LXR and aromatase knock-out mice : animal models providing insight into human diseases

Abstract

In our characterization of mouse strains carrying mutations in nuclear receptor and aromatase genes we have found phenotypes which resemble four human diseases: benign prostatic hyperplasia (BPH), Sjögren s syndrome (SS), amyotrophic lateral sclerosis (ALS), and exocrine pancreatic insufficiency. The work in this thesis was designed to understand the roles of LXRalpha, LXRbeta and aromatase in development of these diseases and to investigate whether these animal models help us (1) to understand the etiology of the corresponding human diseases and (2) to develop new therapeutic approaches for the treatment of these diseases.

Paper I: Male LXRbeta-/- mice develop motor neuron disease with age. Because the LXR ligand, beta-sitosterol, has been implicated in ALS, we examined the possibility that LXRbeta-/- mice were more susceptible than their wild type (WT) littermates to the toxic effects of beta-sitosterol. Three weeks of beta-sitosterol treatment of 8-month-old LXRbeta-/- male mice resulted in ALS-Parkinson s like syndrome, characterized by death of the tyrosine hydroxylase-positive dopaminergic neurons in the brain and death of large motor neurons in the spinal cord. WT mice were not affected by beta-sitosterol. In LXRbeta-/- mice, there was activation of microglia and accumulation of intracellular protein aggregates in motor neurons, abnormalities which have been observed in ALS patients. The level of 24-hydroxycholesterol, the main excretory metabolite of cholesterol synthesized in the brain, was increased upon beta-sitosterol treatment in mutant but not in WT mice. Thus loss of LXRbeta renders mice more susceptible to the neurotoxic effects of beta-sitosterol and we suggest that, in human populations where the incidence of ALS is unusually high, there may be genetic alterations in LXRbeta gene signaling pathways.

Paper II: In this study we found that as LXRalpha-/- mice age, they develop smooth-muscle actin-positive stromal overgrowth around the ducts of the ventral prostate (VP) and numerous fibrous nodules growing into the ducts and causing obstruction and extreme dilatation of the ducts. We investigated whether the stromal hyperplasia in LXRalpha-/-mice could help in understanding the etiology of human BPH. We found proliferation in the epithelial cells but not in the accumulated stromal cells in the VP of LXRalpha-/- mice. Expression of the transcription repressor of E-cadherin, Snail and the intracellular mediator of TGF-beta, Smad 2/3, was increased. We postulate that loss of LXRalpha from the prostatic epithelium results in increased epithelial proliferation and epithelial to mesenchymal transition, resulting in excessive stromal accumulation. These findings suggest that LXRalpha-/- mice may be a useful model to study prostatic stromal hyperplasia.

Paper III: LXRbeta-/-mice are resistant to developing obesity on a high fat diet. As they age, these mice develop pancreatic exocrine insufficiency. By electron microscopy, abnormalities in the pancreatic ducts as well as in the pancreatic secretion were found. Particularly interesting was the presence within the ducts of lamellar structures similar to those seen in cystic fibrosis. With a specific antibody we demonstrated the presence of LXRbeta in the nuclei of epithelial cells lining the pancreatic ducts. We therefore searched for LXR-regulated genes that might influence the density of pancreatic secretion. We found that aquaporin-1 is an LXR-regulated gene highly expressed in the pancreatic ductal epithelium of wild type mice but not LXRbeta-/- mice. Thus LXRbeta plays an important role in controlling pancreatic juice secretion through the regulation of water transport, and the observed resistance to a high fat diet in these LXRbeta-/-mice appears to be due to fat malabsorption secondary to pancreatic insufficiency.

Paper IV: As they age, mice which cannot synthesize estrogen (aromatase knock-out mice, ArKO mice), develop severe autoimmune exocrinopathy resembling Sjögren s syndrome (SS), which is not observed in ERalpha-/- or ERbeta-/-mice. Many characteristics typical of human SS were found in these mice: There is B cell hyperplasia in the hematopoietic organs with severe immune cell infiltration in the salivary glands and kidneys, proteolytic fragments of alpha-fodrin in the salivary glands and anti-alpha-fodrin antibodies in the serum. When mice were raised on a phytoestrogen-free diet, there was a mild but significant incidence of infiltration of B lymphocytes in WT mice and severe destructive autoimmune lesions in ArKO mice. In age-matched WT mice fed a diet containing normal levels of phytoestrogen, there were no autoimmune lesions. These findings suggest that estrogen deficiency results in a lympho-proliferative autoimmune disease resembling SS and suggest that estrogen might have clinical value in the prevention or treatment of this disease.