Isoform-specific requirement for Akt1 in the developmental regulation of cellular metabolism during lactation.

The metabolic demands and synthetic capacity of the lactating mammary gland exceed that of any other tissue, thereby providing a useful paradigm for understanding the developmental regulation of cellular metabolism. By evaluating mice bearing targeted deletions in Akt1 or Akt2, we demonstrate that Akt1 is specifically required for lactating mice to synthesize sufficient quantities of milk to support their offspring. Whereas cellular proliferation, differentiation, and apoptosis are unaffected, loss of Akt1 disrupts the coordinate regulation of metabolic pathways that normally occurs at the onset of lactation. This results in a failure to upregulate glucose uptake, Glut1 surface localization, lipid synthesis, and multiple lipogenic enzymes, as well as a failure to downregulate lipid catabolic enzymes. These findings demonstrate that Akt1 is required in an isoform-specific manner for orchestrating many of the developmental changes in cellular metabolism that occur at the onset of lactation and establish a role for Akt1 in glucose metabolism.

Hormone-induced protection against mammary tumorigenesis is conserved in multiple rat strains and identifies a core gene expression signature induced by pregnancy.

Women who have their first child early in life have a substantially lower lifetime risk of breast cancer. The mechanism for this is unknown. Similar to humans, rats exhibit parity-induced protection against mammary tumorigenesis. To explore the basis for this phenomenon, we identified persistent pregnancy-induced changes in mammary gene expression that are tightly associated with protection against tumorigenesis in multiple inbred rat strains. Four inbred rat strains that exhibit marked differences in their intrinsic susceptibilities to carcinogen-induced mammary tumorigenesis were each shown to display significant protection against methylnitrosourea-induced mammary tumorigenesis following treatment with pregnancy levels of estradiol and progesterone. Microarray expression profiling of parous and nulliparous mammary tissue from these four strains yielded a common 70-gene signature. Examination of the genes constituting this signature implicated alterations in transforming growth factor-beta signaling, the extracellular matrix, amphiregulin expression, and the growth hormone/insulin-like growth factor I axis in pregnancy-induced alterations in breast cancer risk. Notably, related molecular changes have been associated with decreased mammographic density, which itself is strongly associated with decreased breast cancer risk. Our findings show that hormone-induced protection against mammary tumorigenesis is widely conserved among divergent rat strains and define a gene expression signature that is tightly correlated with reduced mammary tumor susceptibility as a consequence of a normal developmental event. Given the conservation of this signature, these pathways may contribute to pregnancy-induced protection against breast cancer.

The serine/threonine kinase, Krct, affects endbud morphogenesis during murine mammary gland development.

STK16/Krct (Kinase related to cerevisiae and thaliana) is a ubiquitously expressed member of a unique family of serine/threonine protein kinases that is conserved among all eukaryotes. Despite its cloning 6 years ago to date, the function of this kinase remains unknown. In an attempt to identify a function for Krct, we have generated a doxycycline-dependent transgenic mouse model that permits the inducible overexpression of Krct in the mammary glands of mice treated with tetracycline derivatives. Analysis of these mice reveals that modest overexpression of Krct in the mammary gland during puberty results in duplication of the terminal endbud axis such that multiple, rather than single, budding structures arise at the ends of primary ducts. Supernumerary endbuds in Krct overexpressing mice resemble wild-type terminal endbuds with regard to cellular proliferation rates and localization of cap cells, myoepithelial cells and body cells. However, aberrant transgenic endbuds are surrounded by an increased amount of periductal stroma that in many cases encompasses the entire endbud. These data suggest that Krct may play a role in regulating stromal-epithelial interactions that occur during ductal morphogenesis in the mammary gland.

Identification of LBM180, a lamellar body limiting membrane protein of alveolar type II cells, as the ABC transporter protein ABCA3.

Lamellar bodies are the specialized secretory organelles of alveolar type II (ATII) epithelial cells through which the cell packages pulmonary surfactant and regulates its secretion. Surfactant within lamellar bodies is densely packed as circular arrays of lipid membranes and appears to be the product of several trafficking and biosynthetic processes. To elucidate these processes, we reported previously on the generation of a monoclonal antibody (3C9) that recognizes a unique protein of the lamellar body membrane of 180 kDa, which we named LBM180. We report that mass spectrometry of the protein precipitated by this antibody generated a partial sequence that is identical to the ATP-binding cassette protein, ABCA3. Homology analysis of partial sequences suggests that this protein is highly conserved among species. The ABCA3 gene transcript was found in cell lines of human lung origin, in ATII cells of human, rat, and mouse, as well as different tissues of rat, but the highest expression of ABCA3 was observed in ATII cells. Expression of this transcript was at its maximum prior to birth, and hormonal induction of ABCA3 transcript was observed in human fetal lung at the same time as other surfactant protein transcripts were induced, suggesting that ABCA3 is developmentally regulated. Molecular and biochemical studies show that ABCA3 is targeted to vesicle membranes and is found in the limiting membrane of lamellar bodies. Because ABCA3 is a member of a subfamily of ABC transporters that are predominantly known to be involved in the regulation of lipid transport and membrane trafficking, we speculate that this protein may play a key role in lipid organization during the formation of lamellar bodies.