Mobilization of resources
Many of the intracellular and extracellular changes that are found in cancer cells lead to changes in gene expression and protein metabolism. Activation of these pathways is necessary to mobilize the cellular resources that are necessary for the cancer-cell phenotype. In particular, some of these changes lead to expression of inappropriate gene programmes and activation of metabolic programmes that confer specific advantages to a continuously dividing cancer cell. Among the alterations that are included in this category are those that affect ribosome biosynthesis, expression of differentiation-associated antigens, enzymes involved in nutrient metabolism and enzymes that regulate oxidative potential.
For example, as well as blocking apoptotic pathways, the PI3K–AKT–PTEN pathway might be involved in regulating cell size by activating biosynthetic pathways18,19. Protein phosphatase 2A (PP2A) and the serine/threonine kinase TOR are also thought to be involved in this biosynthetic route; both of these can activate ribosomal S6 kinase (RSK), an important regulator of ribosome assembly. PP2A also activates eukaryotic initiation factor 4E (eIF4E), a protein that is involved in ribosome biogenesis. The identification of gene mutation or loss of each of these molecules in human cancers indicates that these biosynthetic pathways have important functions in cancer pathogenesis.