Anchorage-independent cell growth signature identifies tumors with metastatic potential

Myc-induced microRNAs integrate Myc-mediated cell proliferation and cell fate

The Myc pathway, often deregulated in cancer, is critical in determining cell fate by coordinating a gene expression program that links the control of cell proliferation with cell fate decisions. As such, precise control of the Myc pathway activity must be achieved to ensure faithful execution of appropriate cellular response and to prevent progressing toward a malignant state. With recent highlighted roles of microRNAs (miRNA) as critical components of gene control, we sought to evaluate the extent to which miRNAs may contribute in the execution of Myc function. Combined analysis of mRNA and miRNA expression profiles reveals an integration whereby the Myc-mediated induction of miRNAs leads to the repression of various mRNAs encoding tumor suppressors that block cell proliferation including p21, p27, and Rb. In addition, the proapoptotic PTEN tumor suppressor gene is also repressed by Myc-induced miRNAs, suggesting that Myc-induced miRNAs contribute to the precise control of a transcriptional program that coordinates the balance of cell proliferation and cell death.

Mitochondrial metabolism and ROS generation are essential for Kras-mediated tumorigenicity

Otto Warburg's theory on the origins of cancer postulates that tumor cells have defects in mitochondrial oxidative phosphorylation and therefore rely on high levels of aerobic glycolysis as the major source of ATP to fuel cellular proliferation (the Warburg effect). This is in contrast to normal cells, which primarily utilize oxidative phosphorylation for growth and survival. Here we report that the major function of glucose metabolism for Kras-induced anchorage-independent growth, a hallmark of transformed cells, is to support the pentose phosphate pathway. The major function of glycolytic ATP is to support growth under hypoxic conditions. Glutamine conversion into the tricarboxylic acid cycle intermediate alpha-ketoglutarate through glutaminase and alanine aminotransferase is essential for Kras-induced anchorage-independent growth. Mitochondrial metabolism allows for the generation of reactive oxygen species (ROS) which are required for Kras-induced anchorage-independent growth through regulation of the ERK MAPK signaling pathway. We show that the major source of ROS generation required for anchorage-independent growth is the Q(o) site of mitochondrial complex III. Furthermore, disruption of mitochondrial function by loss of the mitochondrial transcription factor A (TFAM) gene reduced tumorigenesis in an oncogenic Kras-driven mouse model of lung cancer. These results demonstrate that mitochondrial metabolism and mitochondrial ROS generation are essential for Kras-induced cell proliferation and tumorigenesis.

Autophagy and metastasis: another double-edged sword

In order to metastasize, tumor cells must adapt to untoward, stressful microenvironments as they disseminate into the systemic circulation and colonize distant organ sites. Autophagy, a tightly regulated lysosomal self-digestion process that is upregulated during cellular stress, has been demonstrated to suppress primary tumor formation, but how autophagy influences metastasis remains unknown. Autophagy may inhibit metastasis by promoting antitumor inflammatory responses or by restricting the expansion of dormant tumor cells into macrometastases. Conversely, self-eating may promote metastasis by enhancing tumor cell fitness in response to environmental stresses, such as anoikis, during metastatic progression. Because autophagy is titratable, it may serve both prometastatic and antimetastatic functions depending on the contextual demands placed on tumor cells throughout the metastatic process.