Using a new inducible form of phosphatidylinositol 3-kinase (PI 3-kinase) we have found that PI 3-kinase activation has the following effects on cell growth and proliferation. (i) Activation of PI 3-kinase was sufficient to promote entry into S phase of the cell cycle within several hours. This was shown by activation of cyclin-dependent kinase 4 (Cdk4) and Cdk2 and by the induction of DNA synthesis. (ii) PI 3-kinase activation alone was not, however, sufficient to provide for progression through the entire cell cycle. Instead, prolonged activation of PI 3-kinase in the absence of serum stimulation resulted in apoptosis. It is possible that the cells undergo apoptosis because the PI 3-kinase-induced entry into the cell cycle is abnormal. For example, we found that the cyclin E-Cdk2 complex, which normally disappears after entry into S phase of the cell cycle, fails to be downregulated following induction by PI 3-kinase. (iii) Finally, we found that prolonged activation of PI 3-kinase in the presence of serum resulted in cellular changes that resemble those associated with oncogenic transformation. The cells reached high densities, were irregular and refractile in appearance, and formed colonies in soft agar. In contrast, neither PI 3-kinase nor serum stimulation alone could induce these changes. Our results suggest that activation of PI 3-kinase promotes anchorage-independent cell growth and entry into the cell cycle but does not abrogate the growth factor requirement for cell proliferation.Phosphatidylinositol (PI) 3-kinase has been shown to mediate signaling induced by numerous growth factors and tumor antigens. The intracellular levels of the phospholipid products of PI 3-kinase increase in response to stimulation with growth factors or after oncogenic transformation (for reviews, see references 10,11,33,76,80). PI 3-kinase signaling appears to be required for a number of mitogens during the G 1 -to-Sphase transition of the cell cycle (63). Recently, it was demonstrated that PI 3-kinase regulates cell survival in response to various apoptotic stimuli (21, 49).PI 3-kinase is a heterodimeric complex consisting of an 85-kDa regulatory subunit, p85, and a 110-kDa catalytic subunit, p110 (11, 33). The p85 subunit contains two Src homology 2 (SH2) domains, which bind to tyrosine-phosphorylated receptors after stimulation of cells with growth factors and in this manner recruit the p85-p110 complex to the cell membrane. The region between the two SH2 domains of p85, the iSH2 region, mediates the association with p110, and this interaction is required for the enzymatic activity of p110 (37). Based on this observation we generated a chimeric molecule, p110*, in which the iSH2 region of p85 was covalently linked to its binding site at the p110 N terminus by using a flexible hinge region (30). p110* is a constitutively active PI 3-kinase which can activate signaling pathways independent of growth factor stimulation.The generation of constitutively active PI 3-kinase molecules has greatly facilitated the analysis of signa...