIF4E, a eukaryotic protein synthesis initiation factor, is found elevated in all head and neck squamous cell cancers (HNSCC). In a preliminary study using Western blots and a retrospective study using immunohistochemistry, eIF4E elevation in histologically tumor-free surgical margins correlated with a higher local-regional recurrence. We wanted to confirm this hypothesis in a prospective study. PATIENTS AND METHODS: Immunohistochemical analysis of surgical margins and tumors with an antibody to eIF4E was performed on all newly diagnosed HNSCC patients who underwent surgical resection for their disease between January 1996 and December 1997. RESULTS: All 65 patients had elevated levels of eIF4E in the tumors. Thirty-six patients (55%) had elevated eIF4E in histologically tumor-free margins, and 20 of these patients (56%) have had local-regional recu rrences. Twenty-nine patients (45%) had no elevation of eIF4E in the margins, and only two of these patients (6.9%) have had recurrences. Cox regression analysis showed that elevated eIF4E in the margins was an independent prognostic factor (P =.009) for recurrence. The Kaplan-Meier curves for the probability of nonrecurrence were significantly different for positive and negative eIF4E margins (P =. 0001, log-rank test). CONCLUSION: In histologically tumor-free surgical margins, elevated levels of eIF4E predict a significantly increased risk of recurrence. Elevated levels of eIF4E in tumor margins may identify patients who could benefit from additional therapy.

eIF4E expression in tumors: its possible role in progression of malignancies.

A central issue in the study of neoplastic transformation is to understand how proto-oncogene products deregulate normal processes of cell growth and differentiation: an intrinsic aspect of this is to probe the sequence of events leading to altered expression of proto-oncogenes. In the past few years, studies aimed at understanding the regulation and function of protein synthesis initiation factors, eIF4E initially, culminated in the unexpected finding that a moderate overexpression of this factor results in dramatic phenotypic changes, including rapid proliferation and malignant transformation. Conversely, the tumorigenic properties of cancer cells can be strongly inhibited by antisense-RNA against eIF4E, or overexpression of the inhibitory proteins: 4E-BPs. Furthermore, eIF4E is elevated in carcinomas of the breast, head and neck (HNSCC) and prostate, but not in typical benign lesions. This is a strong indication that elevated eIF4E expression may mark a critical transition in cancer progression. Establishing a greater protein synthesis output may be a necessary step for cancer cells in order to sustain their rapid proliferation. However, analysis of cells transformed by eIF4E revealed that the synthesis of only a few proteins was greatly enhanced, while synthesis of most was minimally increased. One possible explanation is that eIF4E causes these effects by specifically increasing the translational efficiency of several oncogene transcripts, leading to overexpression of their products. The feasibility of this hypothesis was confirmed experimentally with the identification of several important products that are specifically upregulated in eIF4E-overexpressing cells. These include: c-Myc, cyclin DI and ODC, which control cycle progression and tumorigenesis; basic fibroblast growth factor (FGF-2) and vascular endothelial growth factor (VEGF), which are powerful promoters of cell growth and angiogenesis. A deeper understanding of the mRNAs that are strongly dependent on excess eIF4E/F for efficient translation will eventually result in fuller understanding of the fundamental role of translational control in different pathophysiological conditions, including malignancy.

Expression of the translation initiation factor eIF4E in the polyp-cancer sequence in the colon.

The eukaryotic translation initiation factor 4E (eIF4E) has been shown to play a key role in cell growth, and several studies have documented an increased expression of eIF4E in a number of solid tumors, including breast, bladder, cervical, and head and neck cancers. This study was done to evaluate the potential role of eIF4E in the polyp-cancer sequence in the colorectum. Eighty-seven cases with lesions in the colorectum with a variety of histopathological diagnoses were randomly selected from the archives of the Pathology Department at Louisiana State University Health Sciences Center-Shreveport. Appropriate sections were selected for immunostaining with eIF4E. The medical records of the patients were reviewed, and demographic information was collected. All statistical analyses were performed using SAS software. A statistically significant relationship was found between the level of eIF4E expression and histological type of lesion: the lowest level of eIF4E expression was found in normal colon tissue, whereas the highest level of eIF4E expression was found in colorectal adenocarcinomas. Carcinomatous lesions were found to have a 43 times higher chance of having a high level of eIF4E expression compared with normal tissue (95% confidence interval, 8.0-213.6, P < 0.0001). In a multivariate analysis, histological type was the only variable that showed a significant relationship with eIF4E expression; no effect was found due to age, gender, race, history of polyps, and family history. The results from this study are consistent with other data from the literature and support the suggestion that eIF4E is strongly involved in colon tumorigenesis. eIF4E might be a useful intermediate biomarker for use in chemoprevention intervention studies in patients with colorectal polyps.

