The present contributions on secretion and targeting of lysosomal enzymes in U937 cells are as follows:
1. The previously described strong enhancement in the secretion of lysozyme and serglycin in the presence of PMA and a minor effect on that of procathepsin D is confirmed.
2. PMA enhances the rate of the secretion of prosaposin and this may explain the induced secretion of a portion of procathepsin D, since at least a fraction of the two precursors are engaged in mutual complexes.
3. PMA is shown to induce the secretion of a high proportion of the cellular ß-hexosaminidase indicating exocytosis of an endosomal/lysosomal compartment.
4. PMA is shown to induce secretion of a portion of partially (intermediate) and fully processed (mature) forms of cathepsin D. This and the observation referred to in preceding statement indicate an effect of PMA distally to the sorting at the TGN.
5. An induction of a fusion of a distal compartment is further supported by finding Lamp-II at the plasma membrane in PMA-treated cells.
6. Tunicamycin, which inhibits N-glycosylation and secondarily the endowment of lysosomal proteins with the M6P marker, enhances the lysosomal targeting of serglycin. This effect is strongly pronounced in the presence of PMA. It can be explained by a participation of CI-MPR in the lysosomal targeting of serglycin. The receptor may have a preference for M6P ligands, that may suppress the binding of serglycin.
7. PMA does not enhance the presence of CI-MPR at the plasma membrane.
8. The effects of PMA appear to be mediated by an activation of phospholipase D and the local generation of phosphatidic acid and diacylglycerol in the membrane. It is proposed that these lipids participate in fusion and fission phenomena of different compartments and a selective enhancement of the secretion of lysosomally targeted products.
9. PMA induces a phosphorylation of several proteins in the soluble as well as the vesicular fractions of U937 cells. One of the selectively phosphorylated proteins was identified as insulin-responsive amino peptidase (IRAP). Further experiments are indicated to explore a possible role of IRAP in the selective secretory effects of PMA
