CELL CYCLE-DEPENDENT MODIFICATIONS IN ACTIVITIES OF PRB-RELATED TUMORSUPPRESSORS AND PROLIFERATION-SPECIFIC CDP CUT HOMEODOMAIN FACTORS INMURINE HEMATOPOIETIC PROGENITOR CELLS/

The histone H4 gene promoter provides a paradigm for defining transcri ptional control operative at the G(1)/S phase transition point in the cell cycle. Transcription of the cell cycle-dependent histone H4 gene is upregulated at the onset of S phase, and the cell cycle control ele ment that mediates this activation has been functionally mapped to a p roximal promoter domain designated Site II. Activity of Site II is reg ulated by an E2F-independent mechanism involving binding of the oncopr otein IRF2 and the multisubunit protein HINF-D, which contains the hom eodomain CDP/cut CDC2, cyclin A, and the tumor suppressor pRb. To addr ess mechanisms that define interactions of Site II regulatory factors with this cell cycle control element, we have investigated these deter minants of transcriptional regulation at the G(1)/S phase transition i n FDC-P1 hematopoietic progenitor cells. The representation and activi ties of histone gene regulatory factors were examined as a function of FDC-P1 growth stimulation. We find striking differences in expression of the pRb-related growth regulatory proteins (pRb/p105, pRb2/p130, a nd p107) following the onset of proliferation. pRb2/p130 is present at elevated levels in quiescent cells and declines following growth stim ulation. By contrast, pRb and p107 are minimally represented in quiesc ent FDC-P1 cells but are upregulated at the G(1)/S phase transition po int. We also observe a dramatic upregulation of the cellular levels of pRb2/p130-associated protein kinase activity when S phase is initiale d. Selective interactions of pRb and p107 with CDP/cut are observed du ring the FDC-P1 cell cycle and suggest functional linkage to competenc y for DNA binding and/or transcriptional activity. These results are p articularly significant in the context of hematopoietic differentiatio n where stringent control of the cell cycle program is requisite for e xpanding the stem cell population during development and tissue renewa l. (C) 1997 Wiley-Liss, Inc.