Objective To investigate the mechanism underlying the WNK4 kinasemediated inhibitory effect on BK channel. Methods Cos-7 cells were cotransfected with BK in combination with either CD4 (control group) or wild type WNK4 (WNK4-WT).Immunostaining and confocal microscopy,chemiluminescence,Western blotting analysis were then employed to determine the BK localization in cells,BK surface expression and total protein level,respectively.To further investigate whether the reduction of BK protein expression is due to an increase in degradation through a lysosomal pathway,BK protein level was determined after treated with bafilomycin A1(Baf A1),a proton pump inhibitor affecting lysosomal degradation. Results Immunostaining and confocal microscopic study showed that BK was localized both in plasma membrane and cytosol in the control group.After cells transfected with WNK4-WT,BK expression was markedly reduced.Chemiluminescent assay found that BK surface expression level was 299.9±18.6 in the control group,whereas it was significantly reduced (148.4±13.7,P＜0.01) in the WNK4-WT group.Western blotting analysis showed that total BK protein level was markedly reduced in the presence of WNK4-WT compared to the control group.WNK4-WT was shown to significantly reduce the BK total protein level (42.3％±15.2％) compared to the control group (100％) (P＜0.01).When the cells was treated with Bafilomycin A1 (Baf A1,0.5 μmol/L),WNK4-mediated reduction in BK protein was reversed (82.2％±12.1％,P＜0.05). Conclusions WNK4 inhibits total and surface protein expression of BK in Cos-7 cells whick is likely due to an increase in BK degradation through a lysosomal pathway.

The efficient clinical treatment of oral squamous cell carcinoma(OSCC)is still a challenge that demands the development of effective new drugs.Phenformin has been shown to produce more potent anti-tumor activities than metformin on different tumors,however,not much is known about the influence of phenformin on OSCC cells.We found that phenformin suppresses OSCC cell proliferation,and promotes OSCC cell autophagy and apoptosis to significantly inhibit OSCC cell growth both in vivo and in vitro.RNA-seq analysis revealed that autophagy pathways were the main targets of phenformin and identified two new targets DDIT4(DNA damage inducible transcript 4)and NIBAN1(niban apoptosis regulator 1).We found that phenformin significantly induces the expression of both DDIT4 and NIBAN1 to promote OSCC autophagy.Further,the enhanced expression of DDIT4 and NIBAN1 elicited by phenformin was not blocked by the knockdown of AMPK but was suppressed by the knockdown of transcription factor ATF4(activation transcription factor 4),which was induced by phenformin treatment in OSCC cells.Mechanistically,these results revealed that phenformin triggers endoplasmic reticulum(ER)stress to activate PERK(protein kinase R-like ER kinase),which phosphorylates the transitional initial factor eIF2,and the increased phosphorylation of eIF2 leads to the increased translation of ATF4.In summary,we discovered that phenformin induces its new targets DDIT4 and especially NIBAN1 to promote autophagic and apoptotic cell death to suppress OSCC cell growth.Our study supports the potential clinical utility of phenformin for OSCC treatment in the future.