Jim L. McManaman Associate Professor Department of Physiology and Biophysics UCHSC at Fitzsimons RC-1 North Tower, P18-5104 PO Box 6511, Mail Stop F8309 Aurora, CO 80045 Phone: (303) 724-3500 Fax: (303) 724-3512 Email: Jim.McManaman@UCHSC.edu

Mechanisms of lipid droplet synthesis

Lipid droplets (LDs) are metabolically active sites of lipid accumulation that form in the cytoplasm of cells of almost all tissues of eukaryotic organisms. In most cell types LDs are considered to be temporary storage sites of triacylglycerol (TAG) and cholesterol esters (ChE). However, elevated LD accumulation occurs in adipose cells and other cells specialized for lipid storage or synthesis such as steroidogenic cells in reproductive tracts and mammary epithelial cells. Importantly elevated LD formation is implicated in many diseases involving abnormalities in lipid metabolism, including atherosclerosis, type II diabetes, cardiac arrhythmias and heart failure and obesity. Proteomic studies (see Figure) by my laboratory and Kathryn Howell's laboratory (1) have shown that a primary protein component of LDs from liver and lactating mammary glands is adipophilin (ADPH). We have found that ADPH plays an important role in regulating LD accumulation in differentiating mammary epithelial cells and several cell culture models. Structure-function studies by our laboratory have shown that specific domains within the ADPH molecule are responsible for targeting it to LDs and for regulating LD accumulation (2). Our current efforts are focused on how ADPH functions in the assembly and secretion of LDs. These investigations utilize molecular approaches in combination with fluorescence and electron microscopy and proteomics in cell culture and in vivo models.

Mechanisms of lipid droplet secretion. Our studies have also linked ADPH to the secretion of LDs by mammary epithelial cells (3). We have developed a model of milk lipid secretion in which ADPH functions to dock LD with the xanthine oxidase/butyrophilin complexes at the apical plasma membrane during milk secretion. We are currently investigating the nature of the interactions between these proteins and how such interactions function to regulate lipid droplet secretion using ADPH knockout mice and adenoviral vectors to alter gene expression in mammary epithelial cells in vivo.

Role of the unfolded protein response (UPR) in regulating mammary epithelial cell apoptosis. A third area of research focuses on understanding the role of endoplasmic reticulum (ER) stress in the initiation of mammary epithelial cell apoptosis at the end of lactation. Our studies indicate that ER stress, triggered by accumulation of proteins within the ER, activates extrusion and apoptosis of mammary epithelial cells. We are focused on identifying the cellular mechanisms controlling ER stress activation and induction of apoptosis in mammary epithelial cells using in vivo and cell culture models.

Selected Publications

(1) Wu, C.C., Howell, K.E., Neville, M.C., Yates, J.R., and McManaman, J.L., Proteomics reveal contribution of endoplasmic reticulum membranes to the lipid secretion process in mammary epithelial cells. Electrophoresis 21: 3470-3482, 2000.

(2) McManaman, J.L., Zabaronick, W., Schaack, J. and Orlicky, D.J. Lipid droplet targeting domains of adipophilin. J. Lipid Res. 44, 668-673, 2003.

(3) McManaman, J.L., Palmer, C.A., Wright, R.M. and Neville, M.C., Functional regulation of xanthine oxidoreductase expression and localization in mammary epithelial cells: Evidence of a role in lipid secretion. J. Physiol., 545, 567-579, 2002.

Latest Publications in PubMed