Second messenger-specific protein kinases in a salt-absorbing intestinal epithelium

An avian sodium-hydrogen exchanger

Intestinal sodium transporters, such as the Na+/H+ exchanger (NHE) are important for Na+ conservation in land birds. In mammals, at least five isoforms of the exchanger, NHEs 1-5, have been cloned, with NHE-1 occurring in epithelial basolateral and nonepithelial cell membranes and NHE-3 being restricted to epithelial apical/brush border membranes. We had demonstrated earlier that chicken intestinal brush border membranes possess NHE activity that functionally resembles mammalian NHE-3. In this study, we used mammalian NHE-1 and NHE-3 probes to examine if chicken enterocytes possess these transporters. Antisera against rat NHE-3 recognized a 97 kDa protein in chicken intestinal brush border membrane, while a NHE-3 cDNA probe failed to recognize any transcript. A NHE-1 antibody failed to recognize any protein in brush border or basolateral membrane, while a NHE-1 cDNA probe recognized a 3.9 kb transcript. Thus, there is more than one NHE isoform in chicken intestine, and our results suggest a novel avian NHE family.

Protein kinase C regulates prairie dog gallbladder ion transport

BACKGROUND

Gallstone formation is characterized by increased biliary calcium (Ca2+) level and altered gallbladder absorption. Recent studies suggest that luminal Ca2+ regulates gallbladder ion transport via intracellular calcium ([Ca2+]ic). Ca2+-calmodulin and protein kinase C (PKC) are two major systems through which [Ca2+]ic carries out second-messenger functions in many cell types. We have previously shown that Ca2+-calmodulin regulates basal gallbladder ion transport in prairie dog. The present study tests the hypothesis that PKC is also essential in regulation of gallbladder ion transport in this model.

METHODS

The role of PKC in regulation of gallbladder ion transport was determined by studying the effects of phorbol esters, synthetic analogues of diacylglycerol, which directly activates PKC. Gallbladders were mounted in Ussing chambers, and standard electrophysiologic parameters were recorded after exposing tissues to either 10(-5) mol/L of 4-alpha-phorbol 12,13-didecanoate (PDD), 4-beta-phorbol 12-myristate 13-acetate, 4-beta-phorbol 12,13-dibutyrate (PDB), or 10(-4) mol/L serotonin. Unidirectional Na+, Cl-, and H2O fluxes were measured before and after treatment with only inactive PDD and most active PDB.

RESULTS

Mucosal and serosal exposure of tissues to either 4-beta-phorbol 12-myristate 13-acetate or PDB resulted in a decrease in short-circuit current and transepithelial potential difference without any change in tissue resistance. Serotonin induced similar changes in gallbladder electrical properties. PDB caused an inhibition of mucosal to serosal fluxes of Na+, Cl-, and H2O, with a decrease in net Na+ absorption, an increase in net Cl- secretion, and a conversion of net H2O absorption to net H2O secretion. Serosal-to-mucosal fluxes of Na+, Cl-, and H2O did not change. Inactive PDD had no effect on either electrophysiologic parameters or ion and water fluxes. Pretreatment of tissues with PKC antagonist 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine blocked the phorbol ester-induced inhibition of ion transport.

CONCLUSION

PKC regulates gallbladder ion transport in the prairie dog by inhibiting Na+ absorption and stimulating Cl- secretion.

Phorbol dibutyrate-specific protein phosphorylation in brush border membranes of chicken enterocytes

