MUDr. Dana Maňasková


Vyhledávání na medicinman.cz
 

Biosyntéza živočišného vitamínu D

7-dehydrocholesterol - Provitamín D3 - (7-DHC) - delta7-Cholesterol - Cholesta-5,7-dien-3-ol, (3ß)- - Provitamin D3 - provitamin D3 - Dehydrocholesterol

  • Zoosterol
  • As a serum cholesterol precursor
  • Also found in the milk of several mammalian species
  • In insects it is a precursor for the hormone ecdysone
    • Required for reaching adulthood
      • Discovered by Nobel-laureate organic chemist Adolf Windaus.
  • Je secernován mazovými žlázami na povrch kůže
    • Dostává ke styku s ultrafialovými paprsky (Wilczek 1978, 24)
    • štěpen UVB zářením [4]
  • Highest concentrations of 7-dehydrocholesterol
    • Epidermal layer of skin - in the keratinocytes [11]
      • Stratum basale
      • Stratum spinosum
        • Production of pre-vitamin D3 greatest in these two layers [7]
  • Vitamin D3 is formed from the precursor steroid 7-dehydrocholesterol (7-DHC)
    • Localized mostly on the plasma membrane of basal epidermal keratinocytes (80% of skin 7-DHC content) [13]
  • 7-Dehydrocholesterol absorbs UV light most effectively at wavelengths between 290 - 320 nm
    • Production of vitamin D3 occur primarily at those wavelength
      • Quantity (intensity)
      • Quality (appropriate wavelength) of the UVB irradiation
      • Quantity of 7-dehydrocholesterol present in the skin
        • Normal circumstances cca 25–50 ug/cm2 of skin available in the stratum spinosum and stratum basale of human skin [7]
  • 7-dehydrocholesterol is photolyzed by ultraviolet light
    • In a 6-electron conrotatory ring-opening electrocyclic reaction
    • Product is previtamin D3 [11]
  • The conversion of 7-dehydrocholesterol to vitamin D3 does not involve enzymes or protein factors
    • Products + side-reaction products in skin
      • Same degree as in an organic solvent (McLaughlin et al., 1982) [12]
  • 10 min of summer sun on the hands and face
    • Sufficient to generate 10 ug of vitamin D3
      • Considered to be the daily requirement (Holick, 1981) [12]
  • 7-dehydrocholesterol can be converted into either cholesterol or vitamin D (cholecalciferol)
    • Conversion of 7-dehydrocholesterol to vitamin D occurs in the skin upon exposure to ultraviolet B light (290–320?nm)
    • Normal sunlight conditions, only around 15% of available 7-dehydrocholesterol will be converted to vitamin D
    • Excess 7-dehydrocholesterol in the skin will be converted to inert compounds for degradation [14]

Pokles

  • Syntéza klesá s věkem !!
    • Tedy i schopnost kůže vyrábět vit. D [17]
  • Statins
    • HMG CoA reductase-inhibitors
      • Inhibit the synthesis of 7-dehydrocholesterol, also inhibit the synthesis of vitamin D [17]


7-Dehydrocholesterol reductase - DHCR7 - delta-7-sterol reductase (EC 1.3.1.21) - 7-DHC reductase - 7-dehydrocholesterol dehydrogenase/cholesterol oxidase - Delta7-sterol reductase

  • Ultimate enzyme of mammalian sterol biosynthesis
  • protein in humans encoded by the DHCR7 gene [7]
  • Terminal enzyme of cholesterol synthesis in the Kandutsch-Russell pathway
    • Converting 7-dehydrocholesterol (7DHC) to cholesterol
      • Using NADPH [7]
      • Removes the C(7-8) double bond introduced by the sterol delta8-delta7 isomerases [7]
    • cholesterol + NADP+ = cholesta-5,7-dien-3beta-ol + NADPH + H+ [16]
  • Important regulatory switch between cholesterol and vitamin D synthesis [12]
  • DHCR7 used to convert 7-dehydrocholesterol to cholesterol
  • Shortage of DHCR7 would decrease the body’s ability to convert 7-dehydrocholesterol into cholesterol
    • Causing a build-up of 7-dehydrocholesterol
      • Could be converted into vitamin D in the skin [14]

Teratogenita

  • Drugs that inhibit DHCR7 resulted in deformities among 10.9% of babies born
  • Drugs that increase DHCR7 expression resulted in deformities among 5.5% [14]

Inhibice DHCR7

  • Drug-induced malformations
  • Inhibitors of the last step of cholesterol biosynthesis
    • AY9944 and BM15766
      • Severely impair brain development [7]
  • Vitamin D3’s inhibition of DHCR7 is indirect
    • Likely to be mediated through a complex pathway
    • Moderate prenatal vitamin D supplementation have reported no adverse effects [14]

