We have previously characterized the trafficking of transferrin (Tf) through HEp2 human carcinoma cells (Ghosh, R. N., D. L. Gelman, and F. R. Maxfield, 1994. J. Cell Sci. 107:2177-2189). Early endosomes in these cells are comprised of both sorting endosomes and recycling compartments, which are distinct separate compartments. Endocytosed Tf initially appears in punctate sorting endosomes that also contain recently endocytosed LDL. After short loading pulses, Tf rapidly sorts from LDL with first-order kinetics (t1/2 approximately 2.5 min), and it enters the recycling compartment before leaving the cell (t1/2 approximately 7 min). Here, we report a second, slower rate for Tf to leave sorting endosomes after HEp2 cells were labeled to steady state with fluorescein Tf instead of the brief pulse used previously. We determined this rate using digital image analysis to measure the Tf content of sorting endosomes that also contained LDL. With an 11-min chase, the Tf in sorting endosomes was 24% of steady-state value. This was in excess of the amount expected (5% of steady state) from the rate of Tf exit after short filling pulses. The excess could not be accounted for by reinternalization of recycled cell surface Tf, implying that either some Tf was retained in sorting endosomes, or that Tf was delivered back to the sorting endosomes from the recycling compartment. The former is unlikely since nearly all sorting endosomes contain detectable Tf after an 11-min chase, even though more than one third of the sorting endosomes were formed during the chase time. Furthermore, while observing living cells by confocal microscopy, we saw vesicle movements that appeared to be fluorescent Tf returning from recycling compartments to sorting endosomes. The slow rate of exit after steady-state labeling was similar to the Tf exit rate from the cell, suggesting an equilibration of Tf throughout the early endosomal system by this retrograde pathway. This retrograde traffic may be important for delivering molecules from the recycling compartment, which is a long-lived organelle, to sorting endosomes, which are transient.

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