Activation of the Ras-ERK pathway inhibits retinoic acid-induced stimulation of tissue transglutaminase expression in NIH3T3 cells.
Retinoic acid (RA) is a potent activator of tissue transglutaminase (TGase) expression, and it was recently shown that phosphoinositide 3-kinase (PI3K) activity was required for RA to increase TGase protein levels. To better understand how RA-mediated TGase expression is regulated, we considered whether co-stimulation of NIH3T3 cells with RA and epidermal growth factor (EGF), a known activator of PI3K, would facilitate the induction or increase the levels of TGase expression. Instead of enhancing these parameters, EGF inhibited RA- induced TGase expression. Activation of the Ras-ERK pathway by EGF was sufficient to elicit this effect, since continuous Ras signaling mimicked the actions of EGF and inhibited RA- induced TGase expression, whereas blocking ERK activity in these same cells restored the ability of RA to up-regulate TGase expression. However, TGase activity is not antagonistic to EGF signaling. The mitogenic and anti-apoptotic effects of EGF were not compromised by TGase overexpression, and in fact, exogenous TGase expression promoted basal cell growth and resistance to serum deprivation-induced apoptosis. Moreover, analysis of TGase expression and GTP binding activity in a number of cell lines revealed high basal TGase GTP binding activity in tumor cell lines U87 and MDAMB231, indicating that constitutively active TGase may be a characteristic of certain cancer cells. These findings demonstrate that TGase may serve as a survival factor and RA- induced TGase expression requires the activation of PI3K but is antagonized by the Ras-ERK pathway.[1]References
- Activation of the Ras-ERK pathway inhibits retinoic acid-induced stimulation of tissue transglutaminase expression in NIH3T3 cells. Antonyak, M.A., McNeill, C.J., Wakshlag, J.J., Boehm, J.E., Cerione, R.A. J. Biol. Chem. (2003) [Pubmed]
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