Differential effects of retroviral long terminal repeats on interleukin-3 gene expression and autocrine transformation

Abstract

Previously we documented the transposition of an intracisternal A particle (IAP) provirus to the interleukin 3 (IL-3) locus which resulted in autocrine transformation. In the present study, the effects of different long terminal repeats (LTRs) on IL-3 gene expression and autocrine transformation were investigated. LTRs from defective IAPs, and replication competent Moloney murine leukemia virus (MoMuLV), human T cell leukemia (HTLV), and immunodeficiency (HIV) viruses, were inserted 5′ of the IL-3 promoter region, and their transforming abilities determined. Addition of the lymphocyte specific (LS) IAP-LTR to the germline IL-3 (gIL3) gene, the IAP-LTR present in the previously described transposition, resulted in a modified IL-3 gene that only infrequently transformed IL-3-dependent cells. In contrast, addition of plasmacytoma (PC) IAP-LTRs to the gIL3 gene, which were isolated from IAPs expressed in plasmacytomas, resulted in modified IL-3 genes that transformed IL-3-dependent cells more readily. The MoMuLV-LTR and the TCRδ enhancer also stimulated high levels of IL-3 expression and autocrine transformation. In contrast, the HTLV-I, HTLV-II and HIV LTRs did not induce significant IL-3 synthesis or autocrine transformation. Consistent with these results, higher levels of CAT expression were observed in cells transiently transfected with PC-IAP-LTR or a TCR enhancer compared with LS-IAP and HTLV LTRs. In summary, the rank order for the effects of different LTRs on IL-3 expression and cell transformation is: TCRδ-enhancer ≈ MoMuLV-LTR > PC-IAP-LTRs ≫ LS-IAP-LTR ≫ HTLV-LTRs ≈ HIV-LTR. These results indicate that the LS-IAP-LTR is very weak at inducing IL-3 gene transcription and additional genetic mutations may be necessary for LS-IAPs to induce autocrine transformation of hematopoietic cells. In contrast, the enhancers contained in PC-IAP-LTRs and TCR enhancers may be more effective in inducing abnormal gene expression and malignant transformation.