Commonalities between pain and memory mechanisms and their meaning for understanding chronic pain

Pain sensing neurons in the periphery (called nociceptors) and the central neurons that receive their projections show remarkable plasticity following injury. This plasticity results in amplification of pain signaling that is now understood to be crucial for the recovery and survival of organisms following injury. These same plasticity mechanisms may drive a transition to a nonadaptive chronic pain state if they fail to resolve following the termination of the healing process. Remarkable advances have been achieved in the past two decades in understanding the molecular mechanisms that underlie pain plasticity following injury. The mechanisms bear a striking resemblance to molecular mechanisms involved in learning and memory processes in other brain regions, including the hippocampus and cerebral cortex. Here those mechanisms, their commonalities and subtle differences, will be highlighted and their role in causing chronic pain will be discussed. Arising from these data is the striking argument that chronic pain is a disease of the nervous system, which distinguishes this phenomena from acute pain that is frequently a symptom alerting the organism to injury. This argument has important implications for the development of disease modifying therapeutics.

Atypical PKC, PKCλ/ι, activates β-secretase and increases Aβ1-40/42 and phospho-tau in mouse brain and isolated neuronal cells, and may link hyperinsulinemia and other aPKC activators to development of pathological and memory abnormalities in Alzheimer's disease

Hyperinsulinemia activates brain Akt and PKC-λ/ι and increases Aβ1-40/42 and phospho-tau in insulin-resistant animals. Here, we examined underlying mechanisms in mice, neuronal cells, and mouse hippocampal slices. Like Aβ1-40/42, β-secretase activity was increased in insulin-resistant mice and monkeys. In insulin-resistant mice, inhibition of hepatic PKC-λ/ι sufficient to correct hepatic abnormalities and hyperinsulinemia simultaneously reversed increases in Akt, atypical protein kinase C (aPKC), β-secretase, and Aβ1-40/42, and restored acute Akt activation. However, 2 aPKC inhibitors additionally blocked insulin's ability to activate brain PKC-λ/ι and thereby increase β-secretase and Aβ1-40/42. Furthermore, direct blockade of brain aPKC simultaneously corrected an impairment in novel object recognition in high-fat-fed insulin-resistant mice. In neuronal cells and/or mouse hippocampal slices, PKC-ι/λ activation by insulin, metformin, or expression of constitutive PKC-ι provoked increases in β-secretase, Aβ1-40/42, and phospho-thr-231-tau that were blocked by various PKC-λ/ι inhibitors, but not by an Akt inhibitor. PKC-λ/ι provokes increases in brain β-secretase, Aβ1-40/42, and phospho-thr-231-tau. Excessive signaling via PKC-λ/ι may link hyperinsulinemia and other PKC-λ/ι activators to pathological and functional abnormalities in Alzheimer's disease.

BMI-related progression of atypical PKC-dependent aberrations in insulin signaling through IRS-1, Akt, FoxO1 and PGC-1α in livers of obese and type 2 diabetic humans

Information on insulin resistance in human liver is limited. In mouse diet-induced obesity (DIO), hepatic insulin resistance initially involves: lipid+insulin-induced activation of atypical protein kinase C (aPKC); elevated Akt activity/activation but selective impairment of compartmentalized Akt-dependent FoxO1 phosphorylation; and increases in gluconeogenic and lipogenic enzymes. In advanced stages, e.g., in hepatocytes of type 2 diabetes (T2D) humans, insulin activation of insulin receptor substrate-1(IRS-1) and Akt fails, further increasing FoxO1-dependent gluconeogenic/lipogenic enzyme expression. Increases in hepatic PGC-1α also figure prominently, but uncertainly, in this scheme. Here, we examined signaling factors in liver samples harvested from human transplant donors with increasing BMI, 20→25→30→35→40→45. We found, relative to lean (BMI=20-25) humans, obese (BMI>30) humans had all abnormalities seen in early mouse DIO, but, surprisingly, at all elevated BMI levels, had decreased insulin receptor-1 (IRS-1) levels, decreased Akt activity, and increased expression/abundance of aPKC-ι and PGC-1α. Moreover, with increasing BMI, there were: progressive increases in aPKC activity and PKC-ι expression/abundance; progressive decreases in IRS-1 levels, Akt activity and FoxO1 phosphorylation; progressive increases in expression/abundance of PGC-1α; and progressive increases in gluconeogenic and lipogenic enzymes. Remarkably, all abnormalities reached T2D levels at higher BMI levels. Most importantly, both "early" and advanced abnormalities were largely reversed by 24-hour treatment of T2D hepatocytes with aPKC inhibitor. We conclude: hepatic insulin resistance in human obesity is: advanced; BMI-correlated; and sequentially involves increased aPKC-activating ceramide; increased aPKC levels and activity; decreases in IRS-1 levels, Akt activity, and FoxO1 phosphorylation; and increases in expression/abundance of PGC-1α and gluconeogenic and lipogenic genes.

Par-complex aPKC and Par3 cross-talk with innate immunity NF-κB pathway in epithelial cells

Components of the Par-complex, atypical PKC and Par3, have been found to be downregulated upon activation of NF-κB in intestinal epithelial cells. To determine their possible role in pro-inflammatory responses we transduced Caco-2 human colon carcinoma cells with constitutively active (ca) PKCι or anti-Par3 shRNA-expressing lentiviral particles. Contrary to previous reports in other cell types, ca-PKCι did not activate, but rather decreased, baseline NF-κB activity in a luminiscence reporter assay. An identical observation applied to a PB1 domain deletion PKCι, which fails to localize to the tight-junction. Conversely, as expected, the same ca-PKCι activated NF-κB in non-polarized HEK293 cells. Likewise, knockdown of Par3 increased NF-κB activity and, surprisingly, greatly enhanced its response to TNFα, as shown by transcription of IL-8, GRO-1, GRO-2 and GRO-3. We conclude that aPKC and Par3 are inhibitors of the canonical NF-κB activation pathway, although perhaps acting through independent pathways, and may be involved in pro-inflammatory responses.

