Gene And Nutrient Interactions

Question 1

Transcriptional control of cholesterol

Answer 1

• SREBP binding protein resides in ER, in low cholesterol moves to golgi via COPII vesicles where cleaved, activated, and moves to nucleus to activate expression of HMG-CoA reductase (rate limiting step of cholesterol synthesis) through binding SRE on DNA
• SREBP kept in ER via INSIG (anchor protein) and SCAP which have SSD (sterol sensing domains)
• in high cholesterol, they anchor SREBP to ER, and in low cholesterol they dissociate
• lanosterol (pre-requisite to cholesterol) can bind and cause free SREBP and can direct towards proteasomal degradation

Question 2

Transcriptional control of glucose metabolism and lipogenesis

Answer 2

• ChoREBP are carb response element binding proteins and bing ChoRE on DNA stimulating lipogenesis in high glucose environments
• 2 domains on alpha isoform: lid and grace. These interact in closed conformation in low glucose environment. Beta isoform only had grace domain, and has indirect role in stabilising alpha form
• when high glucose, lots of glycolysis which produces glucose 6 phosphase which activates the PPP pathway, where metabolites activate phopshatase 2 which removes phopshorylation from ChoREBP alpha, causing open confirmation and movement into nucleus
• binding to ChoRE and stimulation of lipogenesis

Question 3

Transcriptional regulation of vitamin A

Answer 3

• vitamin A enters hepatocytes as retinoid esters on chylomicrons from the gut, inmediately hydrolysed and has 2 fates: storage (in high RA) conditions, LRAT converts to retinoid esters, or, (in low RA) enter oxidation pathway to form retinoic acid which is ligand for RAR and RXR TF (activate liberation of RA). If too much active RA cytochrome P450 can convert to inactive form
• RAR: bind to 9-cis-RA and all-trans-RA
• RXR: bind to 9-cis-RA and can initiate transcription in ligand activated manner or when bound to other nuclear receptors
• these TFs form dimers (homo or hetero) and bind to paired response elements on DNA (RARE or RXRE)
• transcription stimulated when binding of ligand (RA) to TF on RE; this recruits histone acetylase which opens DNA and allows for RNAPII to bind
• if no ligand, then histone deactylase recruited which condenses chromatin

Question 4

Role of RAR and promyelocytic leukaemia

Answer 4

• chromosomal translocations lead to RAR fusion proteins
• PML/RAR fusion leukaemia cells response to vitamin A therapy
• PLZF/RAR fusion do not respond to vitamin A therapy

Question 5

Vitamin D metabolism and regulation of calcium/P homeostasis

Answer 5

• vitamin D3 is hydroxylated in liver and activated in the kidneys to form 1,25(OH)D4 (this process is stimulated by PTH from pituitary gland in response to low Ca and P, stimulating osteoclasts)
• 1,25(OH)D4 binds to VDR (vitamin D receptor) and vitamin enters nucleus where complexes with RXR and is phosphorylated
• phosphorylated complex binds to VDRE and initiates transcription of myriad of genes, including involving immune function (TNFa, IFNy), calcium (osteopontin, osteocalcin), cell proliferation (c-myc, cyclin D3) and cell growth (TGFb, EGF, IGF)

Question 6

Zn gene expression

Answer 6

• Zn involved in metallothionein (hoovers up heavy metals, scavenges ROS) and Zn finger motifs on TF (between cys on beta sheet and his on alpha helix)
• in high cytosolic Zn, MTF1 binds Zn, dimers are phosphorylated, and enter nucleus
• phosphorylates MTF1 dimers bind MRE and initiate transcription of metallothionein

Question 7

Iron post-transcriptional regulation

Answer 7

• IRP1 is Fe binding protein, in low Fe concentrations binds 3’UTR hairpin loop of mRNA, stabilising for translation of transferrin receptor (increasing intake of Fe in cells)
• IRP1 also binds 5’UTR which suppresses translation of ferroportin and ferritin (limits Fe export and storage)
• in high Fe concentrations, IRP1 is bound by O2 and Fe and polyubiquitinated and tagged for proteasomal degradation
• also, in high Fe, Fe can bind to IRP1 causing conformational change and inability to bind to 3’UTR

Question 8

Selenium post-transcriptional regulation of glutathione peroxidase

Answer 8

• GSHpx is enzyme involved in handling lipid peroxides during oxidative stress, contains 4 subunits all of which contain selenocysteine
• 2 types of GSHPx: cytosolic and phospholipid
• in high Se, selenoproteins interact with tegulatory binding protein at 3’UTR allowing translation of GSHPx
• in low Se, phospholipid form is prioritised over cytosolic form