Roads to Stat3 Paved with Cadherins

Abstract

The engagement of cadherins, cell-to-cell adhesion proteins, triggers a dramatic increase in the levels and activity of the Rac/Cdc42 GTPases, through the inhibition of proteasomal degradation. This leads to an increase in transcription and secretion of IL6 family cytokines, activation of their common receptor, gp130, in an autocrine manner and phosphorylation of the signal transducer and activator of transcription-3 (Stat3) on tyrosine-705 by the Jak kinases. Stat3 subsequently dimerizes, migrates to the nucleus and activates the transcription of genes involved in cell division and survival. The Src oncogene also increases Rac levels, leading to secretion of IL6 family cytokines and gp130 activation, which triggers a Stat3-ptyr705 increase. Interestingly, at the same time, Src downregulates cadherins in a quantitative manner, while cadherins are required to preserve gp130 levels for IL6 family signalling. Therefore, a fine balance between Src^527F/Rac/IL6 and Src^527F/cadherin/gp130 levels is in existence, which is required for Stat3 activation. This further demonstrates the important role of cadherins in the activation of Stat3, through preservation of gp130 function. Conversely, the absence of cadherin engagement correlates with low Stat3 activity: In sparsely growing cells, both gp130 and Stat3-ptyr705 levels are very low, despite the fact that cSrc is active in the FAK (focal adhesion kinase)/cSrc complex, which further indicates that the engagement of cadherins is important for Stat3 activation, not just their presence. Furthermore, the caveolin-1 protein downregulates Stat3 through binding and sequestration of cadherins to the scaffolding domain of caveolin-1. We hypothesize that the cadherins/Rac/gp130 axis may be a conserved pathway to Stat3 activation in a number of systems. This fact could have significant implications in Stat3 biology, as well as in drug testing and development.

Keywords: FAK; IL6; Src; Stat3; cadherins; caveolin-1; cell density; gp130; rac1.

Figure 1

The engagement of cadherins from opposing cells increases Rac and Stat3 activity and is required for Src-mediated Stat3 activation and cellular survival (data from [48]). Engagement of cadherins (1) increases Rac (2) levels and activity, leading to Stat3 (7) activation through NFκB (3), IL6 (4), gp130 (5) and Jak (6). Caveolin-1 (8) reduces gp130 and Stat3-ptyr705 by binding to and sequestering cadherins into its scaffolding domain [50]. Src^527F (9) also increases Rac levels and activity, independent from cadherin engagement (see Figure 3A in Ref. [48]). What follows, and its effect upon Stat3, depends upon the levels of Src^527F expression: (A) medium Src^527F levels: activation of Rac leads to the secretion of IL6 family cytokines and the activation of gp130, Jak and Stat3 (see Figure 2B in Ref. [48]). At the same time, gp130 function requires cadherin engagement, but cadherins are downregulated by Src^527F. However, at medium Src^527F levels there is enough residual cadherin/gp130 so as to allow Stat3-ptyr-705 phosphorylation and activation (adapted from [45,46,47,48]). (B) High Src^527F levels: cadherin levels are reduced to nondetectable, and this leads to gp130 downregulation (see Figure 5A,B in Ref. [48]), with a dramatic reduction in Jak and Stat3 activities as a result (see Figure 2 in Ref. [48]). Generated in BioRender.

Figure 2

Cadherin-11, Stat3-ptyr705 and gp130 levels relative to Src^527F. Relative levels of Stat3-ptyr705, cadherin-11 and gp130 were assessed by Western blotting in mouse Balb/c3T3 fibroblasts expressing different amounts of Src^527F, with the highest taken as 100%. Note that Cadherin-11 and gp130 are reduced with increasing Src^527F, while Stat3-ptyr705 is highest at Src^527F levels of approximately 50% and is dramatically reduced at high Src^527F levels. Peak values regarding the cell density of a number of cell lines are shown [48]. Technical detail: Uniform distribution and low cell-to-cell contact at plating is very important. Cells must be passed from a subconfluent petri and vigorously pipetted with a 9 inch pasteur pipette.