Role of tyrosine kinase and phosphotyrosine phosphatase in growth of the intestinal crypt cell (IEC-6) line

Involvement of JAK2 upstream of the PI 3-kinase in cell-cell adhesion regulation by gastrin

The Janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling pathway has been implicated in cell transformation and proliferation. Besides aberrant cell proliferation, loss of cell-cell adhesion during epithelial-mesenchymal transition (EMT) is an important event which occurs during development of epithelial cancers. However, the role of JAK-dependent pathways in this process is not known. We analyzed the involvement of these pathways in the regulation of E-cadherin-dependent cell-cell adhesion by gastrin, a mitogenic factor for gastrointestinal (GI) tract. We identified JAK2/STAT3 as a new pathway in gastrin signaling. We demonstrated that JAK2 functions as an upstream mediator of the phosphatidylinositol 3 (PI 3)-kinase activity in gastrin signaling. Indeed, we observed a coprecipitation of both kinases and an inhibition of gastrin-induced PI 3-kinase activation when JAK2 activity is blocked. We also demonstrated that loss of cell-cell adhesion and the increase in cell motility induced by gastrin required the activation of JAK2 and the PI 3-kinase. Indeed, the modifications in localization of adherens junctions proteins and the migration, observed in gastrin-stimulated cells, were reversed by inhibition of both kinases. These results described the involvement of JAK2 in the modulation of cell-cell adhesion in epithelial cells. They support a possible role of JAK2 in the epithelial-mesenchymal transition which occurs during malignant development.

Phytoestrogens and colorectal cancer prevention

Because colorectal cancer (CRC) is a leading cause of cancer mortality in the Western industrialized world, future chemopreventive strategies will be of high socioeconomic importance. Lower CRC incidences in Far Eastern countries in part may be attributed to high nutritional intake of soy and its abundance in phytoestrogens. Phytoestrogens are plant-derived substances which, due to molecular similarities to endogenous estrogens, distinctly interact with estrogen receptors ERalpha and ERbeta. Both genomic and nongenomic mechanisms have been shown to be responsible for possible anticarcinogenic properties of phytoestrogens, such as induction of apoptosis and inhibition of tyrosine kinases and DNA topoisomerases. This chapter provides a comprehensive review of a variety of studies exploring possible relations between phytoestrogen uptake and CRC risk. While in vitro and animal studies in general are somewhat supportive of a protective role of phytoestrogens against CRC, epidemiological work so far performed does not allow any conclusion on this issue. Studies about mechanisms of phytoestrogenic action against CRC development have been evaluated as well. Because estrogens, vitamin D, and calcium are frequently suggested to be important in CRC prevention, studies concentrating on interactions of phytoestrogens with these substances have been performed. Although support in evidence for a protective effect of phytoestrogens against CRC has increased over the last decade, it is still too early to give a definite recommendation, especially in view of enduring inconsistencies about concentrations most likely to be effective. Further experimental, and particularly epidemiological, studies are required to advance our understanding of the role of phytoestrogens against colon carcinogenesis.

Altered morphology in cultured rat intestinal epithelial IEC-6 cells is associated with alkaline phosphatase expression

Non-transformed, rat intestinal epithelial cells (IEC-6), and human intestinal colonic carcinoma cells (CACO-2) have both been used to study processes of epithelial cell differentiation. However, only CACO-2 cells have been described as spontaneously expressing phenotypic changes of differentiation in culture. We report here that when IEC-6 cells are grown in post-confluent culture, they develop structural changes similar to those seen in cells induced to differentiate by culture on Englebreth-Holm-Swarm (EHS) extracellular matrix proteins. Correlated with this morphological change is loss of nuclear localization of c-myc protein and development of cell surface alkaline phosphatase (ALP) enzymatic activity. Messenger RNAs for liver and intestinal isoforms of ALP were expressed in both pre- and post-confluent cells. Inhibition of ALP activity in post-confluent cells by levamisole indicated the expressed ALP activity to be of the liver isoform. We suggest the expression of ALP activity, which occurs concomitantly with morphological alterations in post-confluent IEC-6 cells, represents increased expression and localization to the cell surface of the liver isoform of ALP. Cultured IEC-6 cells may provide a non-transformed, in vitro alternative to CACO-2 cells for study of epithelial cell differentiation.

Role of autocrine and endocrine gastrin-like peptides in colonic carcinogenesis

Colon carcinogenesis is a multistep process that involves deletions, mutations, and changes in expression of genes that regulate growth, differentiation, and apoptosis. Hyperproliferation can initiate dysplastic growth, resulting in accumulation of genetic defects and progression of colon cancer. Although genetic instability, because of inheritance of specific genetic defects, plays a dominant role in familial cancers, in the majority of sporadic cancers hyperproliferation is likely to play a permissive role in initiation and progression of the disease. Thus factors that regulate growth, differentiation, and apoptosis are likely to play an important role in colon carcinogenesis. Autocrine gastrins, insulin-like growth factor-II, transforming growth factor-alpha, and endocrine gastrins have been implicated in the tumorigenic potential of colon cancer cells. In this article we focus on the role of endocrine and autocrine gastrins in colon cancer and review recent advances that suggest a role of processing intermediates of gastrin in colon carcinogenesis.

