Degradation of protein kinase C alpha and its free catalytic subunit, protein kinase M, in intact human neuroblastoma cells and under cell-free conditions. Evidence that PKM is degraded by mM calpain-mediated proteolysis at a faster rate than PKC

PKCζ activation promotes maturation of cord blood T cells towards a Th1 IFN-γ propensity

A significant number of babies present transiently with low protein kinase C zeta (PKCζ) levels in cord blood T cells (CBTC), associated with reduced ability to transition from a neonatal Th2 to a mature Th1 cytokine bias, leading to a higher risk of developing allergic sensitisation, compared to neonates whose T cells have 'normal' PKCζ levels. However, the importance of PKCζ signalling in regulating their differentiation from a Th2 to a Th1 cytokine phenotype propensity remains undefined. To define the role of PKCζ signalling in the regulation of CBTC differentiation from a Th2 to a Th1cytokine phenotype we have developed a neonatal T cell maturation model which enables the cells to develop to CD45RA- /CD45RO+ T cells while maintaining the Th2 immature cytokine bias, despite having normal levels of PKCζ. The immature cells were treated with phytohaemagglutinin, but in addition with phorbol 12-myristate 13-acetate (PMA), an agonist which does not activate PKCζ. This was compared to development in CBTC in which the cells were transfected to express constitutively active PKCζ. The lack of PKCζ activation by PMA was monitored by western blot for phospho-PKCζ and translocation from cell cytosol to the membrane by confocal microscopy. The findings demonstrate that PMA fails to activate PKCζ in CBTC. The data show that CBTC matured under the influence of the PKC stimulator, PMA, maintain a Th2 cytokine bias, characterised by robust IL-4 and minimal interferon gamma production (IFN-γ), and lack of expression of transcriptional factor, T-bet. This was also reflected in the production of a range of other Th2/Th1 cytokines. Interestingly, introduction of a constitutively active PKCζ mutant into CBTC promoted development towards a Th1 profile with high IFN-γ production. The findings demonstrate that PKCζ signalling is essential for the immature neonatal T cells to transition from a Th2 to a Th1 cytokine production bias.

Cytoplasmic phospholipase A₂ modulation of adolescent rat ethanol-induced protein kinase C translocation and behavior

Ethanol consumption typically begins during adolescence, a developmental period which exhibits many age-dependent differences in ethanol behavioral sensitivity. Protein kinase C (PKC) activity is largely implicated in ethanol-behaviors, and our previous work indicates that regulation of novel PKC isoforms likely contributes to decreased high-dose ethanol sensitivity during adolescence. The cytoplasmic Phospholipase A2 (cPLA2) signaling cascade selectivity modulates novel and atypical PKC isoform activity, as well as adolescent ethanol hypnotic sensitivity. Therefore, the current study was designed to ascertain adolescent cPLA2 activity both basally and in response to ethanol, as well as it's involvement in ethanol-induced PKC isoform translocation patterns. cPLA2 expression was elevated during adolescence, and activity was increased only in adolescents following high-dose ethanol administration. Novel, but not atypical PKC isoforms translocate to cytosolic regions following high-dose ethanol administration. Inhibiting cPLA2 with AACOCF3 blocked ethanol-induced PKC cytosolic translocation. Finally, inhibition of novel, but not atypical, PKC isoforms when cPLA2 activity was elevated, modulated adolescent high-dose ethanol-sensitivity. These data suggest that the cPLA2/PKC pathway contributes to the acute behavioral effects of ethanol during adolescence.

Tear me down: role of calpain in the development of cardiac ventricular hypertrophy

Cardiac hypertrophy develops most commonly in response to hypertension and is an independent risk factor for the development of heart failure. The mechanisms by which cardiac hypertrophy may be reversed to reduce this risk have not been fully determined to the point where mechanism-specific therapies have been developed. Recently, proteases in the calpain family have been implicated in the regulation of the development of cardiac hypertrophy in preclinical animal models. In this review, we summarize the molecular mechanisms by which calpain inhibition has been shown to modulate the development of cardiac (specifically ventricular) hypertrophy. The context within which calpain inhibition might be developed for therapeutic intervention of cardiac hypertrophy is then discussed.

Vasopressin-induced translocation and proteolysis of protein kinase Cα in an amphibian brain: Modulation by corticosterone

In urodele amphibians, the hypothalamic neuropeptide arginine vasotocin and the adrenal steroid corticosterone interact to regulate reproductive behavior by actions in the brain. The present study investigated signal transduction pathways underlying acute effects of vasotocin and corticosterone, presumably mediated via "non-genomic" steroid action, in an amphibian brain. We used Western blot to examine the effects of corticosterone and the vasotocin receptor agonist arginine vasopressin, alone and in combination, on the subcellular localization and proteolytic processing of protein kinase C-alpha (PKCalpha) in tiger salamander brain tissue. Treatment of whole brain minces with vasopressin or vasotocin led to increases in PKCalpha in membrane fractions and concurrent decreases in PKCalpha in cytosolic fractions. Vasopressin or vasotocin treatment also induced the appearance in membrane and cytosolic fractions of a PKCalpha-immunoreactive band that corresponds to PKMalpha, the proteolytically generated, free catalytic subunit of PKCalpha. Treatment with corticosterone alone had no consistent effect on either PKCalpha or PKMalpha in either fraction. However, pretreatment with corticosterone reliably blocked vasopressin-induced increases in cytosolic PKMalpha. These data provide new information about the cellular mechanisms of action of vasopressin and corticosterone in the vertebrate brain and suggest a cellular mechanism by which the two hormones interact to regulate neuronal physiology and behavior.

Calcium influx through L-type channels generates protein kinase M to induce burst firing of dopamine cells in the rat ventral tegmental area

Enhanced activity of the dopaminergic system originating in the ventral tegmental area is implicated in addictive and psychiatric disorders. Burst firing increases dopamine levels at the synapse to signal novelty and salience. We have previously reported a calcium-dependent burst firing of dopamine cells mediated by L-type channels following cholinergic stimulation; this paper describes a cellular mechanism resulting in burst firing following L-type channel activation. Calcium influx through L-type channels following FPL 64176 or (S)-(-)-Bay K8644 induced burst firing independent of dopamine, glutamate, or calcium from the internal stores. Burst firing induced as such was completely blocked by the substrate site protein kinase C (PKC) inhibitor chelerythrine but not by the diacylglycerol site inhibitor calphostin C. Western blotting analysis showed that FPL 64176 and (S)-(-)-Bay K8644 increased the cleavage of PKC to generate protein kinase M (PKM) and the specific calpain inhibitor MDL28170 blocked this increase. Prevention of PKM production by inhibiting calpain or depleting PKC blocked burst firing induction whereas direct loading of purified PKM into cells induced burst firing. Activation of the N-methyl-D-aspartic acid type glutamate or cholinergic receptors known to induce burst firing increased PKM expression. These results indicate that calcium influx through L-type channels activates a calcium-dependent protease that cleaves PKC to generate constitutively active and labile PKM resulting in burst firing of dopamine cells, a pathway that is involved in glutamatergic or cholinergic modulation of the central dopamine system.

