Cholesterol content in tumor tissues is inversely associated with high-density lipoprotein cholesterol in serum in patients with gastrointestinal cancer

BACKGROUND

The authors have previously demonstrated in different experimental models that sustained processes of cellular growth are characterized by alterations of cholesterol metabolism not only in the proliferating tissues but also in the plasma compartment.

METHODS

To evaluate whether alterations of cholesterol metabolism similar to those observed in experimental models are also associated with human cancer, in the present study cholesterol distribution in tumor tissues and lipid composition in the plasma compartment were determined in patients with different types of gastrointestinal cancer.

RESULTS

The results showed that tumor tissues contain increased amounts of cholesterol when compared with the corresponding normal tissues. Intracellular alterations of cholesterol were accompanied by specific changes of cholesterol in the plasma compartment: high-density lipoprotein (HDL) cholesterol was markedly reduced in the serum of patients with gastrointestinal cancer and the lipoprotein profiles showed a decrease in HDL3 fraction, the main HDL subfraction in human serum. The decrease of HDL cholesterol was negatively associated with the clinical stage of the disease. No changes in either total or low-density lipoprotein cholesterol levels were observed.

CONCLUSIONS

A major function attributed to HDL is to maintain normal cell cholesterol homeostasis by removing excess of cholesterol from intracellular pools. Because the use and storage of cholesterol are increased within the tumor tissues during growth, it is possible to hypothesize that low HDL levels observed in patients with gastrointestinal cancer are associated with the increased cholesterol metabolism in proliferating tissues.

Cholesterol metabolism during the growth of a rat ascites hepatoma (Yoshida AH-130)

The metabolism of cholesterol has been investigated in tumour cells, ascitic fluid and blood serum during the growth of an ascites hepatoma (Yoshida AH-130) in the rat. High rates of cholesterol synthesis and elevated free and esterified cholesterol content were observed in tumour cells. During tumour growth, the host animals progressively developed marked changes in the level and distribution of serum cholesterol consisting in an increase of total cholesterol and of a marked reduction of HDL cholesterol (HDL2 subfraction in particular). In agreement with previous observations, these findings indicate that a consistent pattern of altered cholesterol homeostasis develops in relation to normal or neoplastic tissue growth. High synthetic rates and intracellular accumulation of cholesterol are observed in the proliferating cells. Moreover, blood serum cholesterol decreases in the HDL fraction while it increases in LDLs, suggesting that during proliferative processes cholesterol fluxes between tissues and serum lipoproteins are markedly perturbed.

Hepatic cholesterol in lead nitrate induced liver hyperplasia

Wistar rats treated with lead nitrate were used in these experiments to provide evidence of the possible correlation between hyperplasia, induced cholesterol synthesis and the levels of glucose-6-phosphate dehydrogenase (G-6-PD) in the liver. Lead treatment increases liver weight, hepatic cholesterol esters and the relative content of free cholesterol. An increase of the incorporation of tritiated water in free and cholesterol esters was also observed. The effect of lead resulted in an increase of hepatic G-6-PD at all times considered. The correlation between these parameters and hyperplasia are discussed.

Altered pattern of lipid metabolism in patients with lung cancer

Cholesterol distribution in tumoral tissues and lipid composition in the plasma compartment were determined in patients affected by different histologic types of lung cancer. The results showed that tumoral lung tissues contained 2-fold more total cholesterol and 3.5-fold more esterified cholesterol than normal lung tissues. In the patients the alterations in intracellular cholesterol were also associated with peculiar changes in cholesterol distribution in the plasma compartment. Serum high-density lipoprotein (HDL) cholesterol levels were markedly lower in than in controls. No significant changes in other lipid parameters were observed in these patients. We suggest that the reduced levels of serum HDL cholesterol observed in patients with lung tumors may be a consequence of the disease, probably mediated by the greater utilization of cholesterol for new membrane biogenesis and by the accumulation of esterified cholesterol in tumoral tissues.

