JNK1 is downstream of the proximal BCR kinases BTK, SYK, and PI3K. (A) Protein expression of BCR signaling kinases pSYK, SYK, pBTK, BTK, and pAKT and AKT, p-JNK1, total JNK1, total BCL2, and total MCL1 in CLL cells after 8 and 24 h of treatment with the SYK inhibitor entospletinib, the BTK inhibitor ibrutinib, or the PI3K inhibitor idelalisib (CLL#25). (B) Relative expression of p-JNK1 compared with total JNK1 after 4 h of treatment with entospletinib, ibrutinib, and idelalisib (n = 3 CLLs) (unpaired t test, **P < 0.01, ***P < 0.001). (C) Protein expression of SYK, p-JNK1, and total JNK1 in primary human CLL cells after 24 h of siRNA-mediated SYK knockdown (CLL#33 and #30). (D) Knockdown efficiency of SYK (24 h), relative protein expression of p-JNK1 compared with total-JNK1 (24 h), and reduction of CLL cell viability after SYK knockdown compared to control (48 h) (n = 4 different CLLs). (E) Relative BTK phosphorylation compared with relative JNK1 phosphorylation in CLL (n = 19) (raw data in Fig. S1, statistical analysis via Pearson correlation coefficient). (F) BCR inhibitors against BTK (ibrutinib) and SYK (entospletinib) were compared with the JNK1 inhibitor SP600125 in their ability to reduce viability of three CLL patient samples in vitro. Source data are available for this figure: SourceData F5.