Increased Granulocyte-Macrophage Colony-Stimulating Factor mRNA Instability in Cord Versus Adult Mononuclear Cells Is Translation- Dependent and Associated With Increased Levels of A + U-Rich Element Binding Factor

YELOPOIESIS is developmentally immature in the M newborn compared with the adult.’ Whereas circulating levels and proliferative rates of myeloid progenitor cells are elevated in neonatal humans and rats, total body neutrophil storage pools are decreased by 75% in neonatal compared with adult rats.’ Neonatal rats cannot further increase their myeloid progenitor pool size or proliferative rate in response to experimental sepsis as compared with adult animals. This leads to a more rapid depletion of their already reduced neutrophil storage pools, a condition also associated with a fatal outcome in septic human newborns. Coupled with diminished neonatal phagocytic functional capabilities,”‘ these immaturities appear to predispose newborns to depletion of mature effector phagocytes during ovenvhelming bacterial sepsis. The regulatory cytokine granulocyte-macrophage colonystimulating factor (GM-CSF) modulates myeloid proliferation, differentiation, and a~tivation.~ Recombinant GM-CSF can prophylactically enhance the survival of neonatal rats during experimental sepsis.6 Expression of GM-CSF protein and mRNA is sevenfold and fourfold lower, respectively, in umbilical cord compared with adult activated mononuclear cells (MNC).’.* GM-CSF transcript half-life is threefold shorter in cord compared with adult activated MNC,’ whereas the rate of transcription remains comparable in activated cord and adult MNC.9 The cytoplasmic instability of GM-CSF “A, as well as c-myc and c-fos, is regulated in part, by adenosine + uridine-rich elements (AREs) in the 3’-untranslated regions (UTRs).” AREs are present in the 3’-UTRs of many cytokine and proto-oncogene transcripts” and are evolutionarily conserved in the 3’-UTRs of other invertebrate and vertebrate transcripts.” However, functional differences have been found, such as the specific stabilization of transcripts containing the GM-CSF” but not the c-myc or c-fos AREs in tumor cells’‘ and activated T cells.15 Aberrant GM-CSF mRNA stabilization can occur in human acute myeloblastic twice as abundant in cord than in adult MNC extracts. Mobility-shift competition assays and RNaseTl mapping localized the binding site of both complexes to a 52-nucleotide region containing seven of eight AUUUA motifs. Inclusion of AUFI antiserum produced a supershifted complex at 35-fold higher levels in cord than in adult MNC extracts. Extracts from the carcinoma cell line 5637, with extended GM-CSF mRNA half-life, also had very low levels of anti-AUF1 supershifted complex. Anti-AUFI immunoblotting showed significantly higher levels of two AUFI protein isoforms and lower levels of one in cord than in adult MNC or 5637 extracts. These results suggest that destabilization of GM-CSF mRNA in cord MNC is translation-dependent and that increased levels of specific AUFI isoforms in cord MNC may target transcripts for increased degradation, which could account in part for dysregulation of neonatal phagocytic immunity. 0 1996 by The American Society of Hematology. leukemia cellsI6; and ARE disruption, found in autocrine GM-CSF or interleukin-3 (IL-3)-transformed cell lines,17 can result in increased mRNA The human GM-CSF mRNA 3’-UTR (GM-3‘-UTR) has eight AUUUA motifs within a 70-base ARE.” Transcripts of several other myelopoietic cytokines contain an average of four AUUUA motifs within 50-nucleotide AREs. Macrophage colony-stimulating factor (M-CSF),~~ granulocyte colony-stimulating factor (G-CSF)?’ IL-3,22 macrophage inflammatory protein-la (MIP-la),23 and IL-823 transcripts are also less abundant in activated cord MNC compared with adult MNC. A reduced M-CSF mRNA half-life” and comparable transcription rates for G-CSF,9 IL-3; MIP-~cx,~~ and M-CSF” in cord versus adult MNC indicate that these ARE-containing transcripts