Antisense RNA to eIF4E suppresses oncogenic properties of a head and neck squamous cell carcinoma cell line.

OBJECTIVE: The translation initiation factor eIF4E is elevated in all head and neck squamous cell cancers (HNSCCs) and appears to be essential in the progression of solid tumors. Overexpression of eIF4E results in preferential upregulation of two angiogenic factors, vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (FGF-2). We wanted to determine whether reducing eIF4E in a HNSCC cell line could suppress its oncogenic properties and in turn decrease expression of VEGF and FGF-2. METHODS: Levels of eIF4E protein expression were determined in a panel of HNSCC cell lines. An episomal vector containing antisense RNA to eIF4E was used to reduce the eIF4E level in one of these cell lines, FaDu. After a stable transfection, Western blot analysis was performed to determine the level of eIF4E and FGF-2 reduction, while an enzyme-linked immunosorbent assay (ELISA) was used to determine the level of VEGF reduction. In vitro and in vivo experiments were performed to determine whether there was a reversion in the tumorigenic properties of the FaDu cells. RESULTS: All six cell lines had elevated levels of eIF4E compared with Detroit 551, a normal cell line. Reducing eIF4E expression via antisense RNA suppressed both the tumorigenic and angiogenic properties of the FaDu cells, as demonstrated by loss of capacity to grow in soft agar, reduced expression of angiogenic factors, and loss of tumorigenicity in nude mice. CONCLUSIONS: Antisense RNA therapy to eIF4E can potentially be used as adjuvant therapy for head and neck cancers, particularly in cases in which elevated eIF4E is found in the surgical margins.

Expression of eIF4E during head and neck tumorigenesis: possible role in angiogenesis.

OBJECTIVE: The translation initiation factor eIF4E (4E) when overexpressed in mammalian cells results in their oncogenic transformation. 4E facilitates the synthesis of two powerful tumor angiogenic factors (VEGF and FGF-2) by selectively enhancing their translation. 4E is overexpressed not only in all head and neck squamous cell cancers but also in some dysplastic margins. Tumorigenesis in the head and neck is proposed to be a multistep process preceded by clinically evident precancerous lesions. Molecular events underlie the histological changes that herald transformation. We wanted to study the role of 4E in tumorigenesis and further elucidate its causal role in angiogenesis. METHODS: An immunohistochemical analysis with antibodies to 4E, VEGF, and basic (b)-FGF was performed on 115 specimens of the head and neck representing various stages of histological progression of malignancy. This was correlated with mean vessel density (MVD) using factor VIII. RESULTS: There were 41 cases of hyperplasia and low-grade dysplasia, 40 cases of high-grade dysplasia and 34 cases of cancer. There was a significant increase in the percent of cases expressing 4E from low-grade dysplasia through tumor. However, for VEGF and b-FGF the significant increase was only seen between the tumor group and dysplastic groups and no significant increase was noted between low-grade and high-grade dysplasia There was a significant increase in MVD from low- (10.7+/-1) to high-grade grade dysplasia (18.0+/-2.3). This increase was even more striking for the 4E positive cases. CONCLUSION: 4E elevation is correlated with progressive cell transformation in the head and neck. Its correlation with VEGF, b-FGF, and MVD potentiates its possible role in angiogenesis.

Detection of the proto-oncogene eIF4E in larynx and hypopharynx cancers.

BACKGROUND: The proto-oncogene eIF4E has been found to be elevated in head and neck squamous cell carcinomas. In an earlier prospective study overexpression of eIF4E, detected by Western blot analysis, in histologically normal surgical margins correlated with an increased local-regional recurrence rate during a 1-year follow-up. OBJECTIVE: To test the reverse hypothesis that absence of overexpression of eIF4E in the surgical margins is a predictor for long-term survival in patients with squamous cell carcinoma of the head and neck. DESIGN: A retrospective analysis was performed on 31 patients who underwent surgery for squamous cell carcinoma of the larynx or hypopharynx. Immunohistochemical analysis was used to detect eIF4E on paraffin embedded sections of the tumor and the histologically negative surgical margins. RESULTS: All 31 patients overexpressed eIF4E in the tumors. Thirteen patients had no detectable level of eIF4E in the margins, and only 1 had a local-regional recurrence. The average disease-free interval in this group of patients was 82.08 months. The remaining 18 patients all overexpressed eIF4E in the surgical margins (eIF4E score range, 5-80). Twelve (67%) of these patients developed a recurrence; the average disease-free interval was 31.95 months. Cox regression analysis showed that eIF4E in the margin (P= .01), nodes (P= .06), site (P= .02), and age (P= .02) had significant effects on the disease-free interval. The Kaplan-Meier survival curves were significantly different for eIF4E-positive and eIF4E-negative margins (P = .002). CONCLUSIONS: eIF4E in the surgical margins is an independent prognostic factor and its absence in surgical margins may predict long-term survival. Detecting eIF4E in the margins may improve survival by determining which patients would benefit from further resection or adjuvant therapy.

