Hepatomas Cholesterol biosynthesis

As early as 1966, Siperstein et al. showed negative feed-back control of cholesterol biosynthesis to be defective in transplantable hepatomas. More recently Bricker Levey (1972) demonstrated unsupressible cholesterol and fatty acid synthesis by cyclic nucleotides in hepatomas and indicated a deletion of this regulatory mechanism of lipogenesis. 

The potential of PET to inhibit cholesterol biosynthesis was determined by the inhibition of the incorporation of [2- H]acetate and R-[2- C]mevalonate into cholesterol (21). Human hepatoma cells (Hep G2) were cultured in DMEM supplemented with 10% fetal calf serum (PCS) in 12-well culture dishes (3x10 cell/well) at 37 C for 48 hours. After transferring to fresh medium, cells were treated with PET and labeled precursors ([2- H]acetate 2.5 pCi R-[2- CJmevalonate, 1.3 pCi). Lovastatin, in hydroxy acid form (5 and 10 pM), was used as a positive control. Incorporation experiment was carried out at 37 °C for 2 hours. After removal of medium and extensive washing, cells were harvested and crude total lipids were collected and saponified. Unlabeled cholesteiyl oleate was used as a carrier in saponification. Cholesterol was recovered by extraction with n-hexane. Radioactivity was measured by using a liquid scintillation counter. 

This study demonstrated that the Pu-Erh tea contained lovastatin in a low, but yet detectable amount. The aqueous extract of Pu-Erh tea (PET) inhibited cholesterol biosynthesis in cultured human hepatoma cells (Hep G2). PET did not affect post-mevalonate events in the cholesterol pathway, since the incorporation of labeled mevalonate into cholesterol was not affected. Direct evidence to support the occurrence of lovastatin in PET was based on extensive purification and identification of lovastatin in its lactone form by mass spectrometry. To enrich lovastatin, PET was solvent extracted to recover lovastatin in lactone form. The content of lovastatin in Pu-Erh tea varied greatly among different batches. The situation is not unexpected, since the preparation procedure of Pu-Erh tea involves natural fermentation. The growth of Aspergillus and production of lovastatin in Pu-Erh tea during the fermentation and storage is not under control. 

Patients with familial hypercholesterolaemia exhibit lower levels of plasma cholesterol after an operation for portacaval anastomosis, and it has now been shown in rats that such an operation causes an increase in HMG-CoA reductase and cholesterol 7a -hydroxylase activities. Many transplantable human and rodent hepatomas do not control the rate of sterol biosynthesis and HMG-CoA reductase levels in response to dietary cholesterol as normal liver cells do. However, certain hepatoma cells have now been found that, although lacking feedback regulation of choles-terologenesis in vivo, retain their regulatory ability in vitro It thus appears that malignant transformation is not necessarily linked to the loss of regulation by the cell of HMG-CoA reductase activity or sterol synthesis. 

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