抑癌基因MIIP加速EGFR蛋白质降解并抑制肺癌细胞生长的分子机制

Epidermal growth factor receptor (EGFR) and its downstream signaling are key regulators of tumor cell growth, and overexpression of EGFR is commonly observed in various types of cancers. Protein turnover is an important factor to determine the EGFR protein expression level. Migration and invasion inhibitory protein (MIIP) was found to inhibit cancer cell growth. However, the underlying mechanisms were not fully understood. In our previous study, we found that overexpression of MIIP reduced the intracellular steady level of wild-type EGFR protein in non-small cell lung cancer cells and inhibited cell growth. This finding was further validated by pulse-chase experiments showing that MIIP accelerated both immature and mature wild-type EGFR protein degradation. It is hypothesized that MIIP might affect molecules which playing significant roles in wild-type EGFR protein maturation and degradation, so as to accelerate wild-type EGFR protein degradation, decrease EGFR and downstream signaling activation and inhibit cancer cell growth. In this study, we plan to determine whether the role of MIIP in cell growth inhibition is EGFR-expression dependent, amd explore the role of MIIP in regulating wild-type EGFR protein maturation, transport and degradation by employing techniques such as cell transfection, protein pulse-chase, co-immunoprecipitation and immunofluorescence. The study would elucidate the way how MIIP exhibits its tumor suppressor function, the molecules involved in wild-type EGFR protein degradation, and provide novel targets for non-small cell lung cancer's treatment in addition to mutant EGFR.

表皮生长因子受体EGFR介导的信号通路是促进肿瘤生长的重要因素，许多肿瘤存在EGFR过表达。细胞EGFR蛋白水平受其降解速度影响。转移和侵袭抑制蛋白MIIP可抑制肿瘤生长，但机制未完全明确。前期研究中，我们发现MIIP高表达可降低非小细胞肺癌中野生型EGFR蛋白质表达并抑制细胞生长；且MIIP促进新生未成熟及成熟野生型EGFR蛋白质降解。据此推测MIIP可能通过影响EGFR成熟及降解的关键调控分子，加速野生型EGFR蛋白质降解，减少生长信号通路激活。我们将通过细胞转染、蛋白质脉冲追踪实验、免疫共沉淀、免疫荧光等方法，观察MIIP依赖于野生型EGFR表达的对非小细胞肺癌的生长调控；探索MIIP影响野生型EGFR蛋白质成熟转运并加速其降解的机制。我们的研究将揭示MIIP发挥抑癌基因功能的机制，明确野生型EGFR蛋白质降解的调控分子，为非小细胞肺癌治疗提供靶向突变EGFR之外的新理论和新靶点。

既往有研究发现转移与侵袭抑制蛋白（MIIP）在细胞增殖和转移上发挥抑制作用。在非小细胞肺癌中，我们发现过表达MIIP可降低EGFR蛋白质表达水平，而并不影响EGFR mRNA表达水平。蛋白酶体或溶酶体抑制剂可分别抑制MIIP促进EGFR蛋白质降解的作用，提示蛋白酶体和溶酶体途径参与了EGFR蛋白质的降解。进一步地，通过35S-蛋氨酸标记的脉冲-追踪实验，我们发现MIIP可加速内质网内的新生EGFR蛋白质通过蛋白酶体途径降解；同时加速进入内吞转运的成熟EGFR蛋白质通过溶酶体途径降解。MIIP引起的EGFR表达下调可进一步抑制EGFR下游Ras/MEK/ERK信号通路的激活，从而最终抑制细胞增殖。MIIP与EGFR蛋白表达的负相关性在非小细胞肺癌组织中得以验证；并且，肿瘤组织中MIIP的高表达可提示IA-IIIA肺腺癌患者的较好预后。通过此研究，我们发现并阐明了MIIP抑制细胞增殖的一个新的机制。