We have found that in normal human kidney proximal tubular (PT) cells, plasma low density lipoproteins (LDL) exerts a concentration dependent decrease in the level of lactosylceramide (LacCer) and the enzyme involved in LacCer biosynthesis, i.e., UDP-galactose: gluco-sylceramide B1->4 galactosyltransferase (GalT-2). However, methyl-LDL which enters cells through a LDL receptor independent pathway stimulated the activity of GalT-2 approximately three fold in normal PT cells. In contrast, in PT cells from patients with familial hypercholesterolemia (FH), and human kidney tumor PT cells which do not have LDL receptors, LDL increased the activity of GalT-2 approximately three fold. This resulted in a 15 fold increase in the cellular level of LacCer in FH PT cells and the synthesis of more complex glycosphingolipids in tumor PT cells. We have purified and partially characterized GalT-2 from human kidney. Antibodies raised against this enzyme specifically immunoprecipitate GalT-2 from cultured normal PT cells. Our overall objects are: 1) to pursue molecular characterization/expression of GalT-2 and to study the effect of LDL, and M-LDL on the levels and the turnover of mRNA in PT cells; 2) to determine the cytolocalization of GalT-2 in normal PT, FH-PT and tumor PT cells; and 3) to study the effects and mechanisms of action of LDL and modified LDL (M-LDL) on the level of GalT-2 in PT cells. Hypothesis. The synthesis (overproduction) and storage of lactosylceramide in FH-PT cells is due to the lack of regulation of GalT- 2 by LDL. Studies are proposed that will elucidate biochemical mechanisms of action of LDL and M-LDL on GalT-2 and its regulation at the molecular level. such studies provide fundamental knowledge regarding how LacCer synthesis is regulated in normal PT cells and the pathophysiology of LDL mediated lack of regulation of GalT-2 in FH-PT and tumor PT cells in man.
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