RESUMO
Two members of the angiopoietin-like family of proteins, ANGPTL3 and ANGPTL4, have been shown to play important roles in modulating lipoprotein metabolism in the body. Both proteins were found to suppress lipoprotein lipase (LPL) activity in vitro as well as in vivo. However, their mechanisms of inhibition remained poorly understood. Using enzyme kinetic analysis with purified recombinant proteins, we have found key mechanistic differences between ANGPTL3 and ANGPTL4. ANGPTL3 reduced LPL catalytic activity but did not significantly alter its self-inactivation rate. In contrast, ANGPTL4 suppressed LPL by accelerating the irreversible inactivation of LPL. Furthermore, heparin was able to overcome the inhibitory effect of ANGPTL3 on LPL but not that of ANGPTL4. Site-directed mutagenesis demonstrated the critical function of Glu(40) in ANGPTL4. In contrast, when cysteine residues involved in disulfide bond formation were mutated to serines, ANGPTL4 retained its activity. Taken together, our data provide a more detailed view of the structure and mechanisms of these proteins. The finding that ANGPTL3 and ANGPTL4 inhibit LPL activity through distinct mechanisms indicates that the two proteins play unique roles in modulation of lipid metabolism in vivo.
