Mechanobiology of adipocytes

The expansion of visceral adipose tissue (VAT) is considered to be the most life-threatening form of obesity. It is highly correlated with the risk of developing metabolic complications such as type 2 diabetes, cardiovascular diseases and hypertriglyceridemia. VAT is confined in the abdominal space where it experiences compressive stresses. Here we hypothesise that these compressive stresses act as a mechanoregulatory mechanism to inhibit further increase of intracellular lipids and consequently control visceral adiposity. Furthermore, we hypothesise that in obesity such feedback is disrupted due to an excessive increase of adipocyte size and compressive stress. To test this hypothesis, we engineered a 3D adipose tissue model comprising of a collagen scaffold seeded with 3T3-L1 adipocytes. This model was subjected to compressive stress under static load. We validated that this engineered tissue withstands different loads, resulting in different levels of compression. We found a decrease of intracellular lipid content in a load-dependent manner, suggesting an increased lipolytic rate. This was associated with strong activation of the ERK pathway, that in turn activates important lipases, such as adipose triglyceride lipase (ATGL) and the hormone-sensitive lipase (HSL). Finally, overexpression of cavin-1 after compression suggests an altered adipocyte metabolism. We then investigated how adipocyte hypertrophy influences the effectiveness of this mechanical sensing. We found that 3T3-L1 adipocytes treated with a fatty acid cocktail resulted in an opposite response to the compression, inhibiting the HSL activity. In line with these in vitro results, compression of the gonadal fat in overweight mice also led to an inhibition of the HSL activity. Taken together our results indicate that compressive stress acts as a catalyst for lipolysis in normal adipocytes, suggesting a homeostatic feedback mechanism which prevents further adipocyte expansion. However, in overweight animals, compressive stress appears to deactivate the lipolysis, disrupting the proposed feedback mechanism.Open Acces

Comparison of the Halpha equivalent width of HII regions in a flocculent and a grand design galaxy: possible evidences for IMF variations

We present here a study of the Halpha equivalent widths of the flocculent galaxy NGC 4395 and the grand design galaxy NGC 5457. A difference between the mean values of the Halpha equivalent widths for the two galaxies has been found. Several hypotheses are presented in order to explain this difference: differences in age, metallicity, star formation rate, photon leakage and initial mass function. Various tests and Monte Carlo models are used to find out the most probable cause of this difference. The resultsshow that the possible cause for the difference could be a variation in the initial mass function. This difference is such that it seems to favor a fraction of more massive stars in the grand design galaxy when compared with the flocculent galaxy. This could be due to a change of the environmental conditions due to a density wave.Comment: 29 pages, 19 figures, accepted for publication in Ap

Morphological transformation of NGC 205?

NGC 205 is a dwarf elliptical galaxy which shows many features that are more typical of disk galaxies, and our recent study of the central stellar population has added another peculiarity. In the central regions, star formation has been on-going continuously for a few hundred Myr, until ca. 20 Myr ago, perhaps fed by gas funneled to the center in the course of morphological transformation. In this contribution we use a deep, wide-field image obtained at a scale of 2"/px to show that subtle structures can be detected in and near the body of the dwarf galaxy. The southern tidal tail can be mapped out to unprecedented distances from the center, and we suggest that the northern tail is partially hidden behind a very extended dust lane, or ring, belonging to M31. A spiral pattern emerges across the body of the galaxy, but it might be explained by another M31 dust filament.Comment: 2 pages, 1 figure, poster contributed to IAU Symposium 262, Stellar Populations -- Planning for the Next Decade, G. Bruzual & S. Charlot, ed

Density wave triggered star formation in grand design spirals

In normal spiral galaxies the arms are the main sites for star formation. This is the cause of their optical contrast compared with the rest of the disc. The spiral structure can be observed as a higher concentration of H2 regions, neutral gas (both atomic and molecular via CO), dust and stars than in the interarm disc. It seens generally accepted that, at least in grand design spirals, there are density waves in the discs. However, several questions are not clear yet and still under discussion. An important question could be termed the triggering dilemma (by analogy with the 'winding dilemma' raised in the forties): Is the enhanced star formation in the spiral arms triggered by the passage of a system of density waves or is it simply due to the presence of a higher column density of gas there? In the present work, we use triggering in the same sense as the moderate to strong triggering defined by Elmegreen (1992), that is to say that star formation in the arms occurs at a rate faster than that in the interarm zone, relative to the available placental gas. Our group has designed several tests to elucidate whether or not star formation is triggered in the arms with respect to the interarm region and we summarize one of them, that of the ratio of the star formation efficiency in the arms divided by that of the interarm zone at the same galactocentric distance which we may call the relative massive star formation efficiency, where the efficiency is defined using the ratio of the mass of stars (evaluated via the H alpha flux) to the mass of neutral gas, atomic plus molecular (which must be measured with the adequate angular resolution). If the relative efficiency is of order unity, the star formation is proportional to the mass of gas, if some kind of induced star formation is present, the relative efficiency should be considerably larger than unity

Density waves and star formation in grand design spirals

HII regions in the arms of spiral galaxies are indicators of recent star-forming processes. They may have been caused by the passage of the density wave or simply created by other means near the arms. The study of these regions may give us clues to clarifying the controversy over the existence of a triggering scenario, as proposed in the density wave theory. Using H$\alpha$ direct imaging, we characterize the HII regions from a sample of three grand design galaxies: NGC5457, NGC628 and NGC6946. Broad band images in R and I were used to determine the position of the arms. The HII regions found to be associated with arms were selected for the study. The age and the star formation rate of these HII regions was obtained using measures on the H$\alpha$ line. The distance between the current position of the selected HII regions and the position they would have if they had been created in the centre of the arm is calculated. A parameter, T, which measures whether a region was created in the arm or in the disc, is defined. With the help of the T parameter we determine that the majority of regions were formed some time after the passage of the density wave, with the regions located `behind the arm' (in the direction of the rotation of the galaxy) the zone they should have occupied had they been formed in the centre of the arm. The presence of the large number of regions created after the passage of the arm may be explained by the effect of the density wave, which helps to create the star-forming regions after its passage. There is clear evidence of triggering for NGC5457 and a co-rotation radius is proposed. A more modest triggering seems to exist for NGC628 and non significant evidence of triggering are found for NGC6946.Comment: 10 pages, 20 figures, accepted for publication in A&