Developmental changes in insulin-like growth factor I receptor gene expression in the mouse mammary gland

Fenugreek Stimulates the Expression of Genes Involved in Milk Synthesis and Milk Flow through Modulation of Insulin/GH/IGF-1 Axis and Oxytocin Secretion

We previously demonstrated galactagogue effect of fenugreek in a rat model of lactation challenge, foreshadowing its use in women's breastfeeding management. To assess longitudinal molecular mechanisms involved in milk synthesis/secretion in dams submitted to fenugreek supplementation, inguinal mammary, pituitary glands and plasma were isolated in forty-three rats nursing large 12 pups-litters and assigned to either a control (CTL) or a fenugreek-supplemented (FEN) diet during lactation. RT-PCR were performed at days 12 and 18 of lactation (L12 and L18) and the first day of involution (Inv1) to measure the relative expression of genes related to both milk synthesis and its regulation in the mammary gland and lactogenic hormones in the pituitary gland. Plasma hormone concentrations were measured by ELISA. FEN diet induced 2- to 3-times higher fold change in relative expression of several genes related to macronutrient synthesis (Fasn, Acaca, Fabp3, B4galt1, Lalba and Csn2) and energy metabolism (Cpt1a, Acads) and in IGF-1 receptor in mammary gland, mainly at L12. Pituitary oxytocin expression and plasma insulin concentration (+77.1%) were also significantly increased. Altogether, these findings suggest fenugreek might extend duration of peak milk synthesis through modulation of the insulin/GH/IGF-1 axis and increase milk ejection by activation of oxytocin secretion.

The miR-424(322)/503 cluster orchestrates remodeling of the epithelium in the involuting mammary gland

The mammary gland undergoes continuous remodeling. Llobet-Navas et al. identify the microRNA cluster miR-424(322)/503 as an important regulator of epithelial involution after pregnancy. TGF-β regulates the expression of this miR cluster, which in turn targets BCL-2 and IGF1R. This work suggests a model in which activation of the TGF-β pathway after weaning induces the transcription of the miR-424(322)/503 cluster to down-regulate the expression of key genes.

The mammary gland is a very dynamic organ that undergoes continuous remodeling. The critical regulators of this process are not fully understood. Here we identify the microRNA cluster miR-424(322)/503 as an important regulator of epithelial involution after pregnancy. Through the generation of a knockout mouse model, we found that regression of the secretory acini of the mammary gland was compromised in the absence of miR-424(322)/503. Mechanistically, we show that miR-424(322)/503 orchestrates cell life and death decisions by targeting BCL-2 and IGF1R (insulin growth factor-1 receptor). Furthermore, we demonstrate that the expression of this microRNA cluster is regulated by TGF-β, a well-characterized regulator of mammary involution. Overall, our data suggest a model in which activation of the TGF-β pathway after weaning induces the transcription of miR-424(322)/503, which in turn down-regulates the expression of key genes. Here, we unveil a previously unknown, multilayered regulation of epithelial tissue remodeling coordinated by the microRNA cluster miR-424(322)/503.

Gene expression analysis of protein synthesis pathways in bovine mammary epithelial cells purified from milk during lactation and short-term restricted feeding

The objective of the study was to investigate selected key regulatory pathways of milk protein biosynthesis in primary bovine mammary epithelial cells (MECs) of dairy cows during the first 155 days of lactation. In addition, cows were exposed to feed restriction for a short period (FR) during different stages of lactation (week 4 and 21 pp) to study adjustment processes of molecular protein biosynthesis to metabolic challenge. Morning milk samples from twenty-four Holstein-Friesian cows were collected throughout the experimental period (n = 10 per animal). MEC from raw milk were purified using an immunomagnetic separation technique and used for real-time quantitative PCR analyses. As was seen in transcript abundances of all major milk proteins, mRNA levels of E74-like factor 5 (ELF5), an enhancer of signal transducer and activator of transcription (STAT) action, concomitantly decreased towards mid-lactation. Expression of ELF5 as well as of all milk protein genes showed a similar increase during FR in early lactation. Occasional changes in expression could be seen in other Janus kinase (JAK)/STAT factors and in mammalian target of rapamycin (mTOR) pathway elements. Amino acid transfer and glucose transporter and the β-casein expression were also partially affected. In conclusion, our findings suggest a pivotal role of the transcription factor ELF5 in milk protein mRNA expression with complementary JAK/STAT and mTOR signalling for the regulation of protein biosynthesis in the bovine mammary gland.

Selection and use of reference genes in mouse mammary glands

Obtaining quantitative data concerning gene expression is important for understanding milk synthesis in mammary glands. Quantitative real-time PCR (qRT-PCR) is an efficient tool to calculate gene expression; however, it is necessary to find valid reference genes for normalization of qRT-PCR data. We applied the geNorm software to eight commonly used reference genes to identify the most stable and optimal genes for the mouse mammary gland. Based on this analysis, HPRT, RPL and GAPDH are the most appropriate reference genes for data normalization. We tested the expression of the alpha-lactalbumin and fatty acid synthase genes using these three reference genes, both normalized and non-normalized. The normalized mRNA expression ratio was significantly different from the non-normalized ratio. We recommend the use of these three reference genes for the normalization of qRT-PCR data in gene expression studies of mouse mammary glands.

