Endocrine Abstracts

In addition to their essential role in fetal lung development, glucocorticoids promote late-gestation maturation of the fetal heart as shown by severe functional impairment, structural disorganisation and transcriptome immaturity in fetal hearts of glucocorticoid receptor-deficient (GR-/-) mice. Here we use primary fetal mouse cardiomyocytes (E16-E17.5) to investigate whether these effects of glucocorticoids result from direct actions of GR and whether they are independent of other tissues or systems. In primary murine fetal cardiomyocytes (>98% troponin T-positive cells), both dexamethasone and physiological concentrations of corticosterone dose-dependently up-regulated the expression of mRNA encoding glucocorticoid-induced leucine zipper (GILZ; a classic glucocorticoid-induced gene), myosin heavy chain-α (MyHCα, the major contractile isoform in adult hearts), atrial natriuretic peptide (ANP; increased in late gestation heart) and peroxisome proliferator-activated receptor-α coactivator 1α (PGC-1γ, master mitochondrial regulator), all previously shown to be expressed at lower levels in GR-/- fetal heart. Glucocorticoid-induction was attenuated by siRNA-mediated knock-down of GR and blocked by the receptor antagonist RU-38486 (mifepristone) confirming GR as a crucial mediator. Moreover, glucocorticoid-induction of GILZ and PGC1-1α mRNA was maintained in the presence of cycloheximide (protein biosynthesis blocker), suggesting these genes are direct targets of GR in cardiomyocytes. In contrast transcriptional increases of MHCα and ANP were abolished by cycloheximide indicating their regulation is secondary to new protein synthesis. Additionally, corticosterone induced formation of troponin T-associated α-actin stress fibers and caused a dramatic change in myofibril organisation towards more mature, as shown by staining of α-actinin, a sarcomeric marker for myofibrilar Z-discs. Thus, glucocorticoid treatment of fetal cardiomyocytes in vitro mimics biochemical and structural maturational changes seen in vivo. Inhibitor studies implicate GR and not MR as the major mediator of prenatal glucocorticoid effects in heart.

Declaration of interest: There is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.

Funding: Declaration of Funding: BHF 4-year scholarship FS/08/065.