PMEPA1/TMEPAI isoforms function via its PY and Smad‐interaction motifs for tumorigenic activities of breast cancer cells

PMEPA1 (prostate transmembrane protein, androgen‐induced 1)/TMEPAI (transmembrane prostate androgen‐induced protein) is highly expressed in diverse cancers, including breast, lung and prostate cancers. It consists of four isoforms with distinct extracellular regions (isoforms a‐d). The expression and function of these isoforms are still poorly understood. Hence, we aimed to identify the preferentially expressed isoforms in breast cancer cells and analyze possible differences in tumorigenic functions. In this study, we used 5′ Rapid Amplification of cDNA Ends (RACE) and Western blot analyses to identify the mRNA variants and protein isoforms of TMEPAI and found that TMEPAI isoform d as the major isoform expressed by TGF‐β stimulation in breast cancer cells. We then generated CRISPR/Cas9‐mediated TMEPAI knockout (KO) breast cancer cell lines and used a lentiviral expression system to complement each isoform individually. Although there were no clear functional differences between isoforms, double PPxY (PY) motifs and a Smad‐interaction motif (SIM) of TMEPAI were both essential for colony and sphere formation. Collectively, our results provide a novel insight into TMEPAI isoforms in breast cancer cells and showed that coordination between double PY motifs and a SIM of TMEPAI are essential for colony and sphere formation but not for monolayer cell proliferation. TGF‐β induces TMEPAI isoform d expression. TMEPAI via its SIM inhibits TGF‐β/Smad signaling pathway‐mediated growth inhibitory functions, although TMEPAI via its PY motifs promotes PI3K/AKT signaling pathway‐induced colony and tumor sphere formation. Together, these conditions favor cancer cells growth and progression.