Phorbol ester receptors in bovine luteal cells: relationship to protein kinase C

Effects of Topical Antiglaucoma Medications on Corneal Epithelium as Evaluated by Gene Expression Patterns

PURPOSE

To examine the expression pattern of the stress-related genes c-fos and c-jun, which encode the 2 major components of activator protein (AP)-1, and cyclooxygenase (COX)-2 in rat corneal epithelium treated with topical antiglaucoma medications and benzalkonium chloride (BAK) preservative.

METHODS

Eighty-eight male Wistar rats were used. We instilled antiglaucoma eye drops (0.5% Timoptol, 0.005% Xalatan, or 0.12% Rescula), their chemical constituents (active ingredients), or BAK preservative (0.005%, 0.01%, or 0.02%) in 1 eye of each rat. Fellow eyes served as controls. The eyes were enucleated after various intervals. In situ hybridization and immunohistochemistry were used to detect expression of c-fos, c-jun, and COX-2.

RESULTS

Expression of c-fos, c-jun, and COX-2 was minimally observed in uninjured rat corneal epithelium. Thirty minutes to 1 hour after applying the antiglaucoma eye drops, signals for c-fos and c-jun mRNA were detected in the corneal epithelium. Ninety minutes after applying 0.005% Xalatan, 0.12% Rescula, or their chemical constituents, but not 0.5% Timoptol, COX-2 was detected in corneal epithelium. Expression of c-fos and c-jun seemed more marked with prostaglandins than with timolol. Thirty minutes to 1 hour after instillation of 0.02% BAK preservative, signals for c-fos and c-jun mRNA were detected in the corneal and conjunctival epithelium. COX-2 was not induced by 0.5% Timoptol or BAK preservative. COX-2 mRNA was not affected by applying 0.005% or 0.01% BAK preservative. Proteins of these components were also detected, indicating that each mRNA expression was followed by protein synthesis.

CONCLUSIONS

Corneal and conjunctival epithelial cells are transcriptionally activated transiently at an early phase after topical administration of antiglaucoma medications and BAK preservative. Stimulatory effects of prostaglandin drugs on corneal epithelial cells were more marked than those with timolol. Expression of COX-2 may potentially be involved in inflammatory response in the corneal epithelium.

Induction of c-fos and c-jun messenger ribonucleic acid expression by prostaglandin F2alpha is mediated by a protein kinase C-dependent extracellular signal-regulated kinase mitogen-activated protein kinase pathway in bovine luteal cells

PGF2alpha triggers the demise of the corpus luteum whereby progesterone synthesis is inhibited, the luteal structure regresses, and the estrus cycle resumes. Upon binding to its heterotrimeric G-protein-coupled receptors, PGF2alpha initiates the phospholipase C/diacylglycerol and inositol-1,4,5-trisphosphate/Ca(2+)-protein kinase C (PKC) signaling pathway. More recently, we have demonstrated that PGF2alpha activates extracellular signal-regulated kinase (ERK) mitogen-activated protein (MAP) kinase signaling through a Raf-dependent mechanism in bovine luteal cells. However, the relationship between PKC and ERK activation in PGF2alpha signaling has not been clearly defined. Moreover, the signaling pathway that PGF2alpha uses to regulate gene expression is unknown. In this report, primary cultures of bovine luteal cells were used to address the role of PKC in ERK activation and the signaling pathway for induction of c-fos and c-jun messenger RNA (mRNA) expression in response to PGF2alpha. By using a PKC inhibitor and a PKC-deficient luteal cell model, we observed that phorbol ester-responsive isoforms of PKC were required for ERK phosphorylation and activation by PGF2alpha (1 microM) or phorbol 12-myristate 13-acetate (PMA) (20 nM). In PGF2alpha- and PMA-treated cells, active ERK MAP kinase was localized in the nucleus. PGF2alpha-induced ERK phosphorylation was dose-dependently inhibited by the MEK1 inhibitor PD098059 (1-50 microM). The expression of c-fos and c-jun mRNA in luteal cells was markedly increased by treatment with PGF2alpha (1 microM) or PMA (20 nM) for 30 min. We also observed that activation of ERK MAP kinase was required for the expression of c-fos and c-jun mRNA in response to PGF2alpha and PMA because it was abrogated by blocking the ERK pathway with PD098059. In addition, PGF2alpha and PMA-induced c-fos and c-jun mRNA expression was abolished in the PKC-deficient cells. Taken together, our data demonstrate that a PKC-dependent ERK MAP kinase pathway mediates the expression of c-fos and c-jun mRNA in PGF2alpha-treated bovine luteal cells.