Effects of neurosteroids on glucocorticoid receptor-mediated gene transcription in LMCAT cells--a possible interaction with psychotropic drugs

Aberrant activity of hypothalamic-pituitary-adrenal (HPA) axis is often observed in psychiatric disorders and both antidepressant and antipsychotic drugs are known to ameliorate some deleterious effects of glucocorticoids on brain function. Some neurosteroids possess antidepressant and neuroleptic-like properties and attenuate the stress-activated HPA axis activity. However, intracellular mechanism of neurosteroid interaction with glucocorticoids has not been elucidated. We evaluated effects of some neurosteroids on functional activity of glucocorticoid receptor (GR) in vitro. A combined treatment with antipsychotic drugs and involvement of some protein kinases in allopregnanolone effect on GR function were also studied. The effects of allopregnanolone, its two isomers (5beta-pregnan-3alpha-ol-20-one and 5alpha-pregnan-3beta-ol-20-one) and dehydroepiandrosterone sulfate (DHEAS) on the corticosterone-induced chloramphenicol acetyl transferase (CAT) activity were evaluated in mouse fibroblast cells stably transfected with mouse mammary tumor virus (MMTV)-CAT plasmid. We found that allopregnanolone (1-100 microM) and, to a lesser extent, both its isomers inhibited the GR-mediated gene transcription in a concentration-dependent manner. In contrast, DHEAS at the concentration up to 100 microM was inactive. Further experiments revealed that allopregnanolone and antipsychotic drugs (chlorpromazine and clozapine) showed a moderate, additive inhibitory effect on the GR function. With respect to intracellular mechanism of allopregnanolone action, we showed that this neurosteroid inhibited protein kinase C (PKC) activity, decreased the level of PKCalpha isoenzyme in the membrane fraction and decreased the amount of active phosphorylated form of extracellular signal-regulated kinase-mitogen-activated protein kinase (ERK-MAPK) in LMCAT cells. Since PKC and ERK-MAPK inhibitors attenuate the corticosterone-mediated gene transcription, the above findings suggest that allopregnanolone effect on GR function involves interaction with these kinase pathways. On the other hand, allopregnanolone had no effect on protein kinase A (PKA) activity. These data indicate that pregnanolone derivatives, like antidepressants and antipsychotic drugs, may attenuate some glucocorticoid effects via inhibition of GR-mediated gene transcription. Furthermore, the inhibitory effect of allopregnanolone on the corticosterone-induced gene transcription in LMCAT cells depended on the inhibition of PKC and ERK-MAPK pathways.

Actin-based features negatively regulated by protein kinase C-epsilon

Cells exposed to phorbol 12-myristate 13-acetate (PMA) undergo a choreographed sequence of morphological changes. Some of these, including stimulation of membrane ruffles and the later appearance of stress fibers, rely on remodeling of the actin cytoskeleton. Although this process is poorly understood, it is important, because the same features are affected during oncogenic transformation. PMA also activates protein kinase C (PKC). Enzyme activation is followed by degradation. Either process might affect the remodeling of actin. The present studies determined whether any PKC isozymes were subject to degradation in tracheal epithelial cells by quantifying the amount of each isozyme present after PMA exposure. PKC-epsilon was the only isozyme to show declining content correlated with increased stress fiber accumulation. Stress fibers increased between 5 and 10 h, whereas PKC-epsilon declined to 38% of its starting value (95% confidence interval, 10-68%). The relationship could be fit by the function F(x) = 0.683 x exp[-0.841(x - 0.387)], where F is the frequency of fiber-containing cells and x is PKC-epsilon content. Fiber accumulation was further investigated after knockdown of PKC-epsilon with RNA interference and antisense oligodeoxynucleotide. Knockdown enhanced stress fibers in cells not yet exposed to PMA as well as the final frequency of fiber-containing cells after PMA exposure. With knockdown at both transcriptional and protein levels, approximately 15% of the original content was predicted and achieved, as judged from real-time PCR and PKC-epsilon content measurements. The results suggest that PKC-epsilon negatively regulates stress fibers, either by directly turning over one of their components or by regulating an upstream step affecting fiber organization.

Intermediate-term memory for site-specific sensitization in aplysia is maintained by persistent activation of protein kinase C

Recent studies of long-term synaptic plasticity and long-term memory have demonstrated that the same functional endpoint, such as long-term potentiation, can be induced through distinct signaling pathways engaged by different patterns of stimulation. A critical question raised by these studies is whether different induction pathways either converge onto a common molecular mechanism or engage different molecular cascades for the maintenance of long-term plasticity. We directly examined this issue in the context of memory for sensitization in the marine mollusk Aplysia. In this system, training with a single tail shock normally induces short-term memory (<30 min) for sensitization of tail-elicited siphon withdrawal, whereas repeated spaced shocks induce both intermediate-term memory (ITM) (>90 min) and long-term memory (>24 hr). We now show that a single tail shock can also induce ITM that is expressed selectively at the trained site (site-specific ITM). Although phenotypically similar to the form of ITM induced by repeated trials, the mechanisms by which site-specific ITM is induced and maintained are distinct. Unlike repeated-trial ITM, site-specific ITM requires neither protein synthesis nor PKA activity for induction or maintenance. Rather, the induction of site-specific ITM requires calpain-dependent proteolysis of activated PKC, yielding a persistently active PKC catalytic fragment (PKM) that also serves to maintain the memory in the intermediateterm temporal domain. Thus, two unique forms of ITM that have different induction requirements also use distinct molecular mechanisms for their maintenance.

Protein kinase C modulates pulmonary endothelial permeability: a paradigm for acute lung injury

The intracellular serine/threonine kinase protein kinase C (PKC) has an important role in the genesis of pulmonary edema. This review discusses the PKC-mediated mechanisms that participate in the pulmonary endothelial response to agents involved in lung injury characteristic of the respiratory distress syndrome. Thus the paradigms of PKC-induced lung injury are discussed within the context of pulmonary transvascular fluid exchange. We focus on the signal transduction pathways that are modulated by PKC and their effect on lung endothelial permeability. Specifically, alpha-thrombin, tumor necrosis factor (TNF)-alpha, and reactive oxygen species are discussed because of their well-established roles in both human and experimental lung injury. We conclude that PKC, most likely PKC-alpha, is a primary supporter for lung endothelial injury in response to alpha-thrombin, TNF-alpha, and reactive oxygen species.

