The regulation of cell mass (cell growth) is often tightly coupled to the cell division cycle (cell proliferation). Ribosome biogenesis and the control of rDNA transcription through RNA polymerase I are known to be critical determinants of cell growth. Here we show that granulocytic cells deficient in the c‐MYC antagonist MAD1 display increased cell volume, rDNA transcription and protein synthesis. MAD1 repressed and c‐MYC activated rDNA transcription in nuclear run‐on assays. Repression of rDNA transcription by MAD1 was associated with its ability to interact directly with the promoter of upstream binding factor (UBF), an rDNA regulatory factor. Conversely, c‐MYC activated transcription from the UBF promoter. Using siRNA, UBF was shown to be required for c‐MYC‐induced rDNA transcription. These data demonstrate that MAD1 and c‐MYC reciprocally regulate rDNA transcription, providing a mechanism for coordination of ribosome biogenesis and cell growth under conditions of sustained growth inhibition such as granulocyte differentiation.

Retinoids are potent inducers of cell cycle arrest and differentiation of numerous cell types, notably granulocytes. However the mechanisms by which retinoids mediate cell cycle arrest during differentiation remain unclear. We have used myeloid differentiation to characterize the molecular pathways that couple cell cycle withdrawal to terminal differentiation. Using primary cells from mice deficient for either the cyclin-dependent kinase inhibitor (CDKi) p27 Kip1 , the Myc antagonist Mad1, or both Mad1 and p27 Kip1 , we observed that signals mediated through reti- IntroductionThe roles of retinoids during hematopoiesis, particularly granulopoiesis, have been recognized for many years. Most strikingly, retinoids are powerful inducers of differentiation. 1 This effect is central to the use of all-trans-retinoic acid (ATRA) in the treatment of acute promyelocytic leukemia (APL), a leukemia characterized by a chromosomal translocation always involving retinoic acid receptor ␣ (RAR␣). 2,3 Pharmacologic doses of ATRA are able to relieve the repression caused by the fusion proteins via directly binding and activating them and promoting cell cycle arrest and differentiation of the leukemic blasts. 3 However, the mechanisms by which ATRA mediates cell cycle arrest as well as differentiation remain unclear. An understanding of the molecular basis of this effect will provide important insights into how cell cycle withdrawal is coupled to terminal differentiation and how retinoids interact with the cell cycle.The effects of retinoid ligands are mediated through direct binding to the ligand-binding domain of RARs or retinoid X receptors (RXRs), members of the nuclear hormone receptor superfamily. 4 RARs heterodimerize with RXRs, and this heterodimer binds DNA at retinoic acid response elements (RAREs) in the regulatory sequences of target genes. When unliganded, RAR/RXR heterodimers bind RARE and form repressive complexes through binding SMRT-and NCo-R-containing corepressor complexes. 5 Ligand binding triggers the release of the corepressor complexes and coactivator complexes are recruited to the RAR/ RXR dimer and activate gene transcription. [6][7][8] Three isotypes of RARs and RXRs have each been identified: ␣, ␤, and ␥, with each RAR isotype expressed as a number of isoforms. 4 RAR␣ and RAR␥ are believed to be most important in the regulation of granulopoiesis. 4,9 This is most strikingly demonstrated in progenitor assays of bone marrow cells derived from RAR␣1 Ϫ/Ϫ RAR␥ Ϫ/Ϫ mice. These cells displayed a block in granulopoiesis at the myelocyte stage of development, revealing an absolute requirement for RAR-mediated signaling in the terminal differentiation of granulocytes. 9 Our understanding of the mechanisms through which RAR␣ mediates cell cycle arrest and differentiation of myeloid cells is a question of particular relevance. First, RAR␣, but not RAR␤ or RAR␥, is involved in the pathogenesis of myeloid leukemia. 10 RAR␣ is involved in leukemia both as a fusion partner, most commonly involving promyelocytic leukemia...