Internalization of 125I–IL-13 in CHO-K1 and T98G cells.
CHO-K1 (A) and T98G (B) cells were incubated with 0.5 nmol125I–IL-13 at 4°C for 2 hours. Then the temperature was raised to 37°C, and, at various time intervals, 2 duplicate sets of 50 μL aliquots were taken. One set was incubated with glycine buffer (pH = 2.0) for 10 minutes. The mixture was then centrifuged through phthalate oils, and the radioactivity in the cell pellet (internalized) and in the supernatant (surface-bound plus dissociated) was determined with a gamma counter. The other set of 50 μL aliquots was directly centrifuged through phthalate oils, and the radioactivity observed in the supernatant was used for dissociated 125I–IL-13 values. Surface-bound 125I–IL-13 was determined by subtracting dissociated 125I–IL-13 values from (surface-bound plus dissociated) values. Data are expressed as a percentage of total IL-13 bound at time 0. Open circles, surface IL-13 bound on control cells; closed circles, surface-bound on IL-13Rα2–transfected cells; open diamonds, internalization in control cells; and closed diamonds, internalization in IL-13Rα2–transfected cells. Values are the mean of 2 independent experiments. When not shown, standard deviations are smaller than the symbol.
