Abstract
Control of cell proliferation and differentiation by the retinoblastoma protein (pRb) depends on its interactions with key cellular substrates. Available data indicate that pRb and the transcription factor Pax 8 play a crucial role in the differentiation of thyroid follicular cells. In this study, we show that pRb takes part in the complex assembled on the thyroperoxidase gene promoter acting as a transcriptional coactivator of Pax 8. Accordingly, pRb interacts with and potentiates Pax 8 transcriptional activity. In addition, we show that the downregulation of pRb gene expression, in thyrocytes, through RNA interference results in a reduction of the thyroperoxidase gene promoter activity mediated by the Pax 8-binding site. In agreement with these results and with the ability of the adenoviral protein E1A to bind pRb, we show that E1A downregulates Pax 8 activity and that such inhibition requires the E1A-Rb interaction. Furthermore, we show that the Pax 8/pRb synergy plays a role on the sodium/iodide symporter gene expression as well.
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