This is the first report to reveal a relationship between early postnatal urinary F/E ratio and morbidity in preterm infants. Serum cortisol levels are mainly regulated by the HPA axis. However, during development, the foetal adrenal gland is not highly capable of producing its steroids because of the reduced activity of steroid synthetic enzymes, including 3β-hydroxysteroid dehydrogenase (3βHSD), and foetal adrenal function is dependent on maternal adrenal function and placenta [8,9]. Maternal glucocorticoid concentrations during this period are approximately 5–10 times higher than those of the foetus, mainly due to the activity of 11βHSD type 2 in the placenta, which regulates steroid exposure to the foetus. During development, 11βHSD type 2 is highly expressed in both the foetal and placental tissues [10-13]. However, preterm infants who are born unexpectedly at this time are more likely to have impaired adrenal function, which can be associated with severe neonatal conditions such as hypotension [14-16].
In addition to the regulation of adrenal function by the HPA axis, the intensity of cortisol action in individual cells is precisely controlled by the balance of the activity between the intracellular glucocorticoid activating enzyme 11βHSD type 1 and the inactivating enzyme 11βHSD type 2 [17]. In most cells, 11βHSD types 1 and 2 coexist, with 11βHSD type 1 being highly expressed in the liver, adipose tissue, central nervous system, and skeletal muscle, and 11βHSD type 2 in the renal tubular epithelium, colon, sweat glands, and placenta, which are involved in water and electrolyte metabolism [18]. 11βHSD types 1 and 2 act as pre-receptor gateways for the glucocorticoid receptor (GR), a system that fine-tunes the intensity of glucocorticoid action on a cell-by-cell basis. Since 11βHSD type 1 is predominantly responsible for reactivating cortisone to cortisol in vivo, it can be regarded as a local amplification mechanism of glucocorticoid action. In contrast, 11βHSD type 2 is an enzyme that inactivates cortisol to cortisone and is expressed predominantly in the distal nephron in adults, suppressing the binding of cortisol to the mineralocorticoid receptor (MR) in the kidney. Cortisol also acts as a ligand for the MR and has a comparable affinity to aldosterone. Apparent mineralocorticoid excess syndrome (AME; OMIM #218030) is caused by an abnormality in 11βHSD type 2, which fails to inactivate cortisol in the kidney and binds to the MR, resulting in excessive activation of the MR by glucocorticoids, leading to AME [3,19].
The urinary F/E ratio is used as an alternative indicator of renal 11βHSD type 2 activity in the diagnosis of AME [3]. However, in foetuses and neonates, renal 11βHSD type 2 activity is less than one-tenth of that in adults, and unlike in adults, the foetus converts cortisol to cortisone in many tissues [11]. Therefore, the urinary F/E ratio in the neonatal period may reflect the total activity of 11βHSD types 1 and 2 not only in the kidneys, but also in the whole neonatal body and may be an indicator of local steroid action in the whole body. 11βHSDs are expressed in many foetal cells during this period and, in general, prenatal exposure to endogenous and exogenous glucocorticoids promotes maturation of foetal organs in exchange for overall growth [20,21]. Glucocorticoid exposure in the cells is precisely controlled for each tissue maturation. We hypothesised that urinary F/E ratios at birth could be used to infer local glucocorticoid effects, organ maturation, and adrenal maturation after birth, and to predict the impact on subsequent morbidity.
In the univariate analysis of this study, a higher urinary F/E ratio in the early postnatal period was associated with a longer duration of ventilatory care and a higher incidence of chronic lung disease, symptomatic PDA, and hypotension requiring inotropes and steroids after birth. The results of this study suggest that the high urinary F/E ratio (i.e. low 11βHSD type 2 activity) in preterm infants in the early postnatal period is related to the reduced activity of local steroids in each organ. The fact that hypotension in the early postnatal period requiring steroids was significantly more common in the group with a higher F/E ratio in multivariate analysis supports this conjecture. Since several respiratory and circulatory conditions were affected as revealed by the univariate analysis, it was suggested that the F/E ratio in early neonates may estimate the maturation of each organ of the neonate and the maturation of the HPA axis. The present results indicate that there is no correlation between the F/E ratio in the early postnatal period and placental HSD11b2 expression. Therefore, the relationship between each estimation was independent, and the degree of maternal glucocorticoid transfer had less effect on the HPA axis in infants. In other words, it should be noted that the regulation of the intensity of glucocorticoid action in the local organs of the neonate can affect the infant’s condition. Based on these results, we speculate the following pathophysiology.
First, there may be inadequate glucocorticoid production in response to the infant's medical condition after birth, resulting in relative adrenal insufficiency, which may enhance the action of local glucocorticoids [14,22]. Previous reports have indicated that insufficient adrenal function in preterm infants causes poor neonatal outcomes because of the immaturity of adrenal enzymes in preterm infants [14]. However, the absence of a correlation between the F/E ratio and DHEA, a precursor of cortisol, suggests that the F/E ratio at birth is not only related to adrenal immaturity at birth, but also to poor responsiveness of the HPA axis. The fact that urinary F/E showed an immediate decrease with postnatal age in the present study, similar to that reported by Midgley et al. [23], may be because of the enhanced local action that becomes unnecessary as the HPA axis matures. In order to compensate for the poor responsiveness of the HPA axis, local steroid action may be enhanced, thus reducing 11βHSD type 2 activity. Heckmann et al. reported similar actions in cardiac paediatric surgery. Our results are in agreement with that of these studies [24].
Second, we considered the immaturity of the regulation of 11βHSD type 2 activity in the kidney. As mentioned above, in the kidney, local 11βHSD type 2 inactivates glucocorticoids, which are also ligands for MR, preventing them from incorrectly activating the MR. However, it has been reported that preterm infants have a low capacity to produce aldosterone but a high expression of MR [25-27]. The low renal activity of 11βHSD type 2 in preterm infants (one-tenth of that in adults) may be due to the effect of glucocorticoids on the MR, compensating for the low aldosterone levels. Hypotension requiring steroids in the early postnatal period in preterm infants is not accompanied by classical adrenal insufficiency findings such as hypoglycaemia, hyponatraemia, and hyperkalaemia, while small doses of hydrocortisone are effective [14]. This phenomenon may be due to the effect of glucocorticoids on MR in the kidneys.
In the present study, the relationship between placental 11βHSD type 2 gene expression and neonatal morbidity could not be examined due to the small number of samples. The urinary F/E ratio in the early postnatal period did not correlate with placental 11βHSD type 2 gene expression and did not reflect placental 11βHSD type 2 activity, suggesting that the urinary F/E ratio can be evaluated independently without placental 11βHSD type 2 activity. The urinary F/E ratio is the most convenient means of measuring the activity of 11βHSD types 1 and 2. However, there are several caveats to the use of these urinary steroid ratios: 1) it is the sum of both 11β-HSD activities; 2) the ratio is also influenced by 5α- and 5β-reductase activities; and 3) the ratio does not represent a tissue-specific effect but reflects a systemic effect.
In conclusion, the premature infants suffered several conditions, especially respiratory and circulatory problems, did not only show an increased adrenal stress response, but also an enhancement of glucocorticoid action in local organs due to the reduction in cortisol inactivation. The urinary F/E ratio in the early postnatal period in preterm infants contribute to the understanding of the pathogenesis of the infant condition after birth by estimating the amount of local steroid action in the organs.