Insulin-like growth factor type 1 receptor and insulin receptor isoform expression and signaling in mammary epithelial cells

The insulin receptor (IR) isoforms and the IGF type 1 receptor (IGF-1R) share a high degree of structural homology but differ in ligand binding kinetics and functions. We developed a highly specific quantitative PCR assay to quantify and compare IR-A, IR-B, and IGF-1R expression within an RNA population. We determined receptor expression in primary murine mammary epithelial cells (MECs) during postnatal development. Both IR isoform mRNAs were 3- to 16-fold higher than IGF-1R expression at all developmental times. IR protein was also 3- to 10-fold higher than IGF-1R protein; however, significantly less IGF-1R was found in hybrid receptors at early (49%) vs. late (79%) pregnancy, indicating that the amount of hybrid receptor is developmentally regulated. Despite high IR expression, IGF ligands were more effective than insulin in stimulating the insulin receptor substrate-1/phosphatidylinositol 3-kinase/Akt pathway in acutely isolated MECs from virgin glands. Although approximately 40% of IR transcripts were the IGF-II-sensitive IR-A isoform, IGF-II failed to stimulate IR phosphorylation, and an IGF-1R-specific blocking antibody completely abrogated IGF-II-mediated Akt phosphorylation in the virgin MECs. Taken together, these data suggest that the IGF-1R is more active in signaling than the IR and is the predominant mediator of IGF actions in virgin MECs.

Selective response to insulin versus insulin-like growth factor-I and -II and up-regulation of insulin receptor splice variant B in the differentiated mouse mammary epithelium

The terminal differentiation of the mouse mammary gland epithelium during lactation has been shown to require IGFs and/or superphysiological levels of insulin. It has been suggested that IGF receptor I (IGF-IR), in addition to its well-established role in the mammary gland during puberty and pregnancy, serves as the principal mediator of IGFs at this stage of development. However, our analysis of the expression levels of IGF-IR and the two insulin receptor (IR) splice variants, IR-A and IR-B, has revealed a 3- to 4-fold up-regulation of IR-B transcripts and a 6-fold down-regulation of IGF-IR transcripts and protein during terminal differentiation in the developing mammary gland. IR-B expression was also more than 10-fold up-regulated in murine mammary epithelial cell line HC11 during differentiation in vitro. As already described for the human form, murine IR-B cloned from HC11 exhibited selectivity for insulin as compared with IGFs. When differentiated HC11 cells were stimulated by 10 nm insulin, a concentration that is unable to activate IGF-IR, induction of milk protein and lipid synthetic enzyme gene expression, lactate production, and phosphorylation of Akt were observed. In contrast, on differentiated HC11 cells 10 nm IGF-I or 10 nm IGF-II were able to exert growth-promoting effects only. The lack of response of differentiated cells to low levels of IGFs could not be explained by inactivation of IGFs by IGF binding proteins. Our results suggest a previously unrecognized predominant role for IR-B in the differentiated mammary epithelium.

Growth hormone and insulin-like growth factor-I in the transition from normal mammary development to preneoplastic mammary lesions

Adult female mammary development starts at puberty and is controlled by tightly regulated cross-talk between a group of hormones and growth factors. Although estrogen is the initial driving force and is joined by luteal phase progesterone, both of these hormones require GH-induced IGF-I in the mammary gland in order to act. The same group of hormones, when experimentally perturbed, can lead to development of hyperplastic lesions and increase the chances, or be precursors, of mammary carcinoma. For example, systemic administration of GH or IGF-I causes mammary hyperplasia, and overproduction of IGF-I in transgenic animals can cause the development of usual or atypical hyperplasias and sometimes carcinoma. Although studies have clearly demonstrated the transforming potential of both GH and IGF-I receptor in cell culture and in animals, debate remains as to whether their main role is actually instructive or permissive in progression to cancer in vivo. Genetic imprinting has been shown to occur in precursor lesions as early as atypical hyperplasia in women. Thus, the concept of progression from normal development to cancer through precursor lesions sensitive to hormones and growth factors discussed above is gaining support in humans as well as in animal models. Indeed, elevation of estrogen receptor, GH, IGF-I, and IGF-I receptor during progression suggests a role for these pathways in this process. New agents targeting the GH/IGF-I axis may provide a novel means to block formation and progression of precursor lesions to overt carcinoma. A novel somatostatin analog has recently been shown to prevent mammary development in rats via targeted IGF-I action inhibition at the mammary gland. Similarly, pegvisomant, a GH antagonist, and other IGF-I antagonists such as IGF binding proteins 1 and 5 also block mammary gland development. It is, therefore, possible that inhibition of IGF-I action, or perhaps GH, in the mammary gland may eventually play a role in breast cancer chemoprevention by preventing actions of both estrogen and progesterone, especially in women at extremely high risk for developing breast cancer such as BRCA gene 1 or 2 mutations.

IGF ligand and receptor regulation of mammary development

The insulin-like growth factors, IGF-I and IGF-II, have endocrine as well as autocrine-paracrine actions on tissue growth. Both IGF ligands are expressed within developing mammary tissue throughout postnatal stages with specific sites of expression in the epithelial and stromal compartments. The elucidation of circulating versus local actions and of epithelial versus stromal actions of IGFs in stimulating mammary epithelial development has been the focus of several laboratories. The recent studies addressing IGF ligand function provide support for the hypotheses that (1) the diverse sites of IGF expression may mediate different cellular outcomes, and (2) IGF-I and IGF-II are distinctly regulated and have diverse functions in mammary development. The mechanisms for IGF function likely are mediated, in part, through diverse IGF signaling receptors. The local actions of the IGF ligands and receptors as revealed through recent publications are the focus of this review.