Rb operates in the midst of the cell cycle clock apparatus. Its main role is to act as a signal transducer connecting the cell cycle clock with the transcriptional machinery (Ref.1). It plays an important role in the Rb/ E2F pathway in cell proliferation, cell fate determination, and cancer (Ref.2). Cellular senescence is a stable form of cell cycle arrest that limits proliferation of damaged cells and act as a natural barrier to cancer progression. A distinct heterochromatic structure that accumulates in senescent human fibroblasts, designated as SAHF (Senescence-Associated Heterochromatic Foci). SAHF formation coincides with recruitment of heterochromatin proteins and the Rb protein to E2F-responsive promoters and is associated with the stable repression of E2F target genes. Both SAHF formation and the silencing of E2F target genes depended on the integrity of the Rb pathway and do not occur in reversibly arrested cells (Ref.3). Rb activates transcription of the c-Jun gene through the SP1-binding site within the c-Jun promoter
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