The interruption of atypical PKC signaling and Temozolomide combination therapy against glioblastoma

Current treatment options of glioblastoma include chemotherapy and limited surgical resection. Temozolomide (TMZ) is the current therapeutic choice for chemotherapy. Still, it has severe limitations due to the development of resistance that occurs by genetic modification and constitutive activation of several cell signaling pathways. Therefore, it is essential to develop combination therapy of TMZ with other novel compounds to prevent the development of chemo-resistance. In this study, we used two inhibitors; ICA, an inhibitor of PKC-ι and ζ-Stat, an inhibitor of PKC-ζ. T98G and U87MG glioblastoma cells were treated with either ICA or ζ-stat or TMZ monotherapies, as well as TMZ were combined with either ICA or ζ-stat for five consecutive days. Our in vitro results exhibited that ICA when combined with TMZ, significantly decreased the viability of cancerous cells compared with untreated or TMZ or ICA monotherapies. Additionally, glioblastoma cells were remarkably undergoing apoptosis against the combination treatment of TMZ and ICA nucleotide compared with untreated control cells, as suggested by our Annexin-V/PI flow cytometric analysis. Moreover, the combination of TMZ and ICA also decreased the invasion of glioblastoma cell lines by acting on FAK/Paxillin pathway, as evidenced by scratch assay, transwell invasion assay, Western blot and immunoprecipitation analysis. Furthermore, our in vivo data presented that the combination of ICA and TMZ also reduced glioblastoma tumor growth and volume in mice. These data suggest that atypical PKCs, particularly PKC-ι might be an important therapeutic target as adjuvant therapy in the treatment of glioblastoma.

Electrotonic transmission in the retinal vasculature: inhibitory role of the diabetes/VEGF/aPKC pathway

The deleterious impact of diabetes on the retina is a leading cause of vision loss. Ultimately, the hypoxic retinopathy caused by diabetes results in irreversible damage to vascular, neuronal, and glial cells. Less understood is how retinal physiology is altered early in the course of diabetes. We recently found that the electrotonic architecture of the retinovasculature becomes fundamentally altered soon after the onset of this disorder. Namely, the spread of voltage through the vascular endothelium is markedly inhibited. The goal of this study was to elucidate how diabetes inhibits electrotonic transmission. We hypothesized that vascular endothelial growth factor (VEGF) may play a role since its upregulation in hypoxic retinopathy is associated with sight-impairing complications. In this study, we quantified voltage transmission between pairs of perforated-patch pipettes sealed onto abluminal cells located on retinal microvascular complexes freshly isolated from diabetic and nondiabetic rats. We report that exposure of diabetic retinal microvessels to an anti-VEGF antibody or to a small-molecule inhibitor of atypical PKCs (aPKC) near-fully restored the efficacy of electrotonic transmission. Furthermore, exposure of nondiabetic microvessels to VEGF mimicked, via a mechanism sensitive to the aPKC inhibitor, the diabetes-induced inhibition of transmission. Thus, activation of the diabetes/VEGF/aPKC pathway switches the retinovasculature from a highly interactive operational unit to a functionally balkanized complex. By delimiting the dissemination of voltage-changing vasomotor inputs, this organizational fragmentation is likely to compromise effective regulation of retinal perfusion. Future pharmacological targeting of the diabetes/VEGF/aPKC pathway may serve to impede progression of vascular dysfunction to irreversible diabetic retinopathy.

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.

Occludin induces microvillus formation via phosphorylation of ezrin in a mouse hepatic cell line

Apical and basolateral cell membranes are separated by tight junctions (TJs). Microvilli are limited to the apical cell membrane. TJs and microvilli are the landmarks for epithelial cell polarity. However, the direct relationship between TJ proteins (TJPs) and the components of microvilli remains unclear. In this study, we investigated whether occludin, which is considered to be a functional TJP, is involved in microvillus formation. In occludin knockout mouse hepatic cells (OcKO cells), the microvillus density was less than that in wild-type (WT) cells and the length of microvilli was short. Immunoreactivity of ezrin was decreased in OcKO cells compared with that in WT cells. Although there was no change in the expression level of ezrin, phosphorylation of ezrin was decreased in OcKO cells. The microvillus density and the length of microvilli were increased in OcKO cells by transfection of full-length mouse occludin and COOH-terminal domains of occludin. These results suggested that occludin induced microvillus formation via phosphorylation of ezrin and that the COOH-terminal domain of occludin, which is localized in non-TJ areas, might be able to induce microvilli formation. Our results provide new insights into the function of occludin.

Immunosurveillance, interferon, and autophagic networking in cancer: the PRKCI-ULK2 paradigm

The mechanisms controlling immunosurveillance and immunoevasion often operate simultaneously to the triggering of the oncogenic signaling that results in tumor initiation. The resolution of the balance between anti-cancer immune responses and pro-tumorigenic pathways determines if a tumor cell survives and can remodel the microenvironment to reinforce immunosuppression or is eliminated by the immune system. Cancer cells must endure a toxic and metabolically challenging milieu. In its various forms, autophagy provides a way for transformed cells to survive by promoting catabolism and detoxification. Mounting evidence suggests that the boundaries between cancer immunity and mitogenic and metabolic programs are diffuse, with the same molecules likely serving several diverse roles in immunity and metabolism during tumor initiation and progression. Our recent data provide mechanistic detail and functional relevance of a new paradigm whereby the same signaling elements control immunity and autophagy in cancer.