Caracterización de mybbp como un nuevo gen supresor de tumores

Abstract

Tumor microenvironment is heterogeneous and dynamic due to constant tumor and stromal cells interactions and continuous changes in nutrient distribution that are essential for tumor development. Frequently, there is a local and/or temporal lack of efficient vasculature that leads to glucose, oxygen or other nutrients depletion, increasing the stress levels of tumor cells. Characterization of molecular mechanisms that enable tumor cells to adapt to host microenvironments would allow identifying new targets for cancer therapy. To this end, we have performed a genetic loss of function screen in absence of glucose, identifying MYB binding protein 1A (MYBBP1A). MYBBP1A is a regulator of transcription factors involved in the control of several relevant biological processes. In this work we have analyzed the expression of MYBBP1A in human tumors, confirming that MYBBP1A loss is a genetic event found in renal tumors. In a cohort of 97 patients of renal cancer the MYBBP1A expression is lost or reduced in 8% of tumors. This loss was associated with metastasis and poor prognosis, measured by disease free survival and overall survival. Based on this observation and published data that MYBBP1A is a target of pVHL, which is frequently lost in renal tumors, we have used renal carcinoma cell lines to study the possible role of MYBBP1A as a tumor suppressor. In order to study the molecular mechanism through MYBBP1A would take part in tumorigenesis, we analyzed the effect of MYBBP1A overexpression and downregulation in renal carcinoma cell lines. MYBBP1A overexpression suppresses cell growth in all cell lines used regardless of the molecular context. On the other hand, downregulation of MYBBP1A increases some tumor properties in cell lines that express c-MYB and do not express pVHL. In these cell lines downregulation of MYBBP1A induces c-MYB activation, inducing transcriptional activation of its target genes CD34 and CXCR4, which lead to an increase in cancer stem cell-like phenotype. In addition, we have shown that c-MYB induce the transcription of PGC-1α. Hence, MYBBP1A dowrnregulaion activates PGC-1α direct and indirectly. In a direct manner, reduction of the MYBBP1A expression des-represses PGC-1α activity and indirectly increases PGC-1α mRNA levels through c-MYB activation. Finally, PGC-1α activation leads to a metabolic shift from glycolysis to OXPHOS, which is more efficient to produce ATP under glucose limitations. Therefore, combined effects of MYBBP1A downregulation provide selective advantage over other tumor cells. Furthermore, we analyzed MYBBP1A expression and its correlation to the expression of genes of signaling pathways involved in tumor initiation and development. We observed that MYBBP1A correlated negatively with the expression of genes involved in stem cell phenotype, detecting a subgroup of clear cell renal cell carcinomas with a characteristic expression pattern. We have also found some genes regulated transcriptionally by c-MYB whose expression correlated negatively with MYBBP1A and positively with PGC-1α expression. In addition, we identified a subgroup of tumors, 8% of all tumor samples, with a characteristic expression pattern: low expression of MYBBP1A and high expression of PGC-1α and c-MYB target genes. Finally, we observed that MYBBP1A expression correlated negatively with the expression of tricarboxylic acid cycle genes. We also detected that 9% of tumor samples showed low expression of MYBBP1A and high expression of tricarboxylic acid cycle genes. This bioinformatic analysis supports our experimental conclusions.