Changes in the sensitivity of lipogenesis in rat hepatocytes to hormones and precursors over the diurnal cycle and during longer-term starvation of donor animals

p73 regulates autophagy and hepatocellular lipid metabolism through a transcriptional activation of the ATG5 gene

p73, a member of the p53 tumor suppressor family, is involved in neurogenesis, sensory pathways, immunity, inflammation, and tumorigenesis. How p73 is able to participate in such a broad spectrum of different biological processes is still largely unknown. Here, we report a novel role of p73 in regulating lipid metabolism by direct transactivation of the promoter of autophagy-related protein 5 (ATG5), a gene whose product is required for autophagosome formation. Following nutrient deprivation, the livers of p73-deficient mice demonstrate a massive accumulation of lipid droplets, together with a low level of autophagy, suggesting that triglyceride hydrolysis into fatty acids is blocked owing to deficient autophagy (macrolipophagy). Compared with wild-type mice, mice functionally deficient in all the p73 isoforms exhibit decreased ATG5 expression and lower levels of autophagy in multiple organs. We further show that the TAp73α is the critical p73 isoform responsible for inducing ATG5 expression in a p53-independent manner and demonstrate that ATG5 gene transfer can correct autophagy and macrolipophagy defects in p73-deficient hepatocytes. These data strongly suggest that the p73-ATG5 axis represents a novel, key pathway for regulating lipid metabolism through autophagy. The identification of p73 as a major regulator of autophagy suggests that it may have an important role in preventing or delaying disease and aging by maintaining a homeostatic control.

Regulation of fatty acid and cholesterol synthesis: co-operation or competition?

Fatty acids and sterols originally evolved symbiotically as structural components of cell membranes. In some respects, control of their biosynthetic pathways reflects their mutual interdependence in defining changes in the physicochemical properties of the membranes in response to the changing internal and external cellular environments. In some tissues of higher animals, however, cholesterol and fatty acids have multifunctional roles. In particular, the liver synthesizes these lipids for export as multimolecular complexes in the form of micellar bile components and lipoproteins. Intrahepatic fatty acid and cholesterol synthesis is dependent upon the balance between hepatic output of these complexes and dietary input of fat and cholesterol. Thus physiological control of these synthetic processes is often co-ordinated at both the transcriptional and post-translational levels. On the other hand, changes in flux through major metabolic pathways, particularly during physiological transitions and as a result of genetic manipulation, affects substrate availability for these pathways. Under these circumstances, regulation reflects a compensatory response to ensure that flux through the lipid pathways remains unchanged. These regulatory changes can best be interpreted in terms of a Metabolic Control Analysis approach. In summary, flux through the fatty acid and cholesterol pathways reflects (a) cellular demand for these lipids, (b) a variable availability of substrates, (c) a combination of (a) and (b).

Regulation of VLDL secretion in primary culture of rat hepatocytes: involvement of cAMP and cAMP-dependent protein kinases

When hepatocytes were cultured for 24 h in the presence of forskolin (10(-4) mol l-1) or isobutylmethylxanthine (IBMX, 10(-3) mol l-1), the intracellular cAMP concentration peaked (320-380 pmol mg-1 protein) after 10-20 min of culture. This increase was accompanied by a decrease in the secretion of triacylglycerol, cholesterol and apoprotein B associated with VLDL. After 4 h cAMP levels had returned almost to basal values but the inhibition of VLDL secretion persisted. There was a small intracellular accumulation of triacylglycerol but not of apoprotein B. Addition of forskolin and IBMX together led to a further increase in intracellular cAMP and a further suppression of VLDL output. Similar effects on the secretion of VLDL were also observed after addition of Bt2cAMP. Exposure of cell cultures to glucagon (10(-7) mol l-1) for only 10 min raised cellular cAMP levels to > 200 pmol mg-1 protein, and suppressed VLDL secretion during the next 24 h to < 40% of control. All of the substances tested inhibited de novo synthesis of fatty acids but had little or no effect on cholesterol synthesis and did not inhibit oleate esterification to triacylglycerol. The cAMP-dependent protein kinase antagonist Rp-cAMPS prevented suppression of VLDL triacylglycerol secretion induced by glucagon (10(-7) mol l-1) and abolished glucagon-induced ketogenesis. Rp-cAMPS also inhibited Bt2cAMP (7.5 x 10(-6) mol l-1)-induced suppression of VLDL secretion and enhancement of ketogenesis. It is concluded that rat hepatic VLDL metabolism can be regulated by cAMP and cAMP-dependent protein kinases, and that the initial transient rise in cellular cAMP levels induced by glucagon is sufficient to maintain a long-term inhibitory effect on assembly and secretion of VLDL.

Metabolic zonation of the liver: regulation and implications for liver function

Liver parenchyma shows a remarkable heterogeneity of the hepatocytes along the porto-central axis with respect to ultrastructure and enzyme activities resulting in different cellular functions within different zones of the liver lobuli. According to the concept of metabolic zonation, the spatial organization of the various metabolic pathways and functions forms the basis for the efficient adaptation of liver metabolism to the different nutritional requirements of the whole organism in different metabolic states. The present review summarizes current knowledge about this heterogeneity, its development and determination, as well as about its significance for the understanding of all aspects of liver function and pathology, especially of intermediary metabolism, biotransformation of drugs and zonal toxicity of hepatotoxins.

Retinal pigment epithelial change and partial lipodystrophy

Cuticular drusen and retinal pigment epithelial changes were found incidentally in a 27 year old Lebanese woman during assessment of partial lipodystrophy. Her vision was normal despite involvement of both maculae. The patient had hypocomplementaemia, but serum C3 nephritic factor was absent and renal function was normal. She had impaired glucose tolerance and a continuous infusion of glucose with model assessment (CIGMA) test revealed low normal tissue insulin sensitivity and high normal pancreatic beta cell function. Mild fasting hypertriglyceridaemia (2.0 mmol/l) may have been secondary to impaired insulin sensitivity. Endocrine function was otherwise normal apart from a completely absent growth hormone response to adequate hypoglycaemia. The simultaneous occurrence of partial lipodystrophy and retinal pigmentary epithelial and basement membrane changes appears to be a newly recognized syndrome.

Dependence with nutritional status of the ethanol effects on fatty acid metabolism in rat hepatocytes

1. The effects of ethanol on fatty acid synthesis, esterification and oxidation were studied in hepatocytes isolated from fed and 24 hr fasted rats. 2. [3H]H2O was preferentially incorporated into the glycerol backbone of triglycerides and phospholipids. Addition of ethanol markedly increased the incorporation of this label in both classes of glycerolipids; the increase was higher in fasted rat hepatocytes, both in the glycerol backbone and acyl groups of glycerolipids. 3. Ethanol increased [U-14C]palmitate incorporation into triglycerides only in hepatocytes from fasted rats. 4. [14C]CO2 and total acid soluble product formation from [1-14C]palmitate resulted inhibited by ethanol both in the fed and the fasted state.