Glucocorticoid block of protein kinase C signalling in mouse pituitary corticotroph AtT20 D16:16 cells

Abstract

The regulation of large conductance calcium‐ and voltage‐activated potassium (BK) currents by activation of the protein kinase C (PKC) and glucocorticoid signalling pathways was investigated in AtT20 D16:16 clonal mouse anterior pituitary corticotroph cells. Maximal activation of PKC using the phorbol esters, 4β‐phorbol 12‐myristate, 13‐acetate (PMA), phorbol 12, 13 dibutyrate (PDBu) and 12‐deoxyphorbol 13‐phenylacetate (dPPA) elicited a rapid, and sustained, inhibition of the outward steady‐state voltage‐ and calcium‐ dependent potassium current predominantly carried through BK channels. The effect of PMA was blocked by the PKC inhibitors bisindolylmaleimide I (BIS; 100 nM) and chelerythrine chloride (CHE; 25 μM) and was not mimicked by the inactive phorbol ester analogue 4α‐PMA. PMA had no significant effect on the 1 mM tetraethylammonium (TEA)‐insensitive outward current or pharmacologically isolated, high voltage‐activated calcium current. PMA had no significant effect on steady‐state outward current in cells pre‐treated for 2 h with 1 μM of the glucocorticoid agonist dexamethasone. Dexamethasone had no significant effect on steady‐state outward current amplitude or sensitivity to 1 mM TEA and did not block PMA‐induced translocation of the phorbol ester‐sensitive PKC isoforms, PKCα and PKCε, to membrane fractions. Taken together these data suggest that in AtT20 D16:16 corticotroph cells BK channels are important targets for PKC action and that glucocorticoids inhibit PKC signalling downstream of PKC activation.

Large-conductance calcium-activated potassium channel activity is absent in human and mouse neutrophils and is not required for innate immunity

“…In contrast to iberiotoxin, PMA is not a classical pharmacological tool to study BK channels. The action of PMA is variable, ranging from channel activation (6) to inhibition (46), depending on the cell type. The reported effects were not associated with a rise in [Ca 2ϩ ] i but, rather, with PKC-dependent protein phosphorylation.…”

“…The effects of PMA, a well-known PKC activator (39), on BK channel activity are not clear: inhibition (46) and activation have been reported (6). Ahluwalia et al (1) found that PMA increased [Ca 2ϩ ] i in human neutrophils and, therefore, activated BK channels that are Ca 2ϩ sensitive.…”

Ion Channels and Signaling in the Pituitary Gland

“…In these cells, glucocorticoids rapidly activate BK channels via a nongenomic mechanism (195). Glucocorticoids applied for 2 h also promote de novo BK mRNA and protein synthesis and antagonize PKA-and PKC-dependent inhibition of BK channels (188,196). Further studies revealed that glucocorticoid regulation of BK channels in these cells is mediated by serine/ threonine protein phosphatase (197) and that alternative splicing determines sensitivity of BK channels to glucocorticoids and switches their sensitivity to protein phosphorylation (198,199).…”

Calcium-activated potassium channel of the tobacco hornworm,Manduca sexta: molecular characterization and expression analysis

“…For example, adrenal steroids downregulate BK channel currents in hippocampal neurons (Kerr et al, 1989) and can mediate the inclusion of the stress axis regulated exon (STREX) in adrenal chromaffin cells (Shipston et al, 1996;Xie and McCobb, 1998;Tian et al, 1999). Fluctuations in estrogen and/or progesterone during pregnancy may influence alternative exon splicing in mouse uterine muscle, altering muscle contractility (Benkusky et al, 2000).…”