Degradace DHCR

  • DHCR7 is rapidly turned over
    • Degradation is induced by the cholesterol product itself
      • Resulting in decreased DHCR7 protein levels and activity
        • Resulting in 7DHC accumulation
  • Conditions of high cholesterol
    • DHCR7 degradation is associated with increased Vitamin D production [20]

Absence of functional DHCR7

  • Accumulation of 7DHC
  • Lack of cholesterol
  • Devastating developmental disorder

Smith–Lemli–Opitz syndrome - SLOS - 7-dehydrocholesterol reductase deficiency

  • Inborn error of cholesterol synthesis
  • Autosomal recessive
  • Multiple malformation syndrome
  • By a mutation in the enzyme 7-Dehydrocholesterol reductase [7]
  • Patients' plasma
    • cholesterol levels are generally low
    • 7-dehydrocholesterol concentrations are markedly elevated [15]
    • Plasma total sterols are abnormally low
      • Correlate negatively with the percent of 7-dehydrocholesterol [15]
    • 7-dehydrocholesterol and 8-dehydrocholesterol accumulate in the syndrome [15]

Supplemental cholesterol

  • May be therapeutic in SLOS
    • Might suppress synthesis of both cholesterol and 7-DHC in individuals with SLOS
      • Via inhibition of HMG-CoA reductase [13]
  • Total sterol synthesis in the study was not significantly different from controls
    • cholesterol supplementation did not inhibit synthesis of 7-DHC in SLOS subjects
    • If 7-DHC is toxic and contributes to the pathogenesis of SLOS, alternative strategies in addition to cholesterol supplementation will need to be employed [13]

Statin treatment

  • Can ameliorate the low DHCR7 expression seen with common SLOS mutations
  • Wild-type DHCR7 relatively labile
    • End-product inhibition
      • cholesterol accelerates the proteasomal degradation of DHCR7
        • Decreased protein levels and activity
          • Loss of enzymatic activity results in the accumulation of the substrate 7DHC
            • Leads to an increased production of vitamin D

Lehké mutace DHCR7

  • Evolutionarily favored in Northern climates, including Europe and Asia
  • Selective pressure and a plausible heterozygote advantage for those variants
  • Certain DHCR7 variants were associated with
    • Lower vitamin D levels in the general population
    • Individuals with polycystic ovary syndrome
    • Links vitamin D levels with DHCR7 variants
  • A heterozygote advantage allow most of the body’s 7-dehydrocholesterol to be converted into vitamin D (and not cholesterol)
    • Explain why Northern populations have a higher prevalence of SLOS [14]
      • Reproductive advantage because of the reduced fetal death due to rachitic cephalopelvic disproportion

rs12785878 u genu DHCR7

  • cholesterol synthesis
  • 11q12 (p=2.1x10(-27)
  • Nižší hladiny vit. D [30]
  • Common variants at DHCR7 are strongly associated with circulating 25-OH D [30]
    • Could play a larger role in the normal regulation of vitamin D status than previously recognized [30]
  • Nutrafitness - Lifetest - dostupný test v ČR

Major homozygotes

  • Framingham Heart Study (n=5,656)
    • 79.7 (0.71) Mean 25-OH D levels [30]
  • 1958 British Birth Cohort (n=6,552) [30]
    • 59.6 (0.31) Mean 25-OH D levels [30]

Heterozygotes

  • Framingham Heart Study (n=5,656)
    • 76.3 (0.86) Mean 25-OH D levels [30]
  • 1958 British Birth Cohort (n=6,552) [30]
    • 56.3 (0.30) Mean 25-OH D levels [30]

Minor homozygotes

  • Framingham Heart Study (n=5,656)
    • 71.7 (2.01) Mean 25-OH D levels [30]
  • 1958 British Birth Cohort (n=6,552) [30]
    • 55.7 (0.32) Mean 25-OH D levels [30]

rs7944926 DHCR7/NADSYN1

rs12800438 DHCR7/NADSYN1

rs3794060 DHCR7/NADSYN1

rs4945008 DHCR7/NADSYN1

rs4944957 DHCR7/NADSYN1


Inhibice cílená

  • Pharmaceuticals are being designed to inhibit DHCR7
    • Suggested treatment for hepatitis C
  • Tamoxifen (IC50=12?nm)
  • doxorubicin (IC50=150?nm)
    • Used to treat cancer, typically breast cancer [14]
  • Nafoxidine
    • Known estrogen antagonist
    • Major reaction to the drug consisted of dermatitis (55% of patients) exacerbated by sunlight
      • Effected patients’ dermal response to sunlight exposure
        • Dysregulation of vitamin D synthesis pathways [14]

