Peroxisomal membrane channel Pxmp2 is essential for development of mammary gland epithelium in mice

To understand the functional role of peroxisomal membrane channel Pxmp2, mice with a targeted disruption of the Pxmp2 gene were developed. These mice were viable, grew and bred normally. However, Pxmp2-/- were unable to nurse their pups due to retarded mammary ductal outgrowth associated with reduced proliferation of epithelial cells during puberty. Transplantation experiments established the Pxmp2-/- mammary stroma as a tissue responsible for suppression of epithelial growth. Morphological and biochemical examination revealed the presence of peroxisomes in mammary adipocytes, and functional Pxmp2 was detected in the stroma of WT mice. Comparative microarray analysis of WT and Pxmp2-/- mammary fat pad identified an expanded set of differentially expressed genes involved in the regulation of epithelial development. The data point to the possible role of lipid-sensing receptors in mechanisms linking Pxmp2 deficiency and suppression of mammary epithelial growth. The hypothesis was verified using the PPARα agonist clofibrate, which was able to avert pubertal development of mammary epithelium in WT mice. However, treatment of Pxmp2-/- mice with PPARα antagonist MK886 could only partially restore epithelial growth suggesting that several lipid-sensing or other receptors may be affected by Pxmp2 deficiency. The data reveal impaired mammary gland development as a new category of peroxisomal disorders. Microarray analysis was used to compare the gene expression profiles of mammary fat pads of Pxmp2-/- and WT mice (three mice per group). All mice were ovariectomized at the age of 21 days and injected with estradiol at the age of 31 days to standardize the time of the initiation of puberty. The mice were sacrified 8 h thereafter and samples were collected for RNA extraction.