Treatment with HMG CoA reductase inhibitors, i.e. 25-hydroxycholesterol and mevinolin, inhibited cell growth of the human breast cancer cell line MDA231 in a cell cycle-specific manner by blocking progression through G1. Since 25-hydroxycholesterol, as distinguished from mevinolin, also inhibits steps in mevalonate metabolism, exogenous mevalonate failed to overcome the 25-hydroxycholesterol-induced block. Using 25-hydroxycholesterol we investigated whether protein isoprenylation or protein glycosylation is rate-limiting for G1-progression in MDA231. We thereby found that 25-hydroxycholesterol was efficient in inhibiting N-linked glycosylation, measured by determining the glucosamine incorporation into cellular proteins. In contrast, 25-hydroxycholesterol did not depress the level of protein isoprenylation, measured as incorporation of mevalonate into cellular proteins. Furthermore, tunicamycin (an inhibitor of N-linked glycosylation) inhibited G1-progression of MDA231 in a similar way to 25-hydroxycholesterol. Addition of trans-trans farnesol, which inhibits protein isoprenylation, did not result in any inhibitory effects on MDA231 growth. Our data suggest that N-linked protein glycosylation is rate-limiting in the isoprenoid-regulated cell cycle of human breast cancer cells.

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