Carcinogenesis or oncogenesis or tumorigenesis is literally the 'creation' of cancer. It is a process by which normal cells are transformed into cancer cells. It is characterized by a progression of changes at the cellular, genetic and epigenetic level that ultimately reprogram a cell to undergo uncontrolled cell division, thus forming a malignant mass.
Tumorigenesis is associated with elevated glucose and glutamine consumption, but how cancer cells can sense their levels to activate lipid synthesis is unknown. Cheng et al. revealed that ammonia, released from glutamine, promotes lipogenesis via activation of Sterol regulatory element-binding proteins (SREBPs), Endoplasmic Reticulum-Bound Transcription Factors that play a central role in lipid metabolism. Ammonia activates the dissociation of glucose-regulated, N-glycosylated SREBP-cleavage-activating protein (SCAP) from insulin-inducible gene protein (Insig), an endoplasmic reticulum-retention protein, leading to SREBP translocation and lipogenic gene expression. Notably, 25-hydroxycholesterol blocks ammonia to access its binding site on SCAP. Mutating aspartate D428 to alanine prevents ammonia binding to SCAP, abolishes SREBP-1 activation, and suppresses tumor growth. The study characterizes the unknown role, opposite to sterols, of ammonia as a key activator that stimulates SCAP-Insig dissociation and SREBP-1 activation to promote tumor growth and demonstrates that SCAP is a critical sensor of glutamine, glucose, and sterol levels to precisely control lipid synthesis 1).