The receptor interacting protein 1 mediates a link between NFκB and PI3-kinase signaling

As our knowledge of the molecular architecture of signal transduction pathways advances it is becoming clear that there are extensive interconnections between signaling networks which can no longer be viewed as discrete linear modules. In the context of inflammation induced cancer, cross-talk between the PI3K-Akt-mTOR and the NFκB signaling pathways may be biologically significant and play a role in pathogenesis, as well as resistance to therapy. A number of reciprocal links between the PI3K-Akt-mTOR and NFκB pathways have been reported.^1,2 Here we discuss aspects of the recently reported regulation of the PI3K-Akt-mTOR network by components of the NFκB signaling pathway. We focus on the receptor interacting protein 1 (RIP1) as a major link between these two pro-survival pathways.

RIP1 is best known for its essential role in NFκB activation in response to stimuli such as TNFα, TLR3 and DNA damage.³ RIP1 appears to have both a pro-survival function and an important role in cell death under certain cellular contexts.⁴ RIP1 also has a role in activation of the PI3K-Akt pathway.^5,6 As discussed below, a recent study sheds light on the mechanisms used by RIP1 to stimulate PI3K-Akt activation.⁷

RIP1 appears to activate PI3K-Akt by removing the two major brakes that regulate PI3K-Akt activity⁸ as shown in Figure 1. Firstly, RIP1 downregulates PTEN. Previous studies have indicated that TNFα mediated activation of NFκB results in inhibition of PTEN expression.⁹ However, while increased expression of RIP1 downregulates PTEN at both the protein and mRNA level, inhibition of NFκB fails to block RIP’s effect on PTEN,⁷ suggesting an NFκB independent mechanism.

Figure 1.

A schematic model of RIP1-mediated regulation of the PI3K-Akt pathway. RIP1 downregulates both PTEN and mTOR, thus inhibiting both major negative regulators of this pathway resulting in increased PI3K-Akt activation.

Secondly, RIP1 expression also activates PI3K-Akt in PTEN null cells suggesting an alternative mechanism in activating the PI3K-Akt pathway. This alternative mechanism is likely to be inhibition of a negative feedback loop that extends from mTOR to PI3K-Akt.^8,10 Thus, inhibition of mTOR by Rapamycin or TSC1/2 leads to increased activation of PI3K-Akt. Akt mediated activation of mTOR results in activation of p70S6K, which phosphorylates IRS-1 resulting in inhibition of IRS1. Thus, mTOR-p70S6K mediated inhibition of IRS-1 results in decreased PI3K-Akt activation through decreased activity of the IGF/IRS1 pathway. This negative feedback loop may complicate therapeutic inhibition of mTOR in certain cancers, since increased activation of PI3K-Akt could have unwanted consequences resulting from activation of other Akt targets. Increased expression of RIP1 results in a downregulation of mTOR while silencing RIP1 results in increased mTOR levels. RIP1 inhibits the mTOR promoter and negatively regulates mTOR mRNA levels. RIP1 mediated downregulation of mTOR can be blocked by a dominant negative IκBαM and RIP1 fails to inhibit the mTOR promoter if the NFκB binding consensus sites are point-mutated.⁷ As expected, RIP1 expression results in decreased phosphorylation of p70S6K as well as decreased inhibitory phosphorylation of IRS1, supporting this model. Downregulation of mTOR levels by siRNA knockdown has previously been shown to result in increased Akt activation.¹¹

RIP1 is expressed both constitutively and inducibly in inflammatory states and cancer.^3,12 RIP1 is overexpressed in about 30% of cases of glioblastoma (GBM), the most common primary malignant adult brain tumor in adults, and unfortunately (but importantly) confers a worse prognosis in GBM.¹² Furthermore, the PTEN-PI3K-Akt-mTOR pathway is commonly altered in GBM and RIP1 may play a role in this process, since RIP1 level correlates with Akt activation in GBM.⁷ We theorize that RIP1-mediated dual regulation may lead to a synergistic and potent activation of the PI3K-Akt pathway. For example, it is has been suggested that tumors resulting from TSC1/2 loss are relatively benign since increased activity of the mTOR negative feedback loop inhibits Akt. However, an additional mutation such as deletion of PTEN in the setting of TSC1/2 loss results in aggressive tumors.¹⁰ This study also highlights a novel regulation of mTOR at the transcriptional level by RIP1, that appears to be mediated, at least in part, by an activation of NFκB.⁷

The influence of NFκB signaling components on mTOR are complex and may lead to activation or inhibition of mTOR. The IKK kinases that play a key role in NFκB activation, have been reported to activate mTOR. The TSC1/2 tuberous sclerosis complex is a key negative regulator of mTOR. The IKK kinases appear to activate mTOR by a direct inhibition of the tuberous sclerosis complex (TSC1/2) without a requirement for the transcriptional activity of NFκB. IKKα associates with mTORC1 and may be required for effective induction of Akt-induced mTOR activity.¹³ IKKβ was shown to physically interact with, and to phosphorylate TSC1 resulting in inhibition of TSC1 and activation of mTOR.¹⁴ Thus, the central mechanism of mTOR regulation by IKKs and most other stimuli appears to be via inactivation of TSC1/2. RIP1, on the other hand, inhibits mTOR by downregulating its expression via an NFκB dependent mechanism. Thus, NFκB dependent mechanisms appear to regulate both major brakes on the PI3K-Akt pathway.^7,9 Synergistic cross-talk between the two pathways induced by RIP1 overexpression in cancer may favor the survival of malignant phenotypes while simultaneously conferring increased resistance to treatments.