VGLL4 and glutamine deprivation suppress YAP1/TEAD signaling to attenuate the SCC cancer phenotype

Abstract

Epidermal squamous cell carcinoma (SCC) is an aggressive form of cancer. Previous studies have shown that a transcription factor, YAP1, drives the SCC cancer phenotype by binding to TEAD transcription factors to activate pro-cancer gene expression in the nucleus. However, Vestigial-like family member 4 (VGLL4) can bind TEAD factors to disrupt YAP1/TEAD complex formation, which reduces YAP1/TEAD-dependent transcription and target gene expression. This study shows that VGLL4 overexpression decreases TEAD-dependent transcription in SCC cells, which reduces transcript and protein expression of YAP1-target genes (CTGF, CYR61, Cyclin D1), and pro-cancer collagen genes (COL1A2, COL3A1), resulting in a reduction in cell proliferation, spheroid formation, invasion, and migration. Likewise, we show that VGLL4 overexpression decreases YAP1 activity in tumors, resulting in a decrease in YAP1 target and pro-cancer collagen gene expression and reduced EMT marker protein expression, which is associated with a reduction in tumor growth. These findings suggest VGLL4 acts as a tumor suppressor in SCC by inhibiting YAP1/TEAD signaling, resulting in an attenuation of the SCC phenotype. We also studied the interface between glutamine depletion and YAP1 signaling. Glutamine is an abundant amino acid, and its uptake into the cell and mitochondria is facilitated by solute carrier family 1 member 5 (SLC1A5). Cancer cells can become addicted to glutamine and utilize glutaminolysis for ATP production and biomolecule synthesis. Interestingly, YAP1 has been shown to regulate the expression of glutamine metabolism proteins, including GLS1 and SLC1A5. The present study seeks to understand the role of glutamine metabolism in SCC, determine a mechanism for the impact of glutamine metabolism on YAP1 signaling, and determine how glutamine metabolism can be targeted to treat cancer. SLC1A5 knockdown or treatment with V-9302, an SLC1A5 inhibitor, suppressed cell invasion, proliferation, and spheroid formation and was associated with a reduction in YAP1, YAP1-P, TAZ, TG2, and Cyclin D1 protein levels. Surprisingly, SLC1A5 regulated YAP1/TEAD-dependent transcription and YAP1-target gene expression. V-9302 treatment resulted in a decrease in tumor growth. These results suggest that SCC cancer cells are glutamine addicted, YAP1 signaling is influenced by glutamine deprivation, and glutamine restriction is a putative strategy for inhibiting the SCC cancer phenotype.