Role of Cav1.3 in aldosterone secretion in normal adrenal and aldosterone-producing adenomas (APAs)

30-40% of aldosterone-producing adenomas (APAs) that histologically resemble the zona-glomerulosa (ZG) of normal adrenal cortex may harbour somatic mutations in CACNA1D, which encodes the α1-subunit of an L-type calcium channel, Cav1.3 (1). 19 single-base changes have been reported, each causing amino-acid substitutions that are rarely germline (2). Seven of the reported mutations have been demonstrated to cause a gain-of-function in the electrophysiological properties of the channel. Mutant Cav1.3 channels expressed in tsA-201 cells show an increase in the amplitude of Ca2+ current and/or alter the voltage dependence for activation/inactivation. Some of the mutant channels open at more negative membrane potentials than wild-type channels (1, 3). How these mutations affect the aldosterone secretion from adrenocortical cells has not been investigated. In this study, we report an increase in aldosterone secretion from H295R cells when transfected with a mutant Cav1.3 as compared to wild-type channels. This increase in aldosterone secretion is sensitive to inhibition of Cav1.3 by a commonly used dihydropyridine, nifedipine.

A functional calcium channel requires auxiliary β and α2δ subunits for translocation to cell membrane. The activation and inactivation properties of a channel vary according to which beta subunit is present. Our microarray analyses of APAs and adjacent adrenal cortex shows CACNB2 (β2) to be the most predominant of all four β (β1-β4) subunits in both APA and normal adrenal, with much higher expression in ZG than ZF, zona-fasciculata (4). Here we show up-regulation of CACNB2 in the ZG-like APAs (n=12) when compared to adjacent normal adrenal glands of the same patients. There was no significant difference in the CACNB2 levels in ZF-like APAs and paired adjacent normal adrenal (n=3). We have also determined the predominant CACNB2 isoforms (β2a to β2e), which influences palmitoylation and thereby possibly channel inactivation (5). Elevated CACNB2 levels could explain higher membrane translocation of mutant channels in APAs further enhancing the gain-of-function of mutant Cav1.3 and hence increased aldosterone secretion.

References-

1. Azizan EaB et al. (2013). Nat Genet 45: 1055–1060.

2. Fernandes-Rosa FL et al. (2014). Hypertension 64: 354–361.

3. Scholl UI et al. (2013). Nat Genet 45: 1050–1054.

4. Zhou J et al. (2015). Hypertension 65: 1103–1110.

5. Takahashi SX et al. (2003). Biophys J 84: 3007–3021.