Atmospheric deposition of iron from mining activities in northern Ontario.

Samples of Sphagnum moss were collected at varying distances from two iron-mining and processing centres in northern Ontario, Canada--Atikokan and Sudbury--and analyzed for Fe. All samples were collected in ombrotrophic bogs which receive chemical inputs strictly from atmospheric deposition. Mosses from the Atikokan sites exhibited elevated concentrations of Fe up to 7352 mg kg-1 near the local sources, and declined exponentially with distance to lows of 1443 mg kg-1. Atikokan, a mining and ore-processing centre, caused elevated Fe levels up to 50 km distance from the town, while Sudbury exhibited a greater influence due to the tall stack there. At Sudbury, levels ranged from 2478 mg kg-1 near the town down to 382 mg kg-1 in remote sites. Results from these moss analyses were similar to those obtained from precipitation sampling of previous authors.

Overexpression of eukaryotic protein synthesis initiation factor 4E in HeLa cells results in aberrant growth and morphology.

Eukaryotic protein synthesis initiation factor 4E (eIF-4E) is a 25-kDa polypeptide that binds to the 7-methylguanosine-containing cap of mRNA and participates in the transfer of mRNA to the 40S ribosomal subunit, a step that is rate-limiting for protein synthesis under most cellular conditions. eIF-4E is the least abundant of the initiation factors, is present at approximately 10% of molar concentration of mRNA, and thus may serve as a site of regulation for the recruitment of mRNA into polysomes. Previous studies have indicated that phosphorylation of eIF-4E at Ser-53 is correlated with an increased rate of protein synthesis in a variety of systems in vivo and is required for eIF-4E to become bound to the 48S initiation complex. In this study we show that overexpression of eIF-4E in HeLa cells using an episomally replicating, BK virus-based vector leads to an unusual phenotype: cells grow rapidly, forming densely packed, multilayered foci. They progressively form syncytia, some containing as many as six nuclei, and ultimately lyse 1 month after transfection. Some of these properties are reminiscent of oncogenically transformed cells. Cells transfected with the identical vector expressing a variant of eIF-4E, which contains alanine at position 53 and thus cannot be phosphorylated at the major in vivo site, grow normally. Estimations using the Ala-53 variant or a bacterial chloramphenicol acetyltransferase reporter gene in the same vector indicate that the degree of eIF-4E overexpression is 3- to 9-fold more than the endogenous level. These results suggest that eIF-4E may play a key role in cell cycle progression.

Inhibition of mRNA binding to ribosomes by localized activation of dsRNA-dependent protein kinase.

The initiation of protein synthesis can be regulated in mammalian cells by protein kinases which phosphorylate the alpha subunit of initiation factor eIF-2. This phosphorylation results in a block in the recycling of eIF-2 and in the inhibition of messenger RNA binding to 80S initiation complexes. After eIF-2 alpha is phosphorylated, the mRNA becomes associated with 48S complexes consisting of a 40S ribosomal subunit, eIF-2 (alpha P), GDP and Met-tRNAf. One of the eIF-2 alpha kinases is activated by low concentrations of double-stranded RNA (dsRNA). This kinase (PKds) is present at a basal level in all mammalian cells investigated and its synthesis is induced in cells treated with interferon. The PKds may be involved in the inhibition of translation of viral mRNA in interferon-treated cells infected with RNA viruses, as it is activated by viral replicative complexes. It is not known, however, if the activated PKds preferentially inhibits the translation of viral mRNA when cellular protein synthesis proceeds at a normal rate in infected cells. We now report that mRNA covalently linked to dsRNA is preferentially inhibited from binding to 80S complexes by a localized activation of PKds. This suggests that in interferon-treated cells the binding of some nascent viral mRNAs to functional initiation complexes may be preferentially inhibited by a similar mechanism.

Kinetics of dephosphorylation of eIF-2(alpha P) and reutilization of mRNA.

Phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2) causes mRNA to accumulate in 48 S complexes containing Met-tRNAf and eIF-2(alpha P). When the eIF-2 alpha kinase is inhibited by 2-aminopurine, the mRNA is slowly transferred from 48 to 80 S initiation complexes after an initial lag. The cause of this lag was examined by investigating whether mRNA and Met-tRNAf dissociated from 48 S complexes before binding to 80 S. Both compounds were quantitatively transferred from 48 to 80 S complexes after addition of 2-aminopurine and the eIF-2(alpha P) bound to 48 S complexes was dephosphorylated after an initial lag more slowly than unbound eIF-2(alpha P), which was rapidly dephosphorylated. the eIF-2(alpha P) in isolated 48 S complexes was slowly dephosphorylated by partially purified lysate phosphatases, whereas free eIF-2(alpha P) was readily dephosphorylated. These results indicated that 48 S complexes could directly join to a 60 S ribosomal subunit after eIF-2(alpha P) dephosphorylation. The lag and slow kinetics of dephosphorylation of eIF-2(alpha P) bound to 48 S complexes accounted for the slow transfer of mRNA from 48 to 80 S complexes. Moreover, the mRNA bound to 48 S complexes was more susceptible to cleavage by an endonuclease than mRNA in polyribosomes, as shown by activating the (2'-5')oligo(A)-dependent endonuclease. This finding is discussed in view of the possible role of eIF-2 alpha kinase and endonuclease in the inhibition of viral mRNA translation in interferon-treated cells.

Translational regulation of the synthesis of a major heat shock protein in HeLa cells.

The synthesis of a major heat shock protein (HSP 70) was measured in HeLa cells incubated at 42.5 degrees C and then transferred to 37 degrees C or 30 degrees C. After 90 min, synthesis of HSP 70 decreased by 54 and 85%, respectively, whereas HSP 70 mRNA was reduced at most by 20%. Therefore, the reduced synthesis of HSP 70 could not be accounted for by mRNA turnover. HSP 70 was associated with large polyribosomes (6-10 ribosomes) in cells kept at 42.5 degrees C, but with medium or small polyribosomes in cells transferred to 37 degrees C or 30 degrees C (5-6 or 2-3 ribosomes, respectively). Addition of puromycin to these cells resulted in the release of all ribosomes from HSP 70 mRNA, indicating that they were translationally active. The regulation of HSP 70 synthesis was investigated in cell-free systems prepared from heat-shocked or control cells and incubated at 30 degrees C and 42 degrees C. After 5 min at 42 degrees C, the cell-free system from heat-shocked cells synthesized protein at 3 times the rate of the control cell-free system. This difference was in large part due to synthesis of HSP 70. Addition of HSP mRNA to the control cell-free system stimulated protein synthesis at 42 degrees C, but not at 30 degrees C. These findings suggest that translation of HSP 70 mRNA is specifically promoted at high temperature and repressed during recovery from heat shock by regulatory mechanisms active at the level of initiation.

A novel BK virus-based episomal vector for expression of foreign genes in mammalian cells.

A composite mammalian cell-E. coli shuttle vector was developed based on the human papova virus BK and pSV-neo. The vector contains a dioxin-responsive enhancer (DRE) controlling a mouse mammary tumor virus (MMTV) promoter for the inducible expression of inserted genes. In human cells the vector replicates episomally, presumably utilizing the BKV rather than the SV40 origin, and expresses the BK T/t antigens. A deletion in the late BK region precludes the expression of the core/capsid proteins VP1, VP2, and VP3, thereby preventing the infectious lytic cycle. HeLa cells which were transfected with this vector and selected for resistance to the antibiotic G418 maintained the construct primarily in episomal form during more than one year of continuous culture, with little or no integration into the host genome. Transformed cells cultured in higher concentrations of G418 contained higher copy numbers of the vector. This permits one to vary the dosage of an inserted gene easily and reversibly without the need of conventional amplification techniques and clonal analysis. Using a chloramphenicol acetyl transferase (CAT) reporter gene inserted downstream of the MMTV promoter, we found that CAT expression was greater in clones with higher vector copy number. CAT expression was inducible with 2,3,7,8-tetrachlorodibenzo-p-dioxin, but inducibility was found to be inversely proportional to the copy number. Transformation of bacteria with plasmid molecules retrieved from the mammalian host was efficient, making this vector well adapted for the screening of cDNA libraries for the ability to express a phenotype in mammalian cells. Moreover, DNA sequences were stable during long-term passage in mammalian cells; vector passaged continuously for more than one year retained fully functional bacterial genes for resistance to chloramphenicol and ampicillin.