Growth hormone and aging change rat liver fatty acid binding protein levels

Recent insights into the biological functions of liver fatty acid binding protein 1

Over four decades have passed since liver fatty acid binding protein (FABP)1 was first isolated. There are few protein families for which most of the complete tertiary structures, binding properties, and tissue occurrences are described in such detail and yet new functions are being uncovered for this protein. FABP1 is known to be critical for fatty acid uptake and intracellular transport and also has an important role in regulating lipid metabolism and cellular signaling pathways. FABP1 is an important endogenous cytoprotectant, minimizing hepatocyte oxidative damage and interfering with ischemia-reperfusion and other hepatic injuries. The protein may be targeted for metabolic activation through the cross-talk among many transcriptional factors and their activating ligands. Deficiency or malfunction of FABP1 has been reported in several diseases. FABP1 also influences cell proliferation during liver regeneration and may be considered as a prognostic factor for hepatic surgery. FABP1 binds and modulates the action of many molecules such as fatty acids, heme, and other metalloporphyrins. The ability to bind heme is another cytoprotective property and one that deserves closer investigation. The role of FABP1 in substrate availability and in protection from oxidative stress suggests that FABP1 plays a pivotal role during intracellular bacterial/viral infections by reducing inflammation and the adverse effects of starvation (energy deficiency).

Liver fatty acid-binding protein and obesity

While low levels of unesterified long chain fatty acids (LCFAs) are normal metabolic intermediates of dietary and endogenous fat, LCFAs are also potent regulators of key receptors/enzymes and at high levels become toxic detergents within the cell. Elevated levels of LCFAs are associated with diabetes, obesity and metabolic syndrome. Consequently, mammals evolved fatty acid-binding proteins (FABPs) that bind/sequester these potentially toxic free fatty acids in the cytosol and present them for rapid removal in oxidative (mitochondria, peroxisomes) or storage (endoplasmic reticulum, lipid droplets) organelles. Mammals have a large (15-member) family of FABPs with multiple members occurring within a single cell type. The first described FABP, liver-FABP (L-FABP or FABP1), is expressed in very high levels (2-5% of cytosolic protein) in liver as well as in intestine and kidney. Since L-FABP facilitates uptake and metabolism of LCFAs in vitro and in cultured cells, it was expected that abnormal function or loss of L-FABP would reduce hepatic LCFA uptake/oxidation and thereby increase LCFAs available for oxidation in muscle and/or storage in adipose. This prediction was confirmed in vitro with isolated liver slices and cultured primary hepatocytes from L-FABP gene-ablated mice. Despite unaltered food consumption when fed a control diet ad libitum, the L-FABP null mice exhibited age- and sex-dependent weight gain and increased fat tissue mass. The obese phenotype was exacerbated in L-FABP null mice pair fed a high-fat diet. Taken together with other findings, these data suggest that L-FABP could have an important role in preventing age- or diet-induced obesity.

Cellular signaling, AGE accumulation and gene expression in hepatocytes of lean aging rats fed ad libitum or food-restricted

The effects of food restriction on liver glucagon and vasopressin V1a receptors, on AGE accumulation and on gene expression were investigated in 10- and 30-month-old WAG/Rij female rats fed ad libitum or chronically food-restricted by 30%. The age-related increase in glucagon and vasopressin V1a receptor density, as well as the rise in glucagon-induced cAMP generation was prevented by the restriction. AGE accumulation, characteristic of the aging process, was normalized in food-restricted animals. Gene expression determined with rat Atlas cDNA Expression Arrays containing 1176 cDNA indicates that a few genes exhibited a greater than twofold change in mRNA ratios with age. Most down-regulated genes were related to oxidative metabolism of lipids, and most of the up-regulated genes were concerned with the cell cycle and transcription factors. Chronic food restriction partially prevents these changes in gene expression and induces up- and down-regulation of several mRNAs which are not modified with age in ad libitum fed rats.

