Cholesterol and the steroid hormones part 2: biosynthesis

Cholesterol and the steroid hormones part 2: biosynthesis

1. Sources of cholesterol

   • Diet
     • Receptor mediated uptake uptake of LDL/HDL cholesterol from the circulation
       • LDL deposit cholesterol
       • HDL take up cholesterol from the tissues
   • De novo synthesis (in steroid-secreting cells)
     • Radioactive work established all of synthesised cholesterol's carbon atoms came from acetate
     • Approximately 50% of the body's cholesterol is from de novo biosynthesis
       • 10% from the liver
       • 15% from the intestines
     • Occurs in the cytoplasm and ER of steroid-secreting cells
       • Uses acetate as building blocks - forming acetyl-CoA
         • Series of steps from acetyl-CoA to build the steroid nucleus
           • A key enzymes in this process is HMG-CoA reductase (= rate limiting step)
             • An integral protein of the ER membrane
             • Rate limiting step of cholesterol synthesis - heavily regulated
               • Also a target for pharmaceutical agents (e.g. statins)
                 • Statins contain a region that is analogous to mevaldehyde/mevalonate
                   • Clinical trials show that statins greatly reduce blood cholesterol levels and and reduce the risk of cardiovascular disease
               • Regulation of HGM-CoA reductase
                 • HMG-CoA
                   • HMG-CoA reductase
                     • (Mevaldehyde) -> mevalonate
                       • ->->->-> farnesyl-PP
                         • x2 Squalene
                           • Cholesterol
                             • Oxidised cholesterol
                               • +
                         • Farnesyl-P
                           • +
                         • Pi
                     • (long term regulation) - DEGRADATION of HMG-CoA reductase
                     • (short term regulation) - INACTIVATION of HMG-CoA reductase
                       • +
                         • AMPK
                           • +
                             • High AMP (low ATP)
                     • +
                       • Insulin and thyroid hormones
                     • -
                       • Glucagon and glucocorticoids
                         • Hormonal regulation of HMG-CoA reductase
                       • STATINS
                         • e.g. lovastatin, zocor etc.
           • Acetyl-CoA + acetoacetyl-CoA
             • HMG-CoA synthase
               • -> HMG-CoA
                 • HMG-CoA reductase
                   • (active site intermediate = mevaldehyde) -> mevolanate
                     • Farnesyl-pyrophosphate (PP)
                       • x2 Squalene
                         • Cholesterol
                   • 2NADPH - > 2NADP(+) + HS-CoA
   • Cholesterol-ester stores (in steroid-secreting cells)
     • Steroid-secreting cells
       • Feature lipid droplets
       • Lots of SER
       • Mitochondria with tubular christae
         • Mitochondria appear circular (characteristic)
         • Sites of steroid synthesis
     • Cholesterol is stored in lipid dorplets within the cell as a cholesterol ester (Cholesterol + acetyl-CoA)
       • This is catalysed by ACAT (acetyl-CoA cholesterol acetyl transferase)
         • Acetyl-CoA + Cholesterol -> Cholesterol-ester (CE)
         • When cholesterol is need for steroid synthesis the reverse reaction is catalysed by; CE-hydrolase
           • CE -> Cholesterol + acetate
             • Cholesterol -> steroid biosynthesis pathway
         • ACAT is found in the ER

2. Steroidogenesis

   • Rate limiting step
     • The rate of delivery to the inner mitochondrial membrane
       • Early studies showed that protein synthesis inhibtion also inhibited steroidogenesis - suggesting a rapidly synthesised protein was inportant in this
         • Two candidates;
           • StAR (Steroidogenesis acute regulatory protein)
             • 37kDa precursor in response to cAMP
             • Short half-life
             • Active at the level of the outer mitochondrial membrane
             • Mitochondrial importation of StAR results in the generation of the 30kDa mature StAR protein and terinates its activity
           • PBR (Peripheral benzodiazapine receptor
             • High affinity cholesterol binding protein
             • Located in the outer mitochondrial membrane
               • Abundant where outer and inner membranes contact
             • Mutation/down-regulation: inhibition of steroidogenesis
             • Deletion: lethal phenotype
             • PBR drug ligands affect cholesterol transport into mitochondria
           • Both are needed to get cholesterol across the mitochondrial membrane
             • StAR insertion into the outer mitochondrial membrane allows PBR to form a channel(?)
     • (Surprisingly) not the rate of cholesterol uptake, synthesis by the cell or the activity of the enzyme P450scc...
       • P450 = enzyme that catalyses the conversion of cholesterol into pregnenolone (the first step of steroidogenesis)
   • Enzymes of steroid biosynthesis
     • Mostly members of the CYP gene family
       • Encode cytochrome P450 enzymes - catalyse hydroxylations
         • Found in all steroid hormone producing and metabolising cells
     • Other enzymes are hydroxysteroid dehydrogenases (HSDs)
     • Steroidogenesis map
       • CHOLESTEROL
         • P450scc (side chain cleavage)
           • PREGNENOLONE
             • 3-beta-HSD
               • PROGESTERONE
                 • P450c21 hydroxylase
                   • DEOXYCORTICOSTERONE
                     • P450 11 beta Hydroxylase
                       • CORTICOSTERONE
                         • Aldosterone synthase
                           • ALDOSTERONE
                 • P450 17alpha hydroxylase
             • P450 17alpha hydroxylase
               • 17-alpha -OH- PRENENOLONE
                 • 3-beta-HSD
                   • 17-alpha -OH- PROGESTERONE
                     • P450c21 hydroxylase
                       • DEOXYCORTISOL
                         • P450 11 beta Hydroxylase
                           • CORTISOL
                     • P450 17alpha hydroxylase
                       • ANDROSTENEDIONE
                         • Aromatase
                           • ESTRONE
                             • 17beta-HSD
                               • ESTRADIOL
                 • P450 17, 20 Lyase
                   • DHEA
                     • 3-beta-HSD
                       • • 17beta-HSD
                           • TESTOSTERONE
                             • 5alpha reductase
                               • DIHYDROTESTOSTERONE
                             • Aromatase
       • Red = enzyme; green = progestogens; yellow= mineralocorticoid; blue = androgens and orange = oestrogens
     • In the mitochondria and SER (steroids move betwen these compartments by an unknown mechanism)