IL-6, a key pro-inflammatory cytokine, is upregulated “as a defense mechanism” in order to promote the APR and to initiate homeostasis as quickly as possible after an acute injury. In cases of chronic stress; however, IL-6 changes its role by modulating the leukocytic repertoire resulting in a chronic inflammatory state
. Although the presence of IL-6 is imperative during early injury and acute inflammation, contraction of the inflammatory process must occur in order for the system to regain normal homeostasis and to initiate repair. As a result, IL-6 is also posited to possess anti-inflammatory properties. Evidence to support this latter hypothesis is shown by the enhanced inflammatory responses induced following endotoxin exposure to IL-6−/−
, although, the authors never suggest a biochemical mechanism to explain why this unanticipated phenomenon occurs. HGF, a cytokine widely known for perpetuating liver regeneration, has also been described as having anti-inflammatory properties in cases of persistent inflammation
. Importantly, the expression of HGF is induced by IL-6 while its regulation is controlled via acute phase proteins (urokinase and PAI-1) that are also induced following IL-6 stimulation. Hence, we posited a feedback loop wherein the pro-inflammatory properties ascribed to IL-6 are exhibited through induction of the APR and the anti-inflammatory properties are mediated via HGF that is produced in response to IL-6 stimulation.
Using LPS stimulated primary BMM cell cultures as a source of IL-6, we demonstrate that addition of HGF is in fact anti-inflammatory (). Furthermore, we are able to confirm that the HGF-MET interaction propagates the suppression of cytokine production by using the pharmacological inhibitor of MET, SU11274 (). Other studies have focused on using inhibitors of the PI3K and Akt signaling cascade
, that are down-stream of HGF-MET signaling; however, by using a direct MET kinase inhibitor as well as the macrophage specific MET floxed mice (), we demonstrate that HGF-MET signaling is capable of suppressing inflammation.
Traditionally, GSK3β is known for regulating glycogen synthase and the storage of glycogen into peripheral sites
, but recent evidence suggests this kinase may also function as a key player in modulating inflammation
. It is well known that the activation of NFκB through TLR signaling leads to the transcription of pro-inflammatory cytokines; however, recently data was published indicating that at a more general level, cytokine production is regulated through GSK3β, which in turn regulates NFκB activity
. Hence, GSK3β appears to be a pivotal kinase that serves as a nodal point for both the generation and resolution of the inflammatory response
. Our data demonstrates that treatment BMM with HGF leads to increased phosphorylation and inactivation of GSK3β (Ser 9) () and that this response is sustained, even in the presence of LPS.
Pharmacological inhibitors of PI3K, Akt, and GSK3β induce inactivation of GSK3β. Inactive GSK3β can then promote the association of phospho-CREB (Ser 133) with CBP and sequester the CBP away from NFκB p65 (Ser 276). These signaling changes are associated with a switch from a pro- to anti-inflammatory pathway with a resultant increase in IL-10 production
. Our data shows that using HGF in place of those inhibitors gives similar results. BMM treated with both LPS and HGF demonstrated an increased CBP-phospho-CREB interaction, which was reduced in the presence of the MET kinase inhibitor (). Furthermore, there was an increase in the production of IL-10 () and, a reduction in the nuclear translocation phosphorylated p65. Combined, the data suggest that during an inflammatory response, active HGF may be key in switching the cellular response from a pro- to an anti-inflammatory pathway.
In addition to the canonical pathway, TLR signaling has been shown to weakly activate PI3K. This then leads to anti-inflammatory events by altering the cytokine repertoire
. Hence, it has been postulated that PI3K is the point at which TLR signaling is differentiated from a pro-inflammatory to an anti-inflammatory condition. Our data indicates that in the absence of HGF, TLR signaling promotes the phosphorylation of NFκB along with its translocation to the nucleus and that this correlates with the production of the pro-inflammatory cytokine IL-6. In contrast, in the presence of HGF, GSK3β is phosphorylated (inactive) and TLR stimulation leads to production of the anti-inflammatory cytokine, IL-10. Notably, HGF signaling is well known to signal through the PI3K pathway
. Hence, we propose that under normal circumstances, induction of IL-6 through pro-inflammatory stimuli leads to the eventual production of HGF
. HGF-MET interactions then ultimately result in phosphorylation of GSK3β and in the continued presence pro-inflammatory stimuli, facilitates an increased association of phospho-CREB with CBP. This then suppresses NFκB's transcriptional activity and results in the resolution of the inflammatory response (). Our data describe the intimate interaction between IL-6 and HGF in regulation of inflammation. Hence we propose that HGF acts as an internal rheostat regulating the complex cascade of induction and resolution of inflammation ().
Proposed mechanism of HGF-mediated suppression.