Germline DNA replication timing shapes mammalian genome composition

Mammalian DNA replication is a highly organized and regulated process. Large, Mb-sized regions are replicated at defined times along S-phase. Replication Timing (RT) is thought to play a role in shaping the mammalian genome by affecting mutation rates. Previous analyses relied on somatic RT profiles. However, only germline mutations are passed on to offspring and affect genomic composition. Therefore, germ cell RT information is necessary to evaluate the influences of RT on the mammalian genome. We adapted the RT mapping technique for limited amounts of cells, and measured RT from two stages in the mouse germline - primordial germ cells (PGCs) and spermatogonial stem cells (SSCs). RT in germline cells exhibited stronger correlations to both mutation rate and recombination hotspots density than those of RT in somatic tissues, emphasizing the importance of using correct tissues-of-origin for RT profiling. Germline RT maps exhibited stronger correlations to additional genetic features including GC-content, transposable elements (SINEs and LINEs), and gene density. GC content stratification and multiple regression analysis revealed independent contributions of RT to SINE, gene, mutation, and recombination hotspot densities. Together, our results establish a central role for RT in shaping multiple levels of mammalian genome composition.

Suppression of FoxO1 activity by long-chain fatty acyl analogs

OBJECTIVE

Overactivity of the Forkhead transcription factor FoxO1 promotes diabetic hyperglycemia, dyslipidemia, and acute-phase response, whereas suppression of FoxO1 activity by insulin may alleviate diabetes. The reported efficacy of long-chain fatty acyl (LCFA) analogs of the MEDICA series in activating AMP-activated protein kinase (AMPK) and in treating animal models of diabesity may indicate suppression of FoxO1 activity.

RESEARCH DESIGN AND METHODS

The insulin-sensitizing and anti-inflammatory efficacy of a MEDICA analog has been verified in guinea pig and in human C-reactive protein (hCRP) transgenic mice, respectively. Suppression of FoxO1 transcriptional activity has been verified in the context of FoxO1- and STAT3-responsive genes and compared with suppression of FoxO1 activity by insulin and metformin.

RESULTS

Treatment with MEDICA analog resulted in total body sensitization to insulin, suppression of lipopolysaccharide-induced hCRP and interleukin-6-induced acute phase reactants and robust decrease in FoxO1 transcriptional activity and in coactivation of STAT3. Suppression of FoxO1 activity was accounted for by its nuclear export by MEDICA-activated AMPK, complemented by inhibition of nuclear FoxO1 transcriptional activity by MEDICA-induced C/EBPβ isoforms. Similarly, insulin treatment resulted in nuclear exclusion of FoxO1 and further suppression of its nuclear activity by insulin-induced C/EBPβ isoforms. In contrast, FoxO1 suppression by metformin was essentially accounted for by its nuclear export by metformin-activated AMPK.

CONCLUSIONS

Suppression of FoxO1 activity by MEDICA analogs may partly account for their antidiabetic anti-inflammatory efficacy. FoxO1 suppression by LCFA analogs may provide a molecular rational for the beneficial efficacy of carbohydrate-restricted ketogenic diets in treating diabetes.

Modulation of insulin secretion by fatty acyl analogs

The secretagogue, the incretin-like, and the suppressive activities of long-chain fatty acids (LCFAs) in modulating insulin secretion in vivo and in cultured islets were simulated here by beta,beta'-tetramethyl-hexadecanedioic acid (M16) and alpha,alpha'-tetrachloro-tetradecanedioic acid (Cl-DICA). M16, but not Cl-DICA, serves as a substrate for ATP-dependent CoA thioesterification but is not further metabolized. M16, but not Cl-DICA, acted as a potent insulin secretagogue in islets cultured in basal but not high glucose. Short-term exposure to M16 or Cl-DICA resulted in activation of glucose- but not arginine-stimulated insulin secretion. Long-term exposure to M16, but not to Cl-DICA, resulted in suppression of glucose-, arginine-, and K(+)-stimulated insulin secretion; inhibition of glucose-induced proinsulin biosynthesis; and depletion of islets insulin. beta-Cell mass and islet ATP content remained unaffected. Hence, nonmetabolizable LCFA analogs may highlight discrete LCFA metabolites and pathways involved in modulating insulin secretion, which could be overlooked due to the rapid turnover of natural LCFA.

