Binding of mouse VL30 retrotransposon RNA to PSF protein induces genes repressed by PSF: Effects on steroidogenesis and oncogenesis

Abstract

We describe a mechanism of gene regulation involving formation of a complex between PSF protein and mouse VL30 (mVL30) retrotransposon RNA. PSF represses transcription of the insulin-like growth factor 1 (IGF1)-inducible gene P450scc by binding to an insulin-like growth factor response element (IGFRE) motif in the gene. The complex with mVL30 RNA releases PSF, allowing transcription to proceed. Retrovirally mediated transmission of mVL30 RNA to human tumor cells induced several genes, including oncogenes, which also are induced by IGF1, and promoted metastasis. In mice, steroid synthesis is activated in steroidogenic cells by pituitary hormones, which concomitantly induce transcription of mVL30 RNA in the cells. We showed that steroid synthesis could also be activated in mouse steroidogenic adrenal cells by transfection with cDNA encoding either mVL30 RNA tracts that form a complex with PSF or a small interfering RNA (siRNA) that degrades PSF transcripts. These results suggest that mVL30 RNA regulates steroidogenesis, and possibly other physiological processes of mice, by complex formation with PSF. Retrotransposons such as mVL30 apparently evolved not only as ''junk'' DNA but also as transcriptionally active noncoding DNA that acquired physiological and pathological functions. The complete genome of a mouse VL30 (mVL30) retrotransposon is structurally similar to a retroviral genome, with 5Ј and 3Ј LTRs flanking an internal region containing Ϸ3.7 kb (1). In contrast to the internal region of an infectious retroviral genome that encodes gag, pol, and env proteins, the corresponding region of the mVL30 genome has numerous stop codons in all reading frames that probably block formation of a functional protein. The mouse genome contains multiple copies of transcriptionally active mVL30 DNA, and virtually all mouse cells contain mVL30 RNA although the level varies among different tissues and at different developmental stages (1). Because retroviral vectors for gene transfer and gene therapy usually are produced in packaging cells derived from mouse cells, these cells also contain mVL30 RNA. A remarkable property of retroviral vectors is the capacity to transmit mVL30 RNA from a packaging cell to cells infected by the retrovirus, which synthesize, integrate, and transcribe mVL30 cDNA (1). In an earlier report (2), we showed that retroviral-mediated transfection of tissue factor (TF) cDNA into a nonmetastatic human melanoma cell line increased the metastatic potential of the cells to which mVL30-1 RNA also had been transmitted. The increase in metastatic potential depended on expression of TF protein on the cell surface and also on transcription of mVL30-1 cDNA. Dependence on mVL30-1 RNA was surprising because the RNA seems to lack significant coding potential. Here, we report that mVL30-1 RNA forms a complex with PSF, a multifunctional regulatory protein that binds to RNA in spliceosomes (3, 4) and to an IGFRE motif in the insulin-like growth factor 1 (IGF1)-inducible gene P450ssc that represses transcription of the g...

Role of human noncoding RNAs in the control of tumorigenesis

“…Related studies described a mechanism for reversible regulation of proto-oncogene transcription, involving the protein hPSF that binds to the regulatory region of a proto-oncogene and represses transcription, and VL30-1 RNA, a mouse noncoding retroelement RNA, that binds to hPSF, forming a hPSF/RNA complex that dissociates from a proto-oncogene, activating transcription (2)(3)(4)(5). Because VL30-1 RNA is not encoded in the human genome (7), we screened by affinity chromatography a library of RNA fragments, derived from the RNA of a human tumor cell nucleus, for RNAs that bind to hPSF.…”

“…Because VL30-1 RNA is not encoded in the human genome (7), we screened by affinity chromatography a library of RNA fragments, derived from the RNA of a human tumor cell nucleus, for RNAs that bind to hPSF. The screen isolated 5 such human RNAs that release hPSF from a proto-oncogene and (3). Accordingly, the human hPSF-binding RNA fragments could contain short sequences that bind to hPSF.…”

“…The finding that VL30-1 RNA, a mouse retroelement RNA that is not encoded in the human genome, binds selectively to hPSF protein and reverses repression of proto-oncogene transcription (2)(3)(4)(5), prompted a search for human RNAs that have a similar function as VL30-1 RNA. The procedure involved synthesizing a library of RNA fragments from the nuclear RNA repertoire of a human melanoma line and selecting by affinity chromatography RNA fragments that bind to hPSF.…”

“…The mice were anesthetized and injected s.c. into bilateral sites on the neck with cells in 0.1 mL PBS using a 25-gauge needle. Tumor size was monitored every second or third day, and tumor volume was calculated by the formula 4/3(radius) 3 . The mice were killed at the end of an experiment, and the tumors were sectioned for histology.…”

“…T he protein PSF (1) contains a DNA-binding domain (DBD) that binds to the regulatory region of a proto-oncogene and represses transcription, and 2 RNA-binding domains (RBDs) that bind VL30-1 RNA, releasing PSF from a repressed protooncogene and activating transcription (2)(3)(4)(5). Mouse and human genomes encode homologous PSF proteins with Ϸ95% sequence identity, whereas the VL30-1 gene belongs to a family of mouse noncoding retroelement genes (6) that is not present in the human genome (7).…”