Protein kinase C decreases the hepatocyte growth factor-induced activation of Erk1/Erk2 MAP kinases

Madecassoside suppresses proliferation and invasiveness of HGF-induced human hepatocellular carcinoma cells via PKC-cMET-ERK1/2-COX-2-PGE2 pathway

Recent studies showed that Madecassoside (MAD), a pentacyclic triterpene isolated from Centella asitica (L.), was used as a therapeutic agent in wound healing and also as an anti-inflammatory, anti-oxidative activities and anti-aging agent. However, its role in cancer has not been elucidated. In our present study, hepatocyte growth factor (HGF) induced the phosphorylation of its corresponding receptor cMET, increased expression of cyclo-oxygenase-2 (COX-2) and prostaglandin E2 (PGE2) in human hepatocellular carcinoma (HCC) cells lines (HepG2 and SMMC-77), and this effect was inhibited by MAD in a dose-dependent manner. In addition, MAD exhibited significant anti-proliferative and anti-invasive effect in HGF-induced HepG2 and SMMC-77 cells. Moreover, MAD inhibited the phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and the protein kinase C (PKC) activity in HGF-induced HepG2 and SMMC-77 cells. This conclusion was consistent with the effect of selective COX-2 inhibitor (NS-398) and knockdown of COX-2 by siRNA on attenuating the proliferation and invasiveness potential, and over-expression of COX-2 on abolishing the effects of MAD on proliferation and invasiveness potential, and was also in parallel with the effect of PKC inhibitor (Bisindolylmaleimide) on inhibiting PKC activity, MEK/ERK1/2 inhibitor (PD98059) inhibited MEK/ERK1/2 pathways in HGF-induced HepG2 and SMMC-77 cells. Collectively, MAD could inhibit the HGF-activated proliferation and invasiveness of HCC cells via regulating the activation of cMET-PKC-ERK1/2-COX-2-PGE2 cascade, which indicated that MAD might help control HGF-linked HCC.

PKCε-mediated c-Met endosomal processing directs fluctuant c-Met-JNK-paxillin signaling for tumor progression of HepG2

Hepatocyte growth factor (HGF) induced c-Met signaling play critical roles in the progression of hepatocellular carcinoma (HCC). However, c-Met targeting approaches suffered resistance and side effect, thus identification of more suitable downstream targets is needed. Recently, we demonstrated HGF-induced fluctuant ERK/paxillin signaling within 24h. We further examined the underlying mechanisms for fluctuant c-Met/JNK/paxillin signal cascade within 12h. HGF-induced phosphorylation of c-Met, JNK, and paxillin (Ser178) shared a common fluctuation pattern characterized by an initial peak at 0.5h, a middle drop at 4h, and a later peak at 10h. Dynasore, the inhibitor of dynamin, suppressed HGF-induced c-Met internalization and phosphorylation of JNK and paxillin (Ser178) at 0.5h, indicating that endosome formation is required for initial signal enhancement. Further, depletion of PKCε not only enhanced HGF-induced phosphorylation of JNK and paxillin (Ser178) but also prevented c-Met degradation at 0.5h, suggesting that PKCε mediated c-Met degradation for signal declination. On the other hand, HGF induced colocalizations of both phosphorylated JNK and paxillin with the endosomal recycling protein GGA3 at 10h and depletion of GGA3 abolished membrane recycling of c-Met and phosphorylation of JNK/paxillin at the same time point. Interestingly, HGF induced GGA3 phosphorylation in a PKCε-dependent manner during 0.5-4h, which is associated with c-Met degradation in the same period. Finally, HGF-induced cell migration, invasion and intrahepatic metastasis of HepG2 were prevented by the inhibitors of endocytosis. Our results suggest that critical endosomal components are promising therapeutic targets for preventing HGF-induced progression of HCC.

