Monday, March 12, 2012

Prolactin's Biological Functions

1Prolactin (PRL) Basics¶

1.1Structure¶

• there are structural variants. heterogeniety
• residual modification
  • usually decrease activitity of PRL
  • glycosylated, phosphorylated, sulfated PRL
  • post translational
  • modification on residue differs among species
• cleaved forms:
  • 16kDa and 8kDa (loop is destroyed)
    • cathepsin D-like protease does this.
    • it's a antiangeogenic factor
  • 22kDa. C-terminal s-s loop removed
    • by kallikren, a trypsin like serine protease
    • not well studied
• native PRL: has a loop formed by disulfide bridge.

1.2synthesis and secretion¶

• produced by lactotrophs:
  • comprise 20-50% of anterior pituitary cells, depending on the physiological state
    • 50% when it's lactation phase for the organism
  • also the last anterior pituitary cells to differenaite
  • adulthood prolactinomas: robust proliferative response.
    • no other pituitary adenomas produce the same effect
  • regulated by Pit1 (a transcription factor)
• PRL are also expressed in extrapituitary (periphery) sites
  • ex: brain, thymus, lymph node, endometrium, mammary glands
  • they act as local growth factor and hormone.
  • account for ~20% of prolactin production

1.3Regulation¶

• tonically inhibited by dopamine coming from hypothalamus

1.3.1Hypothaalmus-pituitary axis¶

• hypothalamus controls ant. pituitary
  • hypophyseal portal artery branches into ME by primary plexus where many neurotransmitter/neuro peptides are released into.
  • secondary plexus then dumps into secondary plexus
  • tight control of hypothalamus over pitutiary because only a small amount needs to be secreted to affect pituiary (direct access instead of being diluted by systemic blood)
• PRL is tonically inhibited by hypothalamus. Evidences:
  • PRL level increase when pituitary is detached and placed somewhere else like on top of adrenal gland.
  • stalk transection also leads to increase in PRL
  • lesioning of the ME also increases PRL.
  • (basically any interruption and disconnection)
  • reglulated by dopamine (inhibitor)

1.3.2Dopamine¶

• dopamin is converted from tyrosine by tyrosine hydroxylase and DOPA decarboxylase
  • rate-limiting enzyme: tyrosine hydroxylase
• DA-nergic neurons:
  • TIDA : tuberoinfundibular dopaminergic neurons
    • to ME when primary pelxus is
  • THDA and DOPA: to short portal vessel
• lactotrophs have DA receptors
  • in portal blood 80% of receptors are occupied
  • degree of inhibition of PRL release coorelates with degree of receptor occupancy
• DA agonists make PRL decrease. DA antagonists make PRL increase.
• suckling leads to decrease in DA and consequently increase in PRL

1.3.3prolactin short feedback¶

• there's a negative feedback loop at ME or CNS.
  • prlactin gains access to CSF
  • stimulation on DA neruons, which have PRLR (prolactin receptor)
  • PRLR K mice have 30-100 fold increase in PRL.
• feedback loop dimishes during lactation.

1.3.4Lactation¶

• Suckling leads to :
  • increase in secretion of opiod peptide, endorphin in arcuate nucleus --> suppression of TIDA
  • dimished feedback control
  • increase in PRL release factors in hypothalamus
    • TRH, VIP, oxytocin, neuropeptide Y, neurotensin, histamine
  • release of adrenocorticotropin --> more glucocorticoids released into the blood.
  • increase in number of lactotrophs (~50% of ant. pitutiary cells)
  • suppression of tyrosine hydorxylase (the rate limiting enzyme of DA synthesis)

2Functions¶

• (has ~85 functions) Main outline:

2.1reproudction¶

• lactation
  • alveolar growth
  • milk protein synthesis
• luteotrophic function
  • increase progesterone synthesis
    • also inhibit progesterone degradation
  • potentiate sterodogenic effects of LH (lutenizing hormone)
  • PRLR KO : infertile, lack normal luteal function, decrease ovulation, aberrant oogenesis, implantation failure
    • restored by giving progesterone
• parental behavior
  • nest building, nursing, etc.
  • PRLR KO female mice can't recognize pups (so PRL also has effects on nervous system)

2.2homeostasis¶

• immune function
  • restores thymic function in PRL-deficient dwardf mice (or Pit1-KO mice)
  • B and T cells have PRLR and PRL, suggesting autocrine and paracine functions of PRL
  • PRL potentiates IL2 mitogenic effects (mitosis-promoting) in T cells
  • PRL has overlapping functions with some pathways (may share signaling pathway with cytokines) because PRL KO mice do okay with their immune system.
• osmoregulation
  • stimualtes solute movement for lactation
    • uptake of amino acid and glucose by mammary epithelial cells
    • also promote sodium, potassium and water retention by acting on kidney.

