Figure 1. mTOR is a central effector of growth factor and amino acid signals. Mitogenic and growth signals are represented by insulin or insulin-like growth factor (IGF). The cascade of events starts by activation of the receptor through ligand binding, which leads to phosphorylation of the insulin substrate (IRS). The latter, in turn, binds and activates the phosphatidylinositol 3-kinase (PI3-K). This kinase converts phosphatidylinositol (4,5)-bisphosphate (PtdIns[4,5]P2) into phosphatidylinositol (3,4,5)- trisphosphate (PtdIns[3,4,5]P3), which localizes protein kinase B (PKB, also known as Akt) to the membrane, where it can be phosphorylated and activated by 3- phosphoinositide-dependent kinase 1 (PDK1). Activated PKB phosphorylates tumor suppressor protein tuberous sclerosis 2 (TSC2), resulting in inhibition of the tumor suppressor function of the TSC1–TSC2 complex. Rheb, a small guanine nucleotide phosphatase (GTPase) that is inactivated by the GTPase-activating protein (GAP) activity of TSC2, positively modulates the function of mTOR. In turn, mTOR phosphorylates and inhibits 4E-BP1, thereby derepressing eIF4E and facilitating upregulation of protein synthesis, cell size and cell-cycle progression. In parallel, mTOR, together with PDK1, activates S6K1 and S6K2, which are involved in regulating protein synthesis, cell size, cell-cycle progression, glucose homeostasis and survival of newborns. Activation of mTOR by amino acids is carried out by an ill-defined mechanism. TOP mRNAs, which contain a 50-terminal oligopyrimidine tract and are translationally activated by growth factors and amino acids, direct enhanced synthesis of components of the translational apparatus (including ribosomal proteins and elongation factors among others). The role of mTOR in this mode of regulation remains questionable, however, because inhibition of mTOR by rapamycin leads to conflicting results regarding the translational repression of TOP mRNAs. Arrows indicate activation, bars indicate inhibition, and question marks indicate unknown or questionable pathways.

Regolazione della traduzione generale specifica

Regolazione trascrizionale Regolazione traduzionale

Indicazione di controllo traduzionale northern western -Fe +Fe -Fe +Fe mRNA ferritina ferritina actina mRNA actina

METODI PER STUDIARE IL CONTROLLO TRADUZIONALE Analisi della quantità di specifiche proteine: gel poliacrilammide 2D western blot Confronto tra la variazione della quantità di mRNA e quella del prodotto proteico Analisi della stabilità delle proteine mediante l'uso di inibitori della traduzione Analisi della quantità di mRNA sui polisomi (distribuzione)

Polysome analysis

TOP mRNA translational control in cultured cells