Adipose tissue sensitization to insulin induced by troglitazone and MEDICA 16 in obese Zucker rats in vivo

The putative role played by insulin sensitizers in modulating adipose tissue lipolysis in the fasting state was evaluated in obese conscious Zucker rats treated with troglitazone or beta,beta'-tetramethylhexadecanedioic acid (MEDICA 16) and compared with nontreated lean and obese animals. The rates of appearance (R(a)) of glycerol and free fatty acid (FFA), primary intra-adipose reesterification, and secondary reuptake of plasma FFA in adipose fat were measured using constant infusion of stable isotope-labeled [(2)H(5)]glycerol, [2,2-(2)H(2)]palmitate, and radioactive [(3)H]palmitate. The overall lipolytic flux (R(a) glycerol) was increased 1.7- and 1.4-fold in obese animals treated with troglitazone or MEDICA 16, respectively, resulting in increased FFA export (R(a) FFA) in the troglitazone-treated rats. Primary intra-adipose reesterification of lipolysis-derived fatty acids was enhanced twofold by insulin sensitizers, whereas reesterification of plasma fatty acids was unaffected by either treatment. Despite the unchanged R(a) FFA in MEDICA 16 or the increased R(a) FFA induced by troglitazone, very low density lipoprotein production rates were robustly curtailed. Total adipose tissue reesterification, used as an estimate of glucose conversion to glyceride-glycerol, was increased 1.9-fold by treatment with the insulin sensitizers. Our results indicate that, in the fasting state, insulin sensitizers induce, in vivo, a significant activation rather than suppression of adipose tissue lipolysis together with stimulation of glucose conversion to glyceride-glycerol.

Fatty acid cycling in the fasting rat

Adipose tissue lipolysis and fatty acid reesterification by liver and adipose tissue were investigated in rats fasted for 15 h under basal and calorigenic conditions. The fatty acid flux initiated by adipose fat lipolysis in the fasted rat is mostly futile and is characterized by reesterification of 57% of lipolyzed free fatty acid (FFA) back into adipose triglycerides (TG). About two-thirds of FFA reesterification are carried out before FFA release into plasma, whereas the rest consists of plasma FFA extracted by adipose tissue. Thirty-six percent of the fasting lipolytic flux is accounted for by oxidation of plasma FFA, whereas only a minor fraction is channeled into hepatic very low density lipoprotein-triglycerides (VLDL-TG). Total body calorigenesis induced by thyroid hormone treatment and liver-specific calorigenesis induced by treatment with beta, beta'-tetramethylhexadecanedioic acid (Medica 16) are characterized by a 1.7- and 1.3-fold increase in FFA oxidation, respectively, maintained by a 1.5-fold increase in adipose fat lipolysis. Hepatic reesterification of plasma FFA into VLDL-TG is negligible under both calorigenic conditions. Hence, total body fatty acid metabolism is regulated by adipose tissue as both source and sink. The futile nature of fatty acid cycling allows for its fine tuning in response to metabolic demands.

Sensitization to insulin induced by beta,beta'-methyl-substituted hexadecanedioic acid (MEDICA 16) in obese Zucker rats in vivo

Beta,beta'-methyl-substituted hexadecanedioic acid (MEDICA 16) consists of a nonmetabolizable long-chain fatty acid designed to probe the effect exerted by fatty acids on insulin sensitivity. The effect of MEDICA 16 was evaluated in insulin-resistant Zucker (fa/fa) rats in terms of liver, muscle, and adipose tissue response to clamped euglycemic hyperinsulinemia in vivo. Nontreated Zucker rats were insulin resistant, maintaining basal rates of total-body glucose disposal, glucose production in liver, free fatty acid (FFA) flux into plasma, and FFA reesterification in adipose tissue, irrespective of the insulin levels induced. MEDICA 16 treatment resulted in an insulin-induced decrease in hepatic glucose production, together with an insulin-induced increase in total-body glucose disposal. Intracellular reesterification of lipolysed FFA in adipose tissue was specifically activated by MEDICA 16, resulting in a pronounced decrease in FFA release, with a concomitant decrease in plasma FFA. In conclusion, MEDICA 16 treatment results in the sensitization of liver, muscle, and adipose tissue to insulin in an animal model for obesity-induced insulin resistance.

