Differential expression and translocation of protein tyrosine phosphatase 1B-related proteins in ME-180 tumor cells expressing apoptotic sensitivity and resistance to tumor necrosis factor: potential interaction with epidermal growth factor receptor

Heparin-based hydrogel scaffolding alters the transcriptomic profile and increases the chemoresistance of MDA-MB-231 triple-negative breast cancer cells

The tumor microenvironment plays a critical role in the proliferation and chemoresistance of cancer cells. Growth factors (GFs) are known to interact with the extracellular matrix (ECM) via heparin binding sites, and these associations influence cell behavior. In the present study, we demonstrate the ability to define signals presented by the scaffold by pre-mixing growth factors, such as epidermal growth factor, into the heparin-based (HP-B) hydrogel prior to gelation. In the 3D biomimetic microenvironment, breast cancer cells formed spheroids within 24 hours of initial seeding. Despite higher number of proliferating cells in 2D cultures, 3D spheroids exhibited a higher degree of chemoresistance after 72 hours. Further, our RNA sequencing results highlighted the phenotypic changes influenced by solid-phase GF presentation. Wnt/β-catenin and TGF-β signaling were upregulated in the cells grown in the hydrogel, while apoptosis, IL2-STAT5 and PI3K-AKT-mTOR signaling were downregulated. With emerging technologies for precision medicine in cancer, this nature of fine-tuning the microenvironment is paramount for cultivation and downstream characterization of primary cancer cells and rare circulating tumor cells (CTCs), and effective screening of chemotherapeutic agents.

Signalling pathways involved in paracetamol-induced hepatotoxicity: new insights on the role of protein tyrosine phosphatase 1B

Acute hepatic failure secondary to paracetamol poisoning is associated with high mortality. Paracetamol-induced hepatotoxicity causes oxidative stress that triggers signalling pathways and ultimately leads to lethal hepatocyte injury. We will review the signalling pathways activated by paracetamol in the liver emphasizing the role of protein tyrosine phosphatase 1B (PTP1B) in the balance between cell death and survival in hepatocytes. PTP1B has emerged as a key modulator of the antioxidant system mediated by the nuclear factor erythroid-2-related factor 2 (Nrf2) in hepatic cells in response to paracetamol overdose. Also, this phosphatase modulates the classical survival pathways triggered by the activation of the insulin-like growth factor-I (IGF-I) signalling cascade. Therefore, PTP1B is a novel therapeutic target against paracetamol-induced liver failure.

Tumor necrosis factor inhibits ligand-stimulated EGF receptor activation through a TNF receptor 1-dependent mechanism

Tumor necrosis factor (TNF) and epidermal growth factor (EGF) are key regulators in the intricate balance maintaining intestinal homeostasis. Previous work from our laboratory shows that TNF attenuates ligand-driven EGF receptor (EGFR) phosphorylation in intestinal epithelial cells. To identify the mechanisms underlying this effect, we examined EGFR phosphorylation in cells lacking individual TNF receptors. TNF attenuated EGF-stimulated EGFR phosphorylation in wild-type and TNFR2(-/-), but not TNFR1(-/-), mouse colon epithelial (MCE) cells. Reexpression of wild-type TNFR1 in TNFR1(-/-) MCE cells rescued TNF-induced EGFR inhibition, but expression of TNFR1 deletion mutant constructs lacking the death domain (DD) of TNFR1 did not, implicating this domain in EGFR downregulation. Blockade of p38 MAPK, but not MEK, activation of ERK rescued EGF-stimulated phosphorylation in the presence of TNF, consistent with the ability of TNFR1 to stimulate p38 phosphorylation. TNF promoted p38-dependent EGFR internalization in MCE cells, suggesting that desensitization is achieved by reducing receptor accessible to ligand. Taken together, these data indicate that TNF activates TNFR1 by DD- and p38-dependent mechanisms to promote EGFR internalization, with potential impact on EGF-induced proliferation and migration key processes that promote healing in inflammatory intestinal diseases.

Levels of protein tyrosine phosphatase 1B determine susceptibility to apoptosis in serum-deprived hepatocytes

