Fig. 8.

NGF enhanced the uptake of FITC-dextran and of¹²⁵I-Tfn in PC12 cells. A, PC12 cells were incubated with FITC-dextran for 0–10 min at 37°C in the absence or presence of 2 nm NGF. The amount of internalized FITC-dextran was determined by measuring the absorbance at 490 nm of the lysates of the washed cell pellets. The values are expressed as a percentage of the vehicle-treated samples that were warmed for 10 min. NGF increased the uptake of FITC-dextran by 20 ± 3% (n = 3; p = 0.02) at 5 min and by 60 ± 6% (n = 3; p = 0.01) at 10 min. The increase at 10 min resulted in a value that was 157% of the vehicle-treated control. The error bars represent SEM.B, PC12 cells were incubated with¹²⁵I-Tfn in the absence or presence of 2 nm NGF for 2, 5, 15, or 30 min at 37°C. Then they were chilled (4°C) and quickly pelleted before acid stripping of the surface-bound Tfn. Cell-associated counts represent internalized¹²⁵I-Tfn. The values are expressed as a percentage of the vehicle-treated samples at 30 min. NGF treatment increased the uptake of ¹²⁵I-Tfn by 35 ± 10% (n = 3;p = 0.001) at 5 min to a value that was approximately twice that of the control. By 15 min the increase was 13 ± 1% (n = 3; p = 0.001). Error bars represent SEM. C, TrkA activation was required for the NGF effect on increased endocytosis of¹²⁵I-Tfn. Cells were incubated with ¹²⁵I-Tfn in the absence or presence of 2 nm NGF for 5 min at 37°C. Although NGF induced an increased endocytosis of ¹²⁵I-Tfn in KB PC12 cells (177 ± 6% of the vehicle-treated;n = 3; p = 0.001), it had no significant effect in KB PC12 cells pretreated with 200 nmK252a (110 ± 3%; n = 3;p = 0.08) or in PC12 nnr5 cells (96 ± 3%;n = 3; p = 0.38).