STAT3 negatively regulates CD38 expression. (A-B) RPMI 8226 cells were infected with single-guide (sg) nontarget control (sgNT), sgSTAT3 #1 or #2 expressing lentiviral vector, and cultured with control culture medium (Control), BMSC-sup, or IL-6 (1 ng/mL) for 72 hours. (C-D) RPMI 8226 cells were transfected with scrambled siRNA (si SC) or STAT3 siRNA (si STAT3). These cells were cultured with control culture medium, BMSC-sup, or IL-6 (1 or 5 ng/mL) for 48 hours. After incubation, CD38 expression was measured by flow cytometry (A, C). STAT3 downregulation was confirmed by immunoblotting (B, D). (E-G) RPMI 8226 cells were transfected with the dominant-negative STAT3 (STAT3DN) or constitutively active STAT3 (STAT3C) construct. CD38 protein expression (E) and mRNA level (F) were assessed by flow cytometry (FCM) and qRT-PCR, respectively. (G) Phosphorylated STAT3 (P-STAT3) expression was confirmed by immunoblotting. Relative CD38 MFI was calculated in comparison with MFI of control. Data are shown as mean plus or minus standard error of the mean. *P < .05; **P < .01; ***P < .001. (H) CD38 (x-axis) and STAT3 (y-axis) expression are negatively correlated (r, −0.22; P, 9.8e-05) at diagnosis in 319 newly diagnosed MM patient samples from the IFM/DFCI 2009 study. C, control culture medium; sup, BMSC-sup.