Decrease of PKC precedes other cellular signs of calpain activation in area CA1 of the hippocampus after transient cerebral ischemia

One of the specific features of severe brain injury is an activation of calcium-dependent proteolysis by calpains. We have observed a significant increase of activity as early as 3 h after the insult in a well defined model of delayed ischemic neuronal death in gerbil hippocampus. At 24 h, the enzymatic activity transiently normalized, then increased again, following the place and time of selective cellular death in the CA1 region of hippocampus. The enhanced postischemic proteolysis resulted in concomitant cleavage of calpain-specific endogenous substrates like protein kinase C (PKC), fodrin and microtubule-associated protein-2 (MAP2). These effects were also time-dependent and restricted to the vulnerable, CA1 pyramidal neurons-containing the dorsal part (DP) of the hippocampus. We have also characterized the postischemic changes of six different isoforms of PKC. The vulnerable dorsal part of the hippocampus, but not its relative resistant abdominal part (AbP), exhibited a loss of PKCalpha, beta, gamma, and delta isoforms as early as 3 h after ischemic insult. However, at this time, solely in the soluble fraction of homogenate. Later (72 h), a further loss of the enzyme proteins, comprised the particulate fraction as well and resulted in an about 50% decrease of total PKCs in the vulnerable DP region. In the case of PKCalpha, the immunostaining pattern showed, in addition to the disappearance of the enzyme from the injured area, an extensive translocation into nuclei of the survived, ischemia-resistant neurones. The early decreases of PKC isoforms in the cytosol paralleled the transient calpain activation at 3h postischemia but substantially preceded the proteolysis of any other classical calpain substrates, such as fodrin and MAP2, being evidenced not earlier than 48-72 h after the insult and restricted also to the vulnerable dorsal part. In conclusion, our results of the time-dependent effects of transient global cerebral ischemia on the calpain activity, levels and localization of its several substrates suggest, that calpain-mediated proteolysis is specifically involved in the early (induction) as well as in the late (execution) phases of delayed ischemic neuronal death in the CA1 hippocampus.

Effect of chronic inhibition of calpains in the hippocampus on spatial discrimination learning and protein kinase C

Several behavioral and electrophysiological studies have suggested that a sustained activation of protein kinase C would be required to underlie persistent changes associated with memory formation. Limited proteolysis of PKCs by calpains, calcium-activated proteases, cleaves the catalytic and the regulatory domains, generating a free catalytic fragment termed PKM, constitutively active. In order to investigate the potential physiological importance of this limited proteolysis as a mechanism of PKC activation, we have studied the effect of the calpastatin peptide, a specific calpain inhibitor, on the learning of a spatial discrimination task in a radial maze. Thus, using osmotic micro-pumps, the calpastatin peptide was infused bilaterally into the dorsal hippocampus during the six sessions of training and the probe test. The treatment was shown to facilitate the performance of the mice on the two last training sessions and on the probe test. This behavioral effect was shown to correspond to the reduced calpain activity observed in the hippocampus at the very end of the 7-day infusion of the calpastatin peptide, suggesting a relation between both events. In addition, PKC activity measured immediately after the probe test was notably decreased in the membrane fraction of the hippocampus. Although protein levels of PKCs and calpains quantified by western blot were not affected by calpastatin infusion, we found a noticeable correlation between mu-calpain and PKCgamma levels confirming the particular relationship between both proteins. These results suggest that calpains influence on PKCs activity may affect cellular mechanisms during memory processes.

T-kininogen inhibits fibroblast proliferation in the G(1) phase of the cell cycle

By using synthetic protease inhibitors, several investigators have demonstrated that cysteine proteinases are required for cell proliferation. Kininogens are potent and specific physiological inhibitors of cysteine proteinases. We have used several mouse fibroblast-derived cell lines that express biologically active T-kininogen under the control of the mouse metallothionein promoter to test its effect on cell proliferation. Our results indicate that expression of T-kininogen results in diminished proliferative capacity, as measured by reduced cell numbers, both in logarithmically growing cultures and in G(0) cells induced to proliferate in response to serum. Furthermore, both fluorescence-activated cell sorting (FACS) analysis and incorporation of radioactive precursors into DNA suggest that the cells are unable to progress from G(0) through the S phase of the cell cycle in response to serum stimulation. However, we find that T-kininogen-expressing cell lines are still capable of responding to growth factors present in the serum, both by activating the ERK pathway and by expressing early genes, such as c-Fos and c-Jun. Thus, our results suggest that inhibition of cysteine proteinases by T-kininogen leads to inhibition of cell proliferation between the G(1) and S phases of the cell cycle.

Induction and subcellular localization of protein kinase C isozymes following renal ischemia

BACKGROUND

We have previously reported that the expression of the receptor for activated C kinase (RACK1) is induced post-ischemia/preperfusion injury to the kidney, and activation of protein kinase C (PKC) protects renal cells from hypoxic injury. This study was done to determine whether the induced expression of RACK1 is accompanied by changes in the level of expression and subcellular distribution of PKC isozymes.

METHODS

Ischemia/reperfusion injury resulting in acute renal failure was induced by 60 minutes of bilateral renal artery clamping in rats. The expression levels and translocation of various PKC isozymes between soluble and particulate fractions in whole kidney homogenates were demonstrated by immunoblot analysis. The expression pattern of the various PKC isozymes in the kidney postinjury was performed by immunohistochemistry.

RESULTS

PKC alpha, beta II, and zeta were induced and translocated from the soluble fraction to the particulate fraction post-injury. Immunolocalization showed PKC alpha, beta II, and zeta expression to be induced in the proximal tubule epithelial cell (PTEC) at 0 to 30 minutes post-ischemia/reperfusion injury (IRI). At one-day postinjury, the alpha isozyme was translocated to the plasma membrane of the undamaged PTEC, while it was translocated to the nucleus in damaged PTEC. PKC beta II expression was along the basal and lateral side of the undamaged PTEC, while it was distributed in the cytoplasm of sloughed cells in the damaged PTEC. PKC zeta expression at one day was along the apical side of the damaged PTEC. At seven-days postinjury, the expressions of the alpha and zeta isozymes were localized to the plasma membrane of the regenerating PTEC and the expression of PKC beta II isozyme to certain interstitial cells.

CONCLUSION

The induced expression, translocation, and the intracellular spatial distributions of the enzymes suggest that they may mediate multiple processes during IRI.

