IQGAP1 and RNA Splicing in the Context of Head and Neck via Phosphoproteomics

IQGAP1 (IQ motif-containing GTPase-activating protein
1) scaffolds
several signaling pathways in mammalian cells that are implicated
in carcinogenesis, including the RAS and PI3K pathways that involve
multiple protein kinases. IQGAP1 has been shown to promote head and
neck squamous cell carcinoma (HNSCC); however, the underlying mechanism(s)
remains unclear. Here, we report a mass spectrometry-based analysis
identifying differences in phosphorylation of cellular proteins in vivo and in vitro in the presence or
absence of IQGAP1. By comparing the esophageal phosphoproteome profiles
between Iqgap1+/+ and Iqgap1–/– mice, we identified RNA splicing
as one of the most altered cellular processes. Serine/arginine-rich
splicing factor 6 (SRSF6) was the protein with the most downregulated
levels of phosphorylation in Iqgap1–/– tissue. We confirmed that the absence of IQGAP1 reduced
SRSF6 phosphorylation both in vivo and in
vitro. We then expanded our analysis to human normal oral
keratinocytes. Again, we found factors involved in RNA splicing to
be highly altered in the phosphoproteome profile upon genetic disruption
of IQGAP1. Both the Clinical Proteomic Tumor Analysis Consortium (CPTAC)
and the Cancer Genome Atlas (TCGA) data sets indicate that phosphorylation
of splicing-related proteins is important in HNSCC prognosis. The
Biological General Repository for Interaction Datasets (BioGRID) repository
also suggested multiple interactions between IQGAP1 and splicing-related
proteins. Based on these collective observations, we propose that
IQGAP1 regulates the phosphorylation of splicing proteins, which potentially
affects their splicing activities and, therefore, contributes to HNSCC.
Raw data are available from the MassIVE database with identifier MSV000087770.