Rapid and sustained phosphorylation of a calmodulin-binding protein (CaM-BP100) in NGF-treated PC12 cells

ERK mediates inhibition of Na(+)/H(+) exchange and HCO(3)(-) absorption by nerve growth factor in MTAL

Mitogen-activated protein (MAP) kinases mediate a variety of critical cellular events, but their role in the regulation of epithelial transport is largely undefined. Recently, we demonstrated that nerve growth factor (NGF) inhibits HCO(3)(-) absorption in the rat medullary thick ascending limb (MTAL) through an unusual mechanism: 1) NGF inhibits basolateral membrane Na(+)/H(+) exchange activity, an effect opposite to the stimulation of Na(+)/H(+) exchange by growth factors in other cells; and 2) inhibition of basolateral Na(+)/H(+) exchange results secondarily in inhibition of apical Na(+)/H(+) exchange, thereby inhibiting HCO(3)(-) absorption. In this study, we examined the role of MAP kinases in mediating inhibition by NGF. In tissue strips from the inner stripe of the outer medulla and in microdissected MTALs, NGF increased extracellular signal-regulated kinase (ERK) activity twofold but had no effect on c-Jun NH(2)-terminal kinase (JNK) or p38 MAP kinase activity. The selective MAP kinase kinase (MEK1/2) inhibitors U0126 and PD-98059 abolished the NGF-induced ERK activation and largely eliminated (> or = 60%) the effects of NGF to inhibit basolateral Na(+)/H(+) exchange activity and transepithelial HCO absorption in perfused MTALs. The MEK1/2 inhibitors did not affect inhibition of HCO(3)(-) absorption by bath ethylisopropyl amiloride, indicating that ERK activation is not involved in mediating interaction between the basolateral and apical Na(+)/H(+) exchangers. These results demonstrate that NGF inhibits basolateral Na(+)/H(+) exchange activity and HCO(3)(-) absorption in the MTAL through activation of the ERK signaling pathway. These findings identify a novel action of ERK to inhibit Na(+)/H(+) exchange activity and establish a role for MAP kinase pathways in the acute regulation of Na(+)/H(+) exchange activity and transepithelial acid secretion in renal tubules.

Nucleolin is a protein kinase C-zeta substrate. Connection between cell surface signaling and nucleus in PC12 cells

We have previously shown that protein kinase C (PKC)-zeta is activated and required for nerve growth factor (NGF)-induced differentiation of rat pheochromocytoma PC12 cells (Wooten, M. W., Zhou, G., Seibenhener, M. L., and Coleman, E. S. (1994) Cell Growth & Diff. 5, 395-403; Coleman, E. S., and Wooten, M. W. (1994) J. Mol. Neurosci. 5, 39-57). Here we report the characterization and identification of a 106-kDa nuclear protein as a specific substrate of PKC-zeta. NGF treatment of PC12 cells resulted in translocation of PKC-zeta and coincident phosphorylation of a protein that was localized within the nucleoplasm of nuclei isolated from PC12 cells. Addition of PKC-zeta pseudosubstrate peptide in vitro or myristoylated peptide in vivo diminished phosphorylation of pp106 in a dose-dependent fashion. Likewise, addition of purified PKC-zeta, but neither PKC-alpha nor delta, to nuclear extracts resulted in an incremental increase in the phosphorylation of pp106. Expression of dominant-negative PKC-zeta inhibited NGF-induced phosphorylation of pp106, by comparison overexpression of PKC-zeta enhanced basal phosphorylation without a noticeable effect upon NGF-induced effects. Amino acid sequence analysis of four peptides derived from purified pp106 revealed that this protein was homologous to nucleolin. Using an in vitro reconstitution system, purified nucleolin was likewise shown to be phosphorylated by purified PKC-zeta. The staining intensity of both enzyme and substrate in the nucleus increased upon treatment with NGF. In vivo labeling with 32Pi and stimulation of PC12 cells with NGF followed by immunoprecipitation with anti-nucleolin antibody corroborated the in vitro approach documenting enhanced phosphorylation of nucleolin by NGF treatment. Taken together, the findings presented herein document that nucleolin is a target of PKC-zeta that serves to relay NGF signals from cell surface to nucleus in PC12 cells.

Calmodulin is the divalent cation receptor for rapid endocytosis, but not exocytosis, in adrenal chromaffin cells

Exocytosis and the ensuing rapid endocytosis in adrenal chromaffin cells are both Ca(2+)-dependent phenomena but differ in their divalent cation specificity, implying distinct Ca2+ receptors for the two processes. To ascertain whether calmodulin is the Ca2+ receptor for either process, we blocked its function by introducing calmodulin-binding peptides or anti-calmodulin antibodies into these cells. Exo/endocytosis was followed by measurement of cell membrane capacitance. Rapid endocytosis, but not exocytosis, was abolished by these treatments, indicating that calmodulin is the Ca2+ receptor for rapid endocytosis but is not involved in exocytosis. The principal calmodulin target is not protein phosphatase-2B, as antagonism of this enzyme did not inhibit but accelerated rapid endocytosis. Calmodulin may thus regulate both the rate and extent of rapid endocytosis by distinct pathways.