Differential abilities of phorbol esters in inducing protein kinase C (PKC) down-regulation in noradrenergic neurones

1. The ability of several phorbol ester protein kinase C (PKC) activators (phorbol 12, 13-dibutyrate, PDB; phorbol 12, 13-diacetate, PDA; and 12-deoxyphorbol 13-acetate, dPA) to down-regulate PKC was studied by assessing their effects on electrical stimulation-induced (S-I) noradrenaline release from rat brain cortical slices and phosphorylation of the PKC neural substrate B-50 in rat cortical synaptosomal membranes. 2. In cortical slices which were incubated for 20 h with vehicle, acute application of PDB, PDA and dPA (0.1 - 3.0 microM) enhanced the S-I noradrenaline release in a concentration-dependent manner to between 200 - 250% of control in each case. In slices incubated with PDB (1 microM for 20 h), subsequent acute application of PDB (0.1 - 3.0 microM) failed to enhance S-I release, indicating PKC down-regulation. However, in tissues incubated with PDA or dPA (3 microM) for 20 h, there was no reduction in the facilitatory effect of their respective phorbol esters or PDB (0.1 - 3.0 microM) when acutely applied, indicating that PKC was not down-regulated. This was confirmed using Western blot analysis which showed that PDB (1 microM for 20 h) but not PDA (3 microM for 20 h) caused a significant reduction in PKCalpha. 3. Incubation with PDB for 20 h, followed by acute application of PDB (3 microM) failed to increase phosphorylation of B-50 in synaptosomal membranes, indicating down-regulation. In contrast, tissues incubated with PDA or dPA for 20 h, acute application of their respective phorbol ester (10 microM) or PDB (3 microM) induced a significant increase in B-50 phosphorylation. 4. Acutely all three phorbol esters elevate noradrenaline release to about the same extent, yet PDA and dPA have lower affinities for PKC compared to PDB, suggesting unique neural effects for these agents. This inability to cause functional down-regulation of PKC extends their unusual neural properties. Their neural potency and lack of down-regulation may be related to their decreased lipophilicity compared to other phorbol esters. 5. We suggest that PKC down-regulation appears to be related to binding affinity, where agents with high affinity, irreversibly insert PKC into artificial membrane lipid and generate Ca(2+)-independent kinase activity which degrades and deplete PKC. We suggest that this mechanism may also underlie the ability of PDB to down-regulate PKC in nerve terminals, in contrast to PDA and dPA.

Activation of PKC is sufficient to induce an apoptotic program in salivary gland acinar cells

Accumulating evidence suggests that specific isoforms of PKC may function to promote apoptosis. We show here that activation of the conventional and novel isoforms of PKC with 12-O-tetradecanoyl phorbol-13- ester (TPA) induces apoptosis in salivary acinar cells as indicated by DNA fragmentation and activation of caspase-3. TPA-induced DNA fragmentation, caspase-3 activation, and morphologic indicators of apoptosis, can be enhanced by pretreatment of cells with the calpain inhibitor, calpeptin, prior to the addition of TPA. Analysis of PKC isoform expression by immunoblot shows that TPA-induced downregulation of PKC alpha and PKC delta is delayed in cells pre-treated with calpeptin, and that this correlates with an increase of these isoforms in the membrane fraction of cells. TPA-induced apoptosis is accompanied by biphasic activation of the c-jun-N-terminal kinase (JNK) pathway and inactivation of the extracellular regulated kinase (ERK) pathway. Expression of constitutively activated PKC alpha or PKC delta, but not kinase negative mutants of these isoforms, or constitutively activated PKC epsilon, induces apoptosis in salivary acinar cells, suggesting a role for these isoforms in TPA-induced apoptosis. These studies demonstrate that activation of PKC is sufficient for initiation of an apoptotic program in salivary acinar cells. Cell Death and Differentiation (2000) 7, 1200 - 1209.

Amelioration of denervation-induced atrophy by clenbuterol is associated with increased PKC-alpha activity

Rat soleus muscle was denervated for 3 or 7 days, and total membrane protein kinase C (PKC) activity and translocation and immunocytochemical localization of PKC isoforms were examined. Dietary administration of clenbuterol concomitant with denervation ameliorated the atrophic response and was associated with increased membrane PKC activity at both 3 (140%) and 7 (190%) days. Of the five PKC isoforms (alpha, epsilon, theta, zeta, and mu) detected in soleus muscle by Western immunoblotting, clenbuterol treatment affected only the PKC-alpha and PKC-theta forms. PKC-alpha was translocated to the membrane fraction upon denervation, and the presence of clenbuterol increased membrane-bound PKC-alpha and active PKC-alpha as assayed by Ser(657) phosphorylation. PKC-theta protein was downregulated upon denervation, and treatment with clenbuterol further decreased both cytosolic and membrane levels. Immunolocalization of PKC-theta showed differences for regulatory and catalytic domains, with the latter showing fast-fiber type specificity. The results suggest potential roles of PKC-alpha and PKC-theta in the mechanism of action of clenbuterol in alleviating denervation-induced atrophy.

Protein kinase C-alpha mediates endothelial barrier dysfunction induced by TNF-alpha

We tested the hypothesis that protein kinase C-alpha (PKC-alpha) mediates tumor necrosis factor-alpha (TNF-alpha)-induced alterations in permeability of pulmonary microvessel endothelial monolayers (PEM). The permeability of PEM was assessed by the clearance rate of Evans blue-labeled albumin. PEM lysates were analyzed for PKC-alpha mRNA (Northern cDNA blot), protein (Western immunoblot), and activity (translocation and phosphorylation of myristoylated arginine-rich C kinase substrate). Incubation of PEM with TNF-alpha (1,000 U/ml) for 4 h resulted in increases in 1) PKC-alpha protein, 2) cytoskeletal-associated PKC-alpha, 3) PKC-alpha activity, and 4) permeability to albumin. The TNF-alpha-induced increase in PKC-alpha protein, PKC-alpha activity, and permeability was prevented by a 4-h pretreatment with PKC-alpha antisense oligonucleotide but not by the scrambled nonsense oligonucleotide. The TNF-alpha-induced increase in permeability to albumin was prevented by myristoylated protein kinase C inhibitor (an inhibitor of PKC-alpha/beta, 100 microM) and calphostin (an inhibitor of the classic and novel PKC isotypes, 200 nM). The treatment with calphostin from 0.5 to 3.0 h after TNF-alpha still prevented barrier dysfunction induced by 4 h of TNF-alpha treatment. The data indicate that prolonged activation of PKC-alpha, maintained by a translation-dependent pool of PKC-alpha protein, mediates TNF-alpha-induced increases in endothelial permeability in PEM.

Calpain-PKC inter-relations in mouse hippocampus: a biochemical approach

In previous studies, we isolated and identified a mu-calpain/PKCalpha complex from rabbit skeletal muscle. Here, we have used specific purification procedures in order to study the interactions between mu-calpain and PKC in mouse hippocampus, a brain structure implicated in memory processes. We observed that mu-calpain and conventional PKCs (alpha, betaII and gamma) are co-eluted after anion exchange chromatography. In contrast to our previous results obtained on skeletal muscle, mu-calpain and PKC isoenzymes were dissociated after gel filtration chromatography. Furthermore, mu-calpain induced the proteolytic conversion of PKCalpha, betaII, and gamma into PKMalpha, betaII, and gamma with a preferential hydrolysis of PKCgamma, a specific isoenzyme of the nervous system. Although the mu-calpain/PKC interactions in the hippocampus are quite different from skeletal muscle, our results however, point out the functional importance of these inter-relations. Moreover, as PKCgamma has been involved in the biochemical events underlying learning and memory, the preferential relationship between mu-calpain and PKCgamma promotes the importance of the role that mu-calpain could play in the cellular mechanisms of memory formation.

