Noncanonical suppression of GH-dependent isoforms of cytochrome P450 by the somatostatin analog octreotide

Early expression of requisite developmental growth hormone imprinted cytochromes P450 and dependent transcription factors

The sexually dimorphic expression of cytochromes P450 (CYP) drug metabolizing enzymes has been reported in all species examined. These sex differences are initially expressed during puberty and are solely regulated by sex differences in the circulating growth hormone (GH) profiles. Once established, however, the different male- and female-dependent CYP isoforms are permanent and immutable, suggesting that adult CYP expression requires imprinting. Since the hormone that regulates an adult function is likely the same hormone that imprints the function, we selectively blocked GH secretion in some newborn male rats while others also received a concurrent physiologic replacement of rat GH. Rats were subsequently challenged, peripubertally, with either a masculine-like episodic GH regimen or the GH vehicle alone. The results demonstrate that episodic GH regulation of male-specific CYP2C11 and CYP3A2, as well as female-predominant CYP2C6, are dependent on developmental GH imprinting. Moreover, the induction and/or activation of major components in the signal transduction pathway regulating the expression of the principal CYP2C11 isoform is obligatorily dependent on perinatal GH imprinting without which CYP2C11 and drug metabolism would be permanently and profoundly suppressed. Since there are additional adult metabolic functions also regulated by GH, pediatric drug therapy that is known to disrupt GH secretion could unintentionally impair adult health.

Feminization imprinted by developmental growth hormone

Previously, we identified early developmental exposure to growth hormone (GH) as the requisite organizer responsible for programming the masculinization of the hepatic cytochromes P450 (CYP)-dependent drug metabolizing enzymes (Das et al., 2014, 2017). In spite of the generally held dogma that mammalian feminization requires no hormonal imprinting, numerous reports that the sex-dependent regulation and expression of hepatic CYPs in females are permanent and irreversible would suggest otherwise. Consequently, we selectively blocked GH secretion in a cohort of newborn female rats, some of whom received concurrent GH replacement or GH releasing factor. As adults, the feminine circulating GH profile was restored in the treated animals. Two categories of CYPs were measured. The principal and basically female specific CYP2C12 and CYP2C7; both completely and solely dependent on the adult feminine continuous GH profile for expression, and the female predominant CYP2C6 and CYP2E1 whose expression is maximum in the absence of plasma GH, suppressed by the feminine GH profile but more so by the masculine episodic GH profile. Our findings indicate that early developmental exposure to GH imprints the inchoate CYP2C12 and CYP2C7 in the differentiating liver to be solely dependent on the feminine GH profile for expression in the adult female. In contrast, adult expression of CYP2C6 and CYP2E1 in the female rat appears to require no GH imprinting.

Growth hormone: a newly identified developmental organizer

The sexually dimorphic expression of cytochromes P450 (CYP) drug-metabolizing enzymes has been reported in all species examined. These sex differences are only expressed during adulthood and are solely regulated by sex differences in circulating growth hormone (GH) profiles. Once established, however, the different male- and female-dependent CYP isoform profiles are permanent and immutable, suggesting that adult CYP expression requires imprinting. As the hormone that regulates an adult function is likely the same hormone that imprints the function, we selectively blocked GH secretion in some newborn male rats, whereas others received concurrent physiologic replacement of rat GH. The results demonstrate that adult male GH activation of the signal transduction pathway regulating expression of the principal CYP2C11 isoform is obligatorily dependent on perinatal GH imprinting, without which CYP2C11 and drug metabolism would be permanently and profoundly suppressed. As there are other adult metabolic functions also regulated by GH, pediatric drug therapy known to disrupt GH secretion could unintentionally impair adult health.

Tacrolimus dose requirements in paediatric renal allograft recipients are characterized by a biphasic course determined by age and bone maturation

AIMS

Despite longstanding recognition of significant age-dependent differences in drug disposition during childhood, the exact course and the underlying mechanisms are not known. Our aim was to determine the course and determinants of individual relative dose requirements, during long-term follow-up in children on tacrolimus.

METHODS

This was a cohort study in a tertiary hospital with standardized annual pharmacokinetic (PK) follow-up (AUC_0-12hr ) in recipients of a renal allograft (≤19 years), between 1998 and 2015. In addition, the presence of relevant pharmacogenetic variants was determined. The evolution of dose-corrected exposure was evaluated using mixed models.

RESULTS

A total of 184 PK visits by 43 children were included in the study (median age: 14.6). AUC_0-12h corrected for dose per kg demonstrated a biphasic course: annual increase 4.4% (CI: 0.3-8.7%) until ±14 years of age, followed by 13.4% increase (CI 8.7-18.3%). Moreover, exposure corrected for dose per m² proved stable until 14 years (+0.8% annually; CI: -3.0 to +4.8%), followed by a steep increase ≥14 years (+11%; CI: 7.0-16.0%). Analysis according to bone maturation instead of age demonstrated a similar course with a distinct divergence at TW2: 800 (P = 0.01). Genetic variation in CYP3A4, CYP3A5, and CYP3A7 was associated with altered dose requirements, independent of age.