Stimulace

  • Several anti-psychotic drugs were found to enhance DHCR7 activity
  • Can cause high 7-dehydrocholesterol in the absence of SLOS

Increased DHCR7 activity

  • Would increase the rate of cholesterol synthesis
    • Haloperidol
    • Clozapine
    • Chlorpromazine [14]
  • Increase expression

Cholesterol

Prekalciferol


Expozice UVB (320-280 nm) záření - Dornovo záření

  • 7-Dehydrocholesterol reacts with UVB light at wavelengths between 270 and 300 nm
    • Peak synthesis occurring between 295 and 297 nm [11]
  • Suchozemští obratlovci již cca před 300 milióny let
    • Přesunuli se z oceánu, který byl bohatý na vápník [1]
  • Množství vytvořeného vitamínu D v kůži závisí hlavně na
    • Geografické poloze co nejblíže rovníku
    • Možnosti využívání UV záření v dostatečné míře
  • Prochází sklem jen v 1-8 %
  • Proniká vodou, zvláště mořskou
    • Do hloubky několika metrů
      • Umožněn vznik previtaminu D v těle ryb
  • Fotony ultrafialového záření štěpí B jádro 7-dehydrocholesterolu
    • vzniká cholekalciferol (vit. D3)
  • Sluneční záření vzhledem k regulačním mechanismům syntézy nikdy nevede k hypervitaminose [1]
  • Množství vytvořeného vitamínu D v kůži
    • Závisí hlavně na geografické poloze
      • Co nejblíže rovníku
      • Na možnosti využívání UV záření v dostatečné míře [1]
    • By mělo stačit až na 80 % denní potřeby [1] 90% [5]

Přepočtové tabulky

  • K ploše odhalené kůže
  • času expozice
  • Zeměpisné šířce [6]

Geograficky

  • Centrální Polsko
    • UVB začíná pronikat od 12.4 [1]
  • ČR v létě, kolem 21.6. mezi 10 a 16 h dostatek UVB záření
    • Od 21.6. se slunce začíná obracet k jihu [1]
  • V jižním Polsku
    • UVB záření do 3.9. [1]
  • V Turecku
    • Do konce září [1]
  • V Egyptě
    • Do 20.10 [1]

Test na získání vitamínu D

  • Tužku přiložíme kolmo k zemskému povrchu
  • Pokud stín tužky mnohem kratší, než výška tužky, UVB záření k nám proniká [1]

Minimum expozice

  • Adequate amounts of vitamin D can be produced with moderate sun exposure to the face, arms and legs
  • Averaging 5–30 minutes twice per week
  • Approximately 25% of the time for minimal sunburn
  • The darker the skin, and the weaker the sunlight, the more minutes of exposure are needed. [11]

Extensive sun exposure

  • Produces vitamin D levels equivalent to taking 10,000 IU per day
    • no dangers have ever been observed
  • Ale v kůži je x různých mechanismů proti vzniku nadměrné dávky biologicky aktivní formy vit. D...


Previtamín D3

  • Vznikne pre-vitamin D3 in the skin from 7-DHC
  • Nestabilní
  • Previtamín D3 vykazuje jenom 35% aktivitu cholekalciferolu (Ledvina – Stoklasová – Cerman2004, 519; Velíšek 2002, 49)
  • Spontánně se přemění na svůj izomer - cholekalciferol vitamin D3 in the skin
    • In an antarafacial sigmatropic hydride shift
      • In an organic solvent at room temperature
        • Cca 12 days to complete
      • In the skin
        • About 10 times faster than in an organic solvent [11]

Prolonged exposure to sunlight will not produce toxic amounts of D3

  • UV irradiation further converts pre-D3 to
    • Lumisterol3
      • Inactive
      • Accumulates with continued UV exposure
      • Can be converted back to pre-D3 as pre-D3 levels fall [12]
    • Tachysterol3
      • Does not accumulate with extended UV exposure [12]
  • Photoconversion of D3 itself to
    • Suprasterols I and II
    • 5,6 transvitamin D3 [12]
  • Efficiency of vitamin D production by solar radiation
    • Less than that of monochromatic UVB
      • UVA portion of solar radiation influences the ratio of pre-D3 to lumisterol3 produced [12]



