The alveolar stage of lung development represents a particularly vulnerable period in lung maturation. At birth, failure to successfully adapt to air leads to respiratory distress and may be associated with long-term sequelae (BPD). A growing body of evidence points to roles of GC, RA, and VitD in regulating both lung maturation and inflammation in the newborn period. Our studies focused on the regulation of a single gene, Lgl1, while at the same time providing novel information about the complex interactions of GC, RA, and VitD in the regulation of postnatal lung development.
Information regarding the direct molecular targets of GC and RA in lung development is limited. We were the first to report on the coincident regulation of midkine by GC and RA, providing a potential mechanism for the integration of GC and RA effects on fetal lung development (20
). We now report on the coordinate transcriptional regulation of Lgl1
by GC, RA, and VitD in the postnatal lung. We show that the interaction of GC and RA on Lgl1
expression can be additive or antagonistic and that this interaction is dependent on both duration of exposure and developmental stage. Each of VitD and RA is inhibitory when present alone, but together they have no effect on the induction of Lg1l
by GC, a finding with pharmacological implications.
GC is the most potent known modulator of Lgl1
, stimulating expression over 100 fold at PN4. The sensitivity of Lgl1
to GC may account in part for the high levels of Lgl1
maintained in rat lung (12
) from PN7–PN14 when circulating levels of GC are low (21
). The biphasic effects of 9-cis-RA and ATRA between PN7 and PN10 are also of interest. During this period of increasing levels of active forms of retinoid, these agents can have opposing effects, stimulating Lgl1
expression after a short exposure (24h) and suppressing Lgl1
levels after longer exposure.
Postnatal GC treatment inhibits alveolarization (22
). In animal models, some of the detrimental effects of GC on lung development may be ameliorated by simultaneous RA treatment (23
). However, RA does not normalize Dex-induced changes in lung function (25
). The potential role of VitD as a modulator of Lgl1
in postnatal lung is also of interest. To our knowledge, this is the first report of VitD modulating mesenchymal gene expression in developing lung. We speculate that Lgl1
may be a mesenchymal mediator of VitD effects on epithelial maturation.
GC, RA, and VitD regulation of pulmonary development likely operates via both direct and indirect mechanisms. We believe that this is the first example of coordinate temporal regulation of target gene promoter activity by GC, RA, and VitD during lung development. Interactions between RA and VitD increase combinatorial possibilities for gene regulation of Lgl1
by VDR, RXR, and RAR (26
). In the presence of increased concentrations of these hormones, steric hindrance by VDR/RXR heterodimers may compromise the effects of either agent alone.
The enrichment of consensus binding sites for GR, VDR, RXR, and RAR in the promoter region of genes essential to normal human lung development suggests that these agents are likely to impact multiple critical developmental processes in the lung. The proximity of GR and VDR/RXR or VDR/RAR sites offers a context for complex fine-tuning of such regulatory mechanisms.
Deficiency of Lgl1
in the newborn period is associated with features of neonatal lung injury in rat (12
is an essential gene, required prior to lung formation (14
). Haplo-insufficiency for Lgl1
in knockout mice is associated with a complex respiratory phenotype including delayed histological maturation, features of inflammation in the newborn period and altered lung mechanics at maturity. Hormones that regulate lung development also function as mediators of inflammation. Imbalance of these factors in the newborn period is likely to alter respiratory health. The effects of GC and RA in this period are well established. It is also now becoming clear that VitD is an important modulator of immune / inflammatory response (27
). VDR expression in the lung microenvironment is required
for maximal induction of lung inflammation (27
Specific interventions that augment pulmonary maturation in a temporally appropriate manner have the potential to ameliorate hyperoxic lung injury. Abnormal lung development is also a susceptibility factor for respiratory diseases that present later in life, including COPD and asthma (29
). As such, our findings may also have implications for long-term respiratory health.