Cells are maintained in a quiescent state by members of the retinoblastoma protein family, pRb and p130. Both are phosphoproteins and hypophosphorylated forms of pRb and p130 bind and repress the activity of E2F transcription factors, thereby preventing entry into the cell cycle. Mitogenic stimulation causes activation of cyclin dependent kinases (cdk) that phosphorylate both pRb and p130, thereby releasing E2F factors which stimulate the transcription of a number of genes that are required for DNA synthesis and for regulating the cell cycle. In non-dividing cells, cdks are maintained in an inactive state by cdk inhibitor proteins such as p27(Kip1). The aim of our study was to determine how E2F complexes are regulated during the differentiation of human primary granulosa lutein cells (GLC) of the corpus luteum (CL). The CL is formed in the ovary after ovulation at the terminal stage of folliculogenesis after completion of maturation and differentiation of Graafian follicles. As shown by flow cytometry GLC are not dividing, being predominantly in the G(0)/G(1) phase of the cell cycle and, consistent with this, they contain the cdk inhibitor protein, p27(Kip1), but not E2F-1 which is normally expressed only in proliferating cells. The GLC do express E2F-4, hypophosphorylated pRb, p130 forms 1 and 2 and, surprisingly, hypophosphorylated p107. p107 is normally present only in dividing cells where it regulates E2F activity during the cell cycle. These forms of pRb, p130 as well as p107, together with E2F-4 are all active in that they can bind an E2F DNA-binding site in a pull-down assay. Immunocytochemistry shows that these proteins are expressed in almost all GLC but have different sub-cellular distribution: p107 is concentrated in nucleoli, while p130 and E2F-4 show relatively even nuclear and cytoplasmic distributions. Both pRb and p130 have been implicated previously in repressing E2F activity in many different cell types during cell cycle arrest in G(0)/G(1). We conclude that p107 is active in human primary GLC but its nucleolar localisation would suggest that it represses ribosomal RNA synthesis rather than E2F activity.
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