The alpha isoform of protein kinase C is involved in signaling the response of desmosomes to wounding in cultured epithelial cells

Initiation of reepithelialization upon wounding is still poorly understood. To enhance this understanding, we focus here on changes in the adhesive state of desmosomes of cultured Madin-Darby canine kidney cells in response to wounding of confluent cell sheets. Previous results show that desmosomal adhesion in Madin-Darby canine kidney cells changes from a calcium-dependent state to calcium independence in confluent cell sheets. We show that this change, which requires culture confluence to develop, is rapidly reversed upon wounding of confluent cell sheets. Moreover, the change to calcium dependence in wound edge cells is propagated to cells hundreds of micrometers away from the wound edge. Rapid transition from calcium independence to calcium dependence also occurs when cells are treated with phorbol esters that activate PKC. PKC inhibitors, including the conventional isoform inhibitor Gö6976, cause rapid transition from calcium dependence to calcium independence, even in subconfluent cells. The cellular location of the alpha isoform of PKC correlates with the calcium dependence of desmosomes. Upon monolayer wounding, PKCalpha translocates rapidly to the cell periphery, becomes Triton X-100 insoluble, and also becomes concentrated in lamellipodia. The PKCalpha translocation upon wounding precedes both the increase in PKC activity in the membrane fraction and the reversion of desmosomes to calcium dependence. Specific depletion of PKCalpha with an antisense oligonucleotide increases the number of cells with calcium-independent desmosomes. These results show that PKCalpha participates in a novel signaling pathway that modulates desmosomal adhesion in response to wounding.

Free PKC catalytic subunits (PKM) phosphorylate tau via a pathway distinct from that utilized by intact PKC

Protein kinase C (PKC) is reversibly activated at the plasma membrane by the generation of diacylglycerol (DAG) coupled with the release of Ca2+ from intracellular stores. PKC is also irreversibly activated by calpain-mediated PKC cleavage of the regulatory and catalytic subunits; resultant free PKC catalytic subunits are termed "PKM". Unlike PKC, PKM is co-factor-independent, remains active following diffusion away from the membrane, and can theoretically phosphorylate targets inaccessible to, and inappropriate for, PKC. We examined the downstream consequences of PKC activation by the phorbol ester TPA and by ionophore A23187-mediated calcium influx (which experimentally correspond to DAG-mediated and calpain-mediated activation, respectively) on phosphorylation of the microtubule-associated protein tau. Both methods increased phospho-tau immunoreactivity, and neither was inhibited by lithium or olomoucin (inhibitors of tau kinases GSK-3 beta and cdk5, respectively). The TPA-mediated increase, and not the ionophore-mediated increase, was blocked by co-treatment with the mitogen-activated protein (MAP) kinase kinase inhibitor PD98059. These findings indicate that PKC phosphorylates tau via the MAP kinase pathway, but that PKM can bypass this requirement, therefore demonstrating that distinct intracellular pathways can be mediated by PKC and PKM. PKM generation may therefore trigger one or more additional pathways contributing to tau phosphorylation following inappropriate calcium influx.

Role of protein kinase C in the translational regulation of lipoprotein lipase in adipocytes

The hypertriglyceridemia of diabetes is accompanied by decreased lipoprotein lipase (LPL) activity in adipocytes. Although the mechanism for decreased LPL is not known, elevated glucose is known to increase diacylglycerol, which activates protein kinase C (PKC). To determine whether PKC is involved in the regulation of LPL, we studied the effect of 12-O-tetradecanoyl phorbol 13-acetate (TPA) on adipocytes. LPL activity was inhibited when TPA was added to cultures of 3T3-F442A and rat primary adipocytes. The inhibitory effect of TPA on LPL activity was observed after 6 h of treatment, and was observed at a concentration of 6 nM. 100 nM TPA yielded maximal (80%) inhibition of LPL. No stimulation of LPL occurred after short term addition of TPA to cultures. To determine whether TPA treatment of adipocytes decreased LPL synthesis, cells were labeled with [35S]methionine and LPL protein was immunoprecipitated. LPL synthetic rate decreased after 6 h of TPA treatment. Western blot analysis of cell lysates indicated a decrease in LPL mass after TPA treatment. Despite this decrease in LPL synthesis, there was no change in LPL mRNA in the TPA-treated cells. Long term treatment of cells with TPA is known to down-regulate PKC. To assess the involvement of the different PKC isoforms, Western blotting was performed. TPA treatment of 3T3-F442A adipocytes decreased PKC alpha, beta, delta, and epsilon isoforms, whereas PKC lambda, theta, zeta, micro, iota, and gamma remained unchanged or decreased minimally. To directly assess the effect of PKC inhibition, PKC inhibitors (calphostin C and staurosporine) were added to cultures. The PKC inhibitors inhibited LPL activity rapidly (within 60 min). Thus, activation of PKC did not increase LPL, but inhibition of PKC resulted in decreased LPL synthesis by inhibition of translation, indicating a constitutive role of PKC in LPL gene expression.

Degradation of protein kinase Malpha by mu-calpain in a mu-calpain-protein kinase Calpha complex

In previous studies, we isolated and identified a mu-calpain-PKCalpha complex from rabbit skeletal muscle. At the same time we pointed out that an association between mu-calpain and PKCalpha could occur at the level of the plasma membrane of muscle cells, and that PKCalpha could thus be considered as a potential mu-calpain substrate. In the present study, using the mu-calpain-PKCalpha complex as a model, we report that mu-calpain is activated in the combined presence of physiological calcium concentrations (less than 1 microM) and phosphatidylserine. Furthermore our data also show that: (1) there exists a correlation between the appearance of autolyzed mu-calpain forms and PKCalpha hydrolysis which leads to the formation of PKMalpha; (2) in certain experimental conditions, autolyzed mu-calpain forms are able to hydrolyze PKMalpha independently of the presence of diacylglycerol.

Isoform-specific translocation of protein kinase C following glutamate administration in primary hippocampal neurons

High concentrations of glutamate, the major excitatory neurotransmitter in the mammalian brain, lead to intracellular calcium overload resulting in excitotoxic damage and death of neurons. Since protein kinase C (PKC) is involved in neuronal degeneration resulting from cerebral ischemia and from glutamate excitotoxicity, we investigated the effect of glutamate on changes in the cellular distribution of various PKC isoforms in cultured hippocampal neurons in comparison with the effects elicited by the PKC activator phorbol ester. Out of the expressed PKC isoforms alpha, gamma, epsilon, zeta and lambda only the conventional isoforms PKC alpha and gamma responded to glutamate. Using subcellular fractionation and Western blotting with isoform-specific antibodies and immunocytochemical localization with confocal laser scanning microscopy, we observed that phorbol ester and glutamate have different effects on PKC isoform redistribution: Whereas phorbol ester resulted in translocation of PKC alpha and PKC gamma toward a membrane fraction, the glutamate-mediated rise in intracellular calcium concentration induced a translocation mainly toward a detergent-insoluble, cytoskeletal fraction. Immunocytochemical analysis revealed an isoform-specific translocation following glutamate treatment: PKC gamma was translocated mainly to cytoplasmic, organelle-like structures, whereas PKC alpha redistributed to the plasma membrane and into the cell nucleus. The latter result is of special interest, as it indicates that nuclear PKC may play a role in processes of excitotoxic cell damage.