Hepatic glucose-6-phosphate dehydrogenase, cholesterogenesis, and serum lipoproteins in liver regeneration after partial hepatectomy

Liver regeneration after partial hepatectomy was used as an experimental model for studying mammalian cell division and replication. The rate of cell proliferation in this hyperplastic model was correlated with hepatic de novo synthesis of cholesterol, with the hexose monophosphate shunt pathway of glucose metabolism, and with serum lipoproteins. An increase of hepatic cholesterol esters and of incorporation of tritiated water in cholesterol esters was observed at 24 hr after partial hepatectomy. Partial hepatectomy also resulted in an increase of hepatic glucose-6-phosphate dehydrogenase and in alteration of serum lipoproteins, primarily due to a selective decline in high density lipoprotein fraction.

Serum lipoprotein profile in the Mediterranean variant of glucose-6-phosphate dehydrogenase deficiency

Sardinian males with erythrocyte glucose-6-phosphate dehydrogenase (G-6-PD) deficiency have lower serum levels of total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C) and apolipoprotein B (ApoB), compared to normals. Since the enzyme deficiency is expressed also in nucleated cells, we studied cholesterol (C) and DNA synthesis and LDL-receptor expression in freshly isolated circulating mononuclear cells from normal and G-6-PD-deficient Sardinians. Synthesis of C (as 14C-acetate incorporation) and of DNA (as 3H-thymidine incorporation) was clearly reduced, both in basal state and after PHA stimulation, in G-6-PD-deficient cells compared to normal cells. On the other hand, no clear influence of G-6-PD deficiency on LDL-receptor expression could be demonstrated. The Mediterranean variant of G-6-PD deficiency is characterized, whatever the metabolic mechanism may be, by a serum lipoprotein pattern of reduced atherogenicity.

Opposite pattern of MDR1 and caveolin-1 gene expression in human atherosclerotic lesions and proliferating human smooth muscle cells

Cholesterol esterification and smooth muscle cell (SMC) proliferation are the crucial events in the development of atherosclerotic lesions. The objective of this study was to analyse cholesterol esterification and the expression of MDR1 (multidrug resistance), ACAT (acyl-CoA:cholesterol acyltransferase) and caveolin-1 genes in atherosclerotic and healthy vascular walls, in SMCs obtained from atherosclerotic lesions and saphenous veins. Results demonstrated higher levels of cholesterol esters, ACAT and MDR1 mRNAs and lower levels of caveolin-1 mRNA in atherosclerotic segments compared to adjacent serial sections of the same artery and the corresponding non-atherosclerotic arteries from cadaveric donors. SMCs isolated from atherosclerotic plaques manifested an increased capacity to esterify cholesterol and to grow at a faster rate than SMCs isolated from saphenous veins. In addition, when SMCs from atherosclerotic plaques were cultured in the presence of progesterone, a potent inhibitor of cholesterol esterification, significant growth suppression was observed. An increase in ACAT and MDR1 expression and a concomitant decrease in caveolin-1 expression were also observed in SMCs isolated from atherosclerotic arteries as early as 12 h after serum stimulation. An opposite pattern was found when SMCs were treated with progesterone. These findings support the idea that cholesterol esterification plays a role both in early atherogenesis and in clinical progression of advanced lesions and raise the possibility that the cholesterol ester pathway might directly modulate the proliferation of SMCs.

Correlation between cholesterol esterification, MDR1 gene expression and rate of cell proliferation in CEM and MOLT4 cell lines

A positive correlation between cholesterol esterification and growth rate potential was previously found in our laboratory during the growth of CEM and MOLT4 lymphoblastic cells. In the current study, we investigated whether the rates of cholesterol esters synthesis correlate with changes of acyl-CoAcholesterol acyltransferase (ACAT) mRNA levels and of other genes implied in cholesterol biosynthesis and uptake, such as 3-hydroxy-3-methylglutaryl-CoA (HMGCoA) reductase and low density lipoprotein (LDL) receptor. The results showed that the more rapid growing CEM cells had lower levels of expression of HMGCoA-reductase and LDL receptors compared to MOLT4. By contrast, ACAT mRNA levels were higher in CEM cells, further supporting the concept of a possible involvement of cholesterol esters in the regulation of cell growth and division. In this study, high levels of cholesterol esterification and of expression of ACAT gene were also associated with a markedly increased expression of multidrug resistance (MDR1) gene, suggesting that MDR1 activity might contribute to regulate the rate of cell growth and division by modulating intracellular cholesterol ester levels.