Protein tyrosine phosphatase 1B (PTP1B) is a negative regulator of tyrosine kinase growth factor signaling. To assess the importance of PTP1B in the balance between death and survival in the liver, we have developed immortalized neonatal hepatocyte cell lines lacking (PTP1B(-/-)) or overexpressing (PTP1B(+/+PTP1B)) PTP1B. Early activation of caspase-3 occurred in PTP1B(+/+PTP1B) hepatocytes but was nearly abolished in PTP1B(-/-) cells. At the molecular level, PTP1B overexpression/deficiency altered the balance of pro-(Bim) and anti-(Bcl-x(L)) apoptotic members of the Bcl-2 family upon serum withdrawal. Likewise, cytosolic cytochrome C increased rapidly in PTP1B(+/+PTP1B) hepatocytes whereas it was retained in the mitochondria of PTP1B(-/-) cells. DNA fragmentation and the increase of apoptotic cells induced by serum withdrawal in wild-type (PTP1B(+/+)) hepatocytes were absent in PTP1B(-/-) cells. Conversely, overexpression of PTP1B accelerated DNA laddering and increased the number of apoptotic cells. In serum-deprived PTP1B(+/+PTP1B) hepatocytes, a rapid entry of Foxo1 into the nucleus and an earlier activation of caspase-8 was observed. However, both events were suppressed in PTP1B(-/-) hepatocytes. Moreover, PTP1B deficiency conferred resistance to apoptosis induced by activation of Fas and constitutively active Foxo1. Rescue of PTP 1B in deficient hepatocytes recovered the phenotype of wild-type cells whereas reduction of PTP1B by siRNA suppressed apoptosis. Our results reveal a unique role for PTP1B as a mediator of the apoptotic pathways triggered by trophic factors withdrawal in hepatocytes. This novel mechanism may represent an important target in the design of therapeutic strategies for human liver regeneration after pathological damage as well as for treatment of hepatocarcinomas.

Entamoeba histolytica-induced dephosphorylation in host cells

Activation of host cell protein tyrosine phosphatases (PTPases) and protein dephosphorylation is an important mechanism used by various microorganisms to deactivate or kill host defense cells. To determine whether protein tyrosine dephosphorylation played a role in signaling pathways affecting Entamoeba histolytica-mediated host cell killing, we investigated the involvement of PTPases during the attachment of E. histolytica to target cells. We observed a rapid decrease in cellular protein tyrosine levels in Jurkat cells, as measured with an antiphosphotyrosine monoclonal antibody, following adherence to E. histolytica. Ameba-induced protein dephosphorylation was contact dependent and required intact parasite, since blocking amebic adherence with galactose inhibited tyrosine dephosphorylation and amebic lysates had no effect on phosphotyrosine levels. Moreover, disruption of amebic adherence with galactose promoted recovery of phosphorylation in Jurkat cells, indicating that dephosphorylation precedes target cell death. The evidence suggests that ameba-induced dephosphorylation is mediated by host cell phosphatases. Prior treatment of Jurkat cells with phenylarsine oxide, a PTPase inhibitor, inhibited ameba-induced dephosphorylation. We also found proteolytic cleavage of the PTPase 1B (PTP1B) in Jurkat cells after contact with amebae. The calcium-dependent protease calpain is responsible for PTP1B cleavage and enzymatic activation. Pretreatment of Jurkat cells with calpeptin, a calpain inhibitor, blocked PTP1B cleavage and inhibited ameba-induced dephosphorylation. In addition, inhibition of Jurkat cell PTPases with phenylarsine oxide blocked Jurkat cell apoptosis induced by E. histolytica. These results suggest that E. histolytica-mediated host cell death occurs by a mechanism that involves PTPase activation.

Further evidence for the involvement of epidermal growth factor in the signaling pathway of vitamin B12 (cobalamin) in the rat central nervous system

In order to get further evidence for a mandatory involvement of epidermal growth factor (EGF) in the neutrophic action of vitamin B12 (cobalamin (Cbl)) in the central nervous system (CNS) of the rat, we observed the effects of repeated intracerebroventricular (ICV) microinjections of EGF in rats made Cbl-deficient through total gastrectomy. Morphometric analysis demonstrated a significant reduction in both intramyelinic and interstitial edema in the white matter of the spinal cord (SC) of totally gastrectomized (TGX) rats after treatment. Intramyelinic and interstitial edema are characteristic of Cbl-deficient central neuropathy in the rat. Similar lesions were also present in SC white matter of rats treated with repeated ICV microinjections of specific anti-EGF antibodies without any modification in their Cbl status. These results, together with those of a previous study showing the cessation of EGF synthesis in the CNS of TGX rats, demonstrate that: a) EGF is necessarily involved in the signaling pathway of Cbl in the rat CNS; and b) the lack of a neurotrophic growth factor EGF, and not the mere withdrawal of Cbl, causes or at least contributes to neurodegenerative Cbl-deficient central neuropathy.

