Anti-STXBP4 (full length) polyclonal antibody

p63, a member of the p53 tumor suppressor family, is essential for the development of epidermis as well as other stratified epithelia. Collective evidence indicates that Delta Np63 proteins, the N-terminally deleted versions of p63, are essential for the proliferation and survival of stratified epithelial cells and squamous cell carcinoma cells. But in response to DNA damage, Delta Np63 proteins are quickly downregulated in part through protein degradation. To elucidate the mechanisms by which Delta Np63 proteins are maintained at relatively high levels in proliferating cells but destabilized in response to stress, we sought to identify p63 interactive proteins that regulate p63 stability. We found that Stxbp4 and RACK1, two scaffold proteins, play central roles in balancing Delta Np63 protein levels. While Stxbp4 functions to stabilize Delta Np63 proteins, RACK1 targets Delta Np63 for degradation. Under normal growth conditions, Stxbp4 is indispensable for maintaining high basal levels of Delta Np63 and preventing RACK1-mediated p63 degradation. Upon genotoxic stress, however, Stxbp4 itself is downregulated, correlating with Delta Np63 destabilization mediated in part by RACK1. Taken together, we have delineated key mechanisms that regulate Delta Np63 protein stability in vivo.

Purpose: Expression of the Delta N isoform of p63 (Delta Np63) is a diagnostic marker highly specific for lung squamous cell carcinoma (SCC). We previously found that Syntaxin Binding Protein 4 (STXBP4) regulates Delta Np63 ubiquitination, suggesting that STXBP4 may also be an SCC biomarker. To address this issue, we investigated the role of STXBP4 expression in SCC biology and the impact of STXBP4 expression on SCC prognosis. Experimental Design: We carried out a clinicopathologic analysis of STXBP4 expression in 87 lung SCC patients. Whole transcriptome analysis using RNA-seq was performed in STXBP4-positive and STXBP4-negative tumors of lung SCC. Soft-agar assay and xenograft assay were performed using over-expressing or knockdown SCC cells. Results: Significantly higher levels of STXBP4 expression were correlated with accumulations of Delta Np63 in clinical lung SCC specimens (Spearman rank correlation rho = 0.219). Notably, STXBP4-positive tumors correlated with three important clinical parameters: T factor (P < 0.001), disease stage (P = 0.030), and pleural involvement (P = 0.028). Whole transcriptome sequencing followed by pathway analysis indicated that STXBP4 is involved in functional gene networks that regulate cell growth, proliferation, cell death, and survival in cancer. Platelet-derived growth factor receptor alpha (PDGFR alpha) was a key downstream mediator of STXBP4 function. In line with this, shRNA mediated STXBP4 and PDGFRA knockdown suppressed tumor growth in soft-agar and xenograft assays. Conclusions: STXBP4 plays a crucial role in driving SCC growth and is an independent prognostic factor for predicting worse outcome in lung SCC. These data suggest that STXBP4 (C) 2017 AACR.