Caveolin 2 regulates endocytosis and trafficking of the M1 muscarinic receptor in MDCK epithelial cells.

Clathrin and caveolins are known for their involvement in the internalization of numerous receptors. Here we show that in polarized epithelial Madin-Darby canine kidney cells, both the clathrin machinery and caveolins are involved in the endocytosis and delivery to the plasma membrane (PM) of the M1 muscarinic acetylcholine receptor (mAChR). We initially localized this receptor to the lateral membrane, where it accumulates proximal to the tight junctions. From there it is internalized through the clathrin-mediated pathway. In addition, the receptor may associate on the PM with caveolin (cav) 2 or in intracellular compartments with either cav 2, or monomeric or oligomeric cav 1. Association of the PM M1 mAChR with cav 2 inhibits receptor endocytosis through the clathrin-mediated pathway or retains the receptor in an intracellular compartment. This intracellular association attenuates receptor trafficking. Expression of cav 1 with cav 2 rescues the latter's inhibitory effect. The caveolins stimulate M1 mAChR oligomerization thus maintaining a constant amount of monomeric receptor. These results provide evidence that caveolins play a role in the attenuation of the M1 muscarinic receptor's intracellular trafficking to and from the PM.

The VDAC1-based R-Tf-D-LP4 Peptide as a Potential Treatment for Diabetes Mellitus.

Diabetes mellitus is a metabolic disorder approaching epidemic proportions. Non-alcoholic fatty liver disease (NAFLD) regularly coexists with metabolic disorders, including type 2 diabetes, obesity, and cardiovascular disease. Recently, we demonstrated that the voltage-dependent anion channel 1 (VDAC1) is involved in NAFLD. VDAC1 is an outer mitochondria membrane protein that serves as a mitochondrial gatekeeper, controlling metabolic and energy homeostasis, as well as crosstalk between the mitochondria and the rest of the cell. It is also involved in mitochondria-mediated apoptosis. Here, we demonstrate that the VDAC1-based peptide, R-Tf-D-LP4, affects several parameters of a NAFLD mouse model in which administration of streptozotocin (STZ) and high-fat diet 32 (STZ/HFD-32) led to both type 2 diabetes (T2D) and NAFLD phenotypes. We focused on diabetes, showing that R-Tf-D-LP4 peptide treatment of STZ/HFD-32 fed mice restored the elevated blood glucose back to close to normal levels, and increased the number and average size of islets and their insulin content as compared to untreated controls. Similar results were obtained when staining the islets for glucose transporter type 2. In addition, the R-Tf-D-LP4 peptide decreased the elevated glucose levels in a mouse displaying obese, diabetic, and metabolic symptoms due to a mutation in the obese (ob) gene. To explore the cause of the peptide-induced improvement in the endocrine pancreas phenotype, we analyzed the expression levels of the proliferation marker, Ki-67, and found it to be increased in the islets of STZ/HFD-32 fed mice treated with the R-Tf-D-LP4 peptide. Moreover, peptide treatment of STZ/HFD-32 fed mice caused an increase in the expression of ß-cell maturation and differentiation PDX1 transcription factor that enhances the expression of the insulin-encoding gene, and is essential for islet development, function, proliferation, and maintenance of glucose homeostasis in the pancreas. This increase occurred mainly in the ß-cells, suggesting that the source of their increased number after R-Tf-D-LP4 peptide treatment was most likely due to ß-cell proliferation. These results suggest that the VDAC1-based R-Tf-D-LP4 peptide has potential as a treatment for diabetes.

Beta-tubulin cofactor D and ARL2 take part in apical junctional complex disassembly and abrogate epithelial structure.

In epithelial cells, the apical junctional complex (AJC), composed of tight junctions (TJs) and adherens junctions (AJs), maintains cell-surface polarity by forming a fence that prevents lateral movement and diffusion of proteins and lipids between the apical and basolateral PM and holds the epithelial monolayer intact through cell-cell contacts. Disassembly of this complex is a prime event in development and cell transformation. Maintenance of the AJC has been shown to involve mainly the actin cytoskeleton. Recent findings also point to the involvement of the microtubule (MT) system. Here we show the first evidence that in polarized epithelial MDCK cells, ARF-like protein 2 (ARL2) and beta-tubulin cofactor D, known to be involved in MT dynamics, have a role in disassembly of the AJC followed by cell dissociation from the epithelial monolayer, which is not dependent on MT depolymerization. In addition, we show that beta-tubulin cofactor D is partially localized to the lateral PM through its 15 C-terminal amino acids and intact MTs. ARL2 inhibited beta-tubulin cofactor D-dependent cell dissociation from the monolayer and AJC disassembly. To our knowledge, this is the first evidence that beta-tubulin cofactor D plays a role in cells independent of its presumed role in folding tubulin heterodimers. We conclude that ARL2 and beta-tubulin cofactor D participate in AJC disassembly and epithelial depolarization.

Actin is required for endocytosis at the apical surface of Madin-Darby canine kidney cells where ARF6 and clathrin regulate the actin cytoskeleton.