Cultured cell sublines highly susceptible to prion infection

Cultured cell lines infected with prions produce an abnormal isoform of the prion protein (PrP(Sc)). In order to derive cell lines producing sufficient quantities of PrP(Sc) for most studies, it has been necessary to subclone infected cultures and select the subclones producing the largest amounts of PrP(Sc). Since postinfection cloning can introduce differences between infected and uninfected cell lines, we sought an approach to generate prion-infected cell lines that would avoid clonal artifacts. Using an improved cell blot technique, which permits sensitive and rapid comparison of PrP(Sc) levels in multiple independent cell cultures, we discovered marked heterogeneity with regard to prion susceptibility in tumor cell sublines. We exploited this heterogeneity to derive sublines which are highly susceptible to prion infection and used these cells to generate prion-infected lines without further subcloning. These infected sublines can be compared to the cognate uninfected cultures without interference from cloning artifacts. We also used susceptible cell lines and our modified cell blot procedure to develop a sensitive and reproducible quantitative cell culture bioassay for prions. We found that the sublines were at least 100-fold more susceptible to strain RML prions than to strain ME7 prions. Comparisons between scrapie-susceptible and -resistant cell lines may reveal factors that modulate prion propagation.

TNF-induced signaling in apoptosis

Out of the almost 17 members of the TNF superfamily, TNF is probably the most potent inducer of apoptosis. TNF activates both cell-survival and cell-death mechanisms simultaneously. Activation of NF-kB-dependent genes regulates the survival and proliferative effects pf TNF, whereas activation of caspases regulates the apoptotic effects. TNF-induced apoptosis is mediated primarily through the activation of type I receptors, the death domain of which recruits more than a dozen different signaling proteins, which together are considered part of an apoptotic cascade. This cascade does not, however, account for the role of reactive oxygen intermediates, ceramide, phospholipases, and serine proteases which are also implicated in TNF-induced apoptosis. This cascade also does not explain how type II TNF receptors which lack the death domain, induce apoptosis. Nevertheless, this review of apoptosis signaling will be limited to those proteins that makeup the cascade.

The CLK family kinases, CLK1 and CLK2, phosphorylate and activate the tyrosine phosphatase, PTP-1B

The protein-tyrosine phosphatase PTP-1B is an important regulator of intracellular protein tyrosine phosphorylation, and is itself regulated by phosphorylation. We report that PTP-1B and its yeast analog, YPTP, are phosphorylated and activated by members of the CLK family of dual specificity kinases. CLK1 and CLK2 phosphorylation of PTP-1B in vitro activated the phosphatase activity approximately 3-5-fold using either p-nitrophenol phosphate, or tyrosine-phosphorylated myelin basic protein as substrates. Co-expression of CLK1 or CLK2 with PTP-1B in HEK 293 cells led to a 2-fold stimulation of phosphatase activity in vivo. Phosphorylation of PTP-1B at Ser(50) by CLK1 or CLK2 is responsible for its enzymatic activation. These findings suggest that phosphorylation at Ser(50) by serine threonine kinases may regulate the activation of PTP-1B in vivo. We also show that CLK1 and CLK2 phosphorylate and activate the S. cerevisiae PTP-1B family member, YPTP1. CLK1 phosphorylation of YPTP1 led to a 3-fold stimulation of phosphatase activity in vitro. We demonstrate that CLK phosphorylation of Ser(83) on YPTP1 is responsible for the activation of this enzyme. These findings demonstrate that the CLK kinases activate PTP-1B family members, and this phosphatase may be an important cellular target for CLK action.

Regulation of intestinal tyrosine phosphorylation and programmed cell death by peroxovanadate

Cell suspensions of ileal mucosa undergo a rapid and synchronized form of programmed cell death when cultured in a simple medium at 37 degrees C. Because tyrosine phosphorylation of proteins plays a crucial role in the signal transduction of many cellular processes, we examined its role in intestinal programmed cell death by use of immunoblot and immunohistochemical methods. We observed a 50-70% reduction in tyrosine phosphorylation during the initial 10 min of intestinal epithelial cell culture. We hypothesized that the inhibition of protein tyrosine phosphatases would increase protein tyrosine phosphorylation in these suspensions and decrease programmed cell death. A strong inhibitor of these phosphatases (peroxovanadate) but not a weaker one (sodium orthovanadate) abolished the DNA fragmentation/laddering normally seen in dying enterocytes. Peroxovanadate enhanced protein tyrosine phosphorylation of many intestinal proteins, dramatically increasing the dually phosphorylated and active form of mitogen-activated protein kinase. Immunohistochemistry revealed a particularly high level of increased tyrosine phosphorylation in the intestinal crypts in peroxovanadate-treated mucosa. Kinetic studies indicated that the pivotal time for protein tyrosine phosphatase inhibition occurred within 5 min of ex vivo culture, precisely when protein tyrosine phosphorylation declined. Our data suggest that tyrosine kinase inactivation or tyrosine phosphatase activation may initiate intestinal epithelial cell death.