Tyrosine kinase and MAPK inhibition of TNF-alpha- and EGF-stimulated IEC-6 cell growth

The role of TNF-alpha in modulating intestinal crypt cell growth was examined, in comparison with EGF. Both significantly increased IEC-6 cell proliferation. Neither EGF nor TNF-alpha overcame the inhibitory effect on growth exerted by the tyrosine kinase inhibitor genistein. Immunoblots with phosphotyrosine antibodies showed increased tyrosine phosphorylation of IEC-6 cell proteins in response to EGF and TNF-alpha stimulation. TNF-alpha increased ERK1 and ERK2 MAPK phosphorylation. A MAPK assay confirmed the increased activity upon TNF-alpha stimulation. Selective inhibition of MAPK activation by PD98059 resulted in a dose dependent inhibition of TNF-alpha or EGF-induced IEC-6 cell growth. These findings suggest a role for TNF-alpha in the regulation of intestinal epithelial cell growth and that the mitogenic effect of TNF-alpha requires protein tyrosine phosphorylation and MAPK activation.

Association of Tyr phosphorylation of GTPase-activating protein with mitogenic action of serotonin

We have previously shown that serotonin (5-HT) induces both hyperplasia and hypertrophy of pulmonary artery smooth muscle cells (SMC) but not of endothelial cells (EC) through its high-affinity uptake. The present studies demonstrate rapid enhancement by 5-HT of Tyr phosphorylation of proteins, including p120, which also occurs in SMC but not in EC. The p120 protein was identified as GTPase-activating protein (GAP) by immunoprecipitation. Its phosphorylation occurred within minutes and preceded other events associated with 5-HT-induced mitogenesis. Tyr kinase (TK) and 5-HT uptake inhibitors and 8-bromoadenosine 3',5'-cyclic monophosphate blocked both the 5-HT-induced DNA synthesis and Tyr phosphorylation of GAP. Vanadate elevated DNA synthesis and Tyr phosphorylation of GAP of both control and 5-HT-treated cells. 5-HT failed to alter Tyr phosphorylation of GAP in cellular homogenates, as opposed to intact cells. In the presence of 3-isobutyl-1-methylxanthine, 5-HT inhibited cellular growth, presumably through its action on 5-HT1A or 5-HT4 receptors and elevation of adenosine 3',5'-cyclic monophosphate, but this was not associated with an alteration of Tyr phosphorylation of GAP. Similarly, a 5-HT1 or 5-HT2 receptor agonist failed to stimulate Tyr phosphorylation or DNA synthesis of SMC. Stimulation of cellular proliferation and enlargement produced by 1 microM 5-HT were totally abolished by TK inhibitors that did not affect 5-HT uptake. These data indicate that Tyr phosphorylation of GAP may act as an intermediate signal in 5-HT-induced mitogenesis of SMC which requires cellular internalization of 5-HT rather than its action on a membrane receptor.

Differential responsiveness of proximal and distal colonic mucosa to gastrin

In vivo and in vitro experiments were performed to examine the responsiveness of the proximal and distal colonic mucosa to the growth-promoting action of gastrin. Infusion (osmotic minipump) of gastrin G-17-I (250 ng/kg/h) for 5 days to 4-month-old male Fischer-344 rats resulted in a significant (90-150%) increase in proliferative activity (as assessed by BrdU or PCNA immunoreactivity) in the distal colonic mucosa. In contrast, gastrin caused no apparent change in proliferative activity in the proximal colon. Because tyrosine kinases (Tyr-ks) are thought to be critically involved in regulating the trophic action of gastrin, responsiveness of isolated colonocytes from both segments of the colon to gastrin (1 x 10(-9) M) was also examined. Exposure of isolated colonocytes from the distal, but not from the proximal, colon to gastrin for 2 min resulted in a significant (73%) stimulation in Tyr-k activity. This was also accompanied by a marked rise in phosphorylation of at least six membrane proteins with M, of 55, 60, 70, 94, and 170 kDa. Tyr-k activity induced by gastrin in colonocytes from the distal colon was inhibited by tyrphostin (3.2 microM) but not by staurosporine (20 nM). In colonocytes from the distal colon, gastrin also stimulated phospholipase C (PLC) activity, which could also be inhibited by tyrphostin, but not by staurosporine. We conclude that mucosa of the distal, but not the proximal, colon responds to the trophic action of gastrin. Tyr-ks are thought to be involved in the regulation of this process.

Evidence for coupling of phosphotyrosine phosphatase to gonadotropin-releasing hormone receptor in ovarian carcinoma membrane

BACKGROUND

Gonadotropin-releasing hormone (Gn-RH) receptor (Gn-RHR) has been demonstrated in epithelial ovarian carcinoma (Imai et al., Cancer 1994; 74:2555-61). To examine whether Gn-RHR mediates direct antiproliferative effects, we attempted to determine stimulatory regulation by Gn-RH of phosphotyrosine phosphatase (PTP) activity in plasma membranes isolated from ovarian carcinoma samples.

METHODS

Surgically removed ovarian carcinomas were screened for Gn-RHR expression prior to plasma membrane isolation. The phosphotyrosine level was observed by: (1) immunoblotting of membrane extracts with antiphosphotyrosine antibodies, and (2) dephosphorylation from 32P-labeled membrane protein. Membrane PTP activity was determined using the synthetic substrate p-nitrophenyl in a spectrophotometric assay.

RESULTS