PKC mediates fluctuant ERK-paxillin signaling for hepatocyte growth factor-induced migration of hepatoma cell HepG2

Hepatocyte growth factor (HGF) is critical for triggering metastasis of hepatocellular carcinoma cell (HCC). Extracellular signal-regulated kinase (ERK) mediates HGF-induced cell migration via focal adhesion signaling. Protein kinase C (PKC) is a negative regulator of ERK activation, however, both PKC and ERK were required for HGF-induced cell migration. To address this intriguing issue, the signal mechanisms for HGF-induced HepG2 cell migration were investigated in a long-term fashion. HGF-induced phosphorylations of ERK, Src (at Tyr 416) and paxillin (at Ser178 and Tyr31) were up and down for 3 times within 24h. HGF also induced fluctuant PKC activation and Rac degradation. Consistently, HGF induced intermittent actin polarization within 24h, which can be blocked by the inhibitors of PKC (Bisindolymaleimide) and ERK. Inhibitor studies revealed that ERK was required for HGF-induced paxillin phosphorylation at Ser178, whereas PKC and Rac-1 may suppress HGF-induced phosphorylation of ERK and paxillin (at Ser178) and upregulate phosphorylation of paxillin at Tyr31. Based on shRNA technique, PKCα and δ were responsible for suppressing HGF-induced phosphorylation of ERK and paxillin (at Ser178), whereas PKC ε and ζ were required for phosphorylation of paxillin at Tyr31. The HGF-induced fluctuant signaling is reminiscent of c-Met endocytosis. Using Concanavalin A, an inhibitor of endocytosis, we found that c-Met endocytosis was required for PKC to suppress ERK phosphorylation. Moreover, HGF-induced c-Met degradation was also fluctuant, which can be prevented by Bisindolymaleimide. In conclusion, PKC is critical for mediating HGF-induced fluctuant ERK-paxillin signaling during cell migration, probably via triggering endosomal degradation of c-Met.

Role of protein kinase C inhibition in the development of diabetic microvascular complications

Diabetes mellitus affects millions of people in the USA and throughout the world. The global epidemic may be attributed to both the increasing age of the population and the increasing rates of obesity. Diabetes is associated with chronic microvascular (diabetic neuropathy, retinopathy and nephropathy) and macrovascular complications (heart disease, stroke and peripheral vascular disease). Patients with diabetes benefit from a comprehensive approach to prevent complications, including weight loss, smoking cessation, antiplatelet agents, and glycemic and blood pressure control. However, all of these are difficult for patients to achieve and maintain on a daily basis and there is still a risk of developing vascular complications. Specific therapies for diabetic complications targeting molecules that are activated by hyperglycemia are emerging. The aim of this review is to highlight the role of protein kinase Cβ in the development of diabetic microvascular complications while briefly describing clinical experience with the protein kinase Cβ inhibitor ruboxistaurin.

PKCα reduces the lipid kinase activity of the p110α/p85α PI3K through the phosphorylation of the catalytic subunit

The modulation of phosphoinositide 3-kinase (PI3K) activity influences the quality of cellular responses triggered by various receptor tyrosine kinases. Protein kinase C (PKC) has been reported to phosphorylate signalling molecules upstream of PI3K and thereby it may affect the activation of PI3K. Here, we provide the first evidence for a direct effect of a PKC isoenzyme on the activity of PI3K. PKCalpha but not PKCepsilon phosphorylated the catalytic subunit of the p110alpha/p85alpha PI3K in vitro in a manner inhibited by the PKC inhibitor bisindolylmaleimide I (BIM I). The incubation of PI3K with active PKCalpha resulted in a significant decrease in its lipid kinase activity and this effect was also attenuated by BIM I. We conclude that PKCalpha is able to modulate negatively the lipid kinase activity of the p110alpha/p85alpha PI3K through the phosphorylation of the catalytic subunit.

