Calpains, the proteases of two faces controlling the epithelial homeostasis in mammary gland

Calpain-1 and calpain-2 are calcium-dependent Cys-proteases ubiquitously expressed in mammalian tissues with a processive, rather than degradative activity. They are crucial for physiological mammary gland homeostasis as well as for breast cancer progression. A growing number of evidences indicate that their pleiotropic functions depend on the cell type, tissue and biological context where they are expressed or dysregulated. This review considers these standpoints to cover the paradoxical role of calpain-1 and -2 in the mammary tissue either, under the physiological conditions of the postlactational mammary gland regression or the pathological context of breast cancer. The role of both calpains will be examined and discussed in both conditions, followed by a brief snapshot on the present and future challenges for calpains, the two-gateway proteases towards tissue homeostasis or tumor development.

Role of Vitamin A in Mammary Gland Development and Lactation

Vitamin A (all-trans-retinol), its active derivatives retinal and retinoic acid, and their synthetic analogues constitute the group of retinoids. It is obtained from diet either as preformed vitamin A or as carotenoids. Retinal plays a biological role in vision, but most of the effects of vitamin A are exerted by retinoic acid, which binds to nuclear receptors and regulates gene transcription. Vitamin A deficiency is an important nutritional problem, particularly in the developing world. Retinol and carotenoids from diet during pregnancy and lactation influence their concentration in breast milk, which is important in the long term, not only for the offspring, but also for maternal health. In this study, we review the role of vitamin A in mammary gland metabolism, where retinoid signaling is required not only for morphogenesis and development of the gland and for adequate milk production, but also during the weaning process, when epithelial cell death is coupled with tissue remodeling.

The crossroads of breast cancer progression: insights into the modulation of major signaling pathways

Cancer is the disease with highest public health impact in developed countries. Particularly, breast cancer has the highest incidence in women worldwide and the fifth highest mortality in the globe, imposing a significant social and economic burden to society. The disease has a complex heterogeneous etiology, being associated with several risk factors that range from lifestyle to age and family history. Breast cancer is usually classified according to the site of tumor occurrence and gene expression profiling. Although mutations in a few key genes, such as BRCA1 and BRCA2, are associated with high breast cancer risk, the large majority of breast cancer cases are related to mutated genes of low penetrance, which are frequently altered in the whole population. Therefore, understanding the molecular basis of breast cancer, including the several deregulated genes and related pathways linked to this pathology, is essential to ensure advances in early tumor detection and prevention. In this review, we outline key cellular pathways whose deregulation has been associated with breast cancer, leading to alterations in cell proliferation, apoptosis, and the delicate hormonal balance of breast tissue cells. Therefore, here we describe some potential breast cancer-related nodes and signaling concepts linked to the disease, which can be positively translated into novel therapeutic approaches and predictive biomarkers.

Involvement of Different networks in mammary gland involution after the pregnancy/lactation cycle: Implications in breast cancer

Early pregnancy is associated with a reduction in a woman's lifetime risk for breast cancer. However, different studies have demonstrated an increase in breast cancer risk in the years immediately following pregnancy. Early and long-term risk is even higher if the mother age is above 35 years at the time of first parity. The proinflammatory microenvironment within the mammary gland after pregnancy renders an "ideal niche" for oncogenic events. Signaling pathways involved in programmed cell death and tissue remodeling during involution are also activated in breast cancer. Herein, the major signaling pathways involved in mammary gland involution, signal transducer and activator of transcription (STAT3), nuclear factor-kappa B (NF-κB), transforming growth factor beta (TGFβ), and retinoid acid receptors (RARs)/retinoid X receptors (RXRs), are reviewed as part of the complex network of signaling pathways that crosstalk in a contextual-dependent manner. These factors, also involved in breast cancer development, are important regulatory nodes for signaling amplification after weaning. Indeed, during involution, p65/p300 target genes such as MMP9, Capn1, and Capn2 are upregulated. Elevated expression and activities of these proteases in breast cancer have been extensively documented. The role of these proteases during mammary gland involution is further discussed. MMPs, calpains, and cathepsins exert their effect by modification of the extracellular matrix and intracellular proteins. Calpains, activated in the mammary gland during involution, cleave several proteins located in cell membrane, lysosomes, mitochondria, and nuclei favoring cell death. Besides, during this period, Capn1 is most probably involved in the modulation of preadipocyte differentiation through chromatin remodeling. Calpains can be implicated in cell anchoring loss, providing a proper microenvironment for tumor growth. A better understanding of the role of any of these proteases in tumorigenesis may yield novel therapeutic targets or prognostic markers for breast cancer.

