Regulation of SR-BI-mediated selective lipid uptake in Chinese hamster ovary-derived cells by protein kinase signaling pathways.

Scavenger receptor, class B, type I (SR-BI) mediates binding and internalization of a variety of lipoprotein and nonlipoprotein ligands, including HDL. Studies in genetically engineered mice revealed that SR-BI plays an important role in HDL reverse cholesterol transport and protection against atherosclerosis. Understanding how SR-BI's function is regulated may reveal new approaches to therapeutic intervention in atherosclerosis and heart disease. We utilized a model cell system to explore pathways involved in SR-BI-mediated lipid uptake from and signaling in response to distinct lipoprotein ligands: the physiological ligand, HDL, and a model ligand, acetyl LDL (AcLDL). In Chinese hamster ovary-derived cells, murine SR-BI (mSR-BI) mediates lipid uptake via distinct pathways that are dependent on the lipoprotein ligand. Furthermore, HDL and AcLDL activate distinct signaling pathways. Finally, mSR-BI-mediated selective lipid uptake versus endocytic uptake are differentially regulated by protein kinase signaling pathways. The protein kinase C (PKC) activator PMA and the phosphatidyl inositol 3-kinase inhibitor wortmannin increase the degree of mSR-BI-mediated selective lipid uptake, whereas a PKC inhibitor has the opposite effect. These data demonstrate that SR-BI's selective lipid uptake activity can be acutely regulated by intracellular signaling cascades, some of which can originate from HDL binding to murine SR-BI itself.

New phenolic inhibitors of yeast homoserine dehydrogenase.

A relatively unexploited potential target for antimicrobial agents is the biosynthesis of essential amino acids. Homoserine dehydrogenase, which reduces aspartate semi-aldehyde to homoserine in a NAD(P)H-dependent reaction, is one such target that is required for the biosynthesis of Met, Thr, and Ile from Asp. We report a small molecule screen of yeast homoserine dehydrogenase that has identified a new class of phenolic inhibitors of this class of enzyme. X-ray crystal structural analysis of one of the inhibitors in complex with homoserine dehydrogenase reveals that these molecules bind in the amino acid binding region of the active site and that the phenolic hydroxyl group interacts specifically with the backbone amide of Gly175. These results provide the first nonamino acid inhibitors of this class of enzyme and have the potential to be exploited as leads in antifungal compound design.