Diet-induced adiposity leads to early mammary gland development during pregnancy in the rabbit

Epidemiological and animal studies have suggested that alterations to hormonal and metabolic environments during puberty can induce adverse effects on both lactation and mammary tumorigenesis (de Assis et al., 2006). Obese women have an increased risk of failing to initiate successful breastfeeding and can experience a premature cessation of lactation (Riva et al., 1999; Rasmussen et al., 2004). The effects of obesity on lactogenesis were highlighted during early studies in the rat, where this pathological condition was shown to reduce the chances of a successful outcome regarding pregnancy and lactation (Rolls et al., 1984). More recent studies have suggested that impaired mammary gland development and lactogenesis are possible causes of lactation failures and the high mortality rates observed among the pups of obese dams. In cattle, evidence concerning the relationship between growth rate, mammary growth and milk yield has led to the conclusion that an increased growth rate due to high feeding levels before the onset of puberty may impair subsequent pubertal mammary development and milk potential (Sejrsen, et al., 1997). Moreover, high energy feeding decreases mammary epithelial cell proliferation in areas of active ductal expansion at puberty (Davis Rinker et al., 2008). We have further investigated the impact of obesity on mammary gland development. A rabbit model of dietinduced obesity was developed, by feeding female rabbits with a high fat/high sugar (OB) diet (+276% fat and +269% sugar when compared with the control (C) diet), from 8 weeks of age until mid-pregnancy (day 14 of pregnancy, around the age of 24 weeks). Body weight gain between 21 weeks of age and day 14 of pregnancy was significantly higher (+10%) in OB animals, and was mainly associated with the development of more adipose tissue. Mammary gland morphology was altered in the OB group. Alveolar structures had invaded the entire fat pad, whereas they were more clustered in the control group. Moreover, on day 14 of pregnancy, the mammary ducts were made up of a cell monolayer, were dilated and filled with dense products. Numerous microvillosities were located in the apical region of mammary epithelial cells. Electron microscopy analysis revealed that casein micelles were present in the lumen of alveolae. Immunohistochemical studies of HF mammary tissue revealed a more abundant accumulation of the major rabbit milk proteins, alphaS1-casein, kappa-casein and Whey Acidic Protein (WAP), in both the alveolar lumina and secretory ducts. Milk protein synthesis in mammary tissue extracts was quantified by Western blot analyses. A clear accumulation of milk proteins was observed in OB animals but was almost undetectable in the controls. As revealed by BodiPy staining, lipids had also accumulated in the lumens and ducts. Taken together, these analyses revealed an early secretory phenotype in the OB group. These results show that diet-induced obesity, beginning before puberty, alters mammary gland development at mid-pregnancy in the rabbit. On day 14, mammary tissue in obese animals displayed a morphological aspect and functional profile similar to that normally observed at the end of pregnancy. Preliminary data indicated that these abnormalities affecting mammary development are not associated with agalactia at term, but that milk quality is affected and may influence the predisposition of offspring to obesity.