Intracellular proteinases of invertebrates: calcium-dependent and proteasome/ubiquitin-dependent systems

Cytosolic proteinases carry out a variety of regulatory functions by controlling protein levels and/or activities within cells. Calcium-dependent and ubiquitin/proteasome-dependent pathways are common to all eukaryotes. The former pathway consists of a diverse group of Ca(2+)-dependent cysteine proteinases (CDPs; calpains in vertebrate tissues). The latter pathway is highly conserved and consists of ubiquitin, ubiquitin-conjugating enzymes, deubiquitinases, and the proteasome. This review summarizes the biochemical properties and genetics of invertebrate CDPs and proteasomes and their roles in programmed cell death, stress responses (heat shock and anoxia), skeletal muscle atrophy, gametogenesis and fertilization, development and pattern formation, cell-cell recognition, signal transduction and learning, and photoreceptor light adaptation. These pathways carry out bulk protein degradation in the programmed death of the intersegmental and flight muscles of insects and of individuals in a colonial ascidian; molt-induced atrophy of crustacean claw muscle; and responses of brine shrimp, mussels, and insects to environmental stress. Selective proteolysis occurs in response to specific signals, such as in modulating protein kinase A activity in sea hare and fruit fly associated with learning; gametogenesis, differentiation, and development in sponge, echinoderms, nematode, ascidian, and insects; and in light adaptation of photoreceptors in the eyes of squid, insects, and crustaceans. Proteolytic activities and specificities are regulated through proteinase gene expression (CDP isozymes and proteasomal subunits), allosteric regulators, and posttranslational modifications, as well as through specific targeting of protein substrates by a diverse assemblage of ubiquitin-conjugases and deubiquitinases. Thus, the regulation of intracellular proteolysis approaches the complexity and versatility of transcriptional and translational mechanisms.

1,25(OH)2-vitamin D3 affects the subcellular distribution of protein kinase C isoenzymes in muscle cells

Previous studies have shown the involvement of protein kinase C (PKC) in 1,25-dihydroxy-vitamin D3 [1,25(OH)2D3] regulation of DNA synthesis (long-term effect) and Ca2+ channel activity (short-term effect) in cultured myoblasts. Both events mediate stimulation of myoblast cell proliferation and growth by 1,25(OH)2D3. To characterise further the role of PKC in the hormone mode of action in muscle cells, the presence of PKC isoenzymes in chicken embryo myoblasts and changes in their total cell and subcellular levels after treatment (72 h and 5 min) with 1,25(OH)2D3 (1 nM), 12-O-tetradecanoyl phorbol 13-acetate (TPA; 100 nM) and 1,2-dioctanoyl-rac-glycerol (DOG; 50 microM) were investigated. Western blot analysis provided evidence on the expression of PKC alpha, beta and delta isoforms in avian myoblasts. Two immunoreactive bands of 80 kDa (intact molecule) and 50 kDa (catalytic fragment) were detected for each isoenzyme. 1,25(OH)2D3 and DOG, which increased myoblast PKC activity parallel with the stimulation of DNA synthesis and culture growth and the phorbol ester TPA which induced the opposite changes, exerted differential effects on PKC isoenzymes. Long-term (72 h) treatment with 1,25(OH)2D3 and DOG did not change total PKC isoform levels but decreased the 80 kDa species and increased the release of the catalytic fragment of PKC delta and beta, whereas TPA augmented the total amounts of the three PKC isoforms, increasing the band of 80 kDa of PKC beta and delta and the 50 kDa species for PKC alpha. Subcellular distribution studies showed that the 80 kDa molecule is only present in the cytosolic fraction whereas in the particulate fractions the 50 kDa fragments are detected. Increased amounts of the catalytic fragments of PKC beta and delta both in the nucleus and membranes were observed after 72 h treatment with DOG while 1,25(OH)2D3 increases PKC beta in the nucleus and PKC delta in membranes. TPA induced the appearance of the 50 kDa species of PKC alpha in the nuclear and membrane fractions. The phorbol ester also decreased the catalytic fragments of PKC beta and delta in membranes. Increased levels of PKC beta, and to a lesser extent of PKC delta, in membranes and cytosol could be detected after short exposure (5 min) of myoblasts to 1,25(OH)2D3, DOG and TPA. In conclusion, the data indicate the operation in myoblasts of PKC signal transduction pathways mediated by the Ca(2+)-dependent PKCs alpha and beta and the Ca(2+)-independent PKC delta. Moreover, the results suggest that the beta and delta isoforms of PKC could play a role in the regulation of muscle cell metabolism by 1,25(OH)2D3.

The proteolytic cleavage of protein kinase C isotypes, which generates kinase and regulatory fragments, correlates with Fas-mediated and 12-O-tetradecanoyl-phorbol-13-acetate-induced apoptosis

Protein kinase C (PKC) has been implicated in signaling induced by diverse sets of stimuli regulating growth, differentiation, and apoptosis. The present study focused on the fate of PKC isotype proteins during Fas-mediated apoptosis of human leukemic cell lines. Among the PKC isotypes expressed in different cell types, such as Jurkat, HPB-ALL, U937, and HL60, all the nPKC isotypes including nPKCdelta, nPKC epsilon, and nPKCtheta, but not cPKC alpha and betaII and aPKCzeta (n, c, and a represent novel, conventional and atypical, respectively), showed limited proteolytic cleavage during Fas-mediated apoptosis. The limited proteolysis of nPKC isotypes means the disappearance of the intact protein band concomitant with the appearance of two fragments, most likely containing the kinase and regulatory domains, in contrast to the so-called down-regulation known for both cPKC and nPKC isotypes following exposure to stimuli such as 12-O-tetradecanoyl-phorbol 13-acetate (TPA). The time course of Fas-mediated apoptosis in Jurkat cells parallels that of the activation of a 32-kDa cysteine protease (CPP32)-like protease and also closely parallels the proteolytic cleavage of nPKC isotypes. A peptide inhibitor of the CPP32-like protease, Ac-DEVD-CHO, blocked the proteolytic cleavage of nPKC isotypes as well as apoptosis mediated by Fas. Transfection of recombinant protein coding for the catalytic fragment of nPKCdelta to COS1 cells resulted in the apoptotic morphology of cells and nuclei. The effect of TPA on apoptosis depends on the cell type. TPA significantly suppressed Fas-mediated apoptosis in Jurkat, whereas TPA alone caused apoptosis in HPB-ALL, U937, and HL60, only slight apoptosis in Jurkat. The proteolytic fragmentation of nPKC isotypes again closely correlated with the degree of apoptosis even in apoptosis induced by TPA. Separation of TPA-treated cells into apoptotic and non-apoptotic differentiating cells revealed that the proteolytic fragmentation of nPKC isotypes occurs only in apoptotic cells and, in adherent differentiating cells, nPKC isotypes as well as cPKC alpha were down-regulated without the generation of nPKC fragments. These results are consistent with the idea that nPKC isotypes meet two different fates, down-regulation and proteolytic cleavage generating kinase and regulatory fragments, and that the proteolytic cleavage of nPKC isotypes is a step in the signaling pathway involved in Fas-mediated and TPA-induced apoptosis.

