Amino Acids Interfere with the ERK1/2-dependent Control of Macroautophagy by Controlling the Activation of Raf-1 in Human Colon Cancer HT-29 Cells*

Activation of ERK1/2 stimulates macroautophagy in the human colon cancer cell line HT-29 by favoring the phosphorylation of the Gα-interacting protein (GAIP) in an amino acid-dependent manner (Ogier-Denis, E., Pattingre, S., El Benna, J., and Codogno, P. (2000) J. Biol. Chem. 275, 39090–39095). Here we show that ERK1/2 activation by aurintricarboxylic acid (ATA) treatment induces the phosphorylation of GAIP in an amino acid-dependent manner. Accordingly, ATA challenge increased the rate of macroautophagy, whereas epidermal growth factor did not significantly affect macroautophagy and GAIP phosphorylation status. In fact, ATA activated the ERK1/2 signaling pathway, whereas epidermal growth factor stimulated both the ERK1/2 pathway and the class I phosphoinositide 3-kinase pathway, known to decrease the rate of macroautophagy. Amino acids interfered with the ATA-induced macroautophagy by inhibiting the activation of the kinase Raf-1. The role of the Ras/Raf-1/ERK1/2 signaling pathway in the GAIP- and amino acid-dependent control of macroautophagy was confirmed in HT-29 cells expressing the Ras(G12V,T35S) mutant. Similar to the protein phosphatase 2A inhibitor okadaic acid, amino acids sustained the phosphorylation of Ser²⁵⁹, which is involved in the negative regulation of Raf-1. In conclusion, these results add a novel target to the amino acid signaling-dependent control of macroautophagy in intestinal cells.