Accelerated Notch-Dependent Degradation of E47 Proteins in Aged B Cell Precursors Is Associated with Increased ERK MAPK Activation

Abstract

The transcriptional regulator E47, encoded by the E2A gene, is crucial to B lymphopoiesis. In BALB/c senescent mice (∼2 years old), the incidence of E47-expressing pro-B cells in vivo and E47 protein steady state levels in B cell precursors in vitro were reduced. Poor expression of E47 protein was a consequence of accelerated proteasome-mediated turnover and was associated with heightened ubiquitin modification of E2A-encoded proteins in aged B cell precursors. Both MAPK and Notch activity have been previously associated with E2A-encoded protein stability in lymphocytes. Aged B cell precursors exhibited heightened levels of MAPK activity reflected in increased levels of phospho-ERK proteins. Phosphorylation of E2A-encoded proteins was also increased in aged B cell precursors and pharmacologic inhibition of MEK-1 resulted in a partial restoration of their E47 protein. Both Notch proteins and their Delta-like ligands were detected comparably in young and aged B cell precursors. Either inhibition of Notch activation via gamma-secretase or Ab blockade of Notch-Delta-like ligand interactions partially restored E47 expression in aged B cell precursors. We hypothesize that increased MAPK activity promotes phosphorylation of E2A-encoded protein in aged B cell precursors. Subsequently, E2A-encoded proteins undergo ubiquitination and accelerated degradation in a Notch-dependent process. The dysregulation of E2A-encoded protein expression may contribute to the reductions seen in early B lymphopoiesis during murine senescence.

MyoD and E-protein heterodimers switch rhabdomyosarcoma cells from an arrested myoblast phase to a differentiated state

“…However, the E2A proteins have been shown to be posttranscriptionally regulated. For example, E47 degradation is regulated by Notch signaling and MAP kinase activity (King et al 2007). In lymphocytes, E2A proteins regulate proliferation and differentiation (Nutt and Kee 2007), and have been shown to regulate cell cycle factors, such as p57 and CDK6 (Rothschild et al 2006;Schwartz et al 2006).…”

Biochemical and Phosphoproteomic Analysis of the Helix-Loop-Helix Protein E47

“…In mature B cells and aged B cell precursors, Notch-induced phosphorylation of E47 by MAPKs leads to enhanced E47 ubiquitination and degradation (14,28). Additionally, two serine phosphorylation sites in E47 have been shown to be hypophosphorylated in B cell lines, and phosphorylation of these two sites disrupts E47 homodimer formation (43).…”

The MAPK/ERK and PI(3)K Pathways Additively Coordinate the Transcription of Recombination-Activating Genes in B Lineage Cells

“…Activation of the ERK/MAPK pathway in T and B lymphocytes has been shown to modulate the turnover of E2A proteins by phosphorylation of Thr 355 of the E47/E12 that is followed by ubiquitination and subsequent degradation (46)(47)(48). Hence, we next tested whether inhibition of MEK reduces the phosphorylation of E47.…”