RING finger-dependent ubiquitination by PRAJA is dependent on TGF-β and potentially defines the functional status of the tumor suppressor ELF
ELF; Half-life; PRAJA; TGF-β; Ubiquitination
In gastrointestinal cells, biological signals for transforming growth factor-beta (TGF-β) are transduced through transmembrane serine/threonine kinase receptors that signal to Smad proteins. Smad4, a tumor suppressor, is often mutated in human gastrointestinal cancers. The mechanism of Smad4 inactivation, however, remains uncertain and could be through E3-mediated ubiquitination of Smad4/adaptor protein complexes. Disruption of ELF (embryonic liver fodrin), a Smad4 adaptor protein, modulates TGF-β signaling. We have found that PRAJA, a RING-H2 protein, interacts with ELF in a TGF-β-dependent manner, with a fivefold increase of PRAJA expression and a subsequent decrease in ELF and Smad4 expression, in gastrointestinal cancer cell lines (P < 0.05). Strikingly, PRAJA manifests substantial E3-dependent ubiquitination of ELF and Smad3, but not Smad4. Δ-PRAJA, which has a deleted RING finger domain at the C terminus, abolishes ubiquitination of ELF. A stable cell line that overexpresses PRAJA exhibits low levels of ELF in comparison to a Δ-PRAJA stable cell line, where ELF expression is high compared to normal controls. The alteration of ELF and/or Smad4 expression and/or function in the TGF-β signaling pathway may be induced by enhancement of ELF degradation, which is mediated by a high-level expression of PRAJA in gastrointestinal cancers. In hepatocytes, half-life (t1/2) and rate constant for degradation (kD) of ELF is 1.91 h and 21.72 min-1 when coupled with ectopic expression of PRAJA in cells stimulated by TGF-β, compared to PRAJA-transfected unstimulated cells (t1/2 = 4.33 h and kD = 9.6 min-1). These studies reveal a mechanism for tumorigenesis whereby defects in adaptor proteins for Smads, such as ELF, can undergo degradation by PRAJA, through the ubiquitin-mediated pathway. © 2006 Nature Publishing Group All rights reserved.
Saha, T., Vardhini, D., Tang, Y., Katuri, V., Jogunoori, W., Volpe, E., Haines, D., Sidawy, A., Zhou, X., Gallicano, I., Schlegel, R., Mishra, B., & Mishra, L. (2006). RING finger-dependent ubiquitination by PRAJA is dependent on TGF-β and potentially defines the functional status of the tumor suppressor ELF. Oncogene, 25 (5). http://dx.doi.org/10.1038/sj.onc.1209123