Changes in serum and hepatic cholesterol in lead-induced liver hyperplasia

Lead nitrate when injected intravenously as a single dose to male Wistar rats causes a strong hepatic proliferative response followed by reabsorption of excess tissue within 10-14 days. The rate of cell proliferation in this hyperplastic model was positively correlated with hepatic de novo synthesis of cholesterol, stimulation of the hexose monophosphate shunt pathway of glucose metabolism and with alterations in serum lipoproteins.

Hexose monophosphate shunt enzymes in lung tumors from normal and glucose-6-phosphate-dehydrogenase-deficient subjects

Glucose-6-phosphate dehydrogenase (G6PD) and 6-phosphogluconate, dehydrogenase the key enzymes of the hexose monophosphate shunt pathway, were measured in both surrounding and tumoral lung tissues from normal and G6PD-deficient subjects. A significant increase of these enzymatic activities in tumoral tissue was found not only in G6PD-normal patients, but also in G6PD-deficient patients with very low or nonmeasurable G6PD activity in both erythrocytes and normal lung tissue.

Cell growth and cholesterol metabolism in human glucose-6-phosphate dehydrogenase deficient lymphomononuclear cells

Atherosclerosis is an inflammatory-fibroproliferative response of the arterial wall involving a complex set of interconnected events where cell proliferation (lymphomonocytes, and endothelial and smooth-muscle cells) and substantial perturbations of intracellular cholesterol metabolism are considered to be among the main features. Glucose-6-phosphate dehydrogenase (G6PD), the key enzyme of the hexose-monophosphate shunt pathway, is an essential enzyme involved in both cell growth and cholesterol metabolism, raising the question as to whether G6PD deficiency may have metabolic and growth implications in a deficient population. In the present study, we investigated cell growth and cholesterol metabolism in peripheral blood lymphomononuclear cells (PBMC) from G6PD-normal (n = 5) and -deficient (n = 5) subjects stimulated with lectins (phytohaemoagglutinin and Concanavalin A). G6PD activity, DNA ([3H]-thymidine incorporation) cholesterol synthesis and esterification ([14C]-acetate and [14C]-oleate incorporation), and G6PD, HMGCoA reductase and low density lipoprotein (LDL) receptor mRNA levels (RT-PCR) all increased following lectin stimulation in both normal and G6PD-deficient cells. However, these parameters were significantly lower in G6PD-deficient cells (P < 0.05). It is of interest that G6PD-deficient PBMC, which showed lower expression of G6PD and higher expression of the LDL receptor gene than normal PBMC under basal conditions, exhibited an opposite pattern after stimulation: G6PD and HMGCoA reductase being expressed at significantly higher levels in deficient than in normal cells (P < 0.05). We conclude that the reduced capability of G6PD-deficient cells to respond to mitogenic stimuli and to synthesize cholesterol esters may represent favourable conditions for reducing the risk of cardiovascular diseases.

Role of cholesterol synthesis and esterification in the growth of CEM and MOLT4 lymphoblastic cells

CEM and MOLT4 are human T-cell lines isolated from patients with acute cell leukaemia. In culture they show important differences in cholesterol metabolism, CEM being less efficient at synthesizing cholesterol and having a lower activity of 3-hydroxy-3-methylglutaryl-CoA (HMGCoA) reductase. To investigate further the relationship between regulation of intracellular cholesterol metabolism at various steps and rate of cell growth, cholesterol synthesis, esterification and efflux were evaluated in CEM and MOLT4 cells at different times during exponential and stationary growth in vitro. It was shown that, although CEM cells have a lower rate of cholesterol synthesis, they grow at a faster rate than MOLT4 cells. However, CEM cells exhibit an increased capacity to esterify cholesterol associated with a decreased efflux of newly synthesized cholesterol into the medium. These results provide evidence for an association between the capability to synthesize and retain cell cholesterol esters and the growth rate potential.

