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Journal of Virology, July 2000, p. 5776-5787, Vol. 74, No. 13
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

The Long Terminal Repeat of Jaagsiekte Sheep Retrovirus Is Preferentially Active in Differentiated Epithelial Cells of the Lungs

Massimo Palmarini, Shoibal Datta, Reza Omid, Claudio Murgia, and Hung Fan*

Cancer Research Institute and Department of Molecular Biology and Biochemistry, University of California at Irvine, Irvine, California 92697

Received 10 February 2000/Accepted 10 March 2000

Jaagsiekte sheep retrovirus (JSRV) is the etiologic agent of a contagious bronchioloalveolar carcinoma of sheep known as sheep pulmonary adenomatosis (SPA; ovine pulmonary carcinoma). JSRV is unique among retroviruses because it transforms the alveolar type II cells and the nonciliated bronchiolar cells (Clara cells) of the lungs; these cells are where JSRV is specifically expressed in both naturally and experimentally SPA-affected sheep. In this study, we investigated the cell specificity of JSRV expression. By transient-transfection assays of 23 different cell lines with a reporter plasmid driven by the JSRV long terminal repeat (LTR), pJS21-luc, we found that the JSRV LTR is preferentially active in cell lines derived from type II pneumocytes and Clara cells (MLE-15 and mtCC1-2 mouse cell lines). Reporter assays using progressive 5' deletions of pJS21-luc allowed us to establish that the JSRV enhancers are able to activate the JSRV proximal promoter in MLE-15 and mtCC1-2 cells, but they have very low activity in mouse cells of other lineages (e.g., NIH 3T3). The JSRV enhancers are able to activate heterologous promoters in both MLE-15 and 3T3 cells, although optimal activity is achieved in MLE-15 cells only with the homologous JSRV promoter. Thus, JSRV cell-specific LTR activity appears to result from an interaction between the enhancer elements and the JSRV proximal promoter elements. By mutation analysis, we established that an upstream NF-kappa B-like element appears to be responsible for approximately 50% of the JSRV LTR transcriptional activity in MLE-15 cells. Electrophoretic mobility shift assays showed evidence of a factor(s) that binds to this sequence. Antibody supershift experiments indicated that the factor(s) is not related to NF-kappa B component p50 or p52. This factor also appeared to be present in cells that do not support a high level of JSRV expression. Finally the JSRV21 LTR contains putative enhancer binding motifs for transcription factors such as hepatocyte nuclear factor 3 (HNF-3) that are involved in lung-specific gene expression. Cotransfection experiments demonstrated that exogenous HNF-3 is able to enhance the expression of pJS21-luc in NIH 3T3 cells, which normally show minimal enhancer activity for the JSRV LTR.

* Corresponding author. Mailing address: Cancer Research Institute, Bio. Sci. II, University of California Irvine, Irvine, CA 92697. Phone: (949) 824-6631. Fax: (949) 824-4023. E-mail: hyfan{at}uci.edu.

Journal of Virology, July 2000, p. 5776-5787, Vol. 74, No. 13
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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