Hypocholesterolaemic effect of beta beta'-methyl-substituted hexadecanedioic acid (MEDICA 16) in the male hamster

Treatment of cholesterol-fed male hamsters kept on a diet of purina chow with beta beta'-methyl-substituted hexadecanedioic acid (MEDICA 16) resulted in a progressive hypocholesterolaemic effect, amounting to a 50% decrease in the cholesterol content of all plasma lipoproteins. The decrease in plasma cholesterol could be accounted for by activation of plasma-cholesterol efflux through the liver into the bile mediated by MEDICA 16-induced (a) increase of the number of liver LDL receptors, (b) activation of liver neutral cholesteryl ester hydrolase with a concomitant inhibition of liver acyl-CoA cholesterol acyltransferase, resulting in shifting of the liver cholesteryl ester/free-cholesterol cycle in the direction of free cholesterol, and (c) activation of cholesterol efflux from the liver into the bile. The increase in bile cholesterol output was accompanied by an increase in bile phospholipids but not in bile acids. In contrast with rats, MEDICA 16-treatment of male hamsters did not result in a hypotriacylglycerolaemic effect, inhibition of lipogenesis, nor in a substantial decrease in plasma apolipoprotein C-III content.

Triacylglycerol synthesis in cultured rat hepatocytes. The rate-limiting role of diacylglycerol acyltransferase

The limiting role of diacylglycerol acyltransferase with respect to triacylglycerol synthesis in cultured rat hepatocytes was evaluated by following the inhibition of the overall synthetic flux by 2-bromooctanoate acting as an inhibitor of the diacylglycerol acyltransferase step. The flux-control coefficient of diacylglycerol acyltransferase in intact cultured hepatocytes amounted to 0.76 in the presence of saturating glycerol and either palmitate or oleate as the fatty acyl substrates. The flux-control coefficient of diacylglycerol acyltransferase in lysolecithin-permeabilized cultured hepatocytes amounted to 0.80 and 0.99 in the presence of saturating glycerol 3-phosphate and either palmitate or oleate as the fatty acyl substrate, respectively. Hence, triacylglycerol synthesis in liver cells under the experimental conditions employed is rate-limited by the diacylglycerol acyltransferase.

Lipid synthesis in permeabilized cultured rat hepatocytes

Hepatic lipid synthesis was verified and studied in lysolecithin-permeabilized cultured rat hepatocytes and compared to that of intact liver cells. Triacylglycerol synthesis in permeabilized cells incubated in the presence of glycerol 3-phosphate and long chain fatty acids approached that of intact hepatocytes. Similarly, phosphatidylcholine synthesis in permeable cells incubated in the presence of exogenous CDP-choline was similar to that of intact hepatocytes and at the expense of microsomal neutral lipid synthesis. Phosphatidic acid accumulation in lysolecithin-permeabilized liver cells was remarkably increased as compared to that of intact cells, and its synthesis was mostly accounted for by the activity of mitochondrial glycerol-3-phosphate acyltransferase. Mitochondrial-generated phosphatidate was found to migrate to the endoplasmic reticulum, thus establishing a novel lipid esterification pathway which begins in mitochondrial glycerol 3-phosphate acylation and results in microsomal triacylglycerol and phospholipid synthesis. The free access of permeabilized liver cells to substrates and modulators of lipid synthesis, while maintaining an overall synthetic pattern similar to that of intact hepatocytes, makes them a system of choice for studying hepatic lipid synthesis in general and the microsomal/mitochondrial distribution of fluxes in particular.

The hypochylomicronemic effect of beta,beta'-methyl-substituted hexadecanedioic acid (MEDICA 16) is mediated by a decrease in apolipoprotein C-III