Clinical remission is associated with restoration of normal high-density lipoprotein cholesterol levels in children with malignancies

1. Serum lipids and lipoprotein profiles were determined in children affected by different types of malignancies (leukaemias or lymphomas and solid tumours) both before any treatment and after remission of the disease following chemical or surgical therapy. 2. At the time of diagnosis, children bearing tumours showed hypertriglyceridaemia and reduced concentrations of plasma high-density lipoprotein cholesterol levels, the decrease being particularly prominent in patients with haematological tumours. Children bearing solid tumours displayed an increase of total cholesterol, while those with haematological cancer showed decreased phospholipid levels; low-density lipoprotein cholesterol in neoplastic patients was not significantly different from control values. High triacylglycerol and low high-density lipoprotein cholesterol levels were also evident in cancer patients divided according to age into three groups (0-5, 6-10 and 11-15 years) when compared with age-matched control subjects. Similarly, high triacylglycerol and low high-density lipoprotein cholesterol levels were also observed in both male and female children when patients were divided according to sex and compared with corresponding controls. 3. Clinical remission after therapy was accompanied by an increase of high-density lipoprotein cholesterol levels compared with values observed at diagnosis. In contrast, post-treatment levels of triacylglycerol were higher than those observed before therapy. These results support the hypothesis that alterations of high-density lipoprotein cholesterol levels may be related, at least in part, to the rate of tumour growth, while modifications of triacylglycerol levels may be mediated by different mechanisms.

G6PD activity and gene expression in leukemic cells from G6PD-deficient subjects

In the present study we examined gene expression and glucose-6-phosphate dehydrogenase (G6PD) activity in leukemic cells isolated from G6PD normal and deficient subjects. The results have shown that G6PD activity strongly increases in G6PD normal leukemic cells as well as in G6PD deficient leukemic cells when compared to peripheral blood mononuclear cells (PBMC). Higher levels of G6PD gene expression were observed in leukemic cells from G6PD deficient patients compared to G6PD normal. A similar pattern of gene expression was also observed for 3-hydroxy-3-methylglutaryl coenzyme A (HMGCoA) reductase. These results support the hypothesis that G6PD deficient cell, in order to sustain their growth, must respond to the low activity of their mutant enzyme with an increase in quantity through an induction of gene expression.

Hexose monophosphate shunt and cholesterol synthesis in the diabetic and fasting states

The livers of streptozotocin-induced diabetic and fasted rats showed a decreased cholesterol synthesis measured by in vitro incorporation of [2-14C]acetate. A significant decrease of glucose-6-phosphate dehydrogenase (G-6-PD), 6-phosphogluconate dehydrogenase (6-PGD), and pyruvate kinase (PK) was also observed 7 days after administration of streptozotocin. These enzymatic activities were also low in livers of 72 hr fasted animals. An increase of glucose-6-phosphatase (G-6-Pase) was observed consistently in diabetic as well as in fasted rats. Suitable amounts of insulin and refeeding normalized the alterated enzymatic activities in diabetic and in fasted animals, respectively.

Perturbations of triglycerides but not of cholesterol metabolism are prevented by anti-tumour necrosis factor treatment in rats bearing an ascites hepatoma (Yoshida AH-130)