We have demonstrated that phorbol esters such as phorbol dibutyrate (PhE) transiently inhibit Na/H exchange both in intact avian enterocytes and in brush border membrane (BBM) vesicles prepared from enterocytes treated with PhE (Chang et al., 1991, Am. J. Physiol. 260: C1264-C1272). Maximal inhibition occurs at 90 sec and values return to baseline by 15 min. In this study we examined if PhE causes changes in BBM protein phosphorylation by two methods: 1) in situ phosphorylation in which intact cells prelabeled with 32Pi were treated with PhE; 2) in vitro phosphorylation in which BBM, isolated from untreated and PhE-treated enterocytes, were exposed to gamma 32P-ATP. In situ phosphorylation studies showed that, at 90 sec, PhE increases the phosphorylation of BBM proteins of M(r) (pI): 150 (6.5), 89 (approximately 6.2), and 48 (approximately 6.1) kDa which declined to control values at 15 min, suggesting that these may be transport-related substrates. These labeled substrates were recovered in the detergent-insoluble fraction after extraction with 0.1% Triton X-100 overnight. Transient phosphorylation of a number of proteins was also observed when BBM prepared from control or PhE-treated cells were incubated with gamma 32P-ATP +/- 10 nM PhE, phosphatidyl serine, Ca2+, and/or exogenous protein kinase C (PKC). The in vitro phosphoproteins included both Triton-soluble and Triton-insoluble proteins. However, none of these proteins labeled in vitro coincided with those labeled in situ. The decline in phosphorylation with time can be accounted for by phosphatase action as these BBM possess a Ca-dependent phosphatase. In summary, we have demonstrated that the BBM possess PKC-specific substrates which can be visualized by in situ and in vitro phosphorylation. Treatment of intact enterocytes with PhE results in the phosphorylation of three detergent-insoluble proteins with a time course similar to that of PhE inhibition of Na/H transport.

Rectal epithelial expression of protein kinase A phosphorylation of cystic fibrosis transmembrane conductance regulator

BACKGROUND/AIMS

Human rectal epithelium in cystic fibrosis (CF) shows impaired ion transport in response to theophylline or bethanechol, although it possesses regulatory subunits of adenosine cyclic 3',5'-monophosphate (cAMP)-dependent protein kinase (protein kinase A). Protein kinase A-specific phosphorylation of CF transmembrane conductance regulator (CFTR) in rectal tissues of control and CF volunteers was examined in this study.

METHODS

CFTR was evaluated using a polyclonal antiserum (pre-NBF) raised against a peptide corresponding to residues 415-427 of CFTR. Microsomal membranes from normal and CF rectal mucosa and from T-84 cells were incubated with [gamma 32P]-adenosine triphosphate +/- protein kinase A and subjected to immunoblotting with pre-NBF and autoradiography.

RESULTS

Pre-NBF recognized a single band of 180 kilodaltons. Protein kinase A altered phosphorylation of this 180-kilodalton band 1.4-, 2.2- and 0.9-fold in T-84, normal, and CF rectal membranes, respectively. Catalytic activities of protein kinase A, Ca2+ calmodulin protein kinase, or protein kinase C in control and CF tissues were similar.

CONCLUSIONS

cAMP and Ca(2+)-signaling pathways are normal up to the kinases in CF rectal mucosa. Our results suggest differences in CFTR phosphorylation in normal and CF rectal mucosal membranes.

Specific protein phosphorylation occurs in molluscan red blood cell ghosts in response to hypoosmotic stress

The regulation of cellular volume upon exposure to hypoosmotic stress is accomplished by specific plasma membrane permeability changes that allow the efflux of certain intracellular solutes (osmolytes). The mechanism of this membrane permeability regulation is not understood; however, previous data implicate Ca2+ as an important component in the response. The regulation of protein phosphorylation is a pervasive aspect of cellular physiology that is often Ca2+ dependent. Therefore, we tested for osmotically induced protein phosphorylation as a possible mechanism by which Ca2+ may mediate osmotically dependent osmolyte efflux. We have found a rapid increase in 32Pi incorporation into two proteins in clam blood cell ghosts after exposure of the intact cells to a hypoosmotic medium. The osmotic component of the stress, not the ionic dilution, was the stimulus for the phosphorylations. The osmotically induced phosphorylation of both proteins was significantly inhibited when Ca2+ was omitted from the medium, or by the calmodulin antagonist, chlorpromazine. These results correlate temporally with cell volume recovery and osmolyte (specifically free amino acid) efflux. The two proteins that become phosphorylated in response to hypoosmotic stress may be involved in the regulation of plasma membrane permeability to organic solutes, and thus, contribute to hypoosmotic cell volume regulation.