Oncogenic activity of the regulatory subunit p85β of phosphatidylinositol 3-kinase (PI3K)

Abstract

Expression of the regulatory subunit p85β of PI3K induces oncogenic transformation of primary avian fibroblasts. The transformed cells proliferate at an increased rate compared with nontransformed controls and show elevated levels of PI3K signaling. The oncogenic activity of p85β requires an active PI3K-TOR signaling cascade and is mediated by the p110α and p110β isoforms of the PI3K catalytic subunit. The data suggest that p85β is a less effective inhibitor of the PI3K catalytic subunit than p85α and that this reduced level of p110 inhibition accounts for the oncogenic activity of p85β.

Keywords: SH2 domain; inter-SH2 domain; protein–protein interaction.

Fig. 1.

Oncogenic transformation induced in CEF monolayers by the RCAS-mediated expression of human p85β compared with the oncogenic K379E mutant of p85α. Dilutions of RCAS virus indicated on top.

Fig. 2.

Expression of human p85β in CEF enhances cell proliferation. The oncogenic K379E mutant of p85α is used as a positive control and cells expressing wild-type p85α serve as negative control. Equal numbers of cells were seeded and cell number measured after 2 d of growth. The results of 24 measurements are shown.

Fig. 3.

Activation of PI3K signaling in p85β-expressing cells. The Western blot compares phosphorylation of Akt at S473 between untransfected CEF, CEF expressing wild-type or mutant p85α and three clonal isolates of wild-type p85β. Expression of p85β leads to enhanced phosphorylation of Akt.

Fig. 4.

The effect of successive cellular passages (+1 to +5) on the sizes of p85α and p85β. Passage of p85α generates spontaneous, activating C-terminal truncations, whereas the size of p85β is stable.

Fig. 5.

Effect of isoform-specific inhibitors on the efficiency of transformation (focus count with inhibitor divided by focus count without inhibitor) of mutant p85α and wild-type p85β. Mutant p85α is strongly inhibited by the p110α-specific inhibitor A66, and is unaffected by the p110β-specific inhibitor TGX. In contrast, both p110α- and p110β-specific inhibitors reduce focus formation, but do so only partially. Complete inhibition of p85β transformation requires the inhibition of both p110α and p110β.

Fig. 6.

In ovarian cancer, mutations in PIK3CA and up-regulation of PIK3R2 tend to be mutually exclusive. The graph is a summary of results from 231 cases. PIK3CA is altered in 35%, PIK3R1 in 4%, and PIK3R2 in 11% of the cases. Cases are aligned vertically, allowing direct comparison of the status of the three genes in the same case. Green: mutation, red: mRNA up-regulation, gray: no change. Only the cases with alterations in PIK3CA, PIK3R1 or PIK3R2 are shown.