Cholekalciferol - vitamín D3

  • Vitamin D3 vytvořený v pokožce nebo přijmutý potravou se dostává do cirkulace
    • Vázán na protein (DBP, vitamin D-binding protein)
      • Transportuje se do jater
        • Hydroxylován v poloze 25 na 25-hydroxyvitamin D (25-OH-D)
  • Není metabolicky aktivní [4]
  • Vychytáván jaterními buňkami
    • V endoplasmatickém retikulu podléhá hydroxylaci katalyzované enzymem D3-25-hydroxylasou [1]
    • V hepatálních mikrozomech [4]
      • Vznikne 25-hydroxycholekalciferol
  • D3 is preferentially removed from the skin, bound to DBP

D3-25-hydroxylasa

  • Katalyzuje hydroxylaci cholekalciferolu
  • Vzniká 25-hydroxycholekalciferol

CYP27A1 mitochondrial - 25-hydroxylase

Liver

  • Catalyzing a critical step in the bile acid synthesis pathway
  • High capacity, low affinity enzyme
  • 25-hydroxylation is not generally rate limiting in vitamin D metabolism
  • Widely distributed throughout different tissues
    • Highest levels in liver and muscle
      • Also in kidney, intestine, lung, skin, and bone
  • CYP27A1 can hydroxylate vitamin D and related compounds at
    • 24, 25, and 27 positions
    • D2 appears to be preferentially 24-hydroxylated
    • D3 is preferentially 25-hydroxylated
    • The 1alphaOH derivatives of D
      • More rapidly hydroxylated than the parent compounds
      • Differences in biologic activity between D2 and D3 or between 1alphaOHD2 and 1alphaOHD3 [22]

Keratinocytes 25-hydroxylase - CYP27A1

  • Metabolizing vitamin D [12]
  • Expression of like that of CYP27B1 is mitochondrial
  • Increased by vitamin D and UVB irradiation [12]
  • Side chain cleavage enzyme critical for steroidogenesis
  • Also expressed in keratinocytes
  • Could convert vitamin D to 20(OH)D with biologic activity
  • Shown to be regulated by androgens [12]

Mutations in CYP27A1 lead to

  • Cerebrotendinous xanthomatosis
  • Associated with abnormal vitamin D and/or calcium metabolism
    • In some but not all patients [22]
  • Mice CYP27A1 deleted
    • Elevated 25OHD levels
    • Disruption in bile acid synthesis [22]
  • Lack of CYP27B1
    • Decreases the ability of cells to respond by cathelicidin and/or CD14 production [24]

Decrease CYP27A1 expression by


Increased CYP27A1 expression by

  • Dexamethasone
  • Estrogen
    • To male rats increases 25-hydroxylase activity [22]
  • Stimulation of TLR2 by an antimicrobial peptide in macrophages
  • Stimulation of TLR2 in keratinocytes by wounding the epidermis [24]

CYP2R1 - major microsomal 25-hydroxylase

Liver

  • Have 25-hydroxylase activity
  • CYP2R1 25-hydroxylates D2 and D3 equally
  • First described in 2003
    • Many other enzymes with 25-hydroxylase activity in vitro have been described
  • Product of the CYP2R1 human gene [22]
  • Product is released into the plasma

Skin and testes [22]

Keratinocytes

  • Metabolizing vitamin D [12]
  • Expression of like that of CYP27B1 is mitochondrial
  • Increased by vitamin D and UVB irradiation [12]
  • Side chain cleavage enzyme critical for steroidogenesis
  • Could convert vitamin D to 20(OH)D with biologic activity
  • Shown to be regulated by androgens [12]

Fibroblasty

  • More specific 25-hydroxylase
  • Found in dermal fibroblasts [12]
    • Expression in keratinocytes not reported [12]

Mutations and variants CYP2R1

  • CYP2R1 loss is associated with decreased 25OHD levels [22]
  • Mice lacking CYP2R1
    • Reduced 25OHD levels
  • Combined deletion of CYP2R1 and CYP27A1
    • Does not reduce these levels more that about 70% [22]
  • CYP2R1 mutations in family members with rickets
    • Respond to D therapy but suboptimally [22]
  • Nigerian man with a point mutation in CYP2R1
    • History of rickets
  • 133 individuals with type 1 diabetes
  • CYP2R1 polymorphisms
    • Difficult to identify
  • Common variants at the CYP2R1 locus
    • Associated with circulating 25-OH D
      • Strongest evidence to date that CYP2R1 is the enzyme responsible for the critical first step in vitamin D metabolism [30]

rs10741657 u genu CYP2R1

  • Hydroxylation vit.D
  • 11p15 (p=3.3x10(-20)
  • Nižší hl. vit. D [30]
  • Nutrafitness - Lifetest

Major homozygotes

  • Framingham Heart Study (n=5,656)
    • 75.4 (0.87) Mean 25-OH D levels [30]
  • 1958 British Birth Cohort (n=6,552) [30]
    • 56.8 (0.34) Mean 25-OH D levels [30]

