Methods for modulating de novo hepatic lipogenesis by modulating XBP-1 activity
The invention provides methods and compositions for modulating the expression, processing, post-translational modification, stability and/or activity of XBP-1 protein, or a protein in a signal transduction pathway involving XBP-1 to treat dyslipidemias and steatosis disorders. The present invention also pertains to methods for identifying compounds that modulate the expression, processing, post-translational modification, and/or activity of XBP-1 protein or a molecule in a signal transduction pathway involving XBP-1.
1. A method of identifying a compound that is useful in treating and/or preventing dyslipidemia comprising, providing a cell comprising at least one expression construct comprising a DNA molecule encoding an XBP-1 polypeptide and a DNA molecule encoding an IRE-1 polypeptide;
a) contacting the cell with each member of a library of test compounds;
b) determining the activity of the XBP-1 polypeptide in the presence and absence of the test compound;
c) selecting from the library of test compounds a compound of interest that downmodulates the activity of the XBP-1 polypeptide;
d) determining the effect of the compound on de novo hepatic lipogenesis;
e) selecting a compound of interest that downmodulates de novo hepatic lipogenesis as compared to an appropriate control, thereby identifying a compound that is useful in treating and/or preventing dyslipidemia.
2. The method of claim 1 , wherein the effect of the compound on XBP-1 activity is determined by measuring the binding of XBP-1 to IRE-1.
3. The method of claim 1 , wherein the effect of the compound on XBP-1 activity is determined by assaying the activity of IRE-1.
4. The method of claim 3 , wherein the activity of IRE-1 is a kinase activity.
5. The method of claim 3 , wherein the activity of IRE-1 is an endoribonuclease activity.
6. The method of claim 1 , wherein the effect of the compound on XBP-1 activity is determined by assaying XBP-1 and/or IRE-1 protein levels.
7. The method of claim 1 , wherein the XBP-1 polypeptide comprises a transactivation domain and the cell further comprises a vector comprising a regulatory element responsive to the XBP-1 transactivation domain operatively linked to a reporter gene and the effect of the compound on the activity of the polypeptide is determined by evaluating the expression of the reporter gene in the presence and absence of the test compound.
8. The method of claim 1 , wherein the cell comprises a recombinant expression vector encoding a spliced XBP-1 protein; and a vector comprising a regulatory element responsive to the spliced XBP-1 protein operatively linked a reporter gene and the effect of the compound on the activity of the polypeptide is determined by evaluating the expression of the reporter gene in the presence and absence of the test compound.
9. The method of claim 1 , wherein the cell comprises a recombinant expression vector comprising a regulatory element responsive to XBP-1 spliced protein operatively linked a reporter gene, wherein the cell is contacted with an agent that induces ER stress, and the effect of the compound on the activity of the polypeptide is determined by evaluating the expression of the reporter gene in the presence and absence of the test compound.
10. The method of claim 7 , 8 , or 9 , further comprising determining the effect of the identified compound on the ratio of unspliced XBP-1 mRNA to spliced XBP-1 mRNA.
11. The method of claim 7 , 8 , or 9 , further comprising determining the effect of the identified compound on the production of XBP-1 protein.
12. The method of claim 1 , further comprising providing an indicator cell comprising a glucose-dependent promoter operably linked to a reporter gene, wherein the cell in cultured in the presence of glucose, and the effect of the compound on de novo hepatic lipogenesis is determined by evaluating the expression of the reporter gene in the presence and absence of the test compound.
13. The method of claim 1 , wherein the effect of the compound on de novo hepatic lipogenesis is determined by determining the effect of the compound on level of cholesterol and/or triglyceride.
14. The method of claim 1 , wherein determining the ability of the compound to modulate de novo hepatic lipogenesis comprises determining the expression of SREBP and/or chREBP.
15. The method of claim 1 , wherein the ability of the compound to modulate de novo lipogenesis comprises determining the expression of a lipogenic gene.
16. The method of claim 1 , wherein:
the cell further comprises a DNA molecule to which XBP-1 binds, wherein the DNA molecule comprises a promoter selected from the group consisting of a Dgat2 promoter, a Scd1 promoter, an Acc2 promoter; and a PCSK9 promoter; and
the effect of the test compound on de novo hepatic lipogenesis is determined by evaluating the binding of XBP-1 to the DNA molecule in the presence and absence of the test compound.
17. The method of claim 1 , further comprising administering the test compound identified as useful in modulating dyslipidemia to a non-human animal and determining the effect of the identified compound on obesity and/or insulin resistance in the non-human animal.
18. The method of claim 1 , further comprising administering the test compound identified as useful in modulating dyslipidemia to a non-human animal and determining the effect of the compound on at least one of aspartate aminotransferase levels, alanine aminotransferase levels, liver cell inflammation, and the presence of triglyceride vacuoles in liver cells.
19. The method of claim 1 , further comprising determining the effect of the compound on at least one of serum triglycerides, total cholesterol and the distribution of cholesterol among LDL, IDL, and HDL.