Molecular mechanisms and signaling pathways that are activated with blood or plasma from young donors

Akt is an evolutionarily conserved serine/threonine kinase involved in a wide variety of cellular functions, including proliferation, cell survival, differentiation, glucose mobilization, homeostasis, cell migration, and apoptosis. Three isoforms, Akt1,Akt2, and Akt3, are expressed in mammals (Ref.1 , 2 and 3). All three isoforms of Akt share a common structure of three domains. The N-terminus of the protein is a PH (Pleckstrin Homology) domain, which interacts with membrane lipid products such as PIP2 (Phosphatidylinositol-3,4-Bisphosphate) and PIP3 (Phosphatidylinositol-3,4,5-Triphosphate) (Ref.4). Most common Ligands activating Akt include Growth factors, Cytokines, Mitogens and Hormones. Phosphoinositol 3-Kinase (PI3K) is recruited to the phosphotyrosine residues via SH2 domains in the regulatory domain (p85), and is therefore targeted to the inner cell membrane. Binding of the p85 subunit of PI3K to the phosphorylated RTK leads to conformational changes in the catalytic domain of PI3K (p110) and consequent kinase activation. Insulin can also activate PI3K via Insulin Receptor Substrate-1 (IRS1). G-Protein-Coupled Receptor (GPCR) also activates PI3K through Guanine Nucleotide-Binding Protein-Beta (GN-Beta) and Guanine Nucleotide-Binding Protein-Gamma (GN-Gamma) subunits of G-proteins. PI3K then phosphorylates membrane bound PIP2 to generate PIP3. The binding of PIP3 anchors Akt to the plasma membrane and allows its phosphorylation and activation by Phosphoinositide-Dependent Kinase-1 (PDK1). The activity of Akt is negatively regulated by Phosphatase and Tensin Homolog (PTEN), SH2-Containing Inositol Phosphatase (SHIP) and Carboxyl-Terminal Modulator Protein (CTMP) (Ref.5, 6 and 7).