CONCLUSIONS

Children exhibit a biphasic course in tacrolimus disposition characterized by a high and stable drug clearance until a specific phase in pubertal development (TW2: 800 at age: ±14 years), followed by an important decline in relative dose requirements thereafter. Pharmacogenetic variation demonstrated an age/puberty independent effect. We suggest a critical reappraisal of current paediatric dosing algorithms for tacrolimus and drugs with a similar disposition.

Permanent uncoupling of male-specific CYP2C11 transcription/translation by perinatal glutamate

Perinatal exposure of rats and mice to the typically reported 4mg/g bd wt dose of monosodium glutamate (MSG) results in a complete block in GH secretion as well as obesity, growth retardation and a profound suppression of several cytochrome P450s, including CYP2C11, the predominant male-specific isoform--all irreversible effects. In contrast, we have found that a lower dose of the food additive, 2mg/g bd wt on alternate days for the first 9days of life results in a transient neonatal depletion of plasma GH, a subsequent permanent overexpression of CYP2C11 as well as subnormal (mini) GH pulse amplitudes in an otherwise normal adult masculine episodic GH profile. The overexpressed CYP2C11 was characterized by a 250% increase in mRNA, but only a 40 to 50% increase in CYP2C11 protein and its catalytic activity. Using freshly isolated hepatocytes as well as primary cultures exposed to the masculine-like episodic GH profile, we observed normal induction, activation, nuclear translocation and binding to the CYP2C11 promoter of the GH-dependent signal transducers required for CYP2C11 transcription. The disproportionately lower expression levels of CYP2C11 protein were associated with dramatically high expression levels of an aberrant, presumably nontranslated CYP2C11 mRNA, a 200% increase in CYP2C11 ubiquitination and a 70-80% decline in miRNAs associated, at normal levels, with a suppression of CYP2C expression. Whereas the GH-responsiveness of CYP2C7 and CYP2C6 as well as albumin was normal in the MSG-derived hepatocytes, the abnormal expression of CYP2C11 was permanent and irreversible.

Irreversible perinatal imprinting of adult expression of the principal sex-dependent drug-metabolizing enzyme CYP2C11

We proposed to determine whether, like other sexual dimorphisms, drug metabolism is permanently imprinted by perinatal hormones, resulting in its irreversible sex-dependent expression. We treated newborn male rats with monosodium glutamate (MSG), a total growth hormone (GH) blocker, and, using cultured hepatocytes, examined expression of adult CYP2C11, the predominant cytochrome-P450 expressed only in males, as well as the signal transduction pathway by which episodic GH solely regulates the isoform's expression. In addition, adolescent hypophysectomized (hypox) male rats served as controls in which GH was eliminated after the critical imprinting period. Whereas renaturalization of the masculine episodic GH profile restored normal male-like levels of CYP2C11, as well as CYP2C12, in hepatocytes from hypox rats, the cells derived from the MSG-treated rats were completely unresponsive. Moreover, GH exposure of hepatocytes from hypox rats resulted in normal induction, activation, nuclear translocation, and binding to the CYP2C11 promoter of the signal transducers mediating GH regulation of CYP2C11 expression, which dramatically contrasted with the complete unresponsiveness of the MSG-derived hepatocytes, also associated with hypermethylation of GH-response elements in the CYP2C11 promoter. Lastly, neonatal MSG treatment had no adverse effect on postnatal and adult testosterone levels. The results demonstrate that the sexually dimorphic expression of CYP2C11 is irreversibly imprinted shortly after birth by a hormone other than the customary testosterone, but likely by GH.

Growth hormone-independent suppression of growth hormone-dependent female isoforms of cytochrome P450 by the somatostatin analog octreotide

Octreotide is a potent somatostatin analog therapeutically used to treat several conditions including hyper growth hormone secretion in patients with acromegaly. We infused octreotide into female Sprague Dawley rats every 12h for 6 days at levels considerably greater than typical human therapeutic doses. Resulting circulating growth hormone profiles were characterized by ∼25% reduction in plasma levels, including both pulse and interpulse components, but still contained in an otherwise female-like "continuous" secretory profile. The normally elevated feminine expression levels (protein and/or mRNA) of CYP2C12, CYP2A1, CYP2C7 and insulin-like growth factor-1 (IGF-1), all dependent on the continuous feminine growth hormone profile, were dramatically down-regulated. Octreotide suppression of the female-dependent levels of CYPs (cytochromes P450) and IGF-1 could not be explained by the apparently inconsequential alterations in the feminine circulating growth hormone profile. In this regard, somatostatin and its analogs are known to have a myriad of extra-pituitary actions effecting nearly all tissues in the body. Focusing our attention on CYP2C12, accounting for >40% of the total hepatic cytochrome P450 content in the female rat liver, we found a ∼4-fold increase in hepatic ubiquitin-CYP2C12 levels in octreotide treated rats suggesting a possible contributing factor for the >60% suppression of CYP2C12 protein concentrations.