Atypical protein kinase C plays a critical role in protein transport from pre-Golgi intermediates

Overexpression of atypical protein kinase C in HeLa cells facilitates macropinocytosis via Src activation

Atypical protein kinase C (aPKC) is the first recognized kinase oncogene. However, the specific contribution of aPKC to cancer progression is unclear. The pseudosubstrate domain of aPKC is different from the other PKC family members, and therefore a synthetic peptide corresponding to the aPKC pseudosubstrate (aPKC-PS) sequence, which specifically blocks aPKC kinase activity, is a valuable tool to assess the role of aPKC in various cellular processes. Here, we learned that HeLa cells incubated with membrane permeable aPKC-PS peptide displayed dilated heterogeneous vesicles labeled with peptide that were subsequently identified as macropinosomes. A quantitative membrane binding assay revealed that aPKC-PS peptide stimulated aPKC recruitment to membranes and activated Src. Similarly, aPKC overexpression in transfected HeLa cells activated Src and induced macropinosome formation. Src-aPKC interaction was essential; substitution of the proline residues in aPKC that associate with the Src-SH3 binding domain rendered the mutant kinase unable to induce macropinocytosis in transfected cells. We propose that aPKC overexpression is a contributing factor to cell transformation by interacting with and consequently promoting Src activation and constitutive macropinocytosis, which increases uptake of extracellular factors, required for altered cell growth and accelerated cell migration.

Involvement of diacylglycerol kinase γ in modulation of iNOS synthesis in Golgi apparatus of vascular endothelial cells

The aim of this study was to clarify the role of diacylglycerol kinase (DGK)γ in vascular endothelial cells. The mRNA and protein expression of DGKγ and inducible nitric oxide synthase (iNOS) in rat aortic endothelial cells (RAECs) were investigated using RT-PCR, immunocytochemical, and immunoblot analyses. In RAECs, immunoreactivity of DGKγ was detected in the cytoplasm as a tubular or reticular structure. DGKγ immunoreactivity colocalized with those for GM130 and Golgin 97 but not with that for protein disulfide isomerase (PDI). In the presence of brefeldin A, DGKγ immunoreactivity was markedly decreased and displayed an aggregation-like pattern. After treatment of RAECs with nocodazole, DGKγ immunoreactivity was detected in Golgi stacks, which were severely segmented and appeared in vesicular shape. Stimulation with IL-1β increased mRNA expression of DGKγ, which was strongly attenuated by SB203580, a p38 MAPK inhibitor. IL-1β also induced expression of iNOS, which was observed as a tubular structure, and this distribution coincided with DGKγ immunoreactivity. Brefeldin A reduced both iNOS immunoreactivity and DGKγ immunoreactivity. iNOS expression was impaired by DGK inhibitors, R59022 and R59949. These results suggest that DGKγ is upregulated by IL-1β through the p38 MAPK pathway and may be involved in protein trafficking of iNOS in vascular endothelial cells.

Expression and localization of atypical PKC isoforms in liver parenchymal cells

Members of all three classes of the protein kinase C (PKC) family including atypical PKCzeta (PKCzeta) are involved in central functions of liver parenchymal cells. However, expression and localization of PKCiota (PKCiota), the highly homologous atypical PKC (aPKC) isoform, in hepatocytes is unknown to date. PKCzeta and PKCiota were cloned from human and rat liver and fused to fluorescent protein tags (YFP). The sequence of full-length rat PKCiota is not yet known and was cloned from cDNA of hepatocytes by the use of degenerated primers. PKCzeta-YFP and PKCiota-YFP (human and rat) were expressed in HeLa or HEK293 cells and used to test the specificity of seven aPKC antibodies. Two antibodies were PKCiota-specific and two were specific for PKCzeta in immunofluorescence and Western blot analysis. Subcellular localization was analyzed by immunofluorescence in isolated rat and human hepatocytes and liver sections. Low immunoreactivity for aPKCs was found at the sinusoidal membrane and in the cytosol. The highest density of PKCiota as well as PKCzeta was found at the canalicular membrane in co-localization with ABC-transporters, such as bile salt export pump or multidrug resistance-associated protein 2. This topology suggests a specific function of aPKCs at the canalicular membrane in addition to their known role in cell polarity of epithelial cells.

Dynamics of glucose-induced localization of PKC isoenzymes in pancreatic beta-cells: diabetes-related changes in the GK rat