Regulation of delta protein kinase C during rat ovarian differentiation

Studies were undertaken to classify protein kinase C (PKC) forms present in rat corpora lutea and to begin to evaluate their regulation during ovarian differentiation. Hydroxyapatite (HAP) column chromatography of rat luteal tissue revealed the presence of multiple forms of PKC (alpha, beta, delta, zeta). Identification of the PKC isoforms was based upon elution positions from HAP column chromatography and immunoreactivity. The delta PKC isoform was identified as the major Ca(2+)-independent form of PKC present in rat luteal tissue. The Ca(2+)-independent, lipid-dependent phosphorylation of the 80-kDa delta PKC was readily detectable in soluble luteal extracts and was shown to reflect autophosphorylation of delta PKC. To evaluate the regulation of PKC isoforms during ovarian differentiation, PKC protein levels were compared between preovulatory follicle-enriched ovaries and corpora lutea obtained on day 16 of pregnancy. Levels of delta PKC protein were greatly elevated in corpora lutea compared to levels in preovulatory follicles. In contrast, levels of alpha and beta PKC protein remained constant while levels of zeta PKC were slightly higher in the follicular than the luteal extract. Levels of delta PKC mRNA were also higher in corpora lutea than in preovulatory follicles. These results are the first to demonstrate the physiological regulation of delta PKC with follicular differentiation into corpora lutea and implicate a role for this prominent PKC form in the corpus luteum during pregnancy.

Receptors for tumor-promoting phorbol esters in rat ventral prostate

The presence of tumor-promoting phorbol ester receptors in rat prostate was investigated by studying the binding of phorbol diester 12,13-dibutyrate (PDBu) in both soluble and particulate subcellular fractions. Binding of [3H]PDBu to the soluble fraction was optimal after the addition of phosphatidylserine (0.1 mg/ml) and Ca2+ (1 mM). Both subcellular fractions exhibited a single class of PDBu receptor (Kd between 97 and 128 nM) as shown by saturation binding experiments. Phorbol esters with tumor-promoting activity showed a higher affinity for these receptors than did endogenous ligands such as diacylglycerols whereas phorbol esters without tumor-promoting activity were ineffective even at concentrations as high as 1 microM. These properties are highly representative of protein kinase C activity.

Phorbol 12,13-dibutyrate binding to intact human platelets. The role of cytosolic free Ca2+

The role of Ca2+ was examined in regulating the binding of phorbol 12,13-dibutyrate (PdBu) to intact human platelets. Alterations in the cytosolic free Ca2+ concn. [( Ca2+]i), but not extracellular Ca2+, substantially influenced the binding parameters of the phorbol ester. Ca(2+)-depleted platelets demonstrated a significant decline in the maximal binding capacity (Bmax), an increase in equilibrium dissociation constant (Kd) and a decrease in the Hill coefficient (h), suggesting the presence of Ca(2+)-sensitive and Ca(2+)-insensitive populations of PdBu-binding sites. In 1 mM-Ca2+ buffer, thrombin (0.1 NIH unit/ml) and ionomycin (0.5 microM) evoked a rise in [Ca2+]i to approx. 300-500 nM, associated with a significant decline in Kd, but without an apparent effect on Bmax. No effect of thrombin was observed on PdBu binding in Ca(2+)-depleted platelets. Inhibition of protein kinase C (PKC) by H7 was associated with a greater thrombin-evoked [Ca2+]i transient and a decline in Kd. Staurosporine also decreased the Kd for PdBu binding. We propose that this effect of the PKC inhibitors on the Kd was also [Ca2+]i-dependent. These observations in intact platelets indicate that the primary role of agonist- or non-agonist-induced rise in [Ca2+]i is to increase the affinity of PKC for PdBu and, presumably, endogenous diacylglycerol. However, in itself a rise in [Ca2+]i does not increase the Bmax, for PdBu binding.

Calcium-independent phospholipid/diolein-dependent phosphorylation of a soluble ovarian Mr 80,000 substrate protein: biochemical characteristics

Soluble ovarian extracts were incubated with protein kinase effectors in the presence of [gamma 32P]ATP and proteins were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Autoradiograms revealed phosphorylation of an ovarian Mr = 80,000 substrate in the presence of EGTA ([ethylenebis(oxyethylenenitrilo)]tetraacetic acid), phosphatidylserine and 1,2-diolein. In contrast to a classical response pattern to C-kinase effectors, the ovarian Mr = 80,000 phosphorylation was inhibited by 2 x 10(-7) M or greater free Ca2+. The ovarian Mr = 80,000 substrate was distinguished from the myristoylated acidic Mr = 80,000 C-kinase substrate of brain tissue on the basis of heat stability and phosphorylative response to effectors. Phosphorylation of the exogenous substrate myelin basic protein by DEAE-resolved ovarian kinase showed the variant effector dependence, maximal in the presence of EGTA, phosphatidylserine and 1,2-diolein. Finally, the effect of Ca2+ on ovarian Mr = 80,000 [32P]phosphate content could not be accounted for by post-phosphorylation activities, or by DEAE-resolvable or hydroxylapatite-resolvable inhibitory activities.