Our results show that genetic or pharmacological inhibition

Our results show that genetic or pharmacological inhibition of protein kinase CK2, a kinase overexpressed in many different type of cancers [16,17], leads to the downregulation of HSP27 expression in all the four different cell lines tested so far. Therefore, treatment with CK2 inhibitors can prove a valuable strategy in all the conditions where a reduction of HSP27 is desirable. Pertinent to this may be the observation that quercetin, a flavonoid able to inhibit protein kinase CK2 with IC50 in the sub-μM range [37], also downregulates HSP27 expression [38]. Notably, HSP27 expression is under control of the tetrameric form of CK2 holoenzyme, as knockout or knockdown of the β regulatory subunit alone reproduces the same effects of targeting the α/α′ catalytic subunits. This observation discloses the possibility to use more specific drugs that affects only the CK2 tetrameric formation without altering the total CK2 activity [39]. Finally, it should be highlighted that the pharmacological downregulation of HSP27 expression is of widespread interest, as other human diseases would also benefit from it. This especially applies to Cystic Fibrosis (CF) [40] considering that I-BET 151 hydrochloride of phenylalanine at position 508 in Cystic fibrosis transmembrane conductance regulator (F508delCFTR), present in 70–90% of CF patients, maintains at least partially channel activity, but causes the majority of CFTR protein to be sequestered by chaperones. This leads to the recruitment of ubiquitin ligase ultimately resulting in CFTR ubiquitylation and its proteasomal degradation [41]. Consequently F508delCFTR is >99% degraded before it can reach the plasma membrane [42]. HSP27 is one of the chaperones that selectively bind and target F508delCFTR for degradation. Notably, the steady-state levels of F508del CFTR are affected by the modulation of HSP27 expression [40,[43], [44], [45]]. CK2 has been already proposed as a pharmacological target to treat F508del patients in the combinatorial therapy of cysteamine plus epigallocatechin-gallate, but its mechanism of implication in this intricate process is far from being fully understood [46]. Further work will be necessary to verify if HSP27 downregulation could explain, at least partially, the effect of CK2 inhibitors in F508del patients. The following are the supplementary data related to this article.
Acknowledgments This work was supported by Fondazione per la Ricerca sulla Fibrosi Cistica (grant FFC#10/2016 adopted by Gruppo di Sostegno FFC di Seregno and grant FFC#12/2017 adopted by Delegazione FFC di Fabriano Ancona con il Gruppo di Sostegno FFC di Umbertide Città di Castello Perugia) (to M.S.) and by the Associazione Italiana per la Ricerca sul Cancro (AIRC), grant number IG 18756 (to L.A.P.). J.V. was supported by a fellowship of the FFC (to M.S.). We are grateful to Prof. Oriano Marin (CRIBI, University of Padova, Italy) who provided the peptides for CK2 kinase activity assay, and Dr. Anna Cabrelle for her technical support with FACS sorting, HSP27-FLAG plasmid was kindly provided by Prof. Gary Brewer.