3Mechanism of Functions¶

3.1Hormonal control of Lactogenesis¶

• estrogens act on lactotrophs to make them secrete PRL.
• Progesterone inhibits lactogenesis (by inhibiting PRL)
  • after baby is born, progesterone level drops sharply
  • this sharp decrease in progesterone accounts for initiation of lactation
• glucocorticoids or adrenocorticoropin (adrenal hormones) with PRL stimulates lactogenesis (synergistic)
  • right after baby is born there's a sharp increase of glucocorticoids
• prolactin:
  • hypophysectomized mice don't make milk during pregnancy.
    • treated with PRL will restore functions
  • blocking of PRL surge before partuition by Da agonists inhibit lactogenesis
  • insulin, prolactin, hydrocortisol in combination induce cells to make milk proteins

3.2Mammaroy Gland Development¶

• structure of alveolar:
  • lumen surrounded by a layer of epithelial cells, which secrete milk into the lumen
  • myoepithelai cells: muscles cells surrounded epithelial layer. contract upon oxytoxin stimulation -->milk ejection
• prolactin
  • stimulates differentiation of termianl end buds in virgin state
    • ..by acting through progesterone, which promotes differentiation
  • growth and differentiation of alveoli in pregnancy
• during lactation, major cell type :alveoli

• during involution, build up of milk in the gland initiates apoptosis of alveoli structure, which goes back to virgin-like state.

• prolactin KO and PRLR KO have little terminal end buds

  • adding prolactin wouldn't restore structure in PRLP KO
    • because terminal end bud differentiation depends on indirect effect of PRL via progesterone.
• PRLR partial KO have little alveolar growth/differentiation
  • halfploid insufficiency
  • therefore PRL won't restore the function. you need the receptor!

3.3Receptor Signaling Mechanism¶

• PRLR is a Class I cytokine receptor
• prolactin receptor have many homologies with receptors in this family : like GH receptor, IL2 receptor, etc
  • homologies include : Box1/2 (cytoplasm domain), disulfide bond, etc
  • act with JAK kinase, which interact with Box1/2 domains
• STAT:
  • STAT is downstream of JAK. bind to JAK via SH2 domain
  • dimerize upon activation by JAK and go to nucleus to activate transcription via their TA domain (transcription activation)
  • There are 7 of them. Two of them are inhibitory (STAT4_beta and STAT1_beta)
    • inhibit action when dimerize with other STAT
• activation mechanisms
  • JAK2 constitutively bind with PRLR via Box1.
  • activated receptor goes through confirmation change. they dimerzie and allow JAK to phosphorylate (thereby activate) each other
  • activated JAK2 recruits STAT and activate them
  • STAT dimerize and go into nucleus
  • STATs are also target of other pathways
• experiment show tha tEGF don't activate JAK2 like PRL do
  • STAT5a is also not as upregulated when EGF is added.
• JAK KO shows no terminal bud differentiation and has little alveolar structure.
• Prolactin mainly activates JAK@ and STAT5, esp. in mammary gland. but cytokine can activate some pathways of prolactin (-->signal complexity&redundancy)
• gain of function of JAK (with mutation at JH2) leads to lukemia : myeloproliferative disorders
• STAT protein is also targets fr many other tyrosine kinase:
  • ex: target for SAR kinase, which plays a big role in tumoergenesis and metatesis as well.

4Breast Cancer and Prolactin¶

• statistics show that women who breast feed have less chance of getting breast cancer
  • mostly those who breast feed for longer than 6 months get this protection of PRL.
• therefore, even though prolactin is a growth factor for mammary cells, it promotes differentiation --> anticancer/antiproliferative
  • highly differentiated and polarized cells are less likely to turn into cancer cells
  • prolactin makes epithelial cells highly diffentiate (to make milk proteins) and polarized (to transport milk proteins to lumen)
    • factor that induces polarization is cancer-suppressing
    • polariztion comes with tight junction and adhesion molecules
      • PRL KO mice's mammary gland shows little staining of tight junction
      • blocking of JAK2 leads to loose structure. cells become more flat and loosely attached to each other due to less expression of adhesion molecule (Ecad)
• therefore, dual role of PRL in breast carcinogenesis:
  • growth and differentiation
  • suppressor/anti-metastasis activity