Sessile serrated lesions with dysplasia: is it possible to nip them in the bud?

The serrated neoplasia pathway constitutes an "alternative route" to colorectal cancer (CRC), and sessile serrated lesions with dysplasia (SSLDs) are an intermediate step between sessile serrated lesions (SSLs) and invasive CRC in this pathway. While SSLs show indolent growth before becoming dysplastic (> 10-15 years), SSLDs are considered to rapidly progress to either immunogenic microsatellite instable-high (MSI-H) CRC (presumably 75% of cases) or mesenchymal microsatellite stable (MSS) CRC. Their flat shapes and the relatively short window of this intermediate state make it difficult to detect and diagnose SSLDs; thus, these lesions are potent precursors of post-colonoscopy/interval cancers. Confusing terminology and the lack of longitudinal observation data of serrated polyps have hampered the accumulation of knowledge about SSLDs; however, a growing body of evidence has started to clarify their characteristics and biology. Together with recent efforts to incorporate terminology, histological studies of SSLDs have identified distinct dysplastic patterns and revealed alterations in the tumor microenvironment (TME). Molecular studies at the single-cell level have identified distinct gene alterations in both the epithelium and the TME. Mouse serrated tumor models have demonstrated the importance of TME in disease progression. Advances in colonoscopy provide clues to distinguish pre-malignant from non-malignant-SSLs. Recent progress in all aspects of the field has enhanced our understanding of the biology of SSLDs. The aim of this review article was to assess the current knowledge of SSLDs and highlight their clinical implications.

Protein kinase C (PKC)ζ-mediated Gαq stimulation of ERK5 protein pathway in cardiomyocytes and cardiac fibroblasts.

Gq-coupled G protein-coupled receptors (GPCRs) mediate the actions of a variety of messengers that are key regulators of cardiovascular function. Enhanced Gα(q)-mediated signaling plays an important role in cardiac hypertrophy and in the transition to heart failure. We have recently described that Gα(q) acts as an adaptor protein that facilitates PKCζ-mediated activation of ERK5 in epithelial cells. Because the ERK5 cascade is known to be involved in cardiac hypertrophy, we have investigated the potential relevance of this pathway in cardiovascular Gq-dependent signaling using both cultured cardiac cell types and chronic administration of angiotensin II in mice. We find that PKCζ is required for the activation of the ERK5 pathway by Gq-coupled GPCR in neonatal and adult murine cardiomyocyte cultures and in cardiac fibroblasts. Stimulation of ERK5 by angiotensin II is blocked upon pharmacological inhibition or siRNA-mediated silencing of PKCζ in primary cultures of cardiac cells and in neonatal cardiomyocytes isolated from PKCζ-deficient mice. Moreover, upon chronic challenge with angiotensin II, these mice fail to promote the changes in the ERK5 pathway, in gene expression patterns, and in hypertrophic markers observed in wild-type animals. Taken together, our results show that PKCζ is essential for Gq-dependent ERK5 activation in cardiomyocytes and cardiac fibroblasts and indicate a key cardiac physiological role for the Gα(q)/PKCζ/ERK5 signaling axis.

G alpha(q) acts as an adaptor protein in protein kinase C zeta (PKCzeta)-mediated ERK5 activation by G protein-coupled receptors (GPCR).

G(q)-coupled G protein-coupled receptors (GPCR) mediate the actions of a variety of messengers that are key regulators of different cellular functions. These receptors can regulate a highly interconnected network of biochemical routes that control the activity of several members of the mitogen-activated protein kinase (MAPK) family. The ERK5 MAPK has been shown to be activated by G(q)-coupled GPCR via unknown mechanisms. We find that the atypical protein kinase C (PKCzeta), previously reported to interact with the ERK5 activator MEK5 and to be involved in epidermal growth factor-mediated ERK5 stimulation, plays a crucial role in the activation of the ERK5 pathway by G(q)-coupled GPCR. Stimulation of ERK5 by G(q)-coupled GPCR is abolished upon pharmacological inhibition of PKCzeta as well as in embryonic fibroblasts obtained from PKCzeta-deficient mice. Both PKCzeta and MEK5 associate to G alpha(q) upon activation of GPCR, thus forming a ternary complex that seems essential for the activation of ERK5. These data put forward a novel function of G alpha(q) as a scaffold protein involved in the modulation of the ERK5 cascade by GPCR that could be relevant in G(q)-mediated physiological functions.

The death receptor antagonist FAIM promotes neurite outgrowth by a mechanism that depends on ERK and NF-kapp B signaling.

