Peroxisomal membrane channel Pxmp2 in the mammary fat pad is essential for stromal lipid homeostasis and for development of mammary gland epithelium in mice

AbstractTo understand the functional role of the peroxisomal membrane channel Pxmp2, mice with a targeted disruption of the Pxmp2 gene were generated. These mice were viable, grew and bred normally. However, Pxmp2−/− female mice were unable to nurse their pups. Lactating mammary gland epithelium displayed secretory lipid droplets and milk proteins, but the size of the ductal system was greatly reduced. Examination of mammary gland development revealed that retarded mammary ductal outgrowth was due to 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 confirmed the presence of peroxisomes in the mammary fat pad adipocytes, and functional Pxmp2 was detected in the stroma of wild-type mammary glands. Deletion of Pxmp2 led to an elevation in the expression of peroxisomal proteins in the mammary fat pad but not in liver or kidney of transgenic mice. Lipidomics of Pxmp2−/−mammary fat pad showed a decrease in the content of myristic acid (C14), a principal substrate for protein myristoylation and a potential peroxisomal β-oxidation product. Analysis of complex lipids revealed a reduced concentration of a variety of diacylglycerols and phospholipids containing mostly polyunsaturated fatty acids that may be caused by activation of lipid peroxidation. However, an antioxidant-containing diet did not stimulate mammary epithelial proliferation in Pxmp2−/− mice.The results point to disturbances of lipid metabolism in the mammary fat pad that in turn may result in abnormal epithelial growth. The work reveals impaired mammary gland development as a new category of peroxisomal disorders

Autophagy and unfolded protein response (UPR) regulate mammary gland involution by restraining apoptosis-driven irreversible changes

The postnatal mammary gland undergoes repeated cycles of proliferation and cell death, most notably when the fully differentiated (lactating) gland dedifferentiates to a prelactation state. Accumulation of milk proteins in the secretory epithelium creates the stress signal that triggers this process (involution). How this stress is perceived, and the cellular processes that are subsequently activated, remain unclear. We now report that Unfolded Protein Response (UPR), autophagy, and apoptosis related genes cluster separately during lactation and involution in the mouse mammary gland. Time-course experiments in rodents show that autophagy and UPR signaling are tightly co-regulated at the transition from reversible to irreversible involution. Inhibition of autophagy by chloroquine or genetic deletion of one ATG7 allele enhanced progression of mammary involution into the irreversible phase, as characterized by an early/precocious induction of apoptosis. These are the first preclinical in vivo data in support of a clinical trial testing an autophagy inhibitor for prevention of intraductal breast malignancy progression to invasive breast cancer. In marked contrast, stimulation of autophagy by low dose tunicamycin treatment reduced apoptosis and extended the reversible phase of involution by sustaining the secretory epithelium. Autophagy stimulators could be used short-term to promote lactation in women experiencing difficulties or irregularities in nursing. Taken together, these data indicate that UPR and autophagy play a key role in regulating the balance between cell survival and apoptosis during normal mammary gland regression.</p

Systemic alterations play a dominant role in epigenetic predisposition to breast cancer in offspring of obese fathers and is transmitted to a second generation

We previously showed that environmentally-induced epigenetic inheritance of cancer occurs in rodent models. For instance, we reported that paternal consumption of an obesity-inducing diet (OID) increased breast cancer susceptibility in the offspring (F1). Nevertheless, it is still unclear whether programming of breast cancer in daughters is due to systemic alterations or mammary epithelium-specific factors and whether the breast cancer predisposition in F1 progeny can be transmitted to subsequent generations. In this study, we show that mammary glands from F1 control (CO) female offspring exhibit enhanced growth when transplanted into OID females compared to CO mammary glands transplanted into CO females. Similarly, carcinogen-induced mammary tumors from F1 CO female offspring transplanted into OID females has a higher proliferation/apoptosis rate. Further, we show that granddaughters (F2) from the OID grand-paternal germline have accelerated tumor growth compared to CO granddaughters. This between-generation transmission of cancer predisposition is associated with changes in sperm tRNA fragments in OID males. Our findings indicate that systemic and mammary stromal alterations are significant contributors to programming of mammary development and likely cancer predisposition in OID daughters. Our data also show that breast cancer predisposition is transmitted to subsequent generations and may explain some familial cancers, if confirmed in humans

Ectopic expression of syndecan-1 in basal epidermis affects keratinocyte proliferation and wound re-epithelialization

Epidermal proliferation and differentiation can be regulated by soluble morphogens and growth factors. Heparan sulfate proteoglycans (HSPGs) modulate the action of several of these effector molecules, such as members of the fibroblast growth factor (FGF) and Wnt families. Syndecan-1 is a cell-surface proteoglycan that is expressed in differentiating keratinocytes and transiently upregulated in all layers of the epidermis upon tissue injury. To address the role of syndecan-1 in the regulation of keratinocyte proliferation and differentiation, we generated transgenic mice that overexpress syndecan-1 under K14 keratin promoter in the basal layer of the epidermis. We observed epidermal hyperproliferation in newborn transgenic mice, as evidenced by increased number of suprabasal cell layers, elevated proliferating cell nuclear antigen (PCNA) expression in both basal and suprabasal cell layers and by expression of keratin 6 in the interfollicular epidermis. Compared to both wild-type and syndecan-1-null animals, the transgene expression interfered with skin wound healing in adult mice by decreasing cell proliferation in the re-epithelialized epidermis. Thus, syndecan-1 regulates keratinocyte proliferation differently during skin development and in healing wounds