Aging promotes a different phosphatidic acid utilization in cytosolic and microsomal fractions from brain and liver

Among the morphological and biochemical changes taking place in the membranes of aged tissues, we reported in previous studies on alterations in phospholipid synthesis and phospholipid-specific fatty acid composition. Phosphatidic acid (PA) and diacylglycerol (DAG) are central intermediates in phosphoglyceride and neutral lipid biosynthetic pathways and have also recently been implicated in signal transduction. The present paper shows the effect of aging on phosphatidate phosphohydrolase (PAPase) activiy, which operates on phosphatidic acid to synthesize diacylglycerol. Two forms of mammalian PAPase can be indentified on the basis of subcellular localization and enzyme properties, one involved predominantly in lipid synthesis (PAP 1) and the other in signal transduction (PAP 2). Microsomal and cytosolic fractions of brain and liver from 3.5-month-old (adult) and 28.5-month-old (aged) rats were used. PAPase isoform activities were differentiated on the basis of N-ethylmaleimide (NEM) sensitivity and Mg(2+)-dependency. Our results demonstrate that aging caused PAP 2 to increase in brain microsomal fractions but did not affect PAP 1, whereas in brain cytosolic fractions, it caused a strong decrease in PAP 1 (57%). The distribution of enzymes between microsomes and cytosol changed in aged rats with respect to adult rats, showing a translocation of PAP 1 from cytosol to microsomes. In addition, an increase in the production of monoacylglycerol (MAG) was observed in microsomes from aged brain. PAP 2 activity in liver microsomal fractions from aged rats showed no changes with respect to adult rats whereas PAP 1 activity increased 228% in microsomal fractions and 76% in cytosolic fractions in this tissue. The distribution of PAP 1 activity between microsomal and cytosolic fractions in liver tissue was also affected in aged rats, indicating a translocation of this form of the enzyme from cytosolic to microsomal fractions. The production of monoacylglycerol in liver microsomes also increased, whereas there was a decrease in MAG formation from cytosolic fraction. The changes observed in the two PAPase forms in brain and liver of aged rats with respect to adult rats suggest that PA is differently utilized by the PAPase isoforms, probably generating aging-related DAGs different to those present in adults and required for specific cellular functions. The changes observed in liver PAP 1 from aged with respect to adult rats suggest that such changes could be related with modifications in lipid homeostasis induced by age-altered hormonal balance. However, PA-modified utilization during aging through PAP 2 activity could be related to alterations in neural signal transduction mechanisms.

Hormonal regulation of liver fatty acid-binding protein in vivo and in vitro: effects of growth hormone and insulin

Liver fatty acid-binding protein (LFABP) is an abundant protein in hepatocytes that binds most of the long chain fatty acids present in the cytosol. It is suggested to be of importance for fatty acid uptake and utilization in the hepatocyte. In the present study, the effects of bovine GH (bGH) and other hormones on the expression of LFABP and its messenger RNA (mRNA) were studied in hypophysectomized rats and in vitro using primary cultures of rat hepatocytes. One injection of bGH increased LFABP mRNA levels about 5-fold after 6 h, but there was no effect of this treatment on LFABP levels. However, 7 days of bGH treatment increased both LFABP mRNA and LFABP protein levels 2- to 5-fold. Female rats had higher levels of LFABP than male rats. Hypophysectomy of female rats, but not that of male rats, decreased LFABP levels markedly. Treatment of hypophysectomized rats with bGH for 7 days as two daily injections or as a continuous infusion increased LFABP levels to a similar degree. This finding indicates that the sex difference in the expression of LFABP is not regulated by the sexually dimorphic secretory pattern of GH. Neither insulin nor insulin-like growth factor I treatment of hypophysectomized rats for 6-7 days had any effect on LFABP mRNA or LFABP levels. In vitro, bGH dose-dependently increased the expression of LFABP mRNA, but only in the presence of insulin. Insulin alone had a marked dose-dependent effect on LFABP mRNA levels and was of importance for maintaining the expression of LFABP mRNA during the culture. Incubation with bGH increased LFABP mRNA levels within 3 h. GH had no effect on LFABP mRNA levels in the presence of actinomycin D, indicating a transcriptional effect of GH. Incubation with glucagon in vitro decreased LFABP mRNA levels markedly, indicating that glucagon, in contrast to GH, has an effect opposite that of insulin on LFABP mRNA expression. It is concluded that GH is an important regulator of LFABP in vivo and in vitro. In contrast to the effect of GH on insulin-like growth factor I mRNA, the presence of insulin was a prerequisite for the effect of GH on LFABP mRNA expression in vitro. The results emphasize the role of GH in the regulation of hepatic fatty acid metabolism.