Sexual Dimorphism in the Liver Impact on drug metabolism disease and cancer Arlin Rogers MIT Division of Comparative Medicine BE 450 April 25 2005 Gender differences in hepatic function Lipid metabolism Microsomal smooth endoplasmic reticulum Mitochondrial Peroxisomal Steroid metabolism Sex steroids and other cholesterol derivatives Energy production Females have higher mitochondrial function and metabolic activity Sex and the single rat liver Rats exhibit highest liver sexual dimorphism among tested species Females sleep longer when given hexabarbital 1937 Gonadectomy or exogenous steroids affect drug metabolism 1950 s Classical species used in toxicology studies Skewed results Liver dimorphism in non rats Mice Exhibit dimorphism in Cyp genes but some reversed in gender specificity vs rat Less pronouced differences than in rat Other small animals Dimorphism shown but not pronounced Primates non human and human Differences in drug metabolism between sexes known But no gender dimorphic CYP 450 genes shown yet Individual variation masks small gender differences Mechanisms of liver dimorphism Growth hormone periodicity GH is the greatest determinant of liver dimorphism Sex steroids Determine GH periodicity by indirect means Direct action on liver Liver expresses androgen estrogen and progesterone receptors Imprinting Neonatal Peripubertal Hepatocyte nuclear factors Growth hormone regulation GH secreted by pituitary Regulated by neuropeptides from neurons in the hypothalamus Positive feedforward peptides growth hormone releasing hormone GHRH ghrelin Negative feedback peptide somatostatin inhibits GH release by pituitary somatotrophes until a certain threshold is exceeded Sex steroids and GH regulation Both androgens and estrogens stimulate GHRH and ghrelin secretion Hypothalamic arcuate nucleus neurons bear both androgen receptors AR and estrogen receptors ER Only androgens stimulate somatostatin secretion Hypothalamic periventricular nucleus neurons only AR Net effect of sex steroids on GH secretion Females frequent unpredictable release of submaximal GH from pituitary Males diurnally regular large GH boluses followed by long nadirs with undetectable serum GH Control of GH secretion males Figure removed for copyright reasons GH periodicity and genderspecific CYP gene expression Rat Cyp2c11 is malepredominant Cyp2c12 is female Regular GH cycle in males stimulates Cyp2c11 via Jak2 Stat5b signaling Interpeak period determines dimorphic gene expression not wave amplitude Waxman DJ Courtesy of David J Waxman Used with permission GH sex steroids affects on liver Effects of Various Treatments on the Expression of Sex Specific Isoforms of Cytochrome P450 in Rat Liver Treatment Males Females Steroid administration to intact animals Estradiol reduces expression of male isoforms Testosterone reduces expression of female isoforms but increases expression of some male specific isoforms Castration Reduces male specific isoforms Reduces female specific isoforms Castration followed by steroid administration Testosterone increases expression of male isoforms Estradiol restores levels of femalespecific isoforms Hypophysectomy Significantly reduces the level of male specific isoforms Causes expression of male specific isoforms Hypophysectomy followed by steroid administration No effect of estradiol No effect of testosterone Hypophysectomy followed by growth hormone administration Isoform expression reflects pattern of growth hormone secretion Isoform expression reflects pattern of growth hormone secretion The age of the animal at the time of castration determines the effect on the composition of hepatic cytochrome P450 isoforms For example castration does not have an effect if animals are older than five weeks of age Summary Most sex steroid effects mediated through GH Figure by MIT OCW Imprinting Female rats given testosterone approach but do not achieve male phenotype Females are not just males without androgens Certain male specific genes retain male phenotype following post pubertal castration Imprinting also occurs in the neonatal period Mechanisms poorly understood Hepatocyte nuclear factor mediated dimorphism Rat Cyp2a2 Plasma GH CYP2C12 Female specific HNF6 Time hr HNF3 CYP2A2 Plasma GH M A L E Other HNFs also important F E M A L E HNF3 3 5 hr HNF4 CYP8B1 Cyp2d9 Time hr Male specific pY STAT5b Wiwi and Waxman JBC 2005 Figure by MIT OCW Xenobiotic metabolism Drug Oxidation Derivative PHASE I Conjugation Conjugate PHASE II Cytochrome P450 Flavin monooxygenases etc Glutathione S transferases Glucuronidyl transferases etc Figure by MIT OCW Cytochrome P450 Heme thiolate enzymes absorb light at 450 nm when bound with CO Pigment 450 nm Cytochrome P450 1 Superfamily of haemoproteins iron containing 2 Major enzymes of Phase I metabolism 3 Standardized Nomenclature Phase I drug metabolism Monooxygenases or mixed function oxidases Generally add hydroxyl OH group to hydrophobic compounds to increase water solubility and prepare for phase II conjugation Figure by MIT OCW CYP 3 A 4 Family Subfamily Member Nomenclature Human CYP all caps Nonhuman Cyp CYP 450 metabolism human Graph removed for copyright reasons Source Accelrys Software Inc None of these enzymes have known sexual dimorphism P450 species comparison Figure removed for copyright reasons Source Table 2 in Kedderis G L and C A Mugford Sex dependent metabolism of xenobiotics Drug Metab Rev 30 1998 441 498 Implications for toxicology studies in animals Many rodent P450 are dimorphic but not human Figure removed for copyright reasons Source Table 3 in Kedderis G L and C A Mugford Sex dependent metabolism of xenobiotics Drug Metab Rev 30 1998 441 498 Entries for 2A1 2A2 2A4 CYP2C 3A2 and CYP4A are highlighted Dimorphic metabolism in rats Drugs and Chemicals Showing Sex Dependent Differences in Metabolism in Rats Agent Differences Cocaine Males metabolize the agent two times faster than females Diazepam Metabolism is greater in males than females Hexobarbital Metabolism in females is slower resulting in higher blood levels a prolonged sleep time Indinavir Males metabolize the agent three times faster than females Morphine Metabolism is greater in males than females Pentobarbital Metabolism in females is slower resulting in higher blood levels a prolonged sleep time Tolbutamide Metabolism is greater in males than females Not surprising given known high dimorphism of Cyp genes but Kedderis and Mugford 1998 Figure by MIT OCW Humans also show dimorphism