Heterozygotes

  • Framingham Heart Study (n=5,656)
    • 78.6 (0.76) Mean 25-OH D levels [30]
  • 1958 British Birth Cohort (n=6,552) [30]
    • 60.2 (0.32) Mean 25-OH D levels [30]

Minor homozygotes

  • Framingham Heart Study (n=5,656)
    • 81.6 (1.26) Mean 25-OH D levels [30]
  • 1958 British Birth Cohort (n=6,552) [30]
    • 61.1 (0.29) Mean 25-OH D levels [30]

rs2060793 CYP2R1

rs1993116 CYP2R1

rs12794714 CYP2R1

rs10500804 CYP2R1

rs7116978 CYP2R1


Vitamin D-binding protein

  • 52-59 kDA protein
  • Synthesized in the liver
  • Binds and transports vitamin D and its metabolites
    • 25-OH D
    • 1,25(OH)2D
  • Alpha-globulin carrier protein [11]
  • Metabolites produced from Vitamin D2 tend to bind less well to the vitamin D-binding protein [11]
  • Liver produces DBP and albumin
    • Calcidiol to kidneys
    • Calcitriol throughout the body [11]
  • High affinity for actin
    • May serve as a scavenger for actin released into the blood during cell death
  • Activate macrophages and osteoclasts

Zvýšení

  • Estrogeny
    • DBP like other steroid hormone binding proteins is increased by oral (not transdermal) estrogens and pregnancy
  • Glucocorticoids
  • EGF
  • IL-6
  • TGF-beta [22]

Deficit VDBP

  • Production decreased
    • In liver disease
    • TGF-beta [22]
  • Lost in protein losing enteropathies or the nephrotic syndrome
  • Result in low total levels of the vitamin D metabolites
    • Without necessarily being vitamin D deficient
      • Free concentrations may be normal [22]

Knock out mouse in DBP

  • no obvious abnormality was observed
  • Increased turnover in vitamin D
  • Increased susceptibility to osteomalacia on a vitamin D deficient diet
  • Osteopetrosis (indicating failure of osteoclast function) was not found [22]

SNPs in GC gene

  • Associated in changes in 25-OH D concentrations
  • GC haplotypes (GC1S, GC1F, and GC2)
    • Combinations of alleles at these nonsynonymous SNPs
  • GC variants associated with lower 25-OH D concentrations
    • Were strongly related to lower levels of DBP
  • Levels of DBP may affect
    • The delivery of 25-OH D and activated vitamin D (1,25(OH)2D) to target organs
    • Clearance of vitamin D metabolites from the circulation
  • Alterations in quantity or function of DBP
    • Could be accompanied by changes in the relative proportions of free and bound 25-OH D
    • Free fraction being the potential rate-limiting factor for 1,25(OH)2D production

rs2282679 v genu GC - vitamin D binding protein (GC, DBP)

  • 4p12 (overall p=1.9x10(-109) for in GC
  • Měla největší vliv
  • vitamin D transport
  • Lidé se dvěma nepříznivými kopiemi genu měli hladinu vitaminu D o 20 % nižší
    • Influence vitamin D status [30]
    • Variation at these loci identifies individuals of European descent who have substantially elevated risk of vitamin D insufficiency [30]
  • Nutrafitness - Lifetest - dostupný test v ČR

Major homozygotes

  • Framingham Heart Study (n=5,656)
    • 82.6 (0.73) Mean 25-OH D levels [30]
  • 1958 British Birth Cohort (n=6,552)
    • 61.9 (0.33) Mean 25-OH D levels [30]

Heterozygotes

  • Framingham Heart Study (n=5,656)
    • 74.8 (0.81) Mean 25-OH D levels [30]
  • 1958 British Birth Cohort (n=6,552) [30]
    • 57.0 (0.30) Mean 25-OH D levels [30]
  • Strong association of genetic variants at GC with 25-OH D concentrations
    • SNP rs2282679 was strongly associated with DBP (P=4.0 × 10-42)
      • With the minor allele related to lower DBP concentrations [30]

Minor homozygotes

  • Framingham Heart Study (n=5,656)
    • 64.6 (1.79) Mean 25-OH D levels [30]
  • 1958 British Birth Cohort (n=6,552) [30]
    • 52.8 (0.27) Mean 25-OH D levels [30]
  • Strong association of genetic variants at GC with 25-OH D concentrations
    • SNP rs2282679 was strongly associated with DBP (P=4.0 × 10-42)
      • With the minor allele related to lower DBP concentrations [30]

rs4588 (Thr›Lys)