Glucose metabolism affects most major signal pathways in pancreatic beta-cells. Multiple protein kinases, including protein kinase C (PKC) isoenzymes, are involved in these effects; however, their role is poorly defined. Moreover, the dynamics of kinase isoenzyme activation in reference to the biphasic insulin secretion is unknown. In perfused pancreas of Wistar rats, PKCalpha staining was strongly associated with insulin staining, jointly accumulating in the vicinity of the plasma membrane during early first-phase insulin response. The signal declined before the onset of second phase and reappeared during second-phase insulin release as foci, only weekly associated with insulin staining; this signal persisted for at least 15 min after glucose stimulation. In the GK rat, glucose had minimal effect on beta-cell PKCalpha. In control beta-cells, PKCdelta stained as granulated foci with partial association with insulin staining; however, no glucose-dependent translocation was observed. In the GK rat, only minimal staining for PKCdelta was observed, increasing exclusively during early first-phase secretion. In Wistar beta-cells, PKCepsilon concentrated near the nucleus, strongly associated with insulin staining, with dynamics resembling that of biphasic insulin response, but persisting for 15 min after cessation of stimulation. In GK rats, PKCepsilon staining lacked glucose-dependent changes or association with insulin. PKCzeta exhibited bimodal dynamics in control beta-cells: during early first phase, accumulation near the cell membrane was observed, dispersing thereafter. This was followed by a gradual accumulation near the nucleus; 15 min after glucose stimulus, clear PKCzeta staining was observed within the nucleus. In the GK rat, a similar response was only occasionally observed. In control beta-cells, glucose stimulation led to a transient recruitment of PKCtheta, associated with first-phase insulin release, not seen in GK beta-cell. Data from this and related studies support a role for PKCalpha in glucose-induced insulin granule recruitment for exocytosis; a role for PKCepsilon in activation of insulin granules for exocytosis and/or in the glucose-generated time-dependent potentiation signal for insulin release; and a dual function for PKCzeta in initiating insulin release and in a regulatory role in the transcriptional machinery. Furthermore, diminished levels and/or activation of PKCalpha, PKCepsilon, PKCtheta, and PKCzeta could be part of the defective signals downstream to glucose metabolism responsible for the deranged insulin secretion in the GK rat.

Palmitoylation of Inducible Nitric-oxide Synthase at Cys-3 Is Required for Proper Intracellular Traffic and Nitric Oxide Synthesis

A number of cell types express inducible nitric-oxide synthase (NOS2) in response to exogenous insults such as bacterial lipopolysaccharide or proinflammatory cytokines. Although it has been known for some time that the N-terminal end of NOS2 suffers a post-translational modification, its exact identification has remained elusive. Using radioactive fatty acids, we show herein that NOS2 becomes thioacylated at Cys-3 with palmitic acid. Site-directed mutagenesis of this single residue results in the absence of the radiolabel incorporation. Acylation of NOS2 is completely indispensable for intracellular sorting and .NO synthesis. In fact, a C3S mutant of NOS2 is completely inactive and accumulates to intracellular membranes that almost totally co-localize with the Golgi marker beta-cop. Likewise, low concentrations of the palmitoylation blocking agents 2-Br-palmitate or 8-Br-palmitate severely affected the .NO synthesis of both NOS2 induced in muscular myotubes and transfected NOS2. However, unlike endothelial NOS, palmitoylation of inducible NOS is not involved in its targeting to caveolae. We have created 16 NOS2-GFP chimeras to inspect the effect of the neighboring residues of Cys-3 on the degree of palmitoylation. In this regard, the hydrophobic residue Pro-4 and the basic residue Lys-6 seem to be indispensable for palmitoylation. In addition, agents that block the endoplasmic reticulum to Golgi transit such as brefeldin A and monensin drastically reduced NOS2 activity leading to its accumulation in perinuclear areas. In summary, palmitoylation of NOS2 at Cys-3 is required for both its activity and proper intracellular localization.

Rab2 Interacts Directly with Atypical Protein Kinase C (aPKC) ι/λ and Inhibits aPKCι/λ-dependent Glyceraldehyde-3-phosphate Dehydrogenase Phosphorylation

Atypical protein kinase C iota/lambda (PKCiota/lambda) is essential for protein transport in the early secretory pathway. The small GTPase Rab2 selectively recruits the kinase to vesicular tubular clusters (VTCs) where PKCiota/lambda phosphorylates glyceraldehyde-3-phosphate dehydrogenase (GAPDH). VTCs are composed of small vesicles and tubules and serve as transport intermediates that shuttle cargo from the endoplasmic reticulum to the Golgi complex. These structures are the first site of segregation of the anterograde and retrograde pathways. When Rab2 binds to a VTC subcompartment, the subsequent recruitment of PKCiota/lambda and soluble components, including COPI (coatomer and ADP-ribosylation factor), results in the release of retrograde-directed vesicles. Because Rab2 stimulates PKCiota/lambda membrane association in a dose-dependent manner, we investigated whether the two proteins physically interact. Using a combination of in vivo and in vitro assays, we found that Rab2 interacts directly with PKCiota/lambda and that this interaction occurs through the Rab2 amino terminus (residues 1-19) and the PKCiota/lambda regulatory domain. A mutant lacking the PKCiota/lambda binding domain (Rab2N'Delta19) was functionally characterized. In contrast to Rab2, Rab2N'Delta19 failed to recruit PKCiota/lambda to normal rat kidney microsomes in a quantitative binding assay. To determine whether Rab2 modulates the ability of PKCiota/lambda to phosphorylate GAPDH, an in vitro kinase assay was supplemented with Rab2 or Rab2N'Delta19. Rab2 inhibited PKCiota/lambda-dependent GAPDH phosphorylation, whereas no effect was observed when the assay was performed with the aminoterminal truncation mutant. These results suggest that a downstream effector recruited to the VTC stimulates PKCiota/lambda-mediated GAPDH phosphorylation by alleviating the inhibition imposed by Rab2-PKCiota/lambda interaction.