Translation modulation of acid beta-glucosidase in HepG2 cells: participation of the PKC pathway

Acid beta-glucosidase (GCase) is the enzyme deficient in Gaucher disease, a prototypical inherited metabolic error for enzyme and gene therapy. An 80 kDa mammalian cytoplasmic translational control protein (TCP80) modulates GCase translation in vitro and ex vivo by interacting with the 5' coding region of GCase RNA. Ten predicted PKC phosphorylation sites (Ser- or Thr-) are in the TCP80 protein. Phosphorylation of TCP80 in vitro by PKC greatly enhanced its translational inhibitory function using in vitro translation assays; binding of GCase mRNA to TCP80 was unaltered. Conversely, de-phosphorylation of TCP80 reduced its translational inhibitory function. Phosphorylation-related modulation of GCase mRNA translation also was studied in HepG2 cells. GCase expression (protein and activity levels) in HepG2 cells increased (>2-fold) in cells treated with bisindolylmaleimide (BIM), a highly selective PKC specific inhibitor. This correlated with a 90% reduction in TCP80 phosphorylation in the presence of BIM. The amount of TCP80 protein in cytoplasm and its RNA-binding activity were unchanged. These experiments indicate that GCase mRNA translation is modulated by PKC signaling pathways that are mediated through TCP80. These findings indicate potential broader impacts of the TCP/PKC system on expression of this and other genes of therapeutic interest.

PKC controls HGF-dependent c-Met traffic, signalling and cell migration

The growth factor/receptor pair HGF/c-Met exerts control on proliferation, morphogenesis and motility, and through overexpression and mutation is implicated in cancer. Here we have investigated the relationship between receptor signalling and traffic, and its control by specific PKC isotypes. It is shown that c-Met signalling to the ERK cascade occurs within endosomal compartments and that it is in this compartment that PKCepsilon specifically exerts its control on the pathway with the consequent accumulation of ERK in focal complexes. These events are clearly separated from the subsequent microtubule-dependent sorting of c-Met to its perinuclear destination, which is shown to be under the control of PKCalpha. Thus while it is shown that traffic to endosomes is essential for HGF/c-Met to trigger an ERK response, the subsequent traffic and signalling of c-Met controlled by these two PKC isotypes are unconnected events. The dynamic properties conferred by the PKCepsilon control are shown to be essential for a normal HGF-dependent migratory response. Thus PKCs are shown to control both receptor traffic and signal traffic to relay HGF/c-Met responses.

Protein kinase C modulates negatively the hepatocyte growth factor-induced migration, integrin expression and phosphatidylinositol 3-kinase activation

Previously, we reported that, in hepatocyte growth factor (HGF)-induced HepG2 cells, protein kinase C (PKC) decreased the duration of intensive Erk1/Erk2 MAP kinase activation. This study shows that the inhibition of PKC enhanced significantly the HGF-induced integrin expression. Beside the prolonged activation of Erk1/Erk2, the activity of phosphatidylinositol 3-kinase (PI 3K) was required for growth factor-induced integrin expression. PI 3-kinase was activated to a higher extent in response to HGF than to epidermal growth factor (EGF), though the activation was transient in both cases. In EGF-induced cells, PI 3K activation was terminated by the loss of phosphotyrosine docking sites for PI 3K. To the contrary, the decrease of PI 3K activation, which followed the HGF-induced increase was not accompanied by the loss of phosphotyrosine docking sites and was prevented by the inhibition of PKC. The negative modulator effects of PKC on integrin expression and PI 3-kinase activation correlated with its ability to limit the HGF-induced motogen response.

Protein kinase C controls microtubule-based traffic but not proteasomal degradation of c-Met

Upon hepatocyte growth factor stimulation, its receptor c-Met is rapidly internalized via clathrin-coated vesicles and traffics through an early endosomal compartment. We show here that c-Met accumulates progressively in perinuclear compartments, which in part include the Golgi. The c-Met content in the Golgi is principally the newly synthesized precursor form and, to a lesser extent, the internalized, recycling c-Met. By following the trafficking of c-Met inside the cell using a semi-automatic procedure and using inhibition or activation of protein kinase C (PKC) and microtubule depolymerizing agents, we show that PKC positively controls the trans-cytosolic movement of c-Met along microtubules. In parallel to its traffic, internalized c-Met is progressively degraded by a proteasome-sensitive mechanism; the lysosomal pathway does not play a substantial role. Inhibition or promotion of c-Met traffic to the perinuclear compartment does not alter the kinetics of proteasome-dependent c-Met degradation. Thus susceptibility to proteasomal degradation is not a consequence of post-endocytic traffic. The data define a PKC-controlled traffic pathway for c-Met that operates independently of its degradative pathway.

