Nuclear Receptors

Question 1

Cholesterol

Answer 1

• most commonly occurring steroid
  
  • Major component of cell membrane
  • Steroid hormones are synthesized from cholesterol
    
    • 30-50% comes from diet
    • 60% synthesized in liver and intestine

Question 2

Characteristics of Steroid Hormones (4)

Answer 2

• Hydrophobic nature = aided by a family of transport proteins (b/c low intrinsic water solubility in blood)
• “Structurally related but physiologically diverse”
• They have access to all tissues in body and enter cells directly by passive diffusion
• Receptors for steroid hormones act DIRECTLY W/IN NUCLEUS to regulate transcription through their association w/ select genes

Question 3

Transport Proteins

Answer 3

• made in liver then binds specific steroid and transports it to target cell where it diffuses across membrane
• include CBG (corticosteroid binding globulin) for glucocorticoids

Question 4

Signal Transduction Path II

Answer 4

1- Hormone encounters nuclear receptor either in cytoplasm or nucleus

- “Anchoring” of nuclear receptor in cytoplasm likely results from their association w/ large complex of proteins that include some heat shock proteins and immunophilins

2- Once hormone-bound, steroid hormone receptors preferentially accumulate w/in the nucleus traversing nuclear pore complex (special structure embedded in nuclear envelope)

Question 5

Nuclear Pore Complex

Answer 5

special structure embedded in nuclear envelope that hormone receptors bound to hormone steroid travel through

Question 6

Structure of Steroid Hormone Receptor

Answer 6

2 distinct binding domains… (function indep)
1- hormone-binding domain ligand
2-DNA-binding domain
**contains 2 zinc finger motifs that are so named b/c coordination of zinc by 4 cysteine residues

Question 7

Nuclear Receptors

Answer 7

-Superfamily that INCLUDES steroid hormone receptors

• Members of family share common domain of organization and AA homology (particularly in DNA-binding domain of the receptors)
• DNA binding domains more in common than ligand binding domains

Question 8

Do nuclear receptors recognize only 1 HRE sequence?

Answer 8

• NO- a “consensus” HRE can be defined for ea nuclear receptor that represents the most common sequences features of a particular nuclear receptor DNA-inding site
• Some but not all positions w/in a consensus HRE sequence are critical for nuclear receptor binding
• Nuclear receptor specificity for DNA binding can be dictated by minor sequence variation in HRE

Question 9

Do NRs binding to HREs influence transcription on their own?

Answer 9

• No, transcriptional responses of a given gene to a nuclear receptor DNA-binding in vivo is influenced by many other factors
• Accessory, DNA-bound factors

Question 10

HRE

Answer 10

• hormone response elements
• closely related DNA sequences (typically 12-15 base pairs in length) that nuclear receptors bind to
• the “ligand” for the DNA binding portion of NRs

Question 11

HRU

Answer 11

• Hormone Response Unit
• composed of NRs + other DNA-bound transcription factors they cooperate with during transcriptional activation
• These other factors often dictate the SPECIFICITY of transcription

Question 12

Signal Transduction Path I

Answer 12

• 1- stim of hormone prod by releasing hormones or factors that are synthesized and secreted from neuroendocrine cells
• 2- Transport of hormone to its target cell via plasma transport proteins in bloodstream
• 3- Dissociation from plasma transporter protein and diffusion of free hormone across plasma cell membrane
  4- hormone then binds receptor in nucleus or cytoplasm

Question 13

ER-alpha v ER-beta

Answer 13

• ER-alpha and ER-beta are 2 subtypes of estrogen receptors w/ distinct physiological responses to estrogen binding
  
  • ER-alpha
    
    • Stimulation of uterine growth
    • Bone metabolism
    • Mammary gland development
    • Negative feedback in hypothalamus
  • ER-beta
    
    • Apoptosis in prostate cancer cells
    • Enhancement of glucose stimulated insulin secretion (pancreatic ß cells)
    • Folliculogenesis
    • Reduced intestinal inflammation (reduced colon carcinoma risk)

Question 14

Mechanism of Transcriptional Activation by NRs

Answer 14

• 1- NR binding to HRE w/in target gene can enhance transcriptional activity of RNA polymerase II from a linked promoter
• 2- NRs typically cooperate w/ other DNA-bound transcription factors to make up a “hormone response unit” (HRU)
• 3- Transcription factors bound to HRU can influence the cell-type specificity of a hormone response

Question 15

Mechanism of Transcriptional Repression by NRs

Answer 15

-transcription repression by NRs occurs by a variety of mechanisms and does not always require direct DNA-binding by receptor

Question 16

Co-activator Protein

Answer 16

• Co activators = histone acetylation = inc transcription from linked promotors
• Can contain specific enzymatic activity (HAT) OR recruit such enzymes to chromatin of target genes (these enzymes catalyze distinct modifications of histone proteins)

Question 17

Co-repressor Protein

Answer 17

• Co Repressor = deacetylation = dec transcription from linked promotors
• Function to recruit specific enzymatic activities (HDACs)

Question 18

Rubenstein-Taybi Syndrome

Answer 18

-Mutations in CBP coactivator —> defect in the coactivator’s HAT activity —> Rubinstein- Taybi Syndrome (short stature and dev delay and inc risk of brain cancer/leukemia)

Question 19

Orphan NRs

Answer 19

Endogenous nuclear receptors in NR superfamily that do not have a known ligand as of now

Classes- adopted (have low affinity to dietary lipid ligands) and true orphans (unknown ligand)

Question 20

3 Ways NRs are classified

Answer 20

1- hormone binding specificity
2- DNA binding specificity
3- dimerization properties (homodimer, heterdimer, monomer) steroid receptors usually homodimers

Question 21

5 Main Actions of NRs

Answer 21

1- DNA binding
2- Ligand binding
3- Cooperate w/ DNA bound accessory ligaments (leads to tissue specificity)
4- Recruit co-activators or co-repressors
5- Ligand-induced conformational changes