Calpain activity promotes the sealing of severed giant axons

A barrier (seal) must form at the cut ends of a severed axon if a neuron is to survive and eventually regenerate. Following severance of crayfish medial giant axons in physiological saline, vesicles accumulate at the cut end and form a barrier (seal) to ion and dye diffusion. In contrast, squid giant axons do not seal, even though injury-induced vesicles form after axonal transection and accumulate at cut axonal ends. Neither axon seals in Ca2+-free salines. The addition of calpain to the bath saline induces the sealing of squid giant axons, whereas the addition of inhibitors of calpain activity inhibits the sealing of crayfish medial giant axons. These complementary effects involving calpain in two different axons suggest that endogenous calpain activity promotes plasmalemmal repair by vesicles or other membranes which form a plug or a continuous membrane barrier to seal cut axonal ends.

Activation of protein kinase C by intracellular free calcium in the motoneuron cell line NSC-19

The relationship between intracellular free calcium ([Ca2+]i) and the activation of protein kinase C (PKC) and Ca2+/calmodulin-dependent protein kinase II (CaMKII) was investigated in the NSC-19 motoneuron cell line. Increased extracellular calcium ([Ca2+]o) up to 10 mM resulted in sustained elevations of [Ca2+]i. Control cell cultures (1.3 mM [Ca2+]o, [Ca2+]i = 83 +/- 17 nM) contained Ca2+- and PS/DO lipid-dependent PKC activity predominantly in the cytosol. However, elevation of [Ca2+]o up to 5 mM ([Ca2+]i = 232 +/- 24 nM) resulted in almost complete loss of cytosolic PKC activity. Cells incubated in 10 mM [Ca2+]o ([Ca2+]i = 365 +/- 13 nM) showed increased levels of both cytosolic and membrane PKC activity compared to control. These alterations in PKC activity appeared to be translocation-independent, since PKC protein levels were unchanged as demonstrated by Western blotting analysis. When cells were exposed to 25 or 50 mM [Ca2+]o, [Ca2+]i rose transiently to over 600 and 900 nM, respectively, and then returned to near basal values. Under these conditions, total PKC activity decreased, and increased amounts of the catalytic fragment of PKC, protein kinase M, were generated. Extracts from cells exposed to [Ca2+]o between 1.3 and 25 mM did not differ significantly in the levels of measurable CaMKII activity 10 min following the change in [Ca2+]o.

Production and function of lipid second messengers in proliferating and differentiated neuroblastoma cells

Multiple cellular responses are regulated through the generation of lipid second messengers upon activation of phospholipases. One such response concerns the activity of a class of kinase constituting the protein kinase C family. The production of specific molecular species of lipid second messengers may be therefore of prime importance in the activation of a member of the PKC isoforms. Prompted by this possibility we investigated the production of 1,2 diacyl-sn-glycerol (DAG) and phosphatidic acid (PtdOH) in LA-N-1 neuroblastoma cells under various physiological states. 12-0-Tetradecanoylphorbol 13-acetate (TPA) stimulation activated a phospholipase D (PLD) specific for phosphatidylcholine (PtdCho) in proliferating cells and a phospholipase C (PLC) specific for phosphatidylethanolamine (PtdEtn) in retinoic acid (RA) differentiated cells. These separate activations produced different molecular species of DAG or PtdOH. PtdOH was able to stimulate the Ca2+ dependent protein kinase C (PKC) by a mechanism which differed from the action of DAG. PtdOH did not induce the translocation of the PKC to the membrane. Moreover PtdOH, in contrast to DAG, prevented PKC degradation by inhibiting the enzymatic hydrolysis by m-calpain. These observations suggest that the stimulation of cells by agonists elicited the production of specific molecular species of lipid second messengers depending on the physiological status of the cells, and probably on the nature of the stimulus. It seems therefore likely that the generation of specific lipid second messengers may activate specific PKC isoforms resulting in a specific cellular response.

Degradation of inositol 1,4,5-trisphosphate receptors during cell stimulation is a specific process mediated by cysteine protease activity

Inositol 1,4,5-trisphosphate (InsP3) receptors are down-regulated in response to chronic activation of certain cell surface receptors because their degradation is accelerated. Studies on the nature of the down-regulatory process and the protease(s) responsible for receptor degradation are described here. InsP3 receptor down-regulation was not accompanied by parallel changes in the concentrations of several other relevant proteins (endoplasmic reticulum Ca2+-ATPase, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, and protein kinases alpha and epsilon). Thus, the down-regulatory process selectively targets InsP3 receptors for degradation. Furthermore, down-regulation was unaffected by brefeldin A and NH4Cl, indicating that InsP3 receptor degradation occurs without removal of receptors from the endoplasmic reticulum and independently of functional lysosomes. Analysis of InsP3 receptor immunofluorescence confirmed that the receptors are not redistributed prior to or during down-regulation. Finally, of a range of protease inhibitors tested, only N-acetyl-Leu-Leu-norleucinal blocked down-regulation. Thus, cysteine protease activity accounts for InsP3 receptor degradation and analysis of proteolysis in permeabilized cells indicates that this activity is calpain. Thus, InsP3 receptor down-regulation appears to result from the highly selective calpain-mediated degradation of InsP3 receptors. Calpain activity may be stimulated by the high concentrations of Ca2+ that are thought to be found in the vicinity of activated InsP3 receptors.

