PMEPA1/TMEPAI Is a Unique Tumorigenic Activator of AKT Promoting Proteasomal Degradation of PHLPP1 in Triple-Negative Breast Cancer Cells

SIMPLE SUMMARY: TMEPAI/PMEPA1 is known to be highly expressed in various types of cancer, including triple-negative breast cancer (TNBC). Here, we found that TMEPAI-knockout (KO) in TNBC cells showed less tumorigenic abilities and upregulated PHLPP1, which dephosphorylates AKT at Ser473. Additionally, PHLPP1 depletion in TMEPAI-KO cells partially but significantly recovered AKT Ser473 phosphorylation, as well as in vitro and in vivo tumorigenic activities. Moreover, we demonstrated that TMEPAI PPxY (PY) motifs are essential for binding to NEDD4-2, an E3 ubiquitin ligase, and the complex formation of PHLPP1 with NEDD4-2, which leads to the polyubiquitination and proteasomal degradation of PHLPP1. These findings indicate that the PY motifs of TMEPAI induce the degradation of PHLPP1 and maintain AKT Ser473 phosphorylation at high levels to enhance the tumorigenic potentiality of TNBC. Therefore, the putative oncogenic role of TMEPAI may lead to developing TMEPAI based targeted therapy and/or diagnosis of TNBC in future. ABSTRACT: Transmembrane prostate androgen-induced protein (TMEPAI), also known as PMEPA1, is highly expressed in many types of cancer and promotes oncogenic abilities. However, the mechanisms whereby TMEPAI facilitates tumorigenesis are not fully understood. We previously established TMEPAI-knockout (KO) cells from human triple-negative breast cancer (TNBC) cell lines and found that TMEPAI-KO cells showed reduced tumorigenic abilities. Here, we report that TMEPAI-KO cells upregulated the expression of pleckstrin homology (PH) domain and leucine-rich repeat protein phosphatase 1 (PHLPP1) and suppressed AKT Ser473 phosphorylation, which was consistent with TCGA dataset analysis. Additionally, the knockdown (KD) of PHLPP1 in TMEPAI-KO cells partially but significantly rescued AKT Ser473 phosphorylation, as well as in vitro and in vivo tumorigenic activities, thus showing that TMEPAI functions as an oncogenic protein through the regulation of PHLPP1 subsequent to AKT activation. Furthermore, we demonstrated that TMEPAI PPxY (PY) motifs are essential for binding to NEDD4-2, an E3 ubiquitin ligase, and PHLPP1-downregulatory ability. Moreover, TMEPAI enhanced the complex formation of PHLPP1 with NEDD4-2 and PHLPP1 polyubiquitination, which leads to its proteasomal degradation. These findings indicate that the PY motifs of TMEPAI suppress the amount of PHLPP1 and maintain AKT Ser473 phosphorylation at high levels to enhance the tumorigenic potentiality of TNBC.