Regulation of S6 kinase activity in Madin-Darby canine kidney renal epithelial cells

An intracellular calcium signal activates p70 but not p90 ribosomal S6 kinase in liver epithelial cells

In the rat liver epithelial cell lines GN4 and WB, angiotensin II (Ang II) activates the Gq class of regulatory G-proteins, increasing intracellular calcium, protein kinase C activity, and protein tyrosine phosphorylation. We compared the ability of Ang II and other compounds that increase intracellular calcium (i.e. the calcium ionophore A23187 and thapsigargin) or protein kinase C activity (the phorbol ester 12-O-tetradecanoylphorbol-13-acetate) to activate p70 ribosomal S6 kinase (p70(S6K)) and p90 ribosomal S6 kinase (p90(RSK)). In GN4 cells, increasing intracellular calcium stimulated p70(S6K) activity in a rapamycin- and wortmannin- sensitive manner, but did not affect p90(RSK) activity. In contrast, 12-O-tetradecanoylphorbol-13-acetate strongly activated p90(RSK) but only weakly stimulated p70(S6K). The ability of calcium to activate p70(S6K) was confirmed by blocking the A23187-dependent activation through chelation of extracellular calcium with EGTA; the effect of thapsigargin was inhibited by the cell permeant chelator bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester (BAPTA-AM). Similarly, BAPTA-AM prevented the activation of p70(S6K) by Ang II, suggesting that this signal was largely calcium-dependent. In contrast, the Ang II-dependent activation of mitogen-activated protein kinase and p90(RSK) was not inhibited but was enhanced by BAPTA-AM. These results show that in GN4 cells, Ang II selectively activates p70(S6K) through effects on calcium, p90(RSK) through effects on protein kinase C. The activation of p70(S6K) by calcium stimuli or Ang II was independent of calmodulin but correlated well with the activation of the recently identified, nonreceptor calcium-dependent tyrosine kinase (CADTK)/PYK-2. Both calcium- and Ang II-dependent activation of p70(S6K) were attenuated by the tyrosine kinase inhibitor genistein, and activation of p70(S6K) was higher in GN4 than WB cells, correlating with the increased expression and activation of CADTK/PYK-2 in GN4 cells. In summary, these results demonstrate that intracellular calcium selectively activates p70(S6K) in GN4 cells, consistent with increased CADTK/PYK-2 signaling in these cells.

Effects of endothelin on mitogen-activated protein kinase activity and protein synthesis in isolated adult feline cardiac myocytes

The growth-promoting effects of endothelin-1 (ET-1) were examined in adult heart cells. The activity of mitogen-activated protein kinases (MAPKs) was measured in cytosolic extracts of isolated adult feline cardiac myocytes incubated with and without ET-1. Kinase activity was assessed by phosphorylation of the exogenous substrate, myelin basic protein. ET-1 stimulated the activity of MAPK up to 4-fold, with peak activation occurring between five and ten minutes after addition of ET-1. Polyclonal antisera raised against a 14-amino acid sequence of the erk-2 gene product, a MAPK isoform, identified two major bands in cytosolic extracts of the cardiac myocytes. Partial purification of kinase activities using Mono Q anion-exchange chromatography demonstrated two major peaks of myelin basic protein kinase activity. Subsequent immunoblots of the eluted fractions demonstrated that the immunoreactive bands observed in the cytosolic extracts eluted in those fractions possessing kinase activity. Overnight pretreatment of the cardiac myocytes with 100 ng/ml pertussis toxin inhibited the ET-1 stimulated increase in MAPK activity by 50 - 70%, but did not alter stimulation by 100 nM phorbol 12-myristate 13-acetate (PMA). These data suggest that stimulation of MAPK by ET-1 may be mediated by more than one pathway. MAPK has been shown to be activated in the intracellular transmission of growth factor signals. Indicative of a growth effect in this adult heart cell model, myocytes exposed to increasing concentrations of ET-1 demonstrated a dose dependent increase in [3H]-phenylalanine incorporation into cellular protein. This response was blocked by staurosporine and partially inhibited by pretreatment with pertussis toxin, again suggesting the possible involvement of multiple early signals. These data from isolated adult cardiac myocytes further support the hypothesis that ET-1 has a role in the regulation of cardiac growth.

