Quantitative Phosphoproteomics Analysis of ERBB3/ERBB4 Signaling

Abstract

The four members of the epidermal growth factor receptor (EGFR/ERBB) family form homo- and heterodimers which mediate ligand-specific regulation of many key cellular processes in normal and cancer tissues. While signaling through the EGFR has been extensively studied on the molecular level, signal transduction through ERBB3/ERBB4 heterodimers is less well understood. Here, we generated isogenic mouse Ba/F3 cells that express full-length and functional membrane-integrated ERBB3 and ERBB4 or ERBB4 alone, to serve as a defined cellular model for biological and phosphoproteomics analysis of ERBB3/ERBB4 signaling. ERBB3 co-expression significantly enhanced Ba/F3 cell proliferation upon neuregulin-1 (NRG1) treatment. For comprehensive signaling studies we performed quantitative mass spectrometry (MS) experiments to compare the basal ERBB3/ERBB4 cell phosphoproteome to NRG1 treatment of ERBB3/ERBB4 and ERBB4 cells. We employed a workflow comprising differential isotope labeling with mTRAQ reagents followed by chromatographic peptide separation and final phosphopeptide enrichment prior to MS analysis. Overall, we identified 9686 phosphorylation sites which could be confidently localized to specific residues. Statistical analysis of three replicate experiments revealed 492 phosphorylation sites which were significantly changed in NRG1-treated ERBB3/ERBB4 cells. Bioinformatics data analysis recapitulated regulation of mitogen-activated protein kinase and Akt pathways, but also indicated signaling links to cytoskeletal functions and nuclear biology. Comparative assessment of NRG1-stimulated ERBB4 Ba/F3 cells revealed that ERBB3 did not trigger defined signaling pathways but more broadly enhanced phosphoproteome regulation in cells expressing both receptors. In conclusion, our data provide the first global picture of ERBB3/ERBB4 signaling and provide numerous potential starting points for further mechanistic studies.

Fig 1. NRG1-induced proliferation and signaling of Ba/F3 cells expressing ERBB3 and/or ERBB4.

(A) Cells expressing either ERBB4 (upper panel), ERBB4 and ERBB3 (middle panel) or ERBB3 receptors (lower panel) were cultured in medium without ligand, in the presence of 10 ng/ml IL3, or in the presence of 10 or 100 ng/ml NRG1. Cell numbers were counted at the indicated time points. (B) Parental Ba/F3 or Ba/F3 cells expressing ERBB3, ERBB4 or both receptors were incubated in the absence of IL3 and treated with 100 ng/mL NRG1 where indicated. Total cell lysate were then prepared and immunoblotted with phosphoepitope- and protein-specific antibodies for ERBB3, ERBB4, Akt and MEK.

Fig 2. Quantitative phosphoproteomics workflow and experimental design.

Ba/F3 ERBB3/ERBB4 or ERBB4 cells were treated in the three replicate experiments as indicated. Upon lysis and proteolytic digestion, peptides were differentially labeled with the three isotopic variant of mTRAQ and then pooled prior to peptide separation by high pH reversed phase chromatography and IMAC phosphopeptide enrichment. Phosphopeptide fractions were then analyzed by quantitative LC-MS on a LTQ Orbitrap Velos instrument. Lower panel: Characteristic mTRAQ patterns shown for a peptide harboring a NRG1-induced phosphosite in ERBB3/ERBB4 cells, which less strongly up-regulated in the absence of ERBB3 in ERBB4-expressing Ba/F3 cells.

Fig 3. Identification of significantly different phosphorylation sites.

(A) Volcano plot of NRG1-regulated phosphorylation in Ba/F3 cells expressing ERBB3 and ERBB4. (B) Volcano plot comparison of phosphorylation sites in NRG1-treated ERBB3/ERBB4 versus NRG1-treated ERBB4 expressing Ba/F3 cells. In both comparisons, log₁₀-transformed, average phosphosite ratios are plotted against their standard deviations determined from mTRAQ replicate quantifications. Significantly regulated class I sites according to the Global Mean Rank test are depicted in red, all other sites in blue. The dashed grey lines indicate two-fold regulation.

Fig 4. Contribution of ERBB3 to phosphosite regulation.

Scatter plot of the mean ERBB3/ERBB4 ± NRG1 ratios with mean ERBB3/ERBB4 versus ERBB4 ratios from NRG1-treated cells. Reproducibly quantified ERBB3 phosphosites are encircled.

Fig 5. Network of NRG1-regulated phosphoproteins in ERBB3 and ERBB4 expressing Ba/F3 cells.

The SubExtractor program [35] was used for network generation based on the quantitative phosphoproteomics data from this study and known functional and physical interaction provided by STRING. Proteins are colored according to the strength and direction of regulation of their most strongly regulated phosphosite. Blue, down-regulation; red, up-regulation; intensity, magnitude of regulation.