MAPK3 and hepatocellular carcinoma: Moreover, when we knocked down SP1 to inhibit the expression of STK39 or knocked down WNK1 (which is an upstream activator of STK39) to suppress the phosphorylation of STK39 in HCC cells, the phosphorylation of ERK1/2 would be decreased subsequently (Figure 5F and Figure S3F-G) 40, 41.