Rats transplanted with the ascites hepatoma Yoshida AH-130 developed a severely progressive cachexia, characterised by marked alterations in protein and lipid metabolism. In particular, high levels of serum triglycerides and free fatty acids were associated with altered levels and distribution of plasma cholesterol, with increased total and very low-density lipoprotein-low-density lipoprotein (VLDL-LDL) cholesterol and reduced high-density lipoprotein (HDL) cholesterol. The tumour cells showed high rates of cholesterol synthesis and elevated content of free and esterified cholesterol, whereas total cholesterol synthesis was reduced in the host liver. To determine whether these perturbations could be related to the elevation of tumour necrosis factor alpha (TNF-alpha) previously shown in the AH-130 bearers (Tessitore L, Costelli P, Baccino FM 1993, Br J Cancer, 67, 15-23), either anti-TNF polyclonal antibodies or non-immune IgGs were injected daily after tumour transplantation. The anti-TNF treatment neither affected tumour growth nor prevented the serum cholesterol changes, while attenuating the hypertriglyceridaemia and the elevated serum free fatty acid levels. These data indicate that TNF does not appear to be directly involved in the altered cholesterol metabolism in AH-130 hosts, thus supporting the view that cholesterol metabolism and lipid metabolism are regulated differently during tumour growth.

Alterations of lipid and cholesterol metabolism in cachectic tumor-bearing rats are prevented by insulin

The ascites hepatoma Yoshida AH130 causes in the host a rapid and progressive body weight loss, associated with reduced food intake, and protein and lipid hypercatabolism. Because insulin regulates glucose as well as lipid and protein metabolism, we suggest that the observed alterations are at least in part secondary to hypoinsulinemia and/or to the increase of counterregulatory hormones in AH130-bearing rats. To verify this hypothesis, controls with free access to food (n = 4), controls with free access to food plus insulin (107 micromol. kg body wt-1. d-1) (n = 4), controls pair-fed to the tumor-bearing rats (n = 4), pair-fed controls treated with insulin (n= 4), tumor hosts (n = 9), and tumor hosts treated with insulin (n = 6) were used. The Yoshida ascites hepatoma cells ( approximately 10(8) cells/rat) were inoculated intraperitoneally. Daily food intake and body weight were measured; insulin was injected starting the day of tumor implantation for 6 d. The metabolism of both cholesterol and lipids was investigated in tumor cells, and ascitic fluid and blood serum were investigated at the end of treatment. Insulin prevented the reduction of food intake (19 +/- 0.6 vs. 13 +/- 0.4 g/d, P < 0.01; AH130 hosts treated and not treated with insulin, respectively), the loss of body weight (202 +/- 12 vs. 135 +/- 9 g, P < 0.01), lowered the circulating triglycerides (48.3 +/- 4.9 vs. 84.5 +/- 7.1 mmol/L, P < 0.01), and free fatty acids (561 +/- 47 vs. 989 +/- 54 mmol/L (P < 0.01), while corrected the decrease of adipose lipoprotein lipase activity (1,240 +/- vs. 300 +/- pmol FA, P < 0.01) observed in AH130 hosts. Moreover, insulin prevented the decrease in HDL cholesterol (13.2 +/- 0.8 vs. 9.3. +/- 0.7 mmol/L, P < 0.01) and significantly increased hepatic cholesterol synthesis as evaluated by 14C-acetate incorporation into cholesterol, in both liver (3,337 +/- 245 vs. 830 +/- 115 Bq/g, P < 0.01) and AH130 cells (11,676 +/- 1,693 vs. 4,196 +/- 527 Bq/10(6) cells, P < 0.01). Thus insulin treatment ameliorated many metabolic derangements, with a lengthening of rats survival time (7 +/- 1 vs. 11 +/- 1 d, P < 0.05) without significantly stimulating tumor growth. These data, together with our previous observations on the effectiveness of insulin on protein turnover perturbations, suggest that many metabolic alterations occurring during cancer cachexia can be avoided by the administration of this hormone.

Variations of serum lipoproteins during cell proliferation induced by lead nitrate

In the present study serum lipoproteins were investigated during cell proliferation induced by a potent mitogen, lead nitrate. A strong decrease in HDL2 and a concomitant increase in HDL3 were observed in lead-treated rats. The recovery of normal lipoprotein pattern took place together with the regression of hyperplastic process. Since a decrease in HDL also occurs under other conditions of cell growth, we hypothesize that a decrease in HDL, mainly in HDL2 subfraction, may represent a generalized phenomenon related to massive cell proliferation.

