Fig. 1

Activation of oncogenic K-Ras induces CD137 expression in cancer cells. a T-Rex/K-Ras cells were incubated with 100 ng/mL doxycycline for the indicated time to induce K-Ras^G12V expression. CD137 mRNA was measured by real-time PCR. b Immunostaining of CD137 in human pancreatic cancer cells (tissue microarray). c The levels of soluble CD137 in the sera of 10 healthy subjects and 10 pancreatic cancer patients were quantified by ELISA (enzyme-linked immunosorbent assay). d, e T-Rex/K-Ras cells incubated without (OFF) or with (ON) doxycycline for 48 h or 2 months were stained with control antibody (IgG cont) or specific antibody against human CD137. Cell surface CD137 was analyzed by flow cytometry (representative of three separate experiments). f Expression of CD137 in HPNE cells stably transfected with K-Ras (HPNE/K-Ras) in comparison with parental HPNE cells, measured by real-time PCR. g, h Cell surface CD137 in Panc-1 and SW1990 cell lines was analyzed by flow cytometry (representative of three separate experiments). Data are presented as mean ± SEM (standard error of the mean) of three repeated experiments. Statistical analysis: two-tailed unpaired t-test for panels a, c, e and f. **P < 0.01; ***P < 0.001. CD137 tumor necrosis factor receptor superfamily member 9, PCR polymerase chain reaction