LOW-DENSITY-LIPOPROTEIN RECEPTOR AND 3-HYDROXY-3-METHYLGLUTARYL COENZYME-A REDUCTASE GENE-EXPRESSION IN HUMAN MONONUCLEAR LEUKOCYTES IS REGULATED COORDINATELY AND PARALLELS GENE-EXPRESSION IN HUMAN LIVER

The liver plays a key regulatory role in cholesterol metabolism. Two p roteins are central in this role; the LDL receptor and 3-hydroxy-3-met hylglutaryl CoA reductase (HMG CoA reductase), the rate-limiting enzym e in cholesterol biosynthesis. In the current investigation, we have u sed a sensitive nonradioactive method to study the regulation of LDL r eceptor and HMG CoA reductase mRNA levels in liver biopsy samples and freshly isolated mononuclear leukocytes from 13 patients who underwent cholecystectomy for gallstones. mRNA copy numbers were determined by PCR amplification of reverse-transcribed RNA using synthetic RNA as an internal standard. Incorporation of digoxigenin-11-dUTP during amplif ication allowed direct detection and quantitation of mRNA levels by ch emiluminescence. These experiments showed that the average number of L DL receptor mRNA molecules in liver (21 +/- 3 X 10(4)/mu g Of RNA) and mononuclear leukocytes (24 +/- 3 X 10(4)/ mu g of RNA) are indistingu ishable, whereas the number of HMG CoA reductase molecules in liver (1 07 +/- 15 X 10(4)/mu g of RNA) is smaller than that in mononuclear leu kocytes (158 +/- 21 X 10(4)/mu g of RNA, P < 0.05). These numbers corr espond to an average of 1-6 copies of LDL receptor mRNA and 5-42 copie s of HMG CoA reductase mRNA per cell. There was a significant correlat ion between the numbers of LDL receptor (P = 0.0005) and HMG CoA reduc tase (P = 0.003) mRNA molecules in liver and mononuclear leukocytes. F urthermore, the numbers of copies of HMG CoA reductase and LDL recepto r mRNA were correlated with each other in both liver (P = 0.02) and mo nonuclear leukocytes (P = 0.01), consistent with coordinate regulation . These data demonstrate that the mechanisms which regulate mRNA level s in liver and mononuclear cells are similar and suggest that freshly isolated mononuclear cells can be used to predict HMG CoA reductase an d LDL receptor mRNA levels in liver.