Sexual dimorphism in the regulation of cell turnover during liver hyperplasia

Mitogen-activated protein kinase signaling and its association with oxidative stress and apoptosis in lead-exposed hepatocytes

Lead toxicity has become a serious public health concern all over the world. Previous studies have shown that lead induces biochemical and structural changes in liver. However, although lead is known to alter liver functions, the underlying molecular mechanisms of hepatotoxicity are not yet clear. We hypothesized that a correlation exists between oxidative stress, apoptosis and mitogen-activated protein kinases (MAPKs) in lead-exposed liver. Wistar rats were treated with 0, 0.5%, and 1% lead acetate for 3d, 14d, and 35d and sacrificed the next day. On 4d, oxidative stress and apoptosis were correlated with downregulated expressions of ERK1/2 and p38-MAPKα/β, and upregulated expressions of JNK1/3 in males. In females, the correlation was with downregulated expressions of ERK1/2 and upregulated expressions of p38-MAPKα/β and JNK1/3. On 15d, the correlation was observed with upregulated expressions of p38-MAPKα/β in males and downregulated expressions of p38-MAPKα/β in females. In both sexes, a correlation was observed with upregulated expressions of ERK1/2 and JNK1/3 in 1% groups. On 36d, the correlation was observed with downregulated expressions of p38-MAPKα/β in males and their upregulated expressions in females. Time-dependent increase in lipid peroxidation on 15d and 36d correlated with upregulated expressions of p38-MAPKα/β in females and ERK1/2 in 1% groups in both sexes. The lower dose induced more apoptosis up to 15d in females and the higher dose induced in males on 36d. Generally, the female livers had more p38-MAPKα/β than the male livers. On 36d, the female livers showed more p38-MAPKα/β and JNK1/3 than the male livers. In conclusion, although not clearly defined, a correlation exists among oxidative stress, apoptosis, and the MAPKs in lead-exposed hepatocytes. The sex-dependent effects may be due to differences in hormonal or other physiological mechanisms. In lead-exposed hepatocytes, the apoptosis may be induced via oxidative stress-mediated alterations in the MAPKs.

Sexually dimorphic actions of glucocorticoids provide a link to inflammatory diseases with gender differences in prevalence

Males and females show differences in the prevalence of many major diseases that have important inflammatory components to their etiology. These gender-specific diseases, which include autoimmune diseases, hepatocellular carcinoma, diabetes, and osteoporosis, are largely considered to reflect the actions of sex hormones on the susceptibility to inflammatory stimuli. However, inflammation reflects a balance between pro- and anti-inflammatory signals, and investigation of gender-specific responses to the latter has been neglected. Glucocorticoids are the primary physiological anti-inflammatory hormones in mammals, and synthetic derivatives of these hormones are prescribed as anti-inflammatory agents, irrespective of patient gender. We explored the possibility that sexually dimorphic actions of glucocorticoid regulation of gene expression may contribute to the dimorphic basis of inflammatory disease by evaluating the rat liver, a classic glucocorticoid-responsive organ. Surprisingly, glucocorticoid administration expanded the set of hepatic sexually dimorphic genes. Eight distinct patterns of glucocorticoid-regulated gene expression were identified, which included sex-specific genes. Our experiments also defined specific genes with altered expression in response to glucocorticoid treatment in both sexes, but in opposite directions. Pathway analysis identified sex-specific glucocorticoid-regulated gene expression in several canonical pathways involved in susceptibility to and progression of diseases with gender differences in prevalence. Moreover, a comparison of the number of genes involved in inflammatory disorders between sexes revealed 84 additional glucocorticoid-responsive genes in the male, suggesting that the anti-inflammatory actions of glucocorticoids are more effective in males. These gender-specific actions of glucocorticoids in liver were substantiated in vivo with a sepsis model of systemic inflammation.

