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STAT5 and IL-6 dysregulation in mastocytosis
tyrosine phosphorylation (Figure 6A) and reduced STAT5 serine (Ser780) phosphorylation (Figure 6B), with the effects on STAT5 tyrosine phosphorylation being less pro- nounced than those of the JAK2 and STAT5 inhibitors (Figure 6A). Simultaneous inhibition of both JAK2 and ERK1/2 markedly blunted STAT5 phosphorylation (Figure 6B). In addition, inhibition of MEK/ERK also reduced STAT5A and STAT5B mRNA expression by 16 h, an effect that was not seen by 2 h (Figure 6C). The results implicate
AB
MEK/ERK1/2 as a dual regulator of STAT5 activity (via JAK2-independent phosphorylation) and STAT5 transcrip- tion. In contrast, inhibition of PI3K had no effect on STAT5 phosphorylation or transcription in HMC-1.2 cells (Figure 6A and C), indicating the PI3K pathway regulates IL-6 independently of STAT5.
Combination treatment of inhibitors for STAT5, ERK1/2 and PI3K markedly suppressed constitutive IL-6 expres- sion (Figure 6D, left panel) and release (Figure 6D, right
C
Figure 5. STAT5A and B are key in mediating D816V-KIT-induced persistent IL-6 production. Effect of STAT3 (A) and STAT4 (B) knockdown by lentiviral sh-RNA on their respective targets and on the expression of IL-6 mRNA by HMC-1.2 cells. For sh-STAT4, the effect of two different constructs are shown (#1 and #3). Western blot gels underneath the bar graphs represent the effect of sh-STAT3 (A) or sh-STAT4 (B) constructs on the protein levels of STAT3 and STAT4, respectively. Lysates from duplicate samples are shown. The numbers under each pair are mean±Standard Error of Mean (SEM) of at least three separate experiments and represent fold change in the relative band fluorescence compared to the sh-RNA non-target (control). Actin content was used as loading control. (C and D) Effect of silencing STAT5A, STAT5B or the combination of both STAT5A&B by si-RNA on the expression of STAT5A and STAT5B mRNA (C) and of IL-6 mRNA (D) in HMC-1.2. The blots under the bar graph in (C) show the effect of STAT5 silencing in the protein levels of STAT5A/B and the numbers under the blot, the fold change as in (A) and (B). Of note, the antibody used recognizes both STAT5A and B. *P=0.01, **P<0.01, ***P<0.001 and ****P<0.0001.
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