Phorbol ester-induced migration of HepG2 cells is accompanied by intensive stress fibre formation, enhanced integrin expression and transient down-regulation of p21-activated kinase 1

Previously, we observed that phorbol ester induced more intensive scattering of HepG2 human hepatoma cells than hepatocyte growth factor (HGF). Regulatory components accounting for this intensive migration were studied. Phorbol ester-activated protein kinase C induced the early appearance of a great number of actin stress fibres. Whereas in response to HGF, the activation of phosphatidylinositol 3-kinase initiates the rearrangements of the actin cytoskeleton, in phorbol ester-treated cells, the activation of this enzyme was not required to the actin polymerisation. Activation of Erk1/Erk2 MAP kinases that was essential to the migration had a key role in enhancing the adherence of cells to the extracellular matrix via the increased expression of integrins alpha2, alpha6 and beta1. Protein kinase C stimulated the activation of p21-activated kinase (PAK), as well. However, it also stimulated the selective and transient down-regulation of PAK1, which coincided with the formation of stress fibres.

Increased Sp1 phosphorylation as a mechanism of hepatocyte growth factor (HGF/SF)-induced vascular endothelial growth factor (VEGF/VPF) transcription

Hepatocyte growth factor (HGF/SF)-induced expression of vascular endothelial growth factor (VEGF/VPF) has been implicated in paracrine amplification of angiogenesis, contributing to angiogenic responses during inflammation, wound healing, collateral formation and tumor growth. We have shown previously that HGF/SF-mediated VEGF/VPF expression by keratinocytes is primarily dependent on transcriptional activation, and we mapped the HGF/SF-responsive element to a GC-rich region between bp -88 and -65. Sp1-like factors bind to this element constitutively; however the VEGF/VPF promoter is transactivated by HGF/SF in the absence of induced binding activity. In experimental approaches to clarify molecular mechanisms of Sp1-dependent VEGF/VPF gene transcription, neither HGF/SF-dependent changes in nuclear expression nor in relative DNA binding activity of Sp family members to the indicated element were observed. Thus, HGF/SF was hypothesized to induce VEGF/VPF gene transcription via increased transactivation activity of Sp1 owing to biochemical modification. In immunoprecipitation studies, HGF/SF was found to increase the amount of serine-phosphorylated Sp1, revealing a likely mechanism of HGF/SF-induced VEGF/VPF expression, as phosphorylation may enhance the transcriptional activity of Sp1. The contribution of different signaling molecules to HGF/SF-induced VEGF/VPF transcription was demonstrated by the use of chemical inhibition, of expression of kinase-deficient signaling proteins, and by the use of antisense oligonucleotides. Herein, we provide evidence that PI 3-kinase, MEK1/2 and PKC-zeta play a significant role in HGF/SF-induced VEGF/VPF promoter activation. Together, our results elucidate a critical pathway of paracrine amplification of angiogenesis, suggesting that HGF/SF-induced Sp1 phosphorylation may activate VEGF/VPF promoter activity that requires the contribution of distinct signaling molecules.

Activation of Erk1/Erk2 and transiently increased p53 levels together may account for p21 expression associated with phorbol ester-induced transient growth inhibition in HepG2 cells

In HepG2 cells grown in the presence of serum, enhanced Raf-activation correlated with transient growth inhibition. The activation of Raf was increased either by the phorbol ester-induced activation of protein kinase C (PKC) or by the addition of the PKC inhibitor bisindolylmaleimide I (BIM). Either of these treatments increased the cellular levels of p21 by an Erk1/Erk2 MAP kinase cascade-dependent way, since this increase was prevented by the MEK-inhibitor PD98059. Nevertheless, the growth inhibition correlated with the transient increase of p53 levels as well. Either the activation of PKC with phorbol ester or the addition of BIM to cells growing in serum induced a rapid but transient increase of p53 levels, which preceded growth inhibition. This increase of p53 levels was probably due to the transient stabilisation of p53 and did not require the activation of Erk1/Erk2.