Endogenous reverse transcriptase: a mediator of cell proliferation and differentiation.

Endogenous, non-telomeric Reverse Transcriptase (RT) is encoded by two classes of repeated genomic elements, retrotransposons and endogenous retroviruses, and is an essential component of the retrotransposition machinery of both types of elements. Expression of RT-coding genes is generally repressed in non-pathological, terminally differentiated cells, but is active in early embryos, germ cells, embryo and tumor tissues, all of which have a high proliferative potential. To clarify whether reverse transcription is functionally implicated in control of cell growth, differentiation and in embryogenesis, recent experiments have been undertaken to inactivate the endogenous RT activity. RT was inhibited in normal and transformed cell lines by exposure to nevirapine, a non-nucleosidic RT inhibitor. The endogenous RT was also blocked in murine embryos by microinjection of an anti-RT antibody. Both experimental approaches yielded a dramatic inhibition of proliferation. Murine embryos arrested at pre-implantation stages. Transformed cell lines underwent a significant reduction in the rate of cell growth, concomitant with the induction of differentiation. In addition, RT inhibition induced an extensive reprogramming of the gene expression profile both in cultured cell lines and in preimplantation embryos. From these studies, endogenous RT begins to emerge as a key function with a driving role in normal and pathological developmental processes.