Effects of age on the posttranscriptional regulation of CCAAT/enhancer binding protein alpha and CCAAT/enhancer binding protein beta isoform synthesis in control and LPS-treated livers.
The CCAAT/enhancer binding protein alpha (C/EBPalpha) and CCAAT/enhancer binding protein beta (C/EBPbeta) mRNAs are templates for the differential translation of several isoforms. Immunoblotting detects C/EBPalphas with molecular masses of 42, 38, 30, and 20 kDa and C/EBPbetas of 35, 20, and approximately 8.5 kDa. The DNA-binding activities and pool levels of p42(C/EBPalpha) and p30(C/EBPalpha) in control nuclear extracts decrease significantly whereas the binding activity and protein levels of the 20-kDa isoforms increase dramatically with LPS treatment. Our studies suggest that the LPS response involves alternative translational initiation at specific in-frame AUGs, producing specific C/EBPalpha and C/EBPbeta isoform patterns. We propose that alternative translational initiation occurs by a leaky ribosomal scanning mechanism. We find that nuclear extracts from normal aged mouse livers have decreased p42(C/EBPalpha) levels and binding activity, whereas those of p20(C/EBPalpha) and p20(C/EBPbeta) are increased. However, translation of 42-kDa C/EBPalpha is not down-regulated on polysomes, suggesting that aging may affect its nuclear translocation. Furthermore, recovery of the C/EBPalpha- and C/EBPbeta-binding activities and pool levels from an LPS challenge is delayed significantly in aged mouse livers. Thus, aged livers have altered steady-state levels of C/EBPalpha and C/EBPbeta isoforms. This result suggests that normal aging liver exhibits characteristics of chronic stress and a severe inability to recover from an inflammatory challenge.
Hsieh
C C
CC
Department of Human Biological Chemistry and Genetics, University of Texas Medical Branch, Galveston, Texas 77551, USA.
Xiong
W
W
Xie
Q
Q
Rabek
J P
JP
Scott
S G
SG
An
M R
MR
Reisner
P D
PD
Kuninger
D T
DT
Papaconstantinou
J
J
D000375:Aging / Q000378:metabolism*; D000818:Animals; D015139:Blotting, Southern; D015153:Blotting, Western; D022762:CCAAT-Enhancer-Binding Proteins; D019556:COS Cells; D002457:Cell Extracts; D002467:Cell Nucleus; D004268:DNA-Binding Proteins / Q000378:metabolism*; D005786:Gene Expression Regulation; D005822:Genetic Vectors; D008070:Lipopolysaccharides / Q000494:pharmacology*; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D008970:Molecular Weight; D009687:Nuclear Proteins / Q000378:metabolism*; D009838:Oligodeoxyribonucleotides / Q000378:metabolism; D009961:Orosomucoid / Q000235:genetics; D011132:Polyribosomes / Q000378:metabolism; D011401:Promoter Regions, Genetic; D012323:RNA Processing, Post-Transcriptional; D014162:Transfection
D022762:CCAAT-Enhancer-Binding Proteins; D002457:Cell Extracts; D004268:DNA-Binding Proteins; D008070:Lipopolysaccharides; D009687:Nuclear Proteins; D009838:Oligodeoxyribonucleotides; D009961:Orosomucoid