Phospho-β-catenin expression in primary and metastatic melanomas and in tumor-free visceral tissues, and associations with expression of PD-L1 and PD-L2

beta-catenin (beta cat) is an important downstream effector in the Wnt signaling pathway and plays important roles in the development and progression of many cancers including melanoma. beta cat expression is regulated by GSK-3 beta mediated phosphorylation at positions 33, 37 and 41. In normal cells, phosphorylation at these sites triggers proteasomal degradation, which prevents accumulation of free cytoplasmic beta cat. In cancer cells, stabilized beta-catenin translocates into the nucleus, where it associates with TCF/Lef proteins to activate transcription of genes that promote tumorigenesis and metastasis, including PD-L1. It has been suggested that nuclear phospho beta cat (p beta cat) staining may be diagnostically useful in differentiating primary from metastatic melanoma. Also, a p beta cat peptide (residues 30-39, with only S33 phosphorylated) is naturally presented by melanoma cells as a Tcell target. We evaluated expression of pS33-beta cat in primary and metastatic melanomas by immunohistochemistry and found its expression varied widely but was most commonly cytoplasmic. Nuclear staining was identified in only 18% of metastatic melanomas. Staining with antibodies to pS33-beta cat and pS33/37/T41-beta cat was most intense in mitotic melanoma cells; however, pS33-beta cat intensity was not significantly associated with AJCC stage, tumor location, BRAF mutation status, or immune infiltrates. Yet, PD-L1 and PD-L2 expression by tumor cells were significantly higher in tumors with high pS33-beta cat expression. The low rate of nuclear pS33-beta cat expression suggests that pS33-beta cat may have limited utility for identifying metastatic melanomas. However, high expression in dividing cells and strong associations with PD-L1 and PD-L2 expression may inform future personalized therapies for tumors with high pS33-beta cat expression.