In vivo genome-wide binding of Id2 to E2F4 target genes as part of a reversible program in mice liver

The inhibitor of differentiation Id2, a protein lacking the basic DNA-binding domain, is involved in the modulation of a number of biological processes. The molecular mechanisms explaining Id2 pleiotropic functions are poorly understood. Id2 and E2F4 are known to bind simultaneously to c-myc promoter. To study whether Id2 plays a global role on transcriptional regulation, we performed in vivo genome-wide ChIP/chip experiments for Id2 and E2F4 in adult mouse liver. An Id2-containing complex was bound to a common sequence downstream from the TSS on a subset of 442 E2F4 target genes mainly related to cell development and chromatin structure. We found a positive correlation between Id2 protein levels and the expression of E2F4/Id2 targets in fetal and adult liver. Id2 protein stability increased in fetal liver by interaction with USP1 de-ubiquitinating enzyme, which was induced during development. In adult liver, USP1 and Id2 levels dramatically decreased. In differentiated liver tissue, when Id2 concentration was low, E2F4/Id2 was bound to the same region as paused Pol II and target genes remained transcriptionally inactive. Conversely, in fetal liver when Id2 levels were increased, Id2 and Pol II were released from gene promoters and target genes up-regulated. During liver regeneration after partial hepatectomy, we obtained the same results as in fetal liver. Our results suggest that Id2 might be part of a reversible development-related program involved in the paused-ON/OFF state of Pol II on selected genes that would remain responsive to specific stimuli.

Differential functions of calpain 1 during epithelial cell death and adipocyte differentiation in mammary gland involution

Calpains become activated in the mammary gland early during weaning, cleaving several proteins located mainly in the cell membrane, but also in other organelles such as lysosomes, mitochondria and nuclei. By immunofluorescence and Western blot analysis, we have demonstrated the nuclear translocation of calpain-1 and calpain-2, together with the cleavage of several cytoplasmic nucleoporins in epithelial cells of the lobulo-alveolar compartment. In vivo and in vitro calpain inhibition prevented this nucleoporin degradation. In addition, calpain-1 was also present in the nucleus of non-epithelial mammary tissue cells, concomitant with adipocyte re-differentiation. Calpain-1 was internalized within nuclei and found to be present in the nuclear chromatin-enriched fraction, associated with histone H3. Furthermore, we have demonstrated, both in vivo and in vitro, the cleavage of the N-terminal residue of histone H3 by calpain-1. Calpain-1 co-localized with both H3K4me3 (histone H3 trimethylated at Lys4) and H3K27me3 (histone H3 trimethylated at Lys27) at the nuclear periphery, a bivalent epigenetic signal essential for cell differentiation. Using ChIP assays we could confirm the presence of calpain-1 in the promoters of key genes expressed in adipose tissue, such as Cebpa (CCAAT/enhancer-binding protein α) and Lep (leptin). The results of the present study highlight a dual role for calpain-1 in the weaned gland after the pregnancy/lactation cycle, controlling programmed cell death and participating in the epigenetic programme during adipocyte differentiation.

Calpains mediate epithelial-cell death during mammary gland involution: mitochondria and lysosomal destabilization

Our aim was to elucidate the physiological role of calpains (CAPN) in mammary gland involution. Both CAPN-1 and -2 were induced after weaning and its activity increased in isolated mitochondria and lysosomes. CAPN activation within the mitochondria could trigger the release of cytochrome c and other pro-apoptotic factors, whereas in lysosomes it might be essential for tissue remodeling by releasing cathepsins into the cytosol. Immunohistochemical analysis localized CAPNs mainly at the luminal side of alveoli. During weaning, CAPNs translocate to the lysosomes processing membrane proteins. To identify these substrates, lysosomal fractions were treated with recombinant CAPN and cleaved products were identified by 2D-DIGE. The subunit b(2) of the v-type H(+) ATPase is proteolyzed and so is the lysosomal-associated membrane protein 2a (LAMP2a). Both proteins are also cleaved in vivo. Furthermore, LAMP2a cleavage was confirmed in vitro by addition of CAPNs to isolated lysosomes and several CAPN inhibitors prevented it. Finally, in vivo inhibition of CAPN1 in 72-h-weaned mice decreased LAMP2a cleavage. Indeed, calpeptin-treated mice showed a substantial delay in tissue remodeling and involution of the mammary gland. These results suggest that CAPNs are responsible for mitochondrial and lysosomal membrane permeabilization, supporting the idea that lysosomal-mediated cell death is a new hallmark of mammary gland involution.