Figure 1.
Pitstop 2 inhibits both clathrin-dependent and clathrin-independent endocytosis.
(A) Hela cells were preincubated with DMSO (control) or 20 µM pitstop 2 or pitstop 2-negative control for 15 min. Cells were allowed to internalize Alexa594-Transferrin and antibodies to MHCI for 30 min at 37°C in the presence of DMSO or drugs. Surface bound antibodies were removed by low pH wash to visualize the internalized proteins (Internal) prior to fixation or cells were fixed immediately after the internalization to label for the internal and surface pools (Total) of the protein. Cells were then labeled with secondary antibodies to detect MHCI. (B) Cells preincubated with DMSO or 20 µM pitstop 2 were allowed to internalize antibodies directed towards CD59, CD44, CD98 and CD147 for 30 min and fixed. Surface bound antibodies were removed by acidic wash or blocked with unlabeled goat-anti-mouse IgG. Cells were then incubated with secondary antibodies to detect the internalized cargo proteins. The results shown are representative of three independent experiments. Bar, 10 µm.
Figure 2.
Pitstop 2 inhibits clathrin-independent endocytosis in a dose-dependent manner.
(A) Hela cells were preincubated with DMSO (0.1%) or different doses of pitstop 2 ranging from 5 µM to 30 µM. Cells were then allowed to internalize Alexa594-Transferrin and antibodies to MHCI in the presence or absence of the drug for 30 min and then processed as described in Figure 1. Bars, 10 µm. (B) Total integrated fluorescence intensity of internalized Tfn and MHCI was quantified using Metamorph Software as described in Materials and Methods. The values of the different doses of pitstop 2 were then normalized to DMSO controls (set to 100%). Quantification was done for 60 cells at each dose and the error bars represent the standard deviation from the mean. P-values were calculated from the raw data and compared between DMSO and different doses of pitstop 2. Inhibition of MHCI uptake with pitstop was statistically significant for all doses with p-value <0.001 except for 5 µM pitstop 2 with p-value <0.02. Tfn uptake was also inhibited at 20 and 30 µM with p<0.001. The images and quantification shown are from one experiment; similar results were obtained in two additional experiments.
Figure 3.
Pitstop 2 inhibits SNAP-Tac internalization in transfected HeLa cells.
(A) HeLa cells transiently transfected with SNAP-Tac were preincubated for 10 min with either DMSO (control, top panels) or 20 µM pitstop 2 (bottom panels). Cells were then exposed to BG-S-S-594 and allowed to internalize for 30 minutes at 37°C in the presence or absence of pitstop 2. Cells were then washed with PBS, overlaid with Phenol Red-free DMEM containing 25 mM HEPES, and imaged live. Images in the left column were taken before the addition of TCEP, and represent total labeled SNAP-Tac protein (surface + internalized protein). Images of the same cells taken one min after TCEP treatment are shown in the right column, and represent the internalized pool of SNAP-Tac whose BG-S-S-594 label is protected from TCEP cleavage. (B) Percent internalized SNAP-Tac was calculated for individual cells displayed in (A) under either DMSO treatment or pitstop 2 treatment conditions. Small squares represent calculations of individual cells (12 cells measured for DMSO, 16 cells measured for pitstop 2 treatment). Horizontal bars represent the mean percent internalization for each group of cells (13.8% +/−1.6% for DMSO, 3.1% +/−1.1% for pitstop 2). This graph shows the quantification of a single endocytosis experiment. Data pooled from at least five matched sets (DMSO and pitstop 2) of comparable experiments and using two different pitstop 2 stocks demonstrated a similar inhibition of SNAP-Tac internalization in the presence of pitstop 2.
Figure 4.
Pitstop 2 inhibits MHCI uptake in other cells lines.
(A) BEAS-2B cells were preincubated with DMSO or 20 µM pitstop 2. Cells were then incubated with Alexa-594 Tfn and antibodies to MHCI in the presence of DMSO or the drugs for 30 min and then processed as described in Figure 2. Lower panel represents the quantification of the integrated intensity of internalized proteins (Tfn and MHCI). The total integrated intensity of each cell was quantified as described in Materials and Methods. The mean fluorescence intensity, as arbitrary units, is plotted with standard deviation for 50 cells in both control and pitstop-treated cells. Both Tfn and MHCI internalization were inhibited at 20 µM pitstop 2 (p<0.001). (B) COS-7 cells were allowed to internalize Alexa-488 Tfn and antibodies to MHCI in the absence or presence of pitstop 2, and quanitified as described in (A). Statistical analysis done for 50 cells with p<0.01 for Tfn inhibition and p<0.001 for MHCI inhibition. The images and quantification shown are from one experiment and similar to that observed in 2 additional experiments. Bars, 10 µm.
Figure 5.
Pitstop 2 inhibition is not dependent upon AP2 or clathrin.
Hela cells were treated with control siRNA (A) or siRNA to deplete cells of μ2 subunit of AP2 (B) or clathrin heavy chains (C) as described in Materials and Methods. (A–C) Cells were preincubated with DMSO or 20 µM pitstop 2 for 15 min. Alexa488-Tfn and antibodies to MHCI were allowed to internalize for 30 min and processed as in Figure 1. Bars, 10 µm. (D) Detergent lysates of Hela cells treated with control siRNA, μ2 siRNA or CHC siRNA were separated by SDS-PAGE and immunoblotted with antibodies against proteins indicated at the left. The gel shows the level of knock-down from one representative experiment. The means and standard deviation of three separate experiments are plotted in the lower panel showing levels to be 14% and 12% of control levels for μ2 and clathrin heavy chain, respectively.
Figure 6.
Pitstop 2 affects PM surface dynamics but shiga toxin entry is not blocked.
(A) Hela cells expressing SNAP-Tac were preincubated with DMSO or 20 µM pitstop 2 for 15 min. Cells were labeled with BG-488 for 30 min at 37°C in the presence or absence of the drug and then imaged in the confocal microscope. Fluorescence recovery after photobleaching (FRAP) of surface SNAP-Tac was performed. Images were taken at 2 sec intervals, starting 10 sec before photobleaching, followed by imaging for a total time of 2 min after photobleaching. Corrected relative fluorescence intensities of surface SNAP-Tac in the FRAP region is represented in the graph. The graph was representative of 15 individual FRAP analyses from 2 independent experiments. (B) Hela cells pretreated with DMSO or pitstop 2 were allowed to internalize 568-labeled shiga toxin (140 ng/ml) and antibodies to CD98 for 30 min. Cells were fixed and labeled with 488-labeled secondary antibodies to detect internalized CD98. This experiment was repeated 2 additional times. Bars, 10 µm.