MDR1 gene expression in normal and atherosclerotic human arteries(1)

Recent studies have shown that a membrane p-glycoprotein, encoded by MDR1 gene, is involved in the transport of free cholesterol from the plasma membrane to endoplasmic reticulum, the site of cholesterol esterification by acyl-CoA:cholesterol acyltransferase (ACAT). Moreover, results deriving from our previous studies have shown that the rate of cell proliferation was positively correlated with cholesteryl ester levels as well as with ACAT and MDR1 gene expression. In this study, lipid content and the expression of the genes involved in cholesterol metabolism such as hydroxy-methylglutaryl coenzyme A reductase (HMGCoA-R), low-density lipoprotein receptor (LDL-R), ACAT and MDR1 have been investigated in control and atherosclerotic arteries. The results have shown that the levels of cholesteryl ester increase with the age of cadaveric donors in arteries prone to atherosclerosis (abdominal aorta, superficial femoral artery) and become predominant in advanced atherosclerotic lesions. The mRNA levels of ACAT and MDR1 showed the same age correlation, reaching the highest values in atherosclerotic specimens. These results suggest that MDR1 may be involved in the accumulation of intracellular cholesterol ester levels found in atherosclerotic lesions. Moreover, the levels of HMGCoA-R, LDL-R and ACAT gene expressions progressively increased with the age of cadaveric donors; conversely, in atherosclerotic specimens, the mRNA levels of HMGCoA-R and LDL-R drastically decreased while ACAT gene expression reached its maximum. These findings suggest a reactivation of normal homeostatic regulation of cholesterol in advanced and complicated lesions.

Lipid metabolism and molecular changes in normal and atherosclerotic vessels

OBJECTIVES

a positive correlation between cholesterol esterification, acyl-CoA:cholesterol acyltransferase (ACAT), multidrug resistance (MDR1) gene expression and atherosclerotic lesions has been shown in human arteries. The objective of this study was to map the expression of MDR1, ACAT genes and the cholesteryl ester content in normal, atherosclerotic and varicose human vessels.

MATERIALS

vascular segments were obtained from seven cadaveric donors, 27 patients undergoing vascular surgery for severe atherosclerotic disease and 11 patients with saphenous vein varicosities.

METHODS

lipid analysis and RT-PCR of MDR1 and ACAT mRNAs were performed.

RESULTS

an increase in cholesteryl ester content and in ACAT and MDR1 expression was demonstrated in relation to the age in the arteries prone to atherosclerosis; this expression was maximal in arteries from symptomatic patients. In resistant arteries and in veins cholesteryl ester accumulation was rare and light, while ACAT and MDR1 expression was not related to the age of the subjects.

CONCLUSIONS

the results showed that an increase in MDR1 and ACAT expression may be responsible for the accumulation of cholesteryl esters as well as for cell growth rate acceleration in vessel sites prone to atherosclerosis.

HDL subfractions as altered in cancer patients

Previous studies from the authors' laboratories have shown that cancer patients are characterized by lower levels of high-density lipoprotein cholesterol (HDL-C) compared with those of normal subjects. HDLs are a complex class of lipoproteins which can be divided mainly into two categories, HDL2 and HDL3, that have not only different lipid and protein composition but also different functions. Therefore, for a better understanding of the metabolism of HDL during tumour growth, the different subfractions of HDL (HDL2 and HDL3) were analysed in the serum of neoplastic patients using a rapid and simple high-performance liquid chromatography (HPLC) method for the analysis. The results obtained showed that serum from neoplastic patients exhibits a peculiar pattern in the distribution of HDL subfractions, consisting of a sharp decrease in HDL3 and a consequent increase of the normal HDL2/HDL3 ratio. It is suggested that evaluation of the HDL subfractions may be of clinical relevance for cancer status and that due to its simplicity, short analytical time and small sample volume required, the HPLC technique used in this study can be easily applied to routine analysis in cancer patients.

Glucose-6-phosphate dehydrogenase (G6PD) activity in tumoral tissues of G6PD-deficient subjects affected by larynx carcinoma

The activity of glucose-6-phosphate dehydrogenase (G6PD), the key enzyme of the hexose monophosphate (HMP) shunt pathway, was measured in both normal and tumoral larynx tissues from normal and G6PD deficient subjects. Significant increases of this enzymatic activity were found in tumoral tissues of both normal and G6PD deficient subjects, who were characterized by very low levels of G6PD activity in erythrocytes as well as in larynx tissue.

Filippo C, Vargiu L, Marceddu T, Sanna L, La Colla P, Dessi S. MDR1, cholesterol certification and cell growth: a comparative study in normal and multidrug-resistant KB cell lines

The product of the MDR1 gene (P-gp) has been implicated in the transport of cholesterol from plasma membrane to endoplasmic reticulum for esterification. In previous studies on leukemia cell lines, we suggested that cholesterol esterification may regulate the rate of cell growth and that the MDR1 gene might be involved in this process by modulating intracellular cholesterol esters levels. To further investigate this matter, the rate of cell growth, cholesterol metabolism, expression of the MDR1 gene, and P-gp activity were compared in KB cell lines displaying differences in expression and function of P-gp (drug-sensitive phenotype versus MDR phenotype). The rate of cell growth correlated with cholesterol esterification in all KB cell lines, whereas the over-expression of MDR1 observed in the MDR cell lines was not always associated with an increased capacity of cells to esterify cholesterol. Two known inhibitors of P-gp activity, progesterone and verapamil, strongly inhibited both cholesterol esterification and cell proliferation in all KB cell lines, but they affected intracellular accumulation of labeled vinblastine only in MDR cell lines. These results further support a role for cholesterol esters in the regulation of cell growth and suggest that the P-gp expressed in MDR KB cells is not involved in the general process leading to cholesterol esterification.

Comparative effects of insulin and refeeding on DNA synthesis, HMP shunt and cholesterogenesis in diabetic and fasted rats

DNA synthesis, cholesterogenesis and the enzymes of the hexosemonophosphate (HMP) shunt pathway were investigated in liver of diabetic rats treated with insulin and in fasted/re-fed rats. Both insulin and refeeding were found to induce liver cell proliferation, accompanied by a remarkable increase in cholesterogenesis. An enhancement of glucose-6-phosphate dehydrogenase (G6PD) and 6-phosphogluconate dehydrogenase (G6PD) activities was also found in insulin-treated diabetic rats and in re-fed rats, supporting the concept that these two enzymes are involved in the proliferative process. Since insulin did not exert the same biochemical effects in a non replicating cell population, such as in insulin-treated normal rats, these studies provide new evidence of a close correlation between DNA, cholesterol synthesis and HMP shunt enzymes during cell proliferation.

Cell cholesterol esters and high-density lipoprotein plasma levels during liver hyperplasia in choline-fed male and female rats

Sexual dimorphism exists in the response of rats to lead nitrate, liver hyperplasia occuring earlier and being more pronounced in males. Excess dietary choline in females shifted the growth pattern towards that of males. To determine whether phosphatidylcholine-induced growth modulations could be related to a derangement of cholesterol metabolism, liver accumulation of cholesterol esters and plasma lipoprotein patterns were investigated. In males, lead-induced liver hyperplasia was associated with increased total cholesterol hepatic content, accumulated cholesterol esters and reduced concentration of plasma High Density Lipoprotein (HDL) cholesterol. Females were less responsive to the liver mitogenic signal of lead nitrate; there was no elevation of cholesterol content nor any marked accumulation of cholesterol esters. This is consistent with the lack of change in the plasma levels of HDL cholesterol. Continuous choline feeding displaced the liver cholesterol ester pattern and plasma HDL cholesterol levels in females, and in parallel that of DNA synthesis, towards those of males. Choline was not observed to have any effect in males. These results suggest that the derangement of phosphatidylcholine metabolism induces growth-related changes in cholesterol turnover; they are consistent with the proposal that the intracellular content of cholesterol esters may have a role in regulating liver growth rates.