Monoclonal antibody to the human insulin receptor, but not insulin, stimulates S6 kinase via human insulin receptors mutated at three major tyrosine autophosphorylation sites

Studies were carried out to examine the role of the major insulin receptor tyrosine autophosphorylation sites in stimulation of S6 kinase activity. For these studies, we employed HTC rat hepatoma cells transfected with and expressing human insulin receptors. In cells transfected with and expressing a large number of normal human insulin receptors (HTC-IR cells), the sensitivity of cells to insulin to stimulate S6 kinase was increased tenfold when compared to untransfected wild type HTC cells (HTC-WT cells). However, in cells transfected with and expressing a large number of mutated human insulin receptors where the tyrosines at three major autophosphorylation sites (1158, 1162, and 1163) were mutated to phenylalanines (HTC-F3 cells), there was no change in insulin sensitivity when compared to HTC-WT cells. We next studied the effect of a human-specific monoclonal antibody to the human insulin receptor, MA-5, on S6 kinase activation. In HTC-WT cells, MA-5 did not interact with endogenous rat insulin receptors and thus did not stimulate S6 kinase. In HTC-IR cells expressing normal human insulin receptors, MA-5 stimulated S6 kinase. Interestingly, MA-5, unlike insulin, was also able to stimulate S6 kinase in HTC-F3 cells expressing mutated receptors. In order to further understand the signaling mechanisms by MA-5 and insulin, two potential intermediate protein kinases were investigated. Neither insulin nor MA-5 appears to activate either microtubule-associated protein 2 (MAP-2) kinase or protein kinase C in these cells. These studies suggest therefore that: 1) insulin and MA-5 may signal S6 kinase activation by independent mechanisms that do not employ either MAP-2 kinase or protein kinase C; and 2) under certain circumstances, S6 kinase appears to be activated by mechanisms that are independent of insulin receptor tyrosine autophosphorylation.

Induction of Na(+)-K(+)-ATPase subunit mRNAs by cycloheximide in a rat liver cell line

Exposure of confluent Clone 9 cells to 40 microM cycloheximide (CHX), a concentration sufficient to inhibit leucine incorporation by 95% within 5 min, coordinately increased the abundances of Na(+)-K(+)-ATPase subunit mRNAs, mRNA alpha 1 and mRNA beta 1. The CHX-induced increases in mRNA alpha 1 and mRNA beta 1 abundances were, respectively, 1.8- and 1.9-fold at 40 min and 3.0- and 3.3-fold at 6 h. Augmented subunit mRNA contents were also observed after exposure to other protein synthesis inhibitors including 100 microM anisomycin and 100 microM emetine. Upon removal of CHX, the rate of leucine incorporation returned to control values within 1 h, but mRNA alpha 1 and mRNA beta 1 content decreased only slowly and were still elevated at 24 h at 1.7- and 1.8-fold the respective control values. Despite the persistence of increased levels of the subunit mRNAs and normalization of the rate of leucine incorporation, Na(+)-K(+)-ATPase activity was unchanged at 3, 6, 24, and 48 h after removal of CHX. In cells "depleted" of protein kinase C (PKC) activity after a 24-h preincubation in the presence of 160 nM 12-O-tetradecanoylphorbol-13-acetate (TPA), mRNA alpha 1 and mRNA beta 1 abundances were still inducible by CHX. It is concluded that exposure of Clone 9 cells to CHX and other inhibitors of protein synthesis results in increased abundances of Na(+)-K(+)-ATPase subunit mRNAs independently of PKC activation.(ABSTRACT TRUNCATED AT 250 WORDS)

cDNA encoding a 59 kDa homolog of ribosomal protein S6 kinase from rabbit liver

We have isolated cDNA molecules encoding a protein with the characteristic sequence elements that are conserved between the catalytic domains of protein kinases. This protein is apparently a serine/threonine kinase and is most closely related to the amino-terminal half of the ribosomal protein S6 kinase II first characterized in Xenopus eggs (42% overall identity and 56% identity in the predicted catalytic domain). However, it clearly differs from S6 kinase II in that it has only one, rather than two predicted catalytic domains and a deduced molecular mass of 59,109 Da. We propose that is may be more related to, or identical, with, the mitogen-inducible S6 kinase of molecular mass 65-70 kDa described in mammalian liver, mouse 3T3 cells and chicken embryos. Remarkable structural features of the cDNA-encoded polypeptide are a section rich in proline, serine and threonine residues that resemble the multiphosphorylation domains of glycogen synthase and phosphorylase kinase alpha subunit, and a characteristic tyrosine residue in the putative nucleotide-binding glycine cluster which, by analogy to cdc2 kinase, is a potential tyrosine phosphorylation site.