Domain analysis reveals striking functional differences between the regulatory subunits of phosphatidylinositol 3-kinase (PI3K), p85α and p85β

Abstract

Our understanding of isoform-specific activities of phosphatidylinositol 3-kinase (PI3K) is still rudimentary, and yet, deep knowledge of these non-redundant functions in the PI3K family is essential for effective and safe control of PI3K in disease. The two major isoforms of the regulatory subunits of PI3K are p85α and p85β, encoded by the genes PIK3R1 and PIK3R2, respectively. These isoforms show distinct functional differences that affect and control cellular PI3K activity and signaling [1–4]. In this study, we have further explored the differences between p85α and p85β by genetic truncations and substitutions. We have discovered unexpected activities of the mutant proteins that reflect regulatory functions of distinct p85 domains. These results can be summarized as follows: Deletion of the SH3 domain increases oncogenic and PI3K signaling activity. Deletion of the combined SH3-RhoGAP domains abolishes these activities. In p85β, deletion of the cSH2 domain reduces oncogenic and signaling activities. In p85α, such a deletion has an activating effect. The deletions of the combined cSH2 and iSH2 domains and also the deletion of the cSH2, iSH2 and nSH2 domains yield results that go in the same direction, generally activating in p85α and reducing activity in p85β. The contrasting functions of the cSH2 domains are verified by domain exchanges with the cSH2 domain of p85β exerting an activating effect and the cSH2 domain of p85α an inactivating effect, even in the heterologous isoform. In the cell systems studied, protein stability was not correlated with oncogenic and signaling activity. These observations significantly expand our knowledge of the isoform-specific activities of p85α and p85β and of the functional significance of specific domains for regulating the catalytic subunits of class IA PI3K.

MicroRNA-106a-5p Inhibited C2C12 Myogenesis via Targeting PIK3R1 and Modulating the PI3K/AKT Signaling

“…In our study, pan-PI3K (p85α) protein levels were repressed by enforced miR-106a-5p agomir in both differentiating and well-differentiated myotubes. PI3K (p85α), encoded by PIK3R1 gene, is a regulatory subunit of PI3Ks and essential for myoblasts proliferation and differentiation [ 24 , 38 ]. Germline deletion of the PIK3R1 gene leads to impaired muscle growth, and a significant reduction in muscle weight and fiber size [ 39 ].…”

RNA-seq Analysis of Wild-Type vs. FOXC2-Deficient Melanoma Cells Reveals a Role for the FOXC2 Transcription Factor in the Regulation of Multiple Oncogenic Pathways

“…Our findings are also consistent with work in other tumor and non-tumor models demonstrating a role for FOXC2 in the activation of PI3K-Akt-mTOR signaling (5,20,21), as we found that several genes associated with the PI3K-Akt signaling pathway (mmu04151) and the mTOR signaling pathway (mmu04150) were upregulated in B16-F1 as compared to its FOXC2-deficient counterpart. These genes include the Pik3r2 gene, which encodes the p85β regulatory subunit of PI3K known to induce oncogenic transformation and cellular proliferation (22,23), and the Insr gene, whose protein product drives various oncogenic activities through PI3K signaling (24). FOXC2 is also well-known for its ability to promote EMT and tumor cell migration/invasion (10,25), and our findings suggest potential mechanisms by which these hallmarks of cancer progression might be regulated by FOXC2 as well.…”

Isoform-specific activities of the regulatory subunits of phosphatidylinositol 3-kinases – potentially novel therapeutic targets

“…For p85α, deletion of the SH3 domain, not including PR1, has a strong activating effect that is seen in cellular transformation and in PI3K signaling. For p85β, the functional consequences of the SH3 truncation are not significant (67).…”

“…Deletion of the combined SH3 and BH domains leads in p85α to loss of the elevated activity seen with the SH3 truncation, and in p85β eliminates the enhanced oncogenic and signaling activities characteristic of the wild type protein (67). These truncations in essence create proteins that closely resemble the splice variants p50α and p55α and the p55γ isoform respectively of the regulatory subunits encoded by PIK3R1 and PIK3R3 .…”

“…The effects of C-terminal deletions on PI3K signaling and on oncogenic transformation in cell culture have been described recently (67). In p85α, deletion of the cSH2 domain induces significant activation of signaling and of oncogenicity.…”