Waterborne Lead Exposure Affects Brain Endocannabinoid Content in Male but not Female Fathead Minnows (Pimephales promelas)

There are several similarities between the behavioral and neurochemical effects of lead (Pb2+) and the cannabinoids. Both Pb2+ exposure and cannabinoid treatment decrease exploratory behavior. Pb2+-induced hyperactivity has been observed in rats and fish. By comparison, cannabinoids increase locomotor activity at higher doses in rats. Moreover, Pb2+ exposure produces learning and memory impairments as do the cannabinoids. Many of the behavioral effects of Pb2+ are thought to be due, in part, to the ability of Pb2+ to either inhibit or mimic the actions of calcium (Ca2+). At low concentrations, Pb2+ enhances basal release of neurotransmitter from presynaptic terminals by increasing intracellular free Ca2+ concentrations. Pb2+ also decreases evoked neurotransmitter release due to blockade of voltage-gated Ca2+ channels. Interestingly, the endocannabinoids (eCBs) including N-arachidonylethanolamine (AEA) and 2-arachidonylglycerol (2-AG) are synthesized in response to increases in intracellular Ca2+ and activate the CB1 receptor that inhibits voltage-gated Ca2+ channels. We tested the hypothesis that waterborne Pb2+ exposure significantly affects whole-brain eCB content in adult male and female fathead minnows (Pimephales promelas). Waterborne Pb2+ exposure (1.0 ppm) resulted in a time-dependent accumulation of Pb2+ in bone in both males and females. Brain AEA and 2-AG content were significantly greater in females compared to males. Pb2+ exposure increased brain AEA content in males at 7 and 14 days of exposure and increased brain 2-AG content at 14 days. Pb2+ exposure had no effect on either brain AEA or 2-AG content in females at any of the time points examined. As eCBs serve as activity-dependent retrograde inhibitors of neurotransmitter release, the increase in brain eCB content would accentuate Pb2+-induced decreases in evoked neurotransmitter release in male but not female fathead minnows.

Cell cholesterol esters and high-density lipoprotein plasma levels during liver hyperplasia in choline-fed male and female rats

Sexual dimorphism exists in the response of rats to lead nitrate, liver hyperplasia occuring earlier and being more pronounced in males. Excess dietary choline in females shifted the growth pattern towards that of males. To determine whether phosphatidylcholine-induced growth modulations could be related to a derangement of cholesterol metabolism, liver accumulation of cholesterol esters and plasma lipoprotein patterns were investigated. In males, lead-induced liver hyperplasia was associated with increased total cholesterol hepatic content, accumulated cholesterol esters and reduced concentration of plasma High Density Lipoprotein (HDL) cholesterol. Females were less responsive to the liver mitogenic signal of lead nitrate; there was no elevation of cholesterol content nor any marked accumulation of cholesterol esters. This is consistent with the lack of change in the plasma levels of HDL cholesterol. Continuous choline feeding displaced the liver cholesterol ester pattern and plasma HDL cholesterol levels in females, and in parallel that of DNA synthesis, towards those of males. Choline was not observed to have any effect in males. These results suggest that the derangement of phosphatidylcholine metabolism induces growth-related changes in cholesterol turnover; they are consistent with the proposal that the intracellular content of cholesterol esters may have a role in regulating liver growth rates.

Transient inactivation of phosphatidylethanolamine N-methyltransferase-2 and activation of cytidine triphosphate: phosphocholine cytidylyltransferase during non-neoplastic liver growth

Phosphatidylethanolamine N-methyltransferase-2 (PEMT2) may contribute to the control of hepatocyte cell division, since its inactivation is associated with several types of liver proliferation including tumorigenesis [Cui, Houweling and Vance (1994) J. Biol. Chem. 269, 24531-24533]. To determine if the inactivation of PEMT2 was involved in non-neoplastic proliferation of hepatocytes, we studied the expression of this enzyme in a model of lead nitrate-induced liver proliferation in vivo in rats. A maximal decrease in PEMT activity (60%) and loss of PEMT2 protein (95%) coincided with maximal DNA synthesis and maximal cytidine triphosphate:phosphocholine cytidylyltransferase activity 36 h and 48 h after lead nitrate stimulation in male and female livers respectively. The decrease in expression of PEMT2 corresponded to a decrease in its mRNA. Compared with males, female rats exhibited a 12 h delay in the peak of DNA synthesis, in cytidylyltransferase activity and in the minimum of PEMT2 expression. Supplementation of the rats with dietary choline shifted the female pattern of PEMT2 inactivation, DNA synthesis and activation of cytidylyltransferase to 12 h earlier so that it was similar to the time frame of the expression of these activities in males. These results are consistent with the proposal that the inactivation of PEMT2 may have a role in the regulation of non-neoplastic growth of liver.