**4. Drugs/toxicants could affect both central and peripheral circadian clock gene expression**

The circadian clock is located in both brain and peripheral tissues [8]. The central clock pacemaker is located in the suprachiasmatic nucleus (SCN) of the hypothalamus, while the peripheral clock is distributed in all peripheral tissues. The liver is the main peripheral tissue under circadian clock regulation [7–9]. Drugs/toxicants could affect both central and peripheral clock gene expression. For example, Mn is a well-known neurotoxicant

In Per1, Per2-deficient mice, the ability of AhR ligand dioxin (TCDD) to induce the Cyp1a1 and Cyp1b1 was enhanced, especially with targeted interruption of Per1 [43]. TCDD induction of Cyp1a1 was 23–43 fold greater during the night time (ZT18) than at the day time (ZT6) in WT mice. However, the diurnal variation in the TCDD induction of Cyp1a1 expression was abolished in Per1ldc, Per2ldc, and Per1ldc/Per2ldc mutant mice, suggesting that Per1, Per2 and their timekeeping function in the circadian clockworks mediate the diurnal variation in TCDD induction of Cyp1a1 [44]. Clock mutant Clk/Clk mice failed to show typical oscillation

of AhR expression, and BaP (an AhR ligand) induction of Cyp1a1 was disrupted [45].

key bile acid synthesis and transport genes, including Cyp7a1 and Ntcp, was lost [47].

when the hepatic GSH level was lowest [52].

effects, which will be discussed below.

20 Circadian Rhythm - Cellular and Molecular Mechanisms

**clock gene expression**

The hepatotoxic potential of antituberculosis drug isoniazid varied when it was administered at ZT1, ZT9, and ZT17, and the toxicity was highest when isoniazid was given at ZT1 [48]. Chlorzoxazone is a CYP2E1 metabolized drug, and its kinetics and half-life were altered with the diurnal variation of CYP2E1 activity. The value of chlorzoxazone half-life in plasma of the light phase group was significantly longer than the dark phase group, with an increase of 6-hydroxychlorzoxazone production [49]. Acute alcohol-induced higher toxicity at ZT13 than ZT1 when Per1 and Per2 were highly expressed. Per1−/− and Per2−/− mice were less susceptible to alcohol hepatotoxicity, especially in Per1 null mice. Per1 null mice had decreased expression of peroxisome proliferators-activated receptor-gamma and its target genes related to lipid metabolism such as Srebp1, fatty acid synthase (Fas), CD36, diacylglycerol O-acyltransferase 2 (Dgat2), AP2, and adipsin [50]. In primary hepatocytes isolated from Clock mutant Clk/Clk mice and WT mice, diethylnitrosamine (DEN) induced apoptosis and cell death were reduced in Clock-deficient mice, probably due to decreased DEN metabolism [51]. Cadmium hepatotoxicity is independent of metabolic activation; while its mortality was high at ZT8 than ZT20

Thus, alterations of diurnal oscillations would affect drug metabolism, efficacy, and toxicity. On the other hand, drugs could target circadian clock gene expressions to produce biological

The circadian clock is located in both brain and peripheral tissues [8]. The central clock pacemaker is located in the suprachiasmatic nucleus (SCN) of the hypothalamus, while the peripheral clock is distributed in all peripheral tissues. The liver is the main peripheral tissue under circadian clock regulation [7–9]. Drugs/toxicants could affect both central and peripheral clock gene expression. For example, Mn is a well-known neurotoxicant

**4. Drugs/toxicants could affect both central and peripheral circadian** 

In Per2−/− mice, bile duct ligation (BDL)-induced liver injury and fibrosis was increased, along with increases in TNFα, TGFβ1, Col1α, and TIMP1 in livers of Per2-null mice as compared to WT mice [46]. In Per1−/− and Per2−/− mice fed on 2% cholestyramine diet, and/or restricted feeding (phase-shift peripheral clock), liver bile acid levels were increased, and the nuclear receptors CAR and PXR were activated, together with the increased serum enzyme AST levels, indicative of liver damage. In these Per1−/− and Per2−/− mice, the circadian expression of

**Figure 2.** Neurotoxicant manganese intoxication produced aberrant expression of circadian clock genes in both central (hypothalamus) and peripheral (liver). Adapted from Li et al. [53].

producing a Parkinson-like syndrome, but it also produces liver injury. In an attempt to examine the effect of Mn on the central and peripheral clocks, rats were given Mn 1 and 5 mg/kg, ip, every 2 days for 1 month, and the hypothalamus and liver were removed to examine the clock gene expression (**Figure 2**). The results showed that Mn-induced aberrant expression of circadian clock genes in both hypothalamus and liver, and liver was more sensitive to Mn-induced decreases in clock gene Bmal1, Per1, and increase in Dbp, indicating that both central and peripheral clocks could be disrupted by drugs/toxicants [53]. Another example is chronic alcohol administration. Chronic alcohol consumption produced disruption of circadian clock gene expression in both central (hypothalamus) and peripheral tissues (liver and colon) [54], and the liver appeared to be more susceptible than brain in alterations of metabolic genes and core molecular clock disruption. In addition to the fatty liver and affected the diurnal oscillations of metabolic genes (alcohol dehydrogenase 1, carnitine palmitoyltransferase 1a, Cyp2e1, Phosphoenolpyruvate carboxykinase 1, pyruvate dehydrogenase kinase 4, Ppargc1a, Ppargc1b and Srebp1c), the diurnal oscillations of core clock genes (Bmal1, Clock, Cry1, Cry2, Per1, and Per2) and clock-controlled genes (Dbp, Hlf, Nocturnin, Npas2, Rev-erbα, and Tef) were altered in livers from ethanol-fed mice. In contrast, ethanol had only minor effects on the expression of core clock genes in the suprachiasmatic nucleus (SCN) [55].
