EFFECT OF LONG-TERM TREATMENT OF 3T3-L1 ADIPOCYTES WITH CHLORATE ON THE SYNTHESIS, GLYCOSYLATION, INTRACELLULAR-TRANSPORT AND SECRETION OF LIPOPROTEIN-LIPASE

Lipoprotein lipase (LPL) is synthesized and glycosylated in the endopl asmic reticulum (ER), transported through the Golgi to the cell surfac e, and finally secreted. To examine the role of heparan sulphate prote oglycans (HSPG) in the synthesis, activity, intracellular transport an d secretion of LPL, 3T3-L1 adipocytes were cultured for 7 days in the presence of 20 mM chlorate, an inhibitor of sulphation of HSPG. Treatm ent of cells with 20 mM chlorate for 7 days caused a 55 % decrease in LPL activity in the intracellular compartment and a 79 % decrease in t he cell-surface compartment. The synthetic rate of LPL in chlorate-tre ated cells was identical with that in control cells as determined by b iosynthetic labelling. The study with endoglycosidase H (endo H) showe d that the treatment with chlorate increased the proportion of LPL sub units which were totally endo H-sensitive. The study with a heparin-Se pharose column showed that 3T3-L1 adipocytes contained three forms of LPL. The first form, accounting for 35 % of the LPL, did not bind to t he heparin-Sepharose column and had little or no activity; the second form, accounting for 32 %, bound to the column and was eluted with 0.4 -0.75 M NaCl but had no activity; the third form, accounting for 33 %, bound to the column and was eluted with 0.8-1.2 M NaCl and had activi ty. In chlorate-treated cells, the first form accounted for 66 % of th e LPL, the second form 15 % and the third form 19 %. When cells were i ncubated for 1 h with brefeldin A, which translocates Golgi proteins t o the ER [J. Lippincott-Schwartz, L. C. Yuan, J. S. Banifacino and R. D. Klausner (1989) Cell 56, 801-813; J. Lippincott-Schwartz, J. Glickm an, J. E. Donaldson, J. Robbins, T. E. Kreis, K. B. Seamen, M. P. Shee tz and R. D. Klausner (1991) J. Cell Biol. 112, 567-577], the chlorate -induced decrease in cellular LPL activity was restored. These finding s indicate that LPL synthesized in chlorate-treated cells can be proce ssed to be fully active, but chlorate-treated cells are unable to tran sport LPL to the Golgi and accumulate inactive LPL with a lower affini ty for heparin in the ER. The treatment with chlorate decreased the pr oportion of LPL subunits that were endo H-resistant, indicating that t he processing of oligosaccharide chains of LPL in the trans-Golgi was impaired in chlorate-treated cells. The amount of S-35-labelled LPL se creted by chlorate-treated cells was identical with that secreted by c ontrol cells, whereas the level of LPL activity in the medium of chlor ate-treated cells was 25 % of that in the medium of control cells, ind icating that most of the LPL secreted by chlorate-treated cells was in active.