1. STAT3
Michael Patel

2. Overview STAT3 is a transcription activator
It homodimerizes and binds to the DNA promoter region
It is associated with oncogenesis, cellular differentiation, and embryogenesis

3. STAT3 Signaling Pathway
Involved in signal transduction pathway
Signal transduction translates external signals into internal cellular responses

4. Stat3 Pathway (cont.)

5. Pathway Components
Cytokines are of the IL-6 (Interleukin 6) family, or very similar to them.
They bind to Glycoprotein 130 (gp130), an important cytokine receptor.
After a gp130 conformational change, Janus kinases (JAK) phosphorylate a STAT3 tyrosine, initiating STAT3 dimerization.

6. Pathway Components (cont.)
MAP and mTor are serine kinases, which phosphorylate S727.
This phosphorylation enhances DNA binding, although it is not essential.

7. Structure STAT3 has three domains: Alpha, Binding, and SH2.
The Binding domain is responsible for binding to DNA.
SH2 is responsible for interacting with the tyrosine kinase.

8. Structure (cont.)
PDB ID: 1bg1

9. SH2 Domain
Includes tyrosine (Y705) that, when phosphorylated, induces dimerization.
The polypeptide strand from residue interacts in trans with the other monomer to form the dimer

10. Phosphotyrosine 705
Y705 shown in blue, rest of binding domain shown surrounding it.
PDB ID: 1bg1

11. Phosphotyrosine 705 (cont.)
Becker, et al. Nature 394, (9 July 1998)

12. DNA Binding STAT3 binds to 9 base sequence: 5’-TTCCCGTAA -3’
However, there may be a high sequence variability: 5’ – TTYN3YAA – 3’ (Y = T or C)
Four loops per monomer are in contact with DNA. (cx not pictured in following slide)
Bases -1, 0, and +1 can be variable because of water-mediated contact.

13. Becker, et al. Nature 394, 145-151(9 July 1998)
DNA Binding (cont.)
Red = Polar Interactions
Blue = Hydrophobic Interactions
Becker, et al. Nature 394, (9 July 1998)

14. Becker, et al. Nature 394, 145-151(9 July 1998)
DNA Binding (cont.)
Becker, et al. Nature 394, (9 July 1998)

15. DNA Binding (cont.) Complete STAT3 dimer bound to DNA.
Becker, et al. Nature 394, (9 July 1998)

16. Regulation and p53
In cancer cells, high amounts of active STAT3 are correlated with a mutant version of p53.
wt versions of p53 are incident with a lack of STAT3 dimerization
Thus, p53 or a downstream phosphatase dependent upon p53 acts as a tyrosine phosphatase, inhibiting dimerization.

17. Pias3 Protein inhibitor of activated Stat3
Binding domain is from residues
Within this domain, the Y94 residue is essential for Pias binding.
Blocks STAT3 DNA binding.

18. A is the lowest energy conformation.
Pias3
A is the lowest energy conformation.
Levy, et al. Blood, 1 April 2006, Vol. 107, No. 7, pp

19. Rac1 Conversely, Rac1 can activate increased amounts of STAT3.
It is associated with increased tyrosine and serine phosphorylation.
Mutation of phenylalanine (F37A) to alanine eliminates binding to STAT3.
Rac1 helps to recruit kinases to STAT3

20. Rac1 (cont.)
Shown are GNP and phenylalanine (red)
PDB ID: 1mh1

21. Oncogenesis
Persistently activated STAT3 is common among many epithelial cell cancers.
Can be the cause of over active kinases or lack of a proper inhibitor.
It is known to have anti-apototic effects.
Specifically, it activates the genes for such apoptosis inhibitors as Bcl-xL, c-Myc, cyclin D1, Vegf, and Survivin.

22. Embryogenesis
STAT3 has been found in mice to be essential for embryogenesis.
Specifically, it is required for embryo implantation in the uterus.
Additionally, deficiencies in pSTAT3 and IL-11, a cytokine, has been found in infertile women.
Demiriadis, et al. Reproductive Biology and Endocrinology 2007, 5:44

23. Possible Treatment Directions
Targeted deletion in skin cells prevents cancer.
One possible treatment involved the injection of antisense oligonucleotides that have no effect on healthy cells.
Platinum containing compounds can disrupt STAT signaling.
Another possibility is use of inhibitors that interact with the tyrosine kinases.