Hyperphosphorylation of Tau and filopodial retraction following microinjection of protein kinase C catalytic subunits

Limited proteolysis of protein kinase C (PKC) by calcium-activated proteolysis cleaves the regulatory and catalytic subunits of PKC, generating a free, constitutively activated kinase ("PKM") that, unlike the intact parent enzyme, is not calcium-dependent, and is not restricted to the plasma membrane. These latter properties leave open the possibility that PKM may have access to, and may therefore phosphorylate, substrates normally unavailable to intact PKC. We examined the potential involvement of such aberrant phosphorylation in certain aspects of the neurodegeneration accompanying Alzheimer's disease by microinjecting PKC and PKM, along with a rhodamine-conjugated dextran tracer, into undifferentiated NB2a/d1 mouse neuroblastoma cells. After 4 hr, cultures were fixed and processed for immunofluorescence with monoclonal antibodies (PHF-1, ALZ-50, Tau-1, AT8) directed against tau in various phosphorylation states followed by fluorescein-conjugated secondary antibodies. Microinjected cells were localized via co-injected rhodamine-conjugated dextran tracer under rhodamine illumination, after which antibody immunoreactivity was examined under fluorescein illumination. Microdensitometric analyses indicated that microinjection of PKC did not increase basal immunofluorescent intensities of the antibodies; by contrast, microinjection of PKM induced three- and twofold increases in PHF-1 and ALZ-50 levels, respectively. By contrast, no significant alteration was observed in AT8 and Tau-1 immunofluorescence following either PKC or PKM microinjection. Whereas undifferentiated NB2a/d1 cells typically elaborate short, filopodia-like neurites, phase-contrast microscopy revealed the absence of filopodia or neurites on PKM-injected cells, while a similar percentage of PKC-injected cells. Cell-free analyses confirmed the ability of PKC, in the presence of necessary co-factors, and PKM to increase PHF-1 and ALZ-50 immunoreactivity; no change was observed in AT8 or Tau-1 immunoreactivity. These findings underscore the possibility that an abnormal amplification in limited PKC proteolysis to generate PKM could, under certain pathological conditions, contribute to neuronal degeneration.

Phospholipids inhibit proteolysis of protein kinase C alpha by mM calcium-requiring calpain

The alpha isoform of protein kinase C (PKC alpha) is rapidly hydrolyzed by mM Ca(2+)-requiring calpain (calcium-activated neutral proteinase) under cell-free conditions (Shea et al, 1994, FEBS Lett. 350:223). In the present study, we demonstrate that this hydrolysis is inhibited by phosphatidyl serine, diacylglycerol, phosphatidyl choline, phosphatidyl inositol, and phosphatidic acid. With the exception of phosphatidic acid, these phospholipids did not directly inhibit calpain activity as evidenced by degradation of [14C]azocasein, suggesting that the nature of inhibition of calpain-mediated PKC alpha degradation is due to an effect of phospholipids on PKC alpha conformation. These findings suggest that m calpain-mediated PKC alpha hydrolysis may be specifically minimized at the plasma membrane, and leave open the possibility that such a mechanism exists in situ. In addition, the unique inhibition of calpain activity by phosphatidic acid suggests the existence of a specific mechanism by which this phospholipid regulates PKC alpha activity.

The role of the calpain-calpastatin system in thyrotropin-releasing hormone-induced selective down-regulation of a protein kinase C isozyme, nPKC epsilon, in rat pituitary GH4C1 cells

We have examined the mechanism for the selective down-regulation of protein kinase C epsilon (nPKC epsilon) in rat pituitary GH4C1 cells responding to thyrotropin-releasing hormone (TRH) stimulation. Among various low molecular weight protease inhibitors examined, only a cysteine protease inhibitor (calpain inhibitor I, N-acetyl-Leu-Leu-norleucinal) blocked the down-regulation of nPKC epsilon. Furthermore, the introduction of a synthetic calpastatin peptide, an exclusively specific inhibitor of calpain, into the cells also reduced the down-regulation, suggesting the involvement of calpain among all the intracellular cysteine proteases in this process. In accordance, we observed TRH-induced translocation of m-calpain from the cytosol to the membrane and the concomitant up-regulation of calpastatin isoforms; presumably, the former represents activation of the protease initiating the kinase degradation, while the latter constitutes a negative feedback system protecting the cells from activated calpain. These results suggest that in GH4C1 cells, TRH mobilizes both protease (m-calpain) and inhibitor (calpastatin) as a strictly regulating system for the nPKC epsilon pathway mediating TRH signals.

Calpain: novel family members, activation, and physiologic function

The current status of calpain research is summarized on the basis of the most recent results. The main points are as follows. (i) Calpain constitutes a large family. (ii) Ca2+ ions cause the dissociation of calpain into subunits and the resulting free 80 kDa subunit is the active form of the enzyme. This dissociation corresponds to the activation of calpain. (iii) Some powerful clues have been obtained that will be helpful for analyzing the physiological function.

Proteolysis of protein kinase C: mM and microM calcium-requiring calpains have different abilities to generate, and degrade the free catalytic subunit, protein kinase M

Limited proteolysis of protein kinase C (PKC) by calpain under cell free conditions cleaves the regulatory and catalytic PKC subunits, generating a free, co-factor independent catalytic subunit, termed PKM. In the present study, we demonstrate distinct differences in the rate, nature, and lipid-sensitivity of PKC and PKM proteolysis by microM and mM calcium-requiring calpain isozymes (mu calpain or m calpain, respectively). PKC is a preferred substrate for m calpain; not even a 100-fold increase in mu calpain was capable of degrading PKC as fast as in calpain. PKM was generated by both m and mu calpains, but was itself rapidly degraded by m calpain and therefore was only transiently detectable. By contrast, PKM was formed but not degraded by mu calpain, and persisted in the presence of mu calpain long after all PKC had been degraded. Phosphatidyl serine (PS) inhibited PKC hydrolysis by m calpain yet enhanced PKC hydrolysis by mu calpain. The ability of either calpain isoenzyme to degrade [14C]azocasein was unaffected by PS, suggesting that the influence of PS was on PKC conformation. These findings point towards distinct roles for mu and m calpain in PKC regulation.

Protein kinase C isoforms in muscle cells and their regulation by phorbol ester and calpain

Objectives were to identify the PKC isoforms in cultured muscle cells, to examine roles of Ca(2+)-dependent proteinases (calpains) in processing of various muscle PKC isozymes and to obtain a mechanistic description of the processing of PKCs by examining the temporal relationships between phorbol ester-dependent translocation of muscle PKCs and calpains between cytosolic and membrane compartments. Using six isoform (alpha, beta, gamma, delta, epsilon, zeta)-specific polyclonal antibodies, PKC alpha, delta and zeta were detected in rat skeletal muscle and in L8 myoblasts and myotubes. PKC alpha and zeta were primarily localized in the cytosolic fraction of L8 myotubes whereas PKC delta was more abundant in the membrane fraction. Phorbol ester (TPA) caused rapid depletion of myotube PKC alpha and PKC alpha and PKC delta isoforms from the cytosolic compartment and rapid appearance of these isoforms in the membrane fraction. However, long-term exposure of myotubes to TPA eventually caused down-regulation of PKCs in the membrane compartment. Down-regulation of PKCs in the membrane fraction was partially blocked by calpain inhibitor II. However, the rapid TPA-dependent cytosolic depletion of PKCs was unaffected by calpain inhibitor. This suggests that calpains may be responsible for membrane-associated down-regulation of PKCs but not for cytosolic depletion. In the final study we assessed the effects of phorbol ester on compartmentation of m-calpain with PKCs in muscle cells. Like the PKCs, TPA caused rapid association of m-calpain with the membrane fraction followed by down-regulation. This demonstrates that phorbol esters cause translocation of both PKCs and calpains to membranes where processing of PKCs may occur via the limited proteolysis exerted by calpains.