Decreased secretion of ApoB follows inhibition of ApoB-MTP binding by a novel antagonist

Apolipoprotein B (apoB) and microsomal triglyceride transfer protein (MTP) are essential for the efficient assembly of triglyceride-rich lipoproteins. Evidence has been presented for physical interactions between these protei ns. To study the importance of apoB-MTP binding in apoB secretion, we have identified a compound, AGI-S17, that inhibited (60-70% at 40 mu M) the bind ing of various apoB peptides to MTP but not to an anti-apoB monoclonal anti body, 1D1, whose epitope overlaps with an MTP binding site in apoB. AGI-S17 had no significant effect on the lipid transfer activity of the purified M TP. In contrast, another antagonist, BMS-200150, did not affect apoB-MTP bi nding but inhibited MTP's Lipid transfer activity. The differential effects of these inhibitors suggest two functionally independent, apoB binding and lipid transfer, domains in MTP. AGI-S17 was then used to study its effect on the Lipid transfer and apoB binding activities of MTP in HepG2 cells. AG I-S17 had no effect on cellular lipid transfer activities, but it inhibited coimmunoprecipitation of apoB with MTP. These studies indicate that AGI-S1 7 inhibits apoB-MTP binding but has no effect on MTP's lipid transfer activ ity. Experiments were then performed to study the effect of inhibition of a poB-MTP binding on apoB secretion in HepG2 cells. AGI-S17 (40 mu M) did not affect cell protein levels but decreased the total mass of apoB secreted b y 70-85%. Similarly, AGI-S17 inhibited the secretion of nascent apoB by 60- 80%, but did not affect albumin secretion. These studies indicate that AGI- S17 decreases apoB secretion most likely by inhibiting apoB-MTP interaction s. Thus, the binding of MTP to apoB may be important for the assembly and s ecretion of apoB-containing lipoproteins and can be a potential target for the development of lipid-lowering drugs. It is proposed that the apoB bindi ng may represent MTP's chaperone activity that assists in the transfer from the membrane to the lumen of the endoplasmic reticulum and in the net lipi dation of nascent apoB, and may be essential for lipoprotein assembly and s ecretion.