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A Third Signaling Pathway for the Type 1 IGF Receptor« Back to Volume 22, Issue 4, December 2006 - Table of Contents The type 1 insulin-like growth factor receptor (IGF-1R) is an a 2 b 2 transmembrane tyrosine kinase receptor. Its best known signaling pathways are the MAP kinase cascade (leading to ERK activation) and the PI-3 kinase/Akt pathway. Both pathways require transducers that directly interact with the IGF-1R b subunits after their autophosphorylation to recruit and activate the respective signaling cascades; for the former it’s shc and grb and for the latter it’s IRS-1 or IRS-2. A third, less well studied signaling pathway after IGF binding to IGF-1R leads to the transient activation of c-Jun N-terminal kinase (JNK). In a recent paper, Yong Lin and colleagues identified the death domain receptor-interacting protein (RIP) as the transducer between IGF-1R and JNK activation. In wild type mouse embryo fibroblasts (MEFs), 100 ng/ml IGF-I stimulated JNK activation, as detected by an in vitro kinase assay using GST-c-Jun(1-79) as substrate. Despite normal expression levels of IGF1R and JNK, RIP-/- cells could not activate JNK in response to IGF-I, as evidenced by both the GST-c-Jun kinase assay and Western immunoblotting for phosphorylated JNK and phosphorylated c-Jun. RIP-/- cells also could not activate JNK in response to tumor necrosis factor (TNF), a known stimulator of JNK activation, but retained the ability to do so in response to interleukin-1 and ultraviolet treatment. While RIP is specific for IGF-1R as a stimulator of JNK, the converse is also true; it is specific in terms of IGF-1R signaling, since both Akt and ERK activation proceeded normally in RIP-/- cells. As proof of principle, reconstitution of RIP in the RIP-/- cells restored their ability to activate JNK after IGF-I treatment. The authors then went on to examine the mechanistic aspects of RIP’s role as a transducer between IGF-1R and JNK. RIP consists of three functional domains: N-terminal kinase domain, an intermediate domain (needed for NF-κB activation), and C-terminal death domain (needed to recruit RIP to the TNF receptor signaling complex). Comparing various RIP mutant constructs to wild type RIP revealed that neither the death domain nor the kinase activity were required for IGF-I-induced JNK activation. PI3 kinase was also not required for IGF-I-induced JNK activation as demonstrated by preincubation of the cells with the PI3 kinase inhibitor, wortmannin, at moderate dose (100 nM). High dose (10 μM) of wortmannin did inhibit JNK activation by IGF-I but not by TNF. Co-immunoprecipitation experiments showed that RIP transiently binds to the b subunit of IGF-1R, but that IGF-I treatment is required first to recruit RIP to the IGF-1R. The minimal IGF-1Rβ binding region of RIP was identified as being within the intermediate domain, a finding consistent with the previous mutant results. Finally, RIP-/- cells showed impaired proliferation relative to wild type in response to IGF-I, a function that was restored by reconstitution of RIP. Thus, the authors elegantly delineated RIP as the transducer between IGF1R and JNK activation; IGF-I activates IGF1R, which recruits RIP and binds to its intermediate domain, and RIP is required for JNK activation (but not ERK or Akt) and full cellular proliferative response to IGF-I. Editor’s CommentRIP was originally identified as a transducer for death domain kinase receptors in response to TNF and TNF-related apoptosis-inducing ligand (TRAIL). It transduces NF- k B activation by TNF, TRAIL, DNA damaging agents and double stranded RNA; JNK, ERK and p38 MAP kinase activation by TNF; and Akt activation by toll-like receptor (TLR)-4 and –6. It is intriguing that JNK activation by IGF-I and the TNF family of cytokines is mediated by the same transducer, RIP. The cross-talk between the mitogenic and survival signaling of IGF and the apoptotic signaling pathways seems to keep getting more and more complex. Perhaps this should not be surprising, since it is the fine balance between the survival/mitogenic and apoptotic signaling pathways that ultimately determine a cell’s fate as it integrates its external cues and stressors. All these layers of checks and balances are what save us from getting cancer. There is yet another level of complexity. This paper shows that IGF-1R can activate JNK via RIP. But JNK can inhibit insulin and IGF signaling by phosphorylating and inhibiting the insulin receptor substrate (IRS)-1.1-3 So there is an internal negative feedback loop as well . Adda Grimberg , MD References - (linked to
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Volume 24 Number 1
May 2008
www.GGHjournal.com
ISSN 1932-9032
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