胞内甲胎蛋白通过调节蛋白应激压力应答影响肝癌发生发展的机制研究

Proteostasis, the maintenance of a healthy proteome, is a key requirement for cell metabolism, organelle biogenesis, stress adaptation, and consequently the long-term viability and well-beingof any cell type and organ. Protein stress, such as misfolded protein aggregates and oxidized proteins，frequently occurs in cancer cells. The resulting aberrant proteins can cause cell death if not properly removed. However, the mechanism that tumor cells response and adapt to protein stress remains unclear. Alpha-fetoprotein is well known as a tumor-specific biomarker, however, its intrinsic physiological role is still unclear, even in hepatocellular carcinogenesis. We found that a strong interaction between AFP and two of the three i-proteasome-specific β (iβ) subunits-β1i and β5i. The β1i maturation is significantly blocked by Knockdown of AFP. Surprisingly, the decreased AFP level also leads to lower level of exosome in the culture supernatant of human hepatocellular carcinoma cells HepG2. Given both the activation of immunoproteasome and the secretion of exosome are deeply involved in clearing aberrant proteins, it could be reasoned that cytoplasmic AFP is a critical regulator in the protein stress response.

恰当的处理错误折叠及氧化的蛋白质导致的蛋白应激压力（protein stress）维持蛋白质稳态（Proteostasis）是细胞存活的必要条件。肿瘤细胞因其生长方式、需持续高速合成蛋白，导致核糖体翻译保真性较低，产生大量错误折叠及氧化的蛋白导致持续性的蛋白应激压力，而肿瘤如何适应这种压力的机制尚未明确。甲胎蛋白(AFP)作为原发性肝癌标志物为人们所熟知，但它的内在生理功能尚有待深入研究。我们前期发现，AFP在胞内结合免疫蛋白酶体β1i和β5i亚基，干扰AFP表达显著抑制β1i活性形式的表达，同时也抑制exosome分泌。鉴于免疫蛋白酶体的活化和exosome分泌都是细胞在蛋白应激压力状态下的重要应对方式，我们拟在此基础上，深入研究AFP在肝癌细胞应对蛋白应激压力维持蛋白质稳态的过程中的作用及机制。研究结果有望为阐释肝癌的发生发展机制提供新观点，并为开发新型抗肿瘤药物提供理论依据。

甲胎蛋白（Alpha-fetoprotein,AFP）作为原发性肝癌的临床血清检测标志物为人们所熟知，研究发现AFP作为一个多功能蛋白，在血清中可以作为载体运输不饱和脂肪酸；并具有雌激素结合活性等生理功能；除了作为分泌蛋白在血清中发挥功能，AFP在胞内也可以结合caspase-3抑制肝癌细胞凋亡，并促进肿瘤细胞迁移和侵袭。我们研究发现AFP在胞内结合免疫蛋白酶体组分β1i，进一步的结果显示AFP招募CREBBP介导了β1i的乙酰化，进而促进β1i由前体切割成为有活性的成熟体，并招募免疫蛋白酶体其他β亚基，促进免疫蛋白酶体的从头合成；而免疫蛋白酶体的活化对于肝癌细胞应对快速增殖中蛋白应激压力发挥重要作用。敲减AFP或β1i显著抑制肝癌细胞的增殖，并增加细胞凋亡。我们也发现AFP通过招募CREBBP促进外泌体的释放，但这一途径和肝癌细胞体外生长无显著相关性。