Downstream Target Of Phosphoinositide 3-kinase Research Articles

Akt/protein kinase B promotes organ growth in transgenic mice.

One of the least-understood areas in biology is the determination of the size of animals and their organs. In Drosophila, components of the insulin receptor phosphoinositide 3-kinase (PI3K) pathway determine body, organ, and cell size. Several biochemical studies have suggested that Akt/protein kinase B is one of the important downstream targets of PI3K. To examine the role of Akt in the regulation of organ size in mammals, we have generated and characterized transgenic mice expressing constitutively active Akt (caAkt) or kinase-deficient Akt (kdAkt) specifically in the heart. The heart weight of caAkt transgenic mice was increased 2.0-fold compared with that of nontransgenic mice. The increase in heart size was associated with a comparable increase in myocyte cell size in caAkt mice. The kdAkt mutant protein attenuated the constitutively active PI3K-induced overgrowth of the heart, and the caAkt mutant protein circumvented cardiac growth retardation induced by a kinase-deficient PI3K mutant protein. Rapamycin attenuated caAkt-induced overgrowth of the heart, suggesting that the mammalian target of rapamycin (mTOR) or effectors of mTOR mediated caAkt-induced heart growth. In conclusion, Akt is sufficient to induce a marked increase in heart size and is likely to be one of the effectors of the PI3K pathway in mediating heart growth.

The akt kinase: molecular determinants of oncogenicity.

The serine-threonine kinase Akt is a downstream target of phosphoinositide 3-kinase (PI 3-kinase); it is activated by the phosphoinositide 3-phosphate-dependent kinases PDK1 and PDK2. Certain mutated forms of Akt induce oncogenic transformation in chicken embryo fibroblast cultures and hemangiosarcomas in young chickens. This ability to transform cells depends on localization of Akt at the plasma membrane and on the kinase activity of Akt. A transdominant negative form of Akt interferes with oncogenic transformation induced by the p3k oncogene, which codes for an activated form of PI 3-kinase. Akt is therefore an essential mediator of p3k-induced oncogenicity.

Lipid Products of Phosphoinositide 3-Kinase Interact with Rac1 GTPase and Stimulate GDP Dissociation

A number of reports suggest that under different conditions leading to cytoskeleton reorganization the GTPase Rac1 and possibly RhoA are downstream targets of phosphoinositide 3-kinase (PI 3-kinase). In order to gain more insight into this particular signaling pathway, we have addressed the question of a possible direct interaction of PI 3-kinase products with the Rho family GTPases RhoA, Rac1, and Cdc42. Using recombinant proteins, we found that Rac1 and, to a lesser extent, RhoA but not Cdc42 were capable to selectively bind to phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) in a mixture of crude brain phosphoinositides. Nucleotide-depleted Rac1 was the most efficient, but the GDP- and GTP-bound forms retained significant PtdIns(3,4,5)P3 binding activity. This protein-lipid association involved electrostatic as well as hydrophobic interactions, since both phosphate groups located at specific positions of the inositol ring and fatty-acyl chains were absolutely required. Based on the sequence of Rac1, two potential binding sites were identified, one at the C terminus and one in the extra alpha-helical domain. Deletion of these two domains resulted in a complete loss of binding to PI 3-kinase products. Finally, PtdIns(3, 4,5)P3 strongly stimulated GDP dissociation from Rac1 in a dose-dependent manner. In agreement, data obtained in intact cells suggest that PtdIns(3,4,5)P3 might target Rac1 to peculiar membrane domains, allowing formation of specific clusters containing not only small GTPases but other partners bearing pleckstrin homology domains such as specific exchange factors required for Rac1 and RhoA activation.