Spatiotemporal dynamics of triglyceride storage in unilocular adipocytes.
Bottom Line: Exogenously added free fatty acids are rapidly adsorbed by mLDs and concurrently get esterified to TG.This process is greatly accelerated by insulin. mLDs transfer their content to the cLD, serving as intermediates that mediate packaging of newly synthesized TG in the large interior of a unilocular adipocyte.This study reveals novel cell biological features that may contribute to the mechanism of adipocyte hypertrophy.
Affiliation: Division of Endocrinology, Diabetes, and Clinical Nutrition, Department of Medicine, Portland, OR 97239.Show MeSH
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Mentions: A close association of mLDs with the cLD and the ER suggests that mLDs may represent intermediary compartments mediating the packaging of newly synthesized TG into the cLD. To verify this hypothesis, we followed the time course of intracellular BODIPY-C12 transport by confocal microscopy. Control (basal) or insulin-treated WAT explants were pulsed with BODIPY-C12 for 15 min and then chased in label-free medium for different time periods. Insulin significantly stimulated the uptake of BODIPY-C12 into adipocytes (Figure 4, A and B). Immediately after an insulin pulse, fluorescence appeared in mLDs scattered around the cLD (Figure 4A, Ins 0h). After an insulin chase, mLD-associated fluorescence gradually decreased, whereas cLD-associated fluorescence concomitantly increased (Figure 4, A and C). Live-cell imaging of BODIPY-C12–labeled WAT explants verified that the disappearance of fluorescence from mLDs is concurrent with the appearance of fluorescence in cLDs (Figure 4D and Supplemental Video S2). Imaging BODIPY-C12–labeled WAT explants at higher spatiotemporal resolution revealed that individual mLDs discharge their fluorescent content with a half-time on the order of 50 s (Figure 4, E, circle, F, solid lines, and G, and Supplemental Video S3). Many mLDs remained static over a 10-min imaging interval (Figure 4, E, square, and F, dotted lines), suggesting that the observed disappearance of mLDs has a probabilistic nature. BODIPY-labeled palmitic acid (BODIPY-C16) and BODIPY-C12 showed similar kinetics of transport in unilocular adipocytes (unpublished data). Collectively, these results confirm a unidirectional transport of BODIPY-labeled lipids between mLDs and cLDs.
Affiliation: Division of Endocrinology, Diabetes, and Clinical Nutrition, Department of Medicine, Portland, OR 97239.