Class I PI3K in oncogenic cellular transformation

Abstract

Class I phosphoinositide 3-kinase (PI3K) is a dimeric enzyme, consisting of a catalytic and a regulatory subunit. The catalytic subunit occurs in four isoforms designated as p110 alpha, p110 beta, p110 gamma and p110 delta. These isoforms combine with several regulatory subunits; for p110 alpha, beta and delta, the standard regulatory subunit is p85, for p110 gamma, it is p101. PI3Ks play important roles in human cancer. PIK3CA, the gene encoding p110 alpha, is mutated frequently in common cancers, including carcinoma of the breast, prostate, colon and endometrium. Eighty percent of these mutations are represented by one of the three amino-acid substitutions in the helical or kinase domains of the enzyme. The mutant p110 alpha shows a gain of function in enzymatic and signaling activity and is oncogenic in cell culture and in animal model systems. Structural and genetic data suggest that the mutations affect regulatory inter- and intramolecular interactions and support the conclusion that there are at least two molecular mechanisms for the gain of function in p110 alpha. One of these mechanisms operates largely independently of binding to p85, the other abolishes the requirement for an interaction with Ras. The non-alpha isoforms of p110 do not show cancer-specific mutations. However, they are often differentially expressed in cancer and, in contrast to p110 alpha, wild-type non-alpha isoforms of p110 are oncogenic when overexpressed in cell culture. The isoforms of p110 have become promising drug targets. Isoform-selective inhibitors have been identified. Inhibitors that target exclusively the cancer-specific mutants of p110 alpha constitute an important goal and challenge for current drug development.

Figure 1

The canonical PI3K signaling pathway. PI3Ks can be activated by RTKs (with or without adaptors such as IRS1) or GPCRs. Ras is an additional positive regulator of PI3K, probably by facilitating membrane localization. The phosphatase PTEN dephosphorylates the product of PI3K, PIP₃ at the 3-positiion and thus acts as the exact enzymatic antagonist of PI3K. PIP₃ initiates downstream signaling by recruiting the serine-threonine kinases AKT and PDK1. PDK1 phosphorylates and thereby activates Akt. Three major signaling branches originate from Akt. Akt-mediated phosphorylation of GSK3β and of FOXO directly and indirectly controls transcriptional activities and cellular growth and survival (blue icons). The signal proceeding through the TSC complex, RHEB, and TOR affects primarily protein synthesis (beige icons). A positive feed-back loop extends from the TOR-RICTOR complex to Akt, resulting in additional activating phosphorylation of Akt. A negative feed-back loop consists of the S6K-mediated phosphorylation of IRS1.

Figure 2

PI3K is a dimeric enzyme. The figure shows the domain structure and domain interaction map of the standard regulatory subunit, p85 and the catalytic subunit, p110 (Huang et al., 2007; Miled et al., 2007; Pacold et al., 2000; Shekar et al., 2005; Walker et al., 1999).

Figure 3

A map of selected cancer-specific gain-of-function mutations in p110α. Suggested mechanisms for the gain of function are listed at the right. The three hot-spot mutations are in red.

Figure 4

The interactions with p85 and with Ras define two distinct molecular mechanisms for the gain of function seen in the hot spot mutations in p110α. The helical domain mutations are largely but not completely independent of binding to p85 but require the interaction with Ras. The kinase domain mutation completely depends on the interaction with p85 but is not affected by a loss of Ras-binding. However, the kinase domain mutation still shows residual signaling activity in the absence of p85-binding.

Figure 5

Cells transformed by the four isoforms of Class I p110 show distinct patterns of constitutive downstream signaling that group p110α–H1047R together with p110δ, as both constitutively activate Akt and downstream components of the pathway. In contrast, p110β–and p110γ–transformed cells do not show this constitutive activation of Akt, but this deficiency can be remedied by linking a myristylation signal to the N-terminus of p110.

Figure 6

(A) Loss of Ras-binding inactivates wild-type p110β and p110γ, but not p110α–H1047R and p110δ. A myristylation signal can substitute for Ras-binding in p110β and p110γ, suggesting that Ras functions as membrane anchor. (B) The dependence on Ras is also reflected by the sensitivity of p110β and p110γ to inhibitors of the MAP kinase pathway.

Figure 6

(A) Loss of Ras-binding inactivates wild-type p110β and p110γ, but not p110α–H1047R and p110δ. A myristylation signal can substitute for Ras-binding in p110β and p110γ, suggesting that Ras functions as membrane anchor. (B) The dependence on Ras is also reflected by the sensitivity of p110β and p110γ to inhibitors of the MAP kinase pathway.

Figure 7

Isoform-selective inhibitors of PI3K. The IC50 values were determined by measuring oncogenic activity in cell culture (Denley et al., 2007).