Effects of CK2β subunit down-regulation on Akt signalling in HK-2 renal cells.

Abstract

The PI3K/Akt pathway is interconnected to protein kinase CK2, which directly phosphorylates Akt1 at S129. We have previously found that, in HK-2 renal cells, downregulation of the CK2 regulatory subunit β (shCK2β cells) reduces S129 Akt phosphorylation. Here, we investigated in more details how the different CK2 isoforms impact on Akt and other signaling pathways. We found that all CK2 isoforms phosphorylate S129 in vitro, independently of CK2β. However, in HK-2 cells the dependence on CK2β was confirmed by rescue experiments (CK2β re-expression in shCK2β HK-2 cells), suggesting the presence of additional components that drive Akt recognition by CK2 in cells. We also found that CK2β downregulation altered the phosphorylation ratio between the two canonical Akt activation sites (pT308 strongly reduced, pS473 slightly increased) in HK-2 cells. Similar results were found in other cell lines where CK2β was stably knocked out by CRISPR-Cas9 technology. The phosphorylation of rpS6 S235/S236, a downstream effector of Akt, was strongly reduced in shCK2β HK-2 cells, while the phosphorylation of two Akt direct targets, PRAS40 T246 and GSK3β S9, was increased. Differently to what observed in response to CK2β down-regulation, the chemical inhibition of CK2 activity by cell treatment with the specific inhibitor CX-4945 reduced both the Akt canonical sites, pT308 and pS473. In CX-4945-treated cells, the changes in rpS6 pS235/S236 and GSK3β pS9 mirrored those induced by CK2β knock-down (reduction and slight increase, respectively); on the contrary, the effect on PRAS40 pT246 phosphorylation was sharply different, being strongly reduced by CK2 inhibition; this suggests that this Akt target might be dependent on Akt pS473 status in HK-2 cells. Since PI3K/Akt and ERK1/2/p90rsk pathways are known to be interconnected and both modulated by CK2, with GSK3β pS9 representing a convergent point, we investigated if ERK1/2/p90rsk signaling was affected by CK2β knock-down and CX-4945 treatment in HK-2 cells. We found that p90rsk was insensitive to any kind of CK2 targeting; therefore, the observation that, similarly, GSK3β pS9 was not reduced by CK2 blockade suggests that GSK3β phosphorylation is mainly under the control of p90rsk in these cells. However, we found that the PI3K inhibitor LY294002 reduced GSK3β pS9, and concomitantly decreased Snail1 levels (a GSK3β target and Epithelial-to-Mesenchymal transition marker). The effects of LY294002 were observed also in CK2β-downregulated cells, suggesting that reducing GSK3β pS9 could be a strategy to control Snail1 levels in any situation where CK2β is defective, as possibly occurring in cancer cells.