O papel de STMN1 e MELK em astrocitoma humano

Abstract

Glioblastoma (GBM) is the most common and aggressive malignant brain tumor in adults and standard treatment consists of surgical resection of the tumor, followed by radiation and chemotherapy with temozolomide. Our group performed the analysis of differently expressed genes between GBM and pilocytic astrocytoma. One of the genes with increased expression in GBMs was MELK (maternal embryonic leucine zipper kinase), which encodes a serine / threonine kinase protein that plays an important role in various cellular processes, such as proliferation, cell cycle, apoptosis, and oncogenesis. A siRNA silenced GBM cell analysis was performed to identify possible genes associated with the MELK pathway. One of the genes identified was STMN1, which encodes stathmin 1, an important cytosolic protein that plays a critical role in mitosis by regulating microtubule dynamics during cell cycle progression and migration. In addition, STMN1 is also involved in other biological processes, such as migrating and differentiating, through the phosphorylation of four serines (S16, S25, S38 and S63) triggers protein activation, weakening its binding to tubulin molecules. FOXM1, an important transcription factor, identified as regulator in the expression of several genes essential for cell cycle progression, including STMN1, was also a target of this study. In addition, FOXM1 is phosphorylated by MELK, regulating mitosis. MELK, STMN1 and FOXM1 are still related to tumor cell resistance to chemotherapy treatment. The aim of this study was to analysize expression levels of MELK, STMN1 and FOXM1 in our cohort astrocytomas of different degrees of malignancy. In addition, these data were validated in silico in larger cohorts of The Cancer Genome Atlas (TCGA). The expression of the three genes increased according to the malignancy of astrocytomas in our cases and in TCGA cases. MELK and STMN1, MELK and FOXM1 and STMN1 and FOXM1 expression levels were positively correlated in our GBM series, as well as in the TCGA series. In addition, the three genes showed higher expression in the proneural GBM subtype than in the Mesenquimal and Clássico subtypes in the case series of TCGA. U87MG and A172 cells silenced with MELK siRNA showed a similar decrease in MELK and STMN1 expression (~90%), but with a lower level of FOXM1 decrease (~50%). Phosphorylated serines of STMN1 were also analyzed after MELK silencing, and there was a decrease in phosphorylation compared to the control. On the other hand, when STMN1 expression was silenced with siRNA, MELK expression did not change and FOXM1 expression decreased by 40%. Silenced cells showed a reduction in cell viability and migration. Additionally, GBM cases were divided based on the expression of high and low STMN1. Of the cases with increased STMN1 expression, 79% showed increased FOXM1 expression, compared to 25% in the low STMN1 expression group. However, these groups showed no difference in survival of patients. The relationship between the three genes and drug resistance was analyzed in temozolamide and vinc...