Editors' ChoiceApoptosis

No Blood Cells Without Cyclin D

Sci. Signal.  19 Aug 2014:
Vol. 7, Issue 339, pp. ec219
DOI: 10.1126/scisignal.2005806

Members of the family of D cyclins (cyclin D1, D2, and D3) are primarily recognized for their importance in stimulating progression through the cell cycle by promoting the phosphorylation-dependent inactivation of the transcriptional repressor Rb. Genetic knockout of all three D cyclins is embryonically lethal in mice due to compromised proliferation of fetal liver hematopoietic cells. Choi et al. created adult mice in which the three forms of cyclin D were specifically and inducibly [with poly(I:C)] knocked out in bone marrow (TKO mice). As expected, proliferation of the hematopoietic stem cells, progenitor cells, and differentiated blood cells in the bone marrow was inhibited after induction. In adults, hematopoietic stem and progenitor cells (HSPCs) are generally quiescent; however, flow cytometric cell sorting (FACS) analysis showed that within 12 hours after induction, half of the stem cell population was lost and that by 48 hours this population was undetectable. By 96 hours, the mature bone marrow blood cell populations were reduced by half. FACS analysis of HPSCs stained for active caspase 3 showed that loss of cyclin D induced apoptosis, and analysis of mature blood cells in the bone marrow stained for apoptotic markers indicated an increase in apoptosis in these cells as well at later time points. Targeted transcriptional analysis for changes in the expression of genes involved in regulation of apoptosis showed that genes encoding the death receptor Fas and its ligand, FasL, were stimulated in the TKO bone marrow cells, and protein analysis confirmed an increase in the cell surface abundance of each of these in the hematopoietic cells of the TKO mice. Fas stimulates the extrinsic apoptotic pathway through activation of caspase 8, and increased caspase 8 cleavage was detected in the bone marrow cells of TKO mice. Administration of a nonspecific caspase inhibitor reduced the loss of bone marrow cells, and administration of a FasL-neutralizing antibody blocked apoptosis of the HSPCs in the TKO mice. A main target of cyclin D is Rb, phosphorylation of which releases its inhibition of E2F transcription factors. Bone marrow cells from the TKO mice had reduced phosphorylation of Rb and reduced expression of E2F target genes. Knockdown of E2F1 in hematopoietic cells resulted in an increase in the mRNAs for Fas and FasL, and reporter gene analysis showed that, of the three E2F family members, overexpression of repressed reporters was controlled by the Fas or FasL promoters. Chromatin immunoprecipitation analysis confirmed that E2F1 was bound to the Fas promoter in HPSCs. Thus, this study indicates that in the bone marrow, the cyclin D family is required not only for the proliferation of hematopoietic cells but also for their survival.

Y. J. Choi, B. Saez, L. Anders, P. Hydbring, J. Stefano, N. A. Bacon, C. Cook, I. Kalaszczynska, S. Signoretti, R. A. Young, D. T. Scadden, P. Sicinski, D-cyclins repress apoptosis in hematopoietic cells by controlling death receptor Fas and its ligand FasL. Dev. Cell 30, 255–267 (2014). [PubMed]