Molecular determinants of breast cancer malignancy

描述（由申请人提供）：肿瘤细胞表达的癌蛋白骨桥蛋白（OPN）增强了其恶性特性，特别是通过影响其生长、侵袭和转移的能力。我们的研究表明，OPN下调肿瘤抑制蛋白Merlin的蛋白水平，可能是通过靶向它在乳腺癌细胞中的降解来实现的。Merlin是一种肿瘤抑制基因，已在各种类型的神经系统肿瘤中得到很好的表征。在神经系统肿瘤中，梅林介导接触抑制、侵袭、细胞生长并阻碍增殖。最近的证据还表明，在上述肿瘤类型中，Merlin蛋白在翻译后修饰（包括akt介导的磷酸化和泛素化）后迅速降解。梅林在乳腺癌中的研究还不够充分。有趣的是，我们还发现Merlin对OPN有负调控作用。基于这些发现，我们假设Merlin和OPN相互调节。这两种蛋白水平之间的平衡对于维持非恶性细胞行为至关重要。Merlin控制未转化细胞的OPN水平。在乳腺癌发病过程中，已知OPN水平升高（由于多种异常信号通路）。我们认为通过OPN信号传导后，活化的磷酸化akt磷酸化了Merlin。磷酸化的Merlin是蛋白酶体降解的靶标。因此，随着乳腺癌的进展，OPN的增加降低了梅林的稳定性，导致细胞中梅林的整体减少。这会破坏Merlin和OPN之间正常的生理平衡。这种恶性循环促进了乳腺癌的恶性发展。我们的总体目标是：(i)阐明Merlin在乳腺癌中的意义；（ii）了解Merlin和OPN在乳腺癌中的相互调节机制。为此，我们提出以下具体目标：

DESCRIPTION (provided by applicant): The oncoprotein, osteopontin (OPN) expressed by tumor cells potentiates their malignant properties, specifically by affecting their ability to grow, invade, and metastasize. Our research shows that OPN downregulates the protein levels of the tumor suppressor protein, Merlin, likely by targeting it for degradation in breast cancer cells. Merlin is a tumor suppressor gene that has been well characterized in various tumor types of the nervous system. In nervous system tumors, Merlin mediates contact inhibition, invasion, cell growth and also impedes proliferation. Recent evidence also implicates that Merlin protein gets degraded rapidly in the above tumor types following post-translation modifications, including Akt-mediated phosphorylation followed by ubiquitination. Merlin is understudied in breast cancer. Interestingly, we also discovered that Merlin exerts a negative regulatory effect on OPN. Based on these findings, we hypothesize that Merlin and OPN reciprocally regulate each other. A balance between the levels of the two proteins is critical to the maintenance of non-malignant cell behavior. Merlin keeps OPN levels under check in untransformed cells. During the pathogenesis of breast cancer OPN levels are known to increase (as a result of multiple aberrant signaling pathways). We propose that following signaling via OPN, activated phospho-Akt phosphorylates Merlin. Phosphorylated Merlin is targeted for proteasomal degradation. Thus, with the advancement of breast cancer, the increase in OPN decreases stability of Merlin, resulting in an overall decreased pool of Merlin in the cells. This disrupts the normal physiological balance between Merlin and OPN. This vicious cycle promotes malignant progression of breast cancer. Our overall objectives are (i) to elucidate the significance of Merlin in breast cancer and, (ii) to understand the mechanism of reciprocal regulation of Merlin and OPN in breast cancer. Towards this, we have proposed the following Specific Aims. Specific Aim 1: Test the hypothesis that by modulating Merlin levels we can regulate the malignant behavior of breast cancer cells. Specific Aim 2: Understand the mechanism of reciprocal regulation of Merlin and OPN in breast cancer. Specific Aim 3: Determine the correlation between expression of OPN and Merlin in patient-derived breast cancer specimens. Expected outcome & Impact: The proposed studies will lead to a better understanding of the role of Merlin in breast cancer progression & will reveal a yin-and-yang relationship between the OPN and Merlin. The ability to potentially use Akt/PI-3-kinase and proteasome inhibitors to stabilize Merlin protein is a treatment modality that can be explored in patients to reinstate the growth-inhibitory activity of Merlin.