Treatment of rats fed a balanced Purina Chow diet with beta,beta'-tetramethyl-substituted hexadecanedioic acid (MEDICA 16) (Bar-Tana, J., Rose-Kahn, G., and Srebnik, M. (1985) J. Biol. Chem. 260, 8404-8410) resulted in an acute 70-80% decrease in plasma chylomicrons-triacylglycerols which was sustained as long as the drug was administered. The hypochylomicronemic effect resulted from an enhanced plasma clearance of chylomicrons whereas their intestinal production and absorption remained unaffected. Chylomicrons-triacylglycerols clearance in MEDICA 16-treated rats was characterized by a fast initial phase lasting for 1-2 min and consisting of elimination of 50-60% of the injected chylomicrons' tracer at a fractional clearance rate of 0.77 +/- 0.27 min-1 as compared to 0.08 +/- 0.01 min-1 in nontreated rats. The fractional clearance rate of chylomicrons-cholesterol ester was similarly affected by MEDICA 16 treatment and amounted to 0.48 +/- 0.05 and 0.05 +/- 0.01 min-1 in MEDICA 16-treated and nontreated rats, respectively. The increased fractional clearance rate of plasma chylomicrons in MEDICA 16-treated rats presumably reflects the primary action of the drug rather than being secondary to the hypochylomicronemic state, since it was similarly observed in MEDICA 16-treated animals made transiently normolipemic by loading them with intestinal lipid. The increase in the fractional clearance rate of plasma chylomicrons resulted from their enhanced uptake by the liver complemented with their activated extrahepatic catabolism. The activation of both catabolic modes in MEDICA 16-treated rats could be accounted for by a 10-fold decrease in the apoC-III content of plasma chylomicrons. No increase was observed in hepatic apoB,E or apoE receptors, nor in the maximal capacity of lipoprotein lipase. The pharmacological reduction of plasma apoC-III may thus offer a treatment mode of choice for selected hyperlipidemic states.

The specificity of triacylglycerol synthesis for medium-chain fatty acids in rat and human adipose preparations

The availability of medium-chain fatty acids as substrates for triacylglycerol synthesis was studied in cultured and suspended rat adipocytes and in pieces of human adipose tissue. Octanoate was virtually excluded from glycerol 3-phosphate esterification while serving as a substrate for diacylglycerol esterification. This specificity was similar to that of cultured rat hepatocytes.

Inhibition of diacylglycerol acyltransferase by 2-bromooctanoate in cultured rat hepatocytes

Triacylglycerol synthesis in cultured rat hepatocytes was inhibited by 2-bromooctanoate with a concomitant accumulation of diacylglycerols. 2-Bromooctanoate inhibition could be ascribed to its thioesterification by medium chain fatty acyl-CoA synthase (Raaka, B.M., and Lowenstein, J.M. (1979) J. Biol. Chem. 254, 6755-6762) with 2-bromooctanoyl-CoA acting as a competitive inhibitor of diacylglycerol acyltransferase. The Ki of 2-bromooctanoyl-CoA was 1.5 microM compared with a Km of 25 microM for the palmitoyl-CoA substrate. Diacylglycerol esterification was also inhibited by C12-C16 2-bromo fatty acids. However, inhibition of triacylglycerol synthesis by long chain 2-bromo fatty acids was accompanied by decreased overall neutral lipid synthesis as a result of inhibition of the long chain fatty acyl-CoA synthase. Since 2-bromooctanoate was a poor inhibitor of the long chain fatty acyl-CoA synthase, it appears to function selectively as an inhibitor of diacylglycerol acyltransferase in cultured rat hepatocytes.

Medium chain fatty acids as specific substrates for diglyceride acyltransferase in cultured hepatocytes

The fatty acyl substrate specificity for de novo diglyceride, triglyceride, and phospholipid synthesis in cultured hepatocytes was evaluated. The fatty acyl availability for diglyceride and phospholipid synthesis is restricted to fatty acids longer than myristic acid with octanoic and decanoic acids being excluded. On the other hand, octanoic and decanoic acids may serve as substrates for the diglyceride acyltransferase. Dodecanoic acid appears to occupy an intermediate position between medium and long chain fatty acids with respect to its availability as substrate for the esterification sites concerned. The availability of octanoate as a specific substrate for the diglyceride acyltransferase reaction in cultured hepatocytes enables the measurement of this activity in situ under conditions of overall triglyceride synthesis.

Enzymes of glucose and fatty acid metabolism in early and term human placenta

The activity of enzymes involved in glycolysis, gluconeogenesis, and lipoenesis in early and term human placenta was determined. A high activity of pyruvate kinase was found, indicating high glycolytic potential. The activity of this enzyme tended to decrease with gestation. The presence of phosphoenolpyruvate carboxylase activity was detected, suggesting the possibility of gluconeogenesis in the placenta. Very low activity of enzymes involved in fatty acid synthesis was found, whereas the activity of the pentose shunt pathway enzymes, glucose-6-phosphogluconate dehydrogenases, was relatively high. This suggested a role of this pathway in the synthesis of lipids other than fatty acids in the placenta. The activities of enzymes in the human placenta and their changes during gestation where compared to previous observations on enzymes in rat placenta.