The heparin-binding proteins apolipoprotein E and lipoprotein lipase enhance cellular proteoglycan production

Apolipoprotein E (apoE) and lipoprotein lipase (LPL), key proteins in the r egulation of lipoprotein metabolism, bind with high affinity to heparin and cell-surface heparan sulfate proteoglycan (HSPG). In the present study, we tested whether the expression of apoE or LPL would modulate proteoglycan ( PG) metabolism in cells. Two apoE-expressing cells, macrophages and fibrobl asts, and LPL-expressing Chinese hamster ovary (CHO) cells were used to stu dy the effect of apoE and LPL on PG production. Cellular PGs were metabolic ally labeled with (35)[S]sulfate for 20 hours, and medium, pericellular PGs , and intracellular PGs were assessed. In all transfected cells, PG levels in the 3 pools increased 1.6- to 3-fold when compared with control cells. I nitial PG production was assessed from the time of addition of radiolabeled sulfate; at 1 hour, there was no difference in PG synthesis by apoE-expres sing cells when compared with control cells. After 1 hour, apoE-expressing cells had significantly greater production of PGs. Total production assesse d with [H-3]glucosamine was also increased. This was due to an increase in the length of the glycosaminoglycan chains. To assess whether the increase in PGs was due to a decrease in PG degradation, a pulse-chase experiment wa s performed. Loss of sulfate-labeled pericellular PGs was similar in apoE a nd control cells, but more labeled PGs appeared in the medium of the apoE-e xpressing cells. Addition of exogenous apoE and anti-human apoE antibody to both non-apoE-expressing and apoE-expressing cells did not alter PG produc tion. Moreover, LPL addition aid not alter cell-surface PG metabolism. Thes e results show that enhanced gene expression of apoE and LPL increases cell ular PG production. We postulate that such changes in vascular PGs can affe ct the atherogenic potential of arteries.