Sterol biosynthesis regulation

Genetic analysis indicates that two of the 10 sad mutants of A. strigosa that we isolated represent different mutant alleles at the Sadi locus.6 These mutants accumulate radiolabelled 2,3-oxidosqualene but not p-amyrin when the roots are fed with 14C-labelled precursor mevalonic acid, suggesting that the triterpenoid pathway is blocked between 2,3-oxidosqualene and P-amyrin.34 The roots of these mutants also lack detectable P-amyrin synthase activity, but, like the wild type and the other mutants, are unimpaired in cycloartenol synthase (CS) activity and sterol biosynthesis.34 The transcript levels for AsbASl are substantially reduced in roots of sadl mutants, while AsCSl transcript levels are unaffected,35 suggesting that the sadl mutants are either mutated in the AsbASl gene itself or in a gene involved in its regulation. 

DEVARENNE, T.P., GHOSH, A., CHAPPELL, J., Regulation of squalene synthase, a key enzyme of sterol biosynthesis in tobacco, Plant Physiol., 2002,129, 1095-1106. 

SREBPs are transcription factors that bind to the sterol regulatory element DNA sequence TCACNCCAC. Unactivated SREBPs are attached to the nuclear envelope and endoplasmic reticulum membranes. In cells with low levels of sterols, SREBPs are cleaved to a water-soluble N-terminal domain that is translocated to the nucleus. These activated SREBPs then bind to specific sterol regulatory element DNA sequences, thus up-regulating the synthesis of enzymes involved in sterol biosynthesis. Sterols in turn inhibit the cleavage of SREBPs and therefore synthesis of additional sterols is reduced through a negative-feedback loop. 

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. 

Vik, A. and Rrne, ). (2001) Upc2p and Fcrri22p, dual regulators of sterol biosynthesis in Saccharomyces cerevisiae. Molecular and Cellular Biology, 21. 6395-6405. 

Because of its predominant role in the regulation of sterol biosynthesis, the first portion of this section will be devoted to the regulation of HMG-CoA reductase. The second portion will review current knowledge of other control points whose importance and roles have been less extensively studied and are less well understood. 

Regulation of HMG-CoA reductase activity by reversible phosphorylation-dephosphorylation thus allows cells to almost instantaneously alter the rate of sterol biosynthesis in response to physiological stimuli. Long-term adjustments (hours or days) are regulated by the quantity of HMG-CoA reductase protein. 

It has also been proposed that hpids, Upid metabolites and cytosolic lipid-binding proteins interact to regulate sterol biosynthesis [199] (see also Chapter 3). Under this model, the accumulation in cells of certain hpids or free hpid intermediates inhibits sterol biosynthetic enzymes. The level of the binding protein for a particular Upid would determine the threshold concentration of Upid needed to produce inhibition [199]. Such a mechanism would allow coordination between the biosynthesis of sterols and other Upids. Fatty acyl-CoAs and Z-protein, the fatty acyl-CoA-binding protein, may modulate HMG-CoA reductase activity [199-201] since physiological... 

HMG-CoA reductase fimctions as the major control point for sterol biosynthesis under most physiological conditions. However, regulation is not achieved solely through changes in the quantity and activity of HMG-CoA reductase. Several other enzymes are also subject to and function in regulatory control. 

In at least one circumstance, however, sterol/isoprenoid biosynthesis is regulated by mechanisms independent of HMG-CoA reductase. When HeLa cells are exposed to dexamethasone, sterol biosynthesis and HMG-CoA synthase activity decrease. 

To summarize, it appears that some regulatory control is exerted over several enzymes of the early portion of the sterol/isoprenoid biosynthetic pathway. The changes in these early enzymes usually parallel those in HMG-CoA reductase and probably represent a cellular conservation process rather than a mechanism for biosynthetic control [82]. The temporal relationship between the change in HMG-CoA reductase activity and that of other early enzymes when CHO cells are incubated in delipidated medium supports this hypothesis [210]. However, it also appears that, under certain circumstances, regulation of these enzymes can serve as a mechanism for control of sterol biosynthesis. 

The identity of the sites responsible for regulating the distribution of intermediates between branches is presently unknown. Interpretation of available evidence is complicated by our incomplete understanding of the late steps of sterol biosynthesis (see Chapter 1). Experiments using various inhibitors of sterol biosynthesis have implicated the following as potential regulatory sites ... 

The emerging picture is that when sterol biosynthesis is suppressed, gross changes in the carbon flux through the sterol/isoprenoid biosynthetic pathway are mediated by HMG-CoA reductase. This decreases the supply of biosynthetic intermediates available for non-sterol products, which normally are synthesized in far smaller quantity than are sterols. To assure a sufficient supply of these intermediates for needed non-sterols such as dolichol and ubiquinone, later steps in the sterol/isoprenoid biosynthetic pathway are also down-regulated [141]. 

Geuns J M C 1975 Regulation of sterol biosynthesis in etiolated mung bean hypocotyl sections. Phytochemistry 14 975-978... 

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