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Regulation of synthesis of lactosylceramide in normal and tumor proximal tubular cells.
We have measured the binding and degradation of low-density lipoprotein (LDL) and LDL-mediated effects on cholesteryl ester (CE) synthesis in cultured normal and tumor proximal tubular (PT) cells. The effects of LDL on the regulation of glycosphingolipid metabolism in cells was pursued employing radioactive precursors, e.g., [3H]serine, and [3H]glucose and by measuring the activity of UDP-galactose: glucosylceramide: B1-4 galactosyltransferase (GalT-2). In normal PT cells, there was a saturable and displaceable binding and degradation of 125I-LDL and a LDL mediated 14-fold stimulation of cholesteryl ester (CE) synthesis. This was accompanied by a suppression (70-80%) of incorporation of [3H]glucose and [3H]serine into GlcCer, LacCer, GbOse3Cer and GbOse4Cer and suppression (70-80%) of GalT-2 activity. In tumor PT cells, displaceable binding and degradation of 125I-LDL was not observed and LDL failed to stimulate CE synthesis. In such cells, LDL exerted a concentration-dependent stimulation of [3H]glucose and [3H]serine incorporation into GSL. Maximum stimulation (250%) of GalT-2 activity in tumor PT cells occurred with 25 micrograms LDL/ml medium. We conclude that LDL taken up via receptor mediated pathway decreases GaIT-2 activity in normal PT cells. In contrast, LDL not taken up via the LDL receptor pathway in tumor PT cells failed to suppress the incorporation of [3H]glucose and [3H]serine into glycosphingolipids and GalT-2 activity leading to a stimulation of lactosylceramide synthesis.
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