Deranged Kv channel regulation in fibroblasts from mice lacking the serum and glucocorticoid inducible kinase SGK1

Coexpression of the serum and glucocorticoid inducible kinase 1 (SGK1) up‐regulates Kv channel activity in Xenopus oocytes and human embryonic kidney cells. To investigate the physiological impact of SGK1 dependent Kv channel regulation, we recorded whole‐cell currents in lung fibroblasts from SGK1 knockout mice (sgk1−/−) and wild‐type littermates (sgk1+/+). Serum‐grown mouse lung fibroblasts (MLF) from both genotypes exhibited voltage‐gated outwardly rectifying K+‐currents with time‐dependent activation (τact∼3 msec), slow inactivation (τinact∼700 msec), use‐dependent inactivation, and (partial) inhibition by K+ channel blockers TEA, 4‐AP, and margatoxin. In serum grown MLF peak Kv current density at +100 mV was significantly lower in sgk1−/− (14 ± 2 pA/pF, n = 13) than in sgk1+/+ (31 ± 4 pA/pF, n = 16). PCR amplification of different Kv1 and Kv3 subunits from mouse fibroblasts demonstrated the expression of Kv1.1‐1.7, Kv3.1, and Kv3.3 mRNA in both sgk1+/+ and sgk1−/− cells. Upon serum deprivation Kv currents almost disappeared in sgk1+/+ (4 ± 1 pA/pF, n = 11) but not in sgk1−/− (10 ± 1 pA/pF, n = 6) MLF. Accordingly, following serum deprivation Kv current density was significantly lower in sgk1+/+ than in sgk1−/−. Stimulation of serum‐depleted cells with dexamethasone (dex) (1 μM, 1 day), IGF‐1 (6.7 μM, 4–6 h) or both, significantly activated Kv currents in sgk1+/+ but not in sgk1−/− MLF. In the presence of both, dex and IGF‐1, the Kv current density was significantly larger in sgk1+/+ (27 ± 3 pA/pF, n = 12) than in sgk1−/− (13 ± 3 pA/pF, n = 10) cells. Similar to MLF, Kv currents were significantly higher in sgk1+/+ mouse tail fibroblasts (MTF). In sgk1+/+ but not sgk1−/− MTF the Kv currents were inhibited upon serum deprivation and reincreased after stimulation of serum deprived MTF with dex (1 μM, 1 day) and afterwards with IGF‐1 (6.7 μM, 4–6 h). According to Fura‐2‐fluorescence capacitative Ca2+ entry was lower in sgk1−/− MTF compared to sgk1+/+ MTF. Upon serum deprivation capacitative Ca2+ entry decreased significantly in sgk1+/+ but not in sgk1−/− MTF. Stimulation of depleted cells with dex (1 μM, 1 day) and afterwards with IGF‐1 (6.7 μM, 4–6 h) reincreased capacitative Ca2+ entry in sgk1+/+ MTF, whereas in sgk1−/− cells it remained unchanged. In conclusion, lack of SGK1 does not abrogate Kv channel activity but abolishes regulation of those channels by serum, glucocorticoids and IGF‐1, an effect influencing capacitative Ca2+ entry. © 2004 Wiley‐Li