  • Rs4588
  • Only 11 bp away from rs7041
  • Direct genotyping of rs4588 in one of our samples (Twins UK)
    • Confirms that it is in linkage disequilibrium (r2>0.99) with multiple associated variants from our genome-wide association study [30]

rs3755967 GC

rs17467825 GC

rs1155563 GC

rs2298850 GC

rs7041 GC

  • Association with circulating 25-OH D [30]

25-hydroxycholekalciferol - 25(OH)D3 - 25-OH-D - calcifediol

  • Tvorba 25–hydroxycholekalciferolu zřejmě není přísně regulována (Ganong 2005, 396) [4]
  • Dlouhý plazmatický poločas eliminace – 2 až 3 týdny
    • Ukazatelem dlouhodobého stavu vitamínu D v organismu [5]
  • Hlavní metabolit vitamínu D3
    • Převažující formou vitamínu D v oběhu
    • Většina jaterních zásob vitamínu
      • Velká část hydroxycholekalciferolu také přechází do žluči
        • Enterohepatální oběh [4]
  • Prekurzorem pro dihydroxylované metabolity
  • V cirkulaci opět navázán na vazebný protein
    • Transportován do ledvin
      • Hydroxylován v poloze 1 na biologicky aktivní metabolit 1,25-dihydroxyvitamin D (1,25-(OH)2-D)
        • Může vznikat i v jiných tkáních, kde je přítomna aktivní 25D-1-hydroxyláza

1 alfa-hydroxyláza - 25-hydroxyvitamin D3 1-alpha-hydroxylase

  • Product of the CYP27B1 human gene
  • mitochondrial oxygenase
    • Catalyzes the hydroxylation of 25-hydroxycholecalciferol (calcifediol) in the 1-alpha position
    • Hydroxylace 25-hydroxycholekalciferolu na 1,25-dihydroxycholekalciferol = kalcitriol
      • Biologicky aktivní = přímo ovlivňuje metabolismus vápníku [1]

Renální 25OHD-1alpha hydroxylase - CYP27B1

  • 25-0H-D je v ledvinách je dále hydroxylován
  • Konvertován v mitochondriích proximálních renálních tubulů [10]
    • Působením enzymu 1-alpha-hydroxylasy
    • Na biologicky aktivní kalcitriol
  • Kromě 1,25-(OH)2-D se v cirkulaci mohou vyskytovat další metabolity
    • Vznikající rovněž v ledvinách
  • Nedostatek akt. vit. D při renálním selhání a nutnost podávání aktivního metabolitu !
    • 1,25(OH)2-D !!!
  • mitochondrial mixed function oxidase
    • Significant homology to other mitochondrial steroid hydroxylases
      • CYP27A1 (39%), CYP24A1 (30%), CYP11A1 (32%), and CYP11ß (33%)
    • mitochondrial P450 enzymes are located in the inner membrane of the mitochondrion
      • Terminal acceptor for electrons transferred from NADPH through ferrodoxin reductase and ferrodoxin

Mutations in this gene

  • Rare autosomal disease of pseudovitamin D deficiency rickets
  • Gene knocked out in animals
    • Retarded growth, rickets, hypocalcemia
    • Hyperparathyroidism
    • Undetectable 1,25(OH)2D
    • Alopecia is not part of this phenotype [22]

Stimulována

  • PTH
  • Růstovým hormonem
  • Prolaktinem (laktogeneze potřebuje vápník)
  • estrogeny
  • Administration of PTH in vivo (50) or in vitro (51,52)
    • Stimulates renal production of 1,25(OH)2D
      • Can be mimicked by
        • CAMP
        • Forskolin [22]
  • PTH activation of protein kinase C (PKC)
  • Synthetic fragments of PTH
    • Lacking the ability to activate adenylate cyclase
    • Stimulate PKC activity
      • Increase 1,25(OH)2D production [22]
    • Direct activation of PKC with phorbol esters
      • Increased 1,25(OH)2D production [22]
  • Calcium modulates the ability of PTH to increase 1,25(OH)2D production
    • Calcium by itself can decrease CYP27B1 activity
    • Block the stimulation by PTH
    • In vivo, calcium can exert its effect in part by reducing PTH secretion [22]
  • Phosphate deprivation
    • Can stimulate CYP27B1 activity in vivo , in vitro
      • Can be blocked by hypophysectomy
        • Partially restored by
          • Growth hormone (GH)
          • Insulin-like growth factor (IGF-I) [22]