Fas apoptosis inhibitory molecule (FAIM) is a protein identified as an antagonist of Fas-induced cell death. We show that FAIM overexpression fails to rescue neurons from trophic factor deprivation, but exerts a marked neurite growth-promoting action in different neuronal systems. Whereas FAIM overexpression greatly enhanced neurite outgrowth from PC12 cells and sympathetic neurons grown with nerve growth factor (NGF), reduction of endogenous FAIM levels by RNAi decreased neurite outgrowth in these cells. FAIM overexpression promoted NF-kappa B activation, and blocking this activation by using a super-repressor I kappa B alpha or by carrying out experiments using cortical neurons from mice that lack the p65 NF-kappa B subunit prevented FAIM-induced neurite outgrowth. The effect of FAIM on neurite outgrowth was also blocked by inhibition of the Ras-ERK pathway. Finally, we show that FAIM interacts with both Trk and p75 neurotrophin receptor NGF receptors in a ligand-dependent manner. These results reveal a new function of FAIM in promoting neurite outgrowth by a mechanism involving activation of the Ras-ERK pathway and NF-kappa B.

DaPKC-dependent phosphorylation of Crumbs is required for epithelial cell polarity in Drosophila.

Both in Drosophila and vertebrate epithelial cells, the establishment of apicobasal polarity requires the apically localized, membrane-associated Par-3-Par-6-aPKC protein complex. In Drosophila, this complex colocalizes with the Crumbs-Stardust (Sdt)-Pals1-associated TJ protein (Patj) complex. Genetic and molecular analyses suggest a functional relationship between them. We show, by overexpression of a kinase-dead Drosophila atypical PKC (DaPKC), the requirement for the kinase activity of DaPKC to maintain the position of apical determinants and to restrict the localization of basolateral ones. We demonstrate a novel physical interaction between the apical complexes, via direct binding of DaPKC to both Crb and Patj, and identify Crumbs as a phosphorylation target of DaPKC. This phosphorylation of Crumbs is functionally significant. Thus, a nonphosphorylatable Crumbs protein behaves in vivo as a dominant negative. Moreover, the phenotypic effect of overexpressing wild-type Crumbs is suppressed by reducing DaPKC activity. These results provide a mechanistic framework for the functional interaction between the Par-3-Par-6-aPKC and Crumbs-Sdt-Patj complexes based in the posttranslational modification of Crb by DaPKC.

Regulation of mature T lymphocyte proliferation and differentiation by Par-4.

The genetic inactivation of the atypical protein kinase C (aPKC) inhibitor, Par-4, gives rise to increased NF-kappaB activation and decreased stimulation of JNK in embryo fibroblasts. Here we have characterized the immunological phenotype of the Par-4(-/-) mice and found that the loss of this gene leads to an increased proliferative response of peripheral T cells when challenged through the TCR. This is accompanied by a higher increase in cell cycle entry and inhibition of apoptosis, with enhanced IL-2 secretion but normal CD25 synthesis. Interestingly, the TCR-triggered activation of NF-kappaB was augmented and that of JNK was severely abrogated. Consistent with previous data from knock outs of different JNKs, NFATc1 activation and IL-4 secretion were augmented in the Par-4-deficient CD4+ T cells, suggesting that the loss of Par-4 drives T-cell differentiation towards a Th2 response. This is compelling evidence that Par-4 is a novel modulator of the immune response through its ability to impact aPKC activity, which translates into lower JNK signaling.

Genetic inactivation of Par4 results in hyperactivation of NF-kappaB and impairment of JNK and p38.

The Par4 gene was first identified in prostate cells undergoing apoptosis after androgen withdrawal. PAR4 was subsequently shown to interact with, and inhibit, atypical protein kinase C isoforms, functioning as a negative regulator of the NF-kappaB pathway. This may explain its pro-apoptotic function in overexpression experiments. To determine the physiological role of PAR4, we have derived primary embryonic fibroblasts (EFs) from Par4(-/-) mice. We show here that loss of PAR4 leads to a reduction in the ability of tumour necrosis factor-alpha (TNF-alpha) to induce apoptosis by increased activation of NF-kappaB. Consistent with recent reports demonstrating the antagonistic actions of NF-kappaB and c-Jun amino-terminal kinase (JNK) signalling, we have found that Par4(-/-) cells show a reduced activation of the sustained phase of JNK and p38 stimulation by TNF-alpha and interleukin 1. Higher levels of an anti-apoptotic JNK-inhibitor protein, X-chromosome-linked inhibitor of apoptosis, in Par4(-/-) EFs might explain the inhibition of JNK activation in these cells.

Role of zeta PKC in B-cell signaling and function.

The atypical protein kinase C isoform, zeta PKC, has been implicated in the control of extracellular signal-regulated kinase (ERK) and nuclear factor (NF)-kappa B pathways. Recent evidence from zeta PKC knock-out mice demonstrates that this kinase is important for NF-kappa B transcriptional activity but not for ERK activation in embryonic fibroblasts. The lack of zeta PKC produces in mice a number of alterations in the development of secondary lymphoid tissues that could be accounted for, at least in part, by defects in B-cell function. Here, we present evidence that the loss of zeta PKC selectively impairs signaling through the B-cell receptor, resulting in inhibition of cell proliferation and survival, as well as defects in the activation of ERK and the transcription of NF-kappa B-dependent genes. Furthermore, zeta PKC-/- mice are unable to mount an optimal T-cell-dependent immune response. Collectively, these results genetically establish a critical role for zeta PKC in B-cell function in vitro and in vivo.