MEVALONIC ACID IS LIMITING FOR N-LINKED GLYCOSYLATION AND TRANSLOCATION OF THE INSULIN-LIKE GROWTH-FACTOR-I RECEPTOR TO THE CELL-SURFACE - EVIDENCE FOR A NEW LINK BETWEEN 3-HYDROXY-3-METHYLGLUTARYL-COENZYME-A REDUCTASE AND CELL-GROWTH

Depletion of mevalonic acid (MVA), obtained by inhibition of 3-hydroxy -3-methylglutaryl-coenzyme A (HMG-CoA) reductase using lovastatin, dep ressed the biosynthesis of dolichyl-phosphate and the rate of N-linked glycosylation and caused growth arrest in the melanoma cell line SK-M EL-2. The growth arrest was partially prevented by addition of high co ncentrations of insulin-like growth factor-1 (IGF-1) to the cells, ind icating that MVA depletion may inhibit cell growth through decreasing the number of IGF-1 receptors (IGF-1R) at the cell surface, Such a dec rease in receptor number might be a result of a lowered translocation of de novo synthesized receptors to the cell membrane which in turn mi ght be a result of a decreased N-linked glycosylation of the receptor proteins, We could also demonstrate that IGF-1R became underglycosylat ed and that the amount of de novo synthesized IGF-1R proteins at the c ell membrane was drastically decreased upon MVA depletion, Analysis of receptor proteins cross-linked with IGF-1, as well as binding assays and immunocytostaining confirmed that the number of functional membran e bound IGF-1R was substantially reduced. The N-linked glycosylation a nd the expression of de novo synthesized IGF-1R proteins at the cell s urface as well as the number of IGF-1 binding sites were completely re stored upon replenishment of MVA. These effects of MVA were efficientl y abrogated by the glycosylation inhibitor tunicamycin, The translocat ion of IGF-1R to the cell membrane was shown to take place just prior to initiation of DNA synthesis in arrested cells stimulated with MVA, Additionally, there was a clear correlation between IGF-1 binding and initiation of DNA synthesis with regard to the MVA dose requirement, I t was confirmed that inhibition of HMG-CoA reductase activity and N-li nked glycosylation also depressed the expression of functional IGF-1R in other cell types (i.e. hepatoblastoma cells and colon cancer cells) , Our data suggest that this mechanism is involved in MVA-regulated ce ll growth.