In epithelial cell lines, apical but not basolateral clathrin-mediated endocytosis has been shown to be affected by actin-disrupting drugs. Using electron and fluorescence microscopy, as well as biochemical assays, we show that the amount of actin dedicated to endocytosis is limiting at the apical surface of epithelia. In part, this contributes to the low basal rate of clathrin-dependent endocytosis observed at this epithelial surface. ARF6 in its GTP-bound state triggers the recruitment of actin from the cell cortex to the clathrin-coated pit to enable dynamin-dependent endocytosis. In addition, we show that perturbation of the apical endocytic system by expression of a clathrin heavy-chain mutant results in the collapse of microvilli. This phenotype was completely reversed by the expression of an ARF6-GTP-locked mutant. These observations indicate that concomitant to actin recruitment, the apical clathrin endocytic system is deeply involved in the morphology of the apical plasma membrane.

Differential effect of bryostatin 1 and phorbol 12-myristate 13-acetate on HOP-92 cell proliferation is mediated by down-regulation of protein kinase Cdelta.

Bryostatin 1 is currently in clinical trials as a cancer chemotherapeutic agent. Although bryostatin 1, like phorbol 12-myristate 13-acetate (PMA), is a potent activator of protein kinase C (PKC), it induces only a subset of those responses induced by PMA and antagonizes others. We report that, in the HOP-92 non-small cell lung cancer line, bryostatin 1 induced a biphasic proliferative response, with maximal proliferation at 1 to 10 nmol/L. This biphasic response mirrored a biphasic suppression of the level of PKCdelta protein, with maximal suppression likewise at 1 to 10 nmol/L bryostatin 1. The typical phorbol ester PMA, in contrast to bryostatin 1, had no effect on the level of PKCdelta and modest suppression of cell proliferation, particularly evident at later treatment times. Flow cytometric analysis revealed changes in the fraction of cells in the G0-G1 and S phases corresponding to the effects on proliferation. Cells overexpressing PKCdelta exhibited a lower rate of cell proliferation compared with control untreated cells and showed neither a proliferative response nor a loss of PKCdelta in response to bryostatin 1. Conversely, treatment with PKCdelta small interfering RNA significantly increased the cellular growth compared with controls. We conclude that the differential effect on cellular proliferation induced by bryostatin 1 compared with PMA reflects the differential suppression of PKCdelta.

Use of a cDNA microarray to determine molecular mechanisms involved in grey platelet syndrome.

The grey platelet syndrome (GPS) is a bleeding disorder of unknown aetiology with phenotypic and genetic heterogeneity. Affected patients exhibit macrothrombocytopenia, decreased alpha-granule content and, sometimes, myelofibrosis. We used microarray technology to investigate changes in gene expression that might reveal mechanisms involved in GPS. The expression of 4900 unique genes and expressed sequence tags was evaluated in fibroblasts from a GPS patient; normal fibroblasts provided the reference standard. Genes that were differentially regulated in the GPS cells were categorized into gene clusters based upon similarity/differences of expression differences. The results showed that genes with functional similarities clustered together. This analysis revealed significant upregulation of selected biological processes involving the production of cytoskeleton proteins, including fibronectin 1, thrombospondins 1 and 2, and collagen VI alpha. These genes appear to play a role in the pathogenesis of GPS. Indeed, Northern blot analyses confirmed that fibronectin, thrombospondin and matrix metalloprotease-2 were overexpressed in GPS fibroblasts compared with normal fibroblasts. Moreover, immunohistochemistry studies revealed robust fibronectin staining in GPS fibroblasts compared with normal ones. Our findings support the feasibility of using cDNA microarray techniques to detect distinctive and informative differences in gene expression patterns relevant to GPS, and suggest that the molecular basis for myelofibrosis in GPS involves upregulation of cytoskeleton proteins.

PKCdelta associates with and is involved in the phosphorylation of RasGRP3 in response to phorbol esters.

RasGRP is a family of guanine nucleotide exchange factors that activate small GTPases and contain a C1 domain similar to the one present in protein kinase C (PKC). In this study, we examined the interaction of RasGRP3 and PKC in response to the phorbol ester PMA. In Chinese hamster ovary or LN-229 cells heterologously expressing RasGRP3, phorbol 12-myristate 13-acetate (PMA) induced translocation of RasGRP3 to the perinuclear region and a decrease in the electrophoretic mobility of RasGRP3. The mobility shift was associated with phosphorylation of RasGRP3 on serine residues and seemed to be PKCdelta-dependent because it was blocked by the PKCdelta inhibitor rottlerin as well as by a PKCdelta kinase-dead mutant. Using coimmunoprecipitation, we found that PMA induced the physical association of RasGRP3 with PKCdelta and, using in situ methods, we showed colocalization of PKCdelta and RasGRP3 in the perinuclear region. PKCdelta phosphorylated RasGRP3 in vitro. Previous studies suggest that ectopic expression of RasGRP3 increases activation of Erk1/2. We found that overexpression of either PKCdelta or RasGRP3 increased the activation of Erk1/2 by PMA. In contrast, coexpression of PKCdelta and RasGRP3 yielded a level of phosphorylation of Erk1/2 similar to that of control vector cells. Our results suggest that PKCdelta may act as an upstream kinase associating with and phosphorylating RasGRP3 in response to PMA. The interaction between RasGRP3 and PKCdelta points to the existence of complex cross-talk between various members of the phorbol ester receptors which can have important impact on major signal transduction pathways and cellular processes induced by phorbol esters or DAG