Inhibována

  • Vlastním produktem
  • fosfáty (např. při renálním selhání s hyperfosforémií)
  • Pravděpodobně i kalciovými ionty
  • Současně se stimuluje hydroxylace v poloze 24
  • FGF23 by osteocyte cells in bone
  • FGF23 inhibit CYP27B1 activity in vivo and in vitro
    • Responsible for
      • Impaired phosphate reabsorption
      • Impaired 1,25(OH)2D production [22]
    • X-linked and autosomal dominant hypophosphatemic rickets
    • Oncogenic osteomalacia
    • FGF23 acts through FGF receptors 1 and 3
      • In conjunction with the coreceptor Klotho [22]
  • High phosphate stimulates FGF23 production from bone
    • Likely the major mechanism by which phosphate leads to decreased CYP27B1 activity [22]
  • 1,25(OH)2D administration
    • Leads to reduction in CYP27B1 activity [22]
  • 1,25(OH)2D
    • Stimulates FGF23 production
    • Inhibits PTH production [22]
  • 24-hydroxylase activity blocked
    • 1,25(OH)2D administration fails to reduce the levels of 1,25(OH)2D produced
      • Apparent feedback regulation of CYP27B1 activity by 1,25(OH)2D appears to be due to its stimulation of CYP24A1 and subsequent catabolism [22]

Extrarenální

  • Demonstrated in
    • Anephric humans
    • Anephric pigs [12]
  • Kidney is generally considered the major source of circulating levels of 1,25(OH)2D
    • Extrarenal CYP27B1 activities providing for local needs under normal circumstances
    • Extrarenal sources can lead to increased 1,25(OH)2D and calcium levels in some pathologic conditions

Místa vzniku

  • Expression of CYP27B1
    • Highest in epidermal keratinocytes
    • Kidney also expresses this enzyme in the renal tubules
    • Brain, placenta, testes, intestine, lung, breast, macrophages,
    • Lymphocytes, parathyroid gland, osteoblasts and chondrocytes
  • Placenta
  • Aktivované makrofágy a monocyty a dendritické buňky při [6]
    • Tuberkulóze
    • Nehodgkinských lymfomech
    • Sarkoidóze
  • Keratinocytes metabolizing vitamin D
    • 1alpha-hydroxylase (CYP27B1) to 1,25(OH)2D [12]
      • Regulated and coupled to the differentiation of these cells
      • As keratinocytes differentiate their production of 1,25(OH)2D declines
        • Higher levels of CYP27B1 in the stratum basale than in the suprabasal layers
      • Change in activity = change in expression of the gene [12]
    • Human keratinocytes rapidly and extensively convert 25(OH)D to 1,25(OH)2D
      • Not clear how much of the 1,25(OH)2D produced by the epidermis actually enters the circulation
        • Km for the 25(OH)D to CYP27B1 in keratinocytes is estimated to be 5 × 10-8 M
          • Lower than that estimated for the kidney [12]
    • Production of 1,25(OH)2D by isolated keratinocytes in culture
      • Confirmed using intact pig skins perfused with 25(OH)D [12]
  • 1,25-D is synthesized in cells of the immune system
    • T cells
    • Antigen-presenting cells
    • Macrophages
      • Often infected by the Th1 pathogens
  • Extrarenal enzyme located in macrophages
    • Major role in certain granulomatous conditions (e.g., sarcoidosis)
      • Uncontrolled elevations of blood 1,25-(OH)2D3 levels
  • Respiratory epithelial cells - primary lung epithelial cells
    • Defense against airborne pathogens
    • Express high baseline levels of activating 1 alfa-hydroxylase
    • Low levels of inactivating 24-hydroxylase
    • Increased expression of genes by the Vitamin D Receptor

Regulace

  • Není regulována parathormonem, ale spíše faktory, jako:
    • IFN-gama
  • Suprese
    • Maturací dendritických buněk [6]
  • If renal production of 1,25(OH)2D is normal
    • Circulating levels of 1,25(OH)2D appear to be sufficient to limit the contribution from epidermal production
      • Likely due to the induction of 25(OH)D-24-hydroxylase (CYP24A1) in the keratinocyte by 1,25(OH)2D
        • Catabolizes the endogenously produced 1,25(OH)2D before it leaves the cell in which it is produced [12]
  • Extrarenal production tends to be stimulated by
    • IFN-g
    • TNF-a more effectively than PTH
    • May be less inhibited by calcium, phosphate, and 1,25(OH)2D depending on the tissue [22]

Obcházení renální regulace

Alfacalcidol drug

  • 25-hydroxylation in the liver will produce calcitriol as the active metabolite
  • Will produce greater effects than other vitamin D precursors in patients with kidney disease
    • Who have loss of the renal 1-alpha-hydroxylase [10]

24,25–dihydrocholekalciferol

  • Vzniká snížením syntézy kalcitriolu
    • Také metabolit vitamínu D3
    • Méně účinný pro regulaci hladiny vápníku
    • Ovlivňuje diferenciaci chrupavčitých buněk, maturaci chondrocytů růstové ploténky
    • Určitou roli při hojení zlomenin [4]

Kalcitriol - 1 alfa,25-dihydroxycholekalciferol - 1alpha,25-dihydroxyvitamin D3 - 1,25-dihydroxyvitamin D3 - 1alpha,25-(OH)2D3 - 1,25(OH)2D - 1,25D3

  • Hormonally active metabolite of vitamin D
  • Three hydroxyl groups
    • Cholecalciferol already has one hydroxyl group
    • Product of calcifediol (25-OH vitamin D3), derived from cholecalciferol (vitamin D3)
      • Nejedná se o product of hydroxylations of ergocalciferol (vitamin D2) !!!
    • There are three (1,3,25-triol) in calcitriol [10]
  • Increases the level of calcium (Ca2+) in the blood
    • Increasing
      • Uptake of calcium from the gut into the blood
      • Reabsorption of calcium by the kidneys
      • Release of calcium into the blood from bone [1]
  • Vznik regulován pomocí zpětné vazby plazmatickým Ca2+ a PO43– [4]
    • Auto-regulated in a negative feedback cycle
      • By parathyroid hormone, fibroblast growth factor 23, cytokines, calcium, and phosphate [11]
  • Syntetizován především v ledvinách, ale i z jiných tkání :
    • Lymfatická tkáň
    • Kůže
    • Pankreas
    • Prostata
    • Mléčná žláza
    • Placenta
    • Kostní tkáň
    • Chrupavky
    • Makrofágy
    • Tuberkulózní granulom
    • Sarkoidóza [4]
  • Formation of 1,25-dihydroxyvitamin D3
    • By the action of hydroxylase enzymes in the epidermis
      • 25-hydroxylase (CYP27A1)
      • 25-hydroxyvitamin D3 1-?-hydroxylase (CYP27B1) (Christakos et al., 2016) [12]

24-hydroxyláza - 25(OH)D-24-hydroxylase - CYP24A1

  • Initiates the degration of both 25-OH D and 1,25(OH)2D
  • Activity reduction of calcifediol and calcitriol
    • By hydroxylation at position 24 forming
      • Secalciferol
      • Calcitetrol [11]
  • In the keratinocyte
  • Induced by 1,25(OH)2D [12]
  • Rozloží vit. D dříve než se dostane do cirkulace [12]

Mutations in human CYP24A1

  • Modulate the level of this enzyme in selective tissue

rs2296241

  • Does not change the amino acid sequence
  • Alter protein levels of CYP24A1
  • Functional impact on vitamin D response element binding
  • SelfDecode
    • Importance: 3

Major Allele: G = 54%

  • A meta-analysis found that subjects with GG
    • Had reduced the risk of androgen-related prostate cancer
  • Subjects with a G
    • Had decreased the risk of oral cancer
    • Had a lower risk of oral lichen planus

Minor Allele: A = 46%


rs6013897 v genu CYP24A1

  • 20q13
  • Hydroxylation
  • Genome-wide significant in the pooled sample (p=6.0x10(-10)
  • Nižší hladiny vit. D [30]
  • Nutrafitness - Lifetest

Higher levels of CYP24A1

  • Breaks down excess 1,25-D
  • Associated with poorer survival in lung adenocarcinoma
  • 2011 study, CYP24A1 mRNA was elevated 8-50 fold in lung cancer
    • Compared to normal non-cancerous lung
    • Significantly higher in less severe cancers [24]
  • CYP24A1 expression was increased in
    • Metastatic breast cancer
      • 53.7% in invasive carcinomas
    • 19.0% in benign lesions [24]
  • Deregulation of the Vitamin D signalling and metabolic pathways in breast cancer
    • Favouring tumour progression
  • During mammary malignant transformation
    • Tumour cells lose their ability to synthesize the active form of Vitamin D
      • Respond to VDR-mediated Vitamin D effects
        • Increasing their ability to degrade this hormone [24]

CYP24A1 knockout mouse

  • Fails to produce any 24-hydroxylated metabolites of vitamin D
  • Skeletal lesion is defective mineralization of intramembranous (not endochondral) bone
  • Skeletal abnormality appears to be due to high circulating 1,25(OH)2D levels
  • Mouse with one lacking the VDR corrects the problem [24]

1 alfa, 24,25(OH)2 Vitamin D


O úroveň výše

Poslední aktualizace: 17. 11. 2018 0:34:34