骨转移是前列腺癌（PCa）患者死亡的主要原因。越来越多的证据表明，基质相互作用分子1（STIM1）介导的储存器操作钙（SOCE）的高表达显著激活钙（Ca2+）信号通路，并参与PCa的骨转移的多个步骤。然而，STIM1的调节机制和靶向治疗尚未明确定义。本研究使用液相色谱质谱分析方法，识别出四跨膜蛋白18（TSPAN18）作为STIM1的结合蛋白。通过共免疫沉淀实验，探索TSPAN18抑制STIM1降解的机制。进一步在离体和体内模型中研究TSPAN18在PCa骨转移中的生物学功能。我们发现STIM1与TSPAN18直接相互作用，TSPAN18通过竞争性抑制E3连接酶三元基体结构域蛋白32（TRIM32）介导的STIM1泛素化和降解，提高了STIM1蛋白稳定性。此外，TSPAN18显著刺激Ca2+内流，进而促进了离体和体内PCa细胞的迁移和侵袭，形成了骨转移。临床上，TSPAN18的过表达与STIM1蛋白表达、骨转移和PCa患者的疾病预后呈正相关。综上所述，本研究发现了一个新的STIM1调控机制，即TSPAN18保护STIM1免受TRIM32介导的泛素化，通过激活STIM1-Ca2+信号轴促进PCa的骨转移。因此，TSPAN18可能是阻断PCa骨转移的有吸引力的治疗靶点。© 2023，意大利国家癌症研究所“Regina Elena”。

Bone metastasis is a principal cause of mortality in patients with prostate cancer (PCa). Increasing evidence indicates that high expression of stromal interaction molecule 1 (STIM1)-mediated store-operated calcium entry (SOCE) significantly activates the calcium (Ca2+) signaling pathway and is involved in multiple steps of bone metastasis in PCa. However, the regulatory mechanism and target therapy of STIM1 is poorly defined.Liquid chromatography-mass spectrometry analysis was performed to identify tetraspanin 18 (TSPAN18) as a binding protein of STIM1. Co-IP assay was carried out to explore the mechanism by which TSPAN18 inhibits STIM1 degradation. The biological function of TSPAN18 in bone metastasis of PCa was further investigated in vitro and in vivo models.We identified that STIM1 directly interacted with TSPAN18, and TSPAN18 competitively inhibited E3 ligase tripartite motif containing 32 (TRIM32)-mediated STIM1 ubiquitination and degradation, leading to increasing STIM1 protein stability. Furthermore, TSPAN18 significantly stimulated Ca2+ influx in an STIM1-dependent manner, and then markedly accelerated PCa cells migration and invasion in vitro and bone metastasis in vivo. Clinically, overexpression of TSPAN18 was positively associated with STIM1 protein expression, bone metastasis and poor prognosis in PCa.Taken together, this work discovers a novel STIM1 regulative mechanism that TSPAN18 protects STIM1 from TRIM32-mediated ubiquitination, and enhances bone metastasis of PCa by activating the STIM1-Ca2+ signaling axis, suggesting that TSPAN18 may be an attractive therapeutic target for blocking bone metastasis in PCa.© 2023. Italian National Cancer Institute ‘Regina Elena’.