The effect of chloroquine on lysosomal prolactin receptors in rat liver

PRL receptors have been previously identified in purified rat liver plasma membrane and Golgi vesicle preparations. In this study, we report on PRL receptors located in highly purified lysosome preparations. These lysosomal PRL receptors were characterized using Scatchard analysis and compared to other intracellular and cell surface receptors. We have identified two classes of lysosomes. Lighter lysosome-like vesicles, which are greatly enriched in acid phosphatase activity (the marker enzyme of lysosomes), contain a great deal of binding activity. This PRL binding was only slightly increased by pretreatment of animals with the lysosomotropic agent chloroquine. In contrast, mature lysosomes showed very little binding activity in control animals, but chloroquine treatment increased binding 7- to 8-fold in these mature lysosomes. We suggest that the lysosome-like structures are immature lysosomes (namely prelysosomes) toward which the hormone-receptor complex is internalized: they appear to bear little proteolytic activity. These structures could play a role in PRL receptor recycling. Lysosomal PRL receptors showed curvilinear Scatchard plots, in contrast to plasma membrane and Golgi counterparts, which were linear over the same range of hormone concentrations. The high affinity site in lysosomes had a Kd comparable to the cell surface and Golgi receptors. The number of binding sites per mg protein in prelysosomes and lysosomes was 3 times greater than that in the homogenate, but Golgi preparations were 3 times as rich as lysosomes. The great number of PRL receptors in prelysosomes could be attributed, in large part, to the low affinity sites. The internalization of PRL into rat liver was examined after in vivo injection of [125I]iodoovine PRL. The labeled hormone was found initially in the plasma membrane fraction, after which it localized preferentially in the Golgi fraction, with maximum incorporation 15 min postinjection. Substantial radioactivity was observed in both classes of lysosomes (L-1 and L-2). In contrast to the Golgi fraction, maximum incorporation of [125I]iodoovine PRL in lysosomes occurred at 30 min. This suggests either that during internalization, PRL first reaches Golgi elements and is then transferred to the lysosomal compartment, or that there are two independent pathways of internalization, one rapid toward the Golgi complex (may be a path of receptor recycling) and the other toward lysosomes (probably leading to receptor degradation).