Decreased E47 in senescent B cell precursors is stage specific and regulated posttranslationally by protein turnover.
The E2A-encoded transcription factor E47 is crucial to B lymphopoiesis. Senescent BALB/c mice ( approximately 2 years old) had reduced pre-B cells ex vivo. Pro-B/early pre-B cells from these aged mice, both ex vivo and in vitro, were deficient in E47 protein. In vitro, IL-7 expanded pro-B/early pre-B cells from young BALB/c mice expressed E47 protein that was relatively stable over a 5-h period. Cultured senescent pro-B/early pre-B cells exhibited reduced E47 protein stability with approximately 50-90% loss of E47 over the same time period. Degradation of E47 was effectively blocked by the proteasome inhibitor lactacystin as well as calpain I and II inhibitors; E2A proteins were also shown to undergo ubiquitination. Although senescent B cell precursors expressed less E47 protein, E47 mRNA levels and turnover were normal. Therefore, E47 protein levels are reduced relatively early in B lineage differentiation in senescence and the decline in E47 protein occurs via increased protein degradation by proteasome and, possibly, calpain pathways. In contrast, normal E47 protein levels were observed within the highly reduced pre-B cell pool in aged mice. This suggests that pre-B cells in senescence undergo selection based on E47 expression. Increased degradation rates and lower steady-state levels were also observed for the transcription factors Pax-5/BSAP, Bob-1, and Ikaros, but this was not a general property of all proteins in aged B cell precursors. Therefore, altered turnover of multiple, select proteins crucial to B cell development may contribute to diminished B lymphopoiesis in old age.
Van der Put
Elaine
E
Department of Microbiology and Immunology, University of Miami School of Medicine, Miami, FL 33101, USA.
Frasca
Daniela
D
King
Anne M
AM
Blomberg
Bonnie B
BB
Riley
Richard L
RL
D000818:Animals; D001402:B-Lymphocytes / Q000502:physiology*; D051792:Basic Helix-Loop-Helix Proteins; D002154:Calpain / Q000378:metabolism; D016922:Cellular Senescence / Q000502:physiology*; D003546:Cysteine Endopeptidases / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D005786:Gene Expression Regulation; D006412:Hematopoietic Stem Cells / Q000502:physiology*; D051379:Mice; D009097:Multienzyme Complexes / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex; D011499:Protein Processing, Post-Translational / Q000502:physiology*; D012333:RNA, Messenger / Q000378:metabolism; D051820:TCF Transcription Factors; D058125:Transcription Factor 7-Like 1 Protein; D014157:Transcription Factors / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*
D051792:Basic Helix-Loop-Helix Proteins; D002154:Calpain; D003546:Cysteine Endopeptidases; D004268:DNA-Binding Proteins; D009097:Multienzyme Complexes; D046988:Proteasome Endopeptidase Complex; D012333:RNA, Messenger; D051820:TCF Transcription Factors; D058125:Transcription Factor 7-Like 1 Protein; D014157:Transcription Factors; C407909:TCF3 protein, human; C544912:TCF7L1 protein, human; C544913:Tcf7l1 protein, mouse