Autophagy is a tightly regulated lysosomal self-digestion process that can both promote and impede tumorigenesis. Here, we utilize a three-dimensional (3D) culture model to address how interactions between autophagy and the phosphatidylinositol 3-kinase(PI3K)/Akt/mammalian target of rapamycin pathway impact the malignant behavior of cells carrying a tumor-derived, activating mutation in PI3K (PI3K-H1047R). In this model, autophagy simultaneously mediates tumor-suppressive and -promoting functions within individual glandular structures. In 3D culture, constitutive PI3K activation overcomes proliferation arrest and promotes resistance to anoikis in the luminal space, resulting in aberrant structures with filled lumen. Inhibiting autophagy in PI3K-H1047R structures triggers luminal cell apoptosis, resulting in lumen clearance. At the same time, autophagy gene depletion strongly enhances PI3K-H1047R cell proliferation during 3D morphogenesis, revealing an unexpected role for autophagy in restricting proliferation driven by PI3K activation. Intriguingly, overexpression of the autophagy cargo receptor p62/SQSTM1 in PI3K-H1047R cells is sufficient to enhance cell proliferation, activate the extracellular signal-related kinase/mitogen-activated protein kinase pathway and to promote epidermal growth factor-independent proliferation in 3D culture. Overall, these results indicate that autophagy antagonizes specific aspects of oncogenic PI3K transformation, with the loss of autophagy promoting proliferation. © 2013 Macmillan Publishers Limited.