Feedback inhibition by RALT controls signal output by the ErbB network

Oncogene (2003) 22, 42214234

& 2003 Nature Publishing Group All rights reserved 0950-9232/03 $25.00www.nature.com/oncFeedback inhibition by RALT controls signal output by the ErbB networkSergio Anastasi1,6, Loredana Fiorentino1,6,7, Monia Fiorini1, Rocco Fraioli1, Gianluca Sala1,

L Castellani2,3, Stefano Alema4, Maurizio Alimandi5 and Oreste Segatton,11Regina Elena Cancer Institute, Via Delle Messi dOro, 156, Rome 00158, Italy; 2University of Cassino, Cassino, Italy; 3INFM Sez.

B, University of Rome Tor Vergata, Rome, Italy;4Istituto di Biologia Cellulare, CNR Monterotondo 00016, Italy; 5Department ofExperimental Medicine, University of Rome La Sapienza, Viale Regina Elena 324, Rome 00161, ItalyThe ErbB-2 interacting protein receptor-associated late

transducer (RALT) was previously identied as a feedback inhibitor of ErbB-2 mitogenic signals. We now

report that RALT binds to ligand-activated epidermal

growth factor receptor (EGFR), ErbB-4 and ErbB-

2.ErbB-3 dimers. When ectopically expressed in 32D

cells reconstituted with the above ErbB receptor tyrosine

kinases (RTKs) RALT behaved as a pan-ErbB inhibitor.

Importantly, when tested in either cell proliferation assays

or biochemical experiments measuring activation of ERK

and AKT, RALT affected the signalling activity of

distinct ErbB dimers with different relative potencies.

RALT DEBR, a mutant unable to bind to ErbB RTKs, did

not inhibit ErbB-dependent activation of ERK and AKT,

consistent with RALT exerting its suppressive activity

towards these pathways at a receptor-proximal level.

Remarkably, RALT DEBR retained the ability to

suppress largely the proliferative activity of ErbB-

2.ErbB-3 dimers over a wide range of ligand concentrations, indicating that RALT can intercept ErbB-2.ErbB-3

mitogenic signals also at a receptor-distal level. A

suppressive function of RALT DEBR towards the

mitogenic activity of EGFR and ErbB-4 was detected at

low levels of receptor occupancy, but was completely

overcome by saturating concentrations of ligand. We

propose that quantitative and qualitative aspects of RALT

signalling concur in dening identity, strength and

duration of signals generated by the ErbB network.

Oncogene (2003) 22, 42214234. doi:10.1038/sj.onc.1206516Keywords: ErbB; feedback inhibition; Mig-6; negative

signaling; RALTIntroductionThe family of ErbB receptor tyrosine kinases (RTKs)

consists of four members: ErbB-1 (also referred to as

epidermal growth factor receptor, EGFR) through

ErbB-4. In the human genome, a dozen 10 genes encode

ErbB ligands possessing distinct receptor specicity.

Regardless of the latter, all ErbB ligands trigger cognate

receptors via a common mechanism entailing the

assembly of homo- and heterodimeric receptor combinations (reviewed...