Restraining PI3K: mTOR signalling goes back to the membrane

doi:10.1016/j.tibs.2004.11.003

Trends in Biochemical Sciences

Volume 30, Issue 1, January 2005, Pages 35-42

https://doi.org/10.1016/j.tibs.2004.11.003 Get rights and content

The lipid kinase phosphoinositide 3-kinase (PI3K) is activated in response to various extracellular signals including peptide growth factors such as insulin and insulin-like growth factors (IGFs). Phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P₃] generated by PI3K is central to the diverse responses elicited by insulin, including glucose homeostasis, proliferation, survival and cell growth. The actions of lipid phosphatases have been considered to be the main means of attenuating PI3K signalling, whereby the principal 3-phosphatase – phosphatase and tensin homologue deleted on chromosome 10 (PTEN) – dephosphorylates PtdIns(3,4,5)P₃, reversing the action of PI3K. Recently, however, another pathway of regulation of PI3K has been identified in which activation of PI3K itself is prevented. This finding, together with earlier work, strongly suggests that a major form of negative feedback inhibition of PI3K results from activated growth signalling via mammalian target of rapamycin (mTOR) and the p70 S6 kinase (S6K) – a pathway that could have consequences for the development of type 2 diabetes and tuberous sclerosis complex.

Section snippets

Insulin–PI3K signalling activates mTOR signalling

PtdIns(3,4,5)P₃ generated in the membrane by PI3K recruits members of intracellular signalling pathways containing pleckstrin homology (PH) domains to the plasma membrane, thereby coupling PI3K signals to downstream effector molecules [1]. Activation of one particular effector, protein kinase B (PKB; also known as AKT), seems to be essential not only in mediating the effects of insulin on glucose homeostasis, but also in regulating the profound effects of insulin and IGFs on mTOR signalling and

Targeting the IRS family of adaptors inhibits PI3K

Previous work on activation of PI3K by insulin indicates that negative regulation of PI3K occurs mainly at the level of the proximal tyrosine kinase substrates of the insulin receptor (InR) RTK – namely, the IRS proteins, a family of adaptor proteins that are essential both for PI3K activation and for mediating the pleiotropic effects of insulin 3, 21, 22.

In a pioneering study, Haruta et al. [23] showed that prolonged exposure to insulin leads to a decrease in both the electrophoretic mobility

Serine/threonine phosphorylation and IRS protein function

The idea that the decrease in mobility of IRS-1 resulting from chronic insulin stimulation is at least contributed to by an increase in phosphorylation now seems to be in little doubt. Thus, the decrease in IRS-1 electrophoretic mobility after chronic insulin stimulation is reversed by dephosphorylation with alkaline phosphatase treatment [30] and mimicked by okadaic acid, a protein phosphatase 2A (PP2A) serine/threonine phosphatase inhibitor 28, 31. An increase in serine/threonine

A role for mTOR signalling in PI3K inhibition

The rescuing effect of the mTOR inhibitor rapamycin in reversing the phosphorylation and inhibition of IRS-1 function resulting from prolonged insulin stimulation has suggested that a kinase (or kinases) that mediates mTOR signalling inhibits PI3K activation in addition to JNK and PKC.

mTOR is a component of two distinct large protein complexes that each contain at least three components. One complex is rapamycin sensitive, includes the mTOR kinase itself, raptor 42, 43 and GβL [44], and is

TSC1–TSC2 promotes PI3K activation by inhibiting mTOR signalling

The negative feedback inhibition of PI3K has been most frequently observed in normal cells after prolonged stimulation with insulin. Two early reports intriguingly indicated, however, that cells lacking the TSC1–TSC2 tumour suppressor also fail to activate PKB in response to insulin, suggesting that a similar mechanism might be used to inhibit PI3K activation caused by constitutive mTOR-dependent signalling 48, 49. New studies have helped to clarify the mechanisms involved in this inhibitory

mTOR signalling inhibits IRS transcription

Unlike previous work indicating that mTOR signalling affects only IRS phosphorylation and protein turnover, the studies of Shah et al. [51] and Harrington et al. [50] have surprisingly indicated that TSC1–TSC2 deficiency and the resulting activation of the mTOR pathway lead to a reduction in IRS-1 transcription (Figure 2). A microarray screen [50], confirmed by northern blot analysis 50, 51, revealed that the level of IRS-1 mRNA is reduced in TSC2-deficient cells as compared with

S6K and IRS-1 phosphorylation

The possibility that S6K might be the crucial rapamycin-sensitive effector responsible for directly phosphorylating IRS-1 and thereby inhibiting PI3K activation was first suggested by Tremblay and Marette [29] and has been subsequently investigated by Harrington et al. [50]. A specific serine residue in IRS-1, Ser302, has been shown to be phosphoryated by S6K in vitro [50] (Figure 1). Furthermore, phosphorylation of Ser302 in IRS-1 increases in the absence of TSC1–TSC2 function and is inhibited

Role of negative feedback regulation of PI3K in vivo

The physiological importance of negative feedback from mTOR and S6K to PI3K has been recently given impetus by a new study of S6K1-deficient mice [58]. This study suggests that at least some of the effects of a high-fat diet (HFD) on inhibiting the insulin–PI3K signalling pathway [59] might be mediated by S6K1 activation; thus, S6K1 is emerging as an attractive new therapeutic target in type 2 diabetes.

In an earlier study, S6K1-deficient mice were shown to be hypoinsulinaemic and mildly glucose

Concluding remarks

In this review, we have focused mainly on the role of mTOR signalling in mediating negative feedback on insulin–PI3K signalling and on the principal role of IRS-1 proteins as recipients of negative regulation from kinases such as S6K, which are normally regulated by nutrient–mTOR and PI3K signalling (Figure 3). Through this type of negative feedback regulation, PI3K and the production of PtdIns(3,4,5)P₃ are selectively uncoupled from the stimulatory effects of insulin; this raises the obvious

Acknowledgements

We apologise to those whose work could not be cited owing to space limitations. We thank Brendan Manning, Jameel Shah and George Thomas for sharing data before publication. L.S.H. is supported by the Tuberous Sclerosis Association (UK), R.F.L. is supported by grants from Cancer Research UK and G.M.F. is funded by a studentship from the Institute of Cancer Research.

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