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PubMed2001

Bisphosphonates: an overview with special reference to alendronate.

Bisphosphonates, analogues of pyrophosphate, are potent inhibitors of osteoclast-mediated bone resorption. They are used in the treatment of Paget's disease of bone, hypercalcaemia and osteolytic bone disease of malignancy, primary and secondary hyperparathyroidism, and in osteoporosis. Bisphosphonate treatment causes an early reduction in bone resorption followed by a later reduction in bone formation. The early inhibition of bone resorption induces a reduction in serum calcium which leads to increased parathyroid hormone (PTH), and subsequently an increase in 1,25-dihydroxyvitamin D. The secondary hyperparathyroidism of bisphosphonate treatment also leads to urinary calcium conservation and phosphaturia, and a reduction in serum phosphate. The increase in the PTH following bisphosphonate therapy is a response to the change in serum calcium and can occur even when there is hypercalcaemia, and this can cause confusion in the interpretation of PTH results. The hypocalcaemic response to bisphosphonates is occasionally severe, especially in patients with hypoparathyroidism. The recent elucidation of bisphosphonate action at the cellular level on the mevalonate pathway has led to interest in its effects on lipoprotein metabolism, which may prove to be of clinical significance. Newer and more potent bisphosphonates are currently undergoing clinical trials in malignant bone disease and osteoporosis, and will lead to further advances in the optimal management of these conditions.

Alendronate↗
PubMed2003

Fosmidomycin resistance in adenylate cyclase deficient (cya) mutants of Escherichia coli.

Adenylate cyclase deficient (cya) mutants of E. coli K-12 were found to be resistant to fosmidomycin, a specific inhibitor of the non-mevalonate pathway, just like to fosfomycin. E. coli glpT mutants were resistant to fosfomycin and also to fosmidomycin. This fact shows that fosmidomycin was transported inside via the glycerol-3-phosphate transporter, GlpT. DNA micro-array analysis showed that the transcription of glpT and other genes concerning glycerol utilization were highly dependent on the presence of cAMP.

Adenylyl Cyclases↗
PubMed2005

Functional analysis of two solanesyl diphosphate synthases from Arabidopsis thaliana.

Solanesyl diphosphate (SPP) is regarded as the precursor of the side-chains of both plastoquinone and ubiquinone in Arabidopsis thaliana. We previously analyzed A. thaliana SPP synthase (At-SPS1) (Hirooka et al., Biochem. J., 370, 679-686 (2003)). In this study, we cloned a second SPP synthase (At-SPS2) gene from A. thaliana and characterized the recombinant protein. Kinetic analysis indicated that At-SPS2 prefers geranylgeranyl diphosphate to farnesyl diphosphate as the allylic substrate. Several of its features, including the substrate preference, were similar to those of At-SPS1. These data indicate that At-SPS1 and At-SPS2 share their basic catalytic machinery. Moreover, analysis of the subcellular localization by the transient expression of green fluorescent protein-fusion proteins showed that At-SPS2 is transported into chloroplasts, whereas At-SPS1 is likely to be localized in the endoplasmic reticulum in the A. thaliana cells. It is known that the ubiquinone side-chain originates from isopentenyl diphosphate derived from the cytosolic mevalonate pathway, while the plastoquinone side-chain is synthesized from isopentenyl diphosphate derived from the plastidial methylerythritol phosphate pathway. Based on this information, we propose that At-SPS1 contributes to the biosynthesis of the ubiquinone side-chain and that At-SPS2 supplies the precursor of the plastoquinone side-chain in A. thaliana.

Alkyl and Aryl Transferases↗
PubMed2004

HMG-CoA reductase inhibitors and the risk of vertebral fracture.

UNLABELLED: Statins inhibit an enzyme in the mevalonate pathway and therefore may affect bone. In this first study on both symptomatic and nonsymptomatic vertebral fractures in the elderly (N = 3469), we show that long-term statin use is significantly associated with a 50% lower vertebral fracture risk. Randomized trials on statins and fractures, carried out in populations at risk for fractures, are needed. INTRODUCTION: Statins are cholesterol-lowering agents that could potentially affect bone. Previous studies on statin use and fracture risk reported contradictory results and did not include both symptomatic and nonsymptomatic vertebral fractures. MATERIALS AND METHODS: To examine the association between statin use, vertebral fractures, and lumbar spine BMD, we performed a prospective population-based cohort study in men and women (N = 3469) > or =55 years of age. These individuals had both baseline and follow-up spinal X-rays available. Statin use was obtained from detailed computerized pharmacy data, and the total number of days of exposure before second X-ray was calculated. A multivariate logistic regression model was fitted to calculate odds ratios and CIs. RESULTS: During a mean follow-up of 6.5 years, 176 incident vertebral fractures occurred. There were 508 statin users and 16 exposed cases. The adjusted relative risk for incident vertebral fracture in users of statins (compared with nonusers) was 0.58 (95% CI, 0.34-0.99). The relative risk decreased on higher cumulative use to 0.52 (95% CI, 0.28-0.97) for use for more than 365 days during the study period. Use of (the hydrophilic statin) pravastatin and use of nonstatin cholesterol-lowering drugs was not significantly associated with vertebral fracture risk. Statin use was not significantly associated with lumbar spine BMD. CONCLUSION: Statin use is associated with a lower risk of vertebral fracture. Randomized clinical trials in a population at risk for fracture are needed to examine this association.

Aged↗
PubMed2006

Effect of monoterpenes on the formation and activation of osteoclasts in vitro.

UNLABELLED: Monoterpenes, present in aromatic plants, are known to inhibit bone resorption in vivo. In this in vitro study, they inhibited the activation of osteoclasts only at high concentrations but inhibited the formation at much lower concentrations. Therefore, monoterpenes may act in vivo directly on osteoclastogenesis. INTRODUCTION: Monoterpenes are the major components of essential oils, which are formed in many plants. Typically, they are found in herbs and certain fruits. When fed to rats, they inhibit bone resorption by an unknown mechanism. In this study, their effect on the activity and formation of osteoclasts in vitro was studied. MATERIALS AND METHODS: The effect of monoterpenes on the development of osteoclasts was studied in co-cultures of bone marrow cells and osteoblasts and in cultures of spleen cells grown with colony stimulating factor (CSF)-1 and RANKL. In cultures of primary osteoblasts, alkaline phosphatase activity and levels of mRNA encoding RANKL and osteoprotegerin (OPG) mRNA (RT-PCR), and in osteoblast and spleen cell cultures, lactate dehydrogenase activity, a measure of toxicity, were determined. The activity of isolated rat osteoclasts was determined by counting the osteoclasts with actin rings using histofluorometry. RESULTS: The monoterpenes inhibited the formation of osteoclasts more strongly in co-cultures (> or = 1 microM) than in cultures of spleen cells (> or = 10 microM). They had a minor effect on osteoblasts. Toxic effects were not observed. The inhibition of the formation of osteoclasts was not reversed by the addition of farnesol and geranylgeraniol, excluding an effect of the monoterpenes through the mevalonate pathway. A high concentration of 1 mM was required to inhibit the activation of osteoclasts. This effect, shown for menthol and borneol, was reversible. CONCLUSIONS: The results suggest that the monoterpenes inhibit bone resorption in vivo through a direct effect on the formation of osteoclasts acting mainly on the hemopoietic cells.

Animals↗
PubMed1998

Heterocycle-containing bisphosphonates cause apoptosis and inhibit bone resorption by preventing protein prenylation: evidence from structure-activity relationships in J774 macrophages.

Recent evidence suggests that bisphosphonates (BPs) may inhibit bone resorption by mechanisms that lead to osteoclast apoptosis. We have previously shown that BPs also reduce cell viability and induce apoptosis in the macrophage-like cell line J774. To determine whether BPs inhibit osteoclast-mediated bone resorption and affect J774 macrophages by the same molecular mechanism, we examined the potency to reduce J774 cell viability of pairs of nitrogen-containing BPs that differ slightly in the structure of the heterocycle-containing side chain but that differ markedly in antiresorptive potency. In all cases, the most potent antiresorptive BP of each pair also caused the greatest loss of J774 viability, while the less potent antiresorptive BPs were also less potent at reducing J774 cell viability. Similarly, the bisphosphinate, phosphonoalkylphosphinate and monophosphonate analogs of BPs (in which one or both phosphonate groups are modified, giving rise to much less potent or inactive antiresorptive agents) were much less potent or inactive at reducing J774 cell viability. Thus, the structure-activity relationships of BPs for inhibiting bone resorption match those for causing loss of cell viability in J774 cells, indicating that BPs inhibit osteoclast-mediated bone resorption and reduce J774 macrophage viability by the same molecular mechanism. Loss of J774 cell viability after treatment with BPs was associated with a parallel increase in apoptotic cell death. We have recently proposed that nitrogen-containing BPs reduce cell viability and cause J774 apoptosis as a consequence of inhibition of enzymes of the mevalonate pathway and hence loss of prenylated proteins. In this study, the BPs that were potent inducers of J774 apoptosis and potent antiresorptive agents were also found to be effective inhibitors of protein prenylation in J774 macrophages, whereas the less potent BP analogs did not inhibit protein prenylation. This provides strong evidence that BPs with a heterocyclic, nitrogen-containing side chain, such as risedronate, inhibit osteoclast-mediated bone resorption and induce J774 apoptosis by preventing protein prenylation.

Animals↗
PubMedDate not supplied

Studies on the biosynthesis of striatal-type diterpenoids and the biological activity of herical.

Feeding experiments with specifically 13C-labeled glucose disclosed that the diterpenoid part of the striatals/striatins is formed via the mevalonate pathway, whereas the pentose moiety originates either via glucuronic acid (70%) or the pentose phosphate cycle (30%). Application of radioactively labeled herical to cultures of Hericium ramosum demonstrates the pivotal role of this cyathane-xyloside in striatal biosynthesis. Herical inhibits a large spectrum of fungi and bacteria and shows cytotoxic and hemolytic properties.

Anti-Bacterial Agents↗
PubMed2004

Bisphosphonates: preclinical review.

Bisphosphonates effectively inhibit osteoclast-mediated bone resorption and are integral in the treatment of benign and malignant bone diseases. The evolution of bisphosphonates over the past 30 years has led to the development of nitrogen-containing bisphosphonates (N-BPs), which have a mechanism of action different from that of the nonnitrogen-containing bisphosphonates. Studies conducted over the past decade have elucidated the mechanism of action and pharmacologic properties of the N-BPs. N-BPs exert their effects on osteoclasts and tumor cells by inhibiting a key enzyme in the mevalonate pathway, farnesyl diphosphate synthase, thus preventing protein prenylation and activation of intracellular signaling proteins such as Ras. Recent evidence suggests that N-BPs also induce production of a unique adenosine triphosphate analogue (Apppi) that can directly induce apoptosis. Our increased understanding of the pharmacologic effects of bisphosphonates is shedding light on the mechanisms by which they exert antitumor effects. As a result of their biochemical effects on protein prenylation, N-BPs induce caspase-dependent apoptosis, inhibit matrix metalloproteinase activity, and downregulate alpha(v)beta(3) and alpha(v)beta(5) integrins. In addition, zoledronic acid (Zometa; Novartis Pharmaceuticals Corp.; East Hanover, NJ and Basel, Switzerland) exerts synergistic antitumor activity when combined with other anticancer agents. Zoledronic acid also inhibits tumor cell adhesion to the extracellular matrix and invasion through Matrigel trade mark and has antiangiogenic activity. A growing body of evidence from animal models demonstrates that zoledronic acid and other bisphosphonates can reduce skeletal tumor burden and prevent metastasis to bone. Further studies are needed to fully elucidate these biochemical mechanisms and to determine if the antitumor potential of bisphosphonates translates to the clinical setting.

Animals↗
PubMed2006

Statins and renal diseases: from primary prevention to renal replacement therapy.

In glomerular diseases with nephrotic syndrome or protracted severe proteinuria, alterations of the lipid metabolism occur and are characterized mainly by increase of LDL cholesterol and frequently also of triglycerides and by qualitative abnormalities of HDL cholesterol and LDL cholesterol. In all renal diseases, when renal insufficiency develops, hyperlipidemia also occurs, with a near-elective increase in VLDL and intermediate-density lipoprotein cholesterol and a decrease of mature HDL cholesterol. There is clear evidence that these abnormalities may induce cardiovascular complications and, probably, also an accelerated progression of the renal damage. The inhibitors of 3-hydroxy-3-methylglutaryl CoA reductase, the so-called statins, are effective in controlling hypercholesterolemia, even in the more advanced stages of renal failure and in patients who are on maintenance dialysis. This antilipidemic effect of statins combines with other effects--antioxidant, anti-inflammatory, immunomodulatory, and antithrombotic (called "pleiotropic" effects)--as a result of the inhibition of the mevalonate pathway induced by these agents. Also because of these non-lipid-dependent effects, statins could have an antiatherosclerotic and renoprotective effect, which has been demonstrated clearly in vivo on renal cells and in experimental models of nephropathy but is still less evident in human renal diseases. Ongoing large trials will establish more clearly whether such effects are present in renal patients.

Animals↗
PubMed2004

Effect of D-003, a mixture of very high molecular weight aliphatic acids, on prednisolone-induced osteoporosis in Sprague-Dawley rats.

BACKGROUND: Drugs inhibiting cholesterol biosynthesis may affect bone metabolism through inhibition of the mevalonate pathway resulting in the inhibition of protein prenylation required for osteoclast activity. D-003 is a mixture of high molecular weight aliphatic primary acids purified from sugar-cane (Saccharum officinarum) wax, with cholesterol-lowering effects demonstrated in experimental and clinical studies. D-003 inhibits cholesterol biosynthesis through indirect regulation of HMG-CoA reductase activity. A previous study demonstrated that D-003 prevented bone loss and bone resorption on ovariectomy-induced osteoporosis in rats. Corticosteroid-induced osteoporosis is the result of changes affecting calcium homeostasis, but the hallmark of corticosteroid-induced bone loss is the direct effects on bone cells, such as inhibition of osteoblastogenesis, promotion of apoptosis of osteoblasts and osteocytes, and decrease in bone formation. OBJECTIVE: To determine whether D-003 could prevent the bone loss induced with prednisolone in Sprague-Dawley rats. METHODS: Rats were randomly distributed in five groups (ten rats per group): a sham-operated control and four groups orally treated with prednisolone 6 mg/kg for 80 days; a positive control orally treated with vehicle; and three groups orally treated with D-003 at 5, 25 and 200 mg/kg, respectively. Rats were killed, bones removed and histological variables of bone resorption and formation studied for histomorphometry. RESULTS: Compared with the sham group, prednisolone significantly (p < 0.01) reduced trabecular bone volume (TBV), while D-003 significantly (p < 0.001) and dose-dependently prevented the prednisolone-induced reduction of TBV. Treatment with prednisolone lowered (p < 0.001) trabecular thickness (TbTh) and number (TbN), while increasing (p < 0.001) the gap between trabeculae. D-003 (5, 25 and 200 mg/kg/day) significantly (p < 0.001) and dose-dependently prevented the reduction of TbTh and TbN and the increase of trabecular gap induced with prednisolone. Treatment with prednisolone increased both the surface and number of osteoclasts compared with sham (p < 0.001). D-003 (5-200 mg/day), however, prevented this effect (p < 0.001 for all comparisons). D-003 also prevented (p < 0.001) the reduction of osteoblast surface (ObS/BS) induced by prednisolone. Osteonecrotic areas were observed in all positive controls, but in none of the sham animals. Positive controls showed hypertrophy of bone marrow adipocytes and lipid-laden pluripotential stromal cells in bones. A significant and dose-dependent reduction of the frequency of animals showing prednisolone-induced osteo-necrosis was observed across the doses of D-003 (5, 25 and 200 mg/kg) investigated here. CONCLUSIONS: D-003 (5, 25 and 200 mg/kg) prevented trabecular bone loss and femoral neck osteonecrosis induced with prednisolone in Sprague Dawley rats, also increasing osteoblast surface and reducing bone resorption parameters. These results suggest that D-003 could be useful for managing corticosteroid-induced osteoporosis.

Animals↗
PubMed2003

New insights into the molecular mechanisms of action of bisphosphonates.

Bisphosphonates are currently the most important and effective class of anti-resorptive drugs available, but the exact molecular mechanisms by which they inhibit osteoclast-mediated bone resorption have only recently been identified. Due to the targeting of bisphosphonates to bone mineral and the ability of osteoclasts to release bone-bound bisphosphonate, a direct effect on mature osteoclasts appears to be the most important route of action. As a result of recent discoveries concerning their molecular mechanism of action, bisphosphonates can be grouped into two classes. The simple bisphosphonates that closely resemble PPi (such as clodronate, etidronate and tiludronate) can be metabolically incorporated into non-hydrolysable analogues of ATP that accumulate intracellularly in osteoclasts, resulting in induction of osteoclast apoptosis. By contrast, the more potent, nitrogen-containing bisphosphonates (such as pamidronate, alendronate, risedronate, ibandronate and zoledronate) appear to act as analogues of isoprenoid diphosphate lipids, thereby inhibiting FPP synthase, an enzyme in the mevalonate pathway. Inhibition of this enzyme in osteoclasts prevents the biosynthesis of isoprenoid lipids (FPP and GGPP) that are essential for the post-translational farnesylation and geranylgeranylation of small GTPase signalling proteins. Loss of bone-resorptive activity and osteoclast apoptosis is due primarily to loss of geranylgeranylated small GTPases. Identification of FPP synthase as the target of nitrogen-containing bisphosphonates has also helped explain the molecular basis for the adverse effects of these agents in the GI tract and on the immune system.

Animals↗
PubMed2000

Lovastatin induces a pronounced differentiation response in acute myeloid leukemias.

We recently identified HMG-CoA reductase, the rate-limiting enzyme of the mevalonate pathway, as a potential therapeutic target of various retinoic acid responsive cancers. Lovastatin, a competitive inhibitor of HMG-CoA reductase, induced a retinoic acid-like differentiation response followed by extensive apoptosis in neuroblastoma cell lines at relatively low concentrations (<20 microM) of this agent. More recently, we demonstrated that acute myeloid leukemias but not acute lymphocytic leukemias also displayed increased sensitivity to lovastatin-induced apoptosis. In this study, we examined the ability of lovastatin to induce differentiation of acute myeloid leukemic cells and to evaluate the role differentiation may hold in the anti-leukemic properties of this agent. Increased expression of the leukocyte integrins CD11b and CD18 as well as down-regulation of the anti-apoptotic gene bcl-2 are associated with late stage differentiation of the myeloid lineage and retinoic acid induced maturation of acute myeloid leukemic cells. Lovastatin exposure induced increased expression of CD11b and CD18 markers similar to retinoic acid treatment. Following 24 hrs exposure to 20 microM lovastatin, all 7 acute myeloid leukemia cell lines tested showed a decrease in bcl-2 mRNA expression while only 1/5 acute lymphocytic leukemia cell lines showed a similar response. A role for bcl-2 in the apoptotic response of acute myeloid leukemia cells to lovastatin was demonstrated as exogenous constitutive expression of bcl-2 in the AML-5 cell line inhibited apoptosis in a time and dose dependent manner. Thus, lovastatin exposure of acute myeloid leukemia cells induced a differentiation response that may contribute to the therapeutic potential of this agent in the treatment of this disease.

Acute Disease↗
PubMed2005

In vivo immunomanipulation of V gamma 9V delta 2 T cells with a synthetic phosphoantigen in a preclinical nonhuman primate model.

Vgamma9Vdelta2(+) cells represent the major population of gammadelta T cells in primate blood and react in an MHC-unrestricted fashion to a set of low m.w. nonpeptide phosphoantigens. Two types of structurally related agonists have been discovered so far: the natural phosphoantigens (hydroxydimethyl allyl-pyrophosphate or isopentenyl-pyrophosphate (IPP)) acting directly on Vgamma9Vdelta2(+) TCR and aminobisphosphonates, which block the mevalonate pathway in target cells, leading to accumulation of natural phosphoantigens that in turn activate Vgamma9Vdelta2(+) cells. We demonstrate in the cynomolgus monkey that Vgamma9Vdelta2 can be manipulated in vivo with bromohydrin pyrophosphate (BrHPP)/Phosphostim, a potent synthetic agonist for which the mechanism of action is similar to natural phosphoantigens. Although of very short half-life, injection of BrHPP leads to strong activation of Vgamma9Vdelta2, inducing production of a high level of Th1 cytokines. Combination of BrHPP with low-dose rhIL-2 induces specific amplification of effector-memory peripheral Vgamma9Vdelta2 in blood in a dose-dependant manner. This transient response returns to baseline within 10-15 days. Successive infusions of BrHPP and rhIL-2 induce less vigorous expansions, suggesting a progressive exhaustion of the response. As no toxicity is detected with or without IL-2, this scheme represents a promising immunotherapeutic strategy for induction of systemic Th1 cytokines and massive expansion of gammadelta T cell subset with antitumor and anti-infectious properties.

Animals↗
PubMed2005

[Mevastatin induced apoptosis in U266 human myeloma cell line].

Statins have been used successfully in the treatment of hypercholesteremia. Moreover, in vitro studies have shown that statins can trigger apoptosis in a variety of tumor cell lines. In the present study we analysed the effect of mevastatin -- a novel inhibitor of HMG-COA reductase, the rate-limiting enzyme of the mevalonate pathway -- on U266 human myeloma cells. Apoptosis induced by mevastatin was associated with increased caspase activity and depolarisation of mitochondrial membrane. Expression of BCL-2 mRNA and protein was down-regulated, with no change in BAX or BCLxL protein production. The mitochondrial program was supported by caspase-8 and cleaved BID activity. None of the antibodies neutralising death-ligand/death-receptor pathway -- TRAIL-R2Fc, anti-TNF-a, anti FASL (NOK-1) -- influenced the mevastatin-induced apoptosis. Mevastatin also stimulated shedding of syndecan-1 from the surface of myeloma cells.

Antineoplastic Agents↗
PubMed1999

Importance of geranylgeranyl pyrophosphate for mesangial cell DNA synthesis.

BACKGROUND: Farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP) are isoprenoid products of the intracellular mevalonate pathway used for prenylation of several low molecular weight G proteins, including Ras. It is likely that platelet-derived growth factor (PDGF) stimulation of mesangial cell proliferation requires prenylated, low molecular weight G proteins. The purpose of this study was to investigate the dependence of platelet-derived growth factor-stimulated mesangial cell DNA synthesis and cell membrane Ras incorporation on FPP and GGPP. METHODS: Quiescent human mesangial cells were exposed to PDGF (25 ng/ml) to stimulate DNA synthesis. Some cells were also treated with the HMG-CoA reductase inhibitor lovastatin (2.5 to 10.0 microM), which inhibits isoprenoid synthesis, in the presence or absence of exogenous FPP or GGPP. DNA synthesis was assessed by thymidine incorporation, and Western blot analysis was used to measure total cell membrane Ras. RESULTS: Stimulation of mesangial cells with PDGF did not increase total cell membrane Ras. Lovastatin reduced cell membrane Ras, and this was prevented by simultaneous exposure of mesangial cells to exogenous FPP (2.5 to 10.0 microM) or GGPP (1 to 5 microM). Lovastatin also reduced PDGF-stimulated mesangial cell DNA synthesis by 90%, and this was completely prevented by simultaneous exposure of cells to exogenous GGPP (1 microM), but not to FPP. CONCLUSIONS: The results of this study suggest that both FPP and GGPP can provide for mesangial cell membrane Ras localization and that PDGF-stimulated mesangial cell DNA synthesis requires the isoprenoid GGPP.

Blotting, Western↗
PubMedDate not supplied

[Inhibitors of HMG CoA reductase: new modes of action, new indications?].

3-hydroxy-3-methylglutaryl Coenzyme A reductase inhibitors, or statins, are hypocholesterolaemic agents used in the treatment of hypercholesterolaemia and cardiovascular diseases. Their hypocholesterolaemic action results from the inhibition of the intracellular synthesis of cholesterol via the mevalonate pathway. Recent studies have demonstrated that cholesterol is not the only intracellular target of statins. Indeed, statins have been shown to inhibit protein isoprenylation, a post-translational modification involved in membrane localization and protein activity. By inhibiting isoprenylation of Ras protein, statins induce the inhibition of the Ras-AP1 pathway and the decrease in cell proliferation. Similarly, inhibition of Rho protein insoprenylation leads to the disorganization of the cytoskeleton and the induction of the fibrinolytic system. All these effects need to be confirmed in vivo and in the human, and open new areas for these therapeutics agents.

Anticholesteremic Agents↗
PubMed2000

Molecular basis of cholesterol feedback lesion in CNS tumours.

An important feature of malignant transformation of tumours is the loss of cholesterol feedback inhibition mechanism (cholesterol-feedback lesion) that regulates mevalonate pathway recognized to play a crucial role in cellular growth, death and differentiation. Recently, it was shown that Receptor-C(k)-dependent signalling regulates genes involved in maintaining cellular cholesterol homeostasis through a transcription factor sterol response element binding protein (SREBP) having affinity for sterol regulatory element (SRE) present in the promoter region of these genes. The present study revealed that CNS tumours exhibit overexpression of Receptor-C(k) gene product which was accompanied by their inability to express SREBP gene product and this phenomenon has the inherent capacity to initiate the cholesterol feedback lesion in these tumours. Based upon these and our earlier studies, we propose for the first time that this loss of cholesterol feedback control may be responsible for the initiation of these tumours.

Adult↗
PubMed2000

Contribution of steroidogenic factor 1 to the regulation of cholesterol synthesis.

Steroidogenic factor 1 (SF-1) is an orphan member of the nuclear receptor family expressed in steroidogenic tissues, where it has an essential role in the regulation of the steroid hormone biosynthesis, adrenal and gonadal development and endocrine responses fundamental for reproduction. Here we show that SF-1 regulates the transcription of cytosolic 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase gene, which is essential for the endogenous synthesis of cholesterol. We have identified an element located 365 bp upstream of the gene for cytosolic HMG-CoA synthase; SF-1 binds as a monomer to this element and confers SF-1 responsiveness to homologous and heterologous promoters. It has been shown that in tissues with a high demand for cholesterol to be used in steroid synthesis, there is a lack of correlation between the cholesterol levels and the activity of the limiting enzymes of the mevalonate pathway. In accord with those results, we observed that cholesterol synthesis from acetate and either cytosolic HMG-CoA mRNA expression or transcriptional activity were not changed in response to 25-hydroxycholesterol in the SF-1-expressing steroidogenic Leydig tumour MA-10 cells. Moreover, the overexpression of SF-1 in non-steroidogenic CV-1 cells renders them less sensitive to the regulatory effects of cholesterol. This observation led to the hypothesis that in steroidogenic tissues the expression of SF-1 permits high levels of endogenous synthesis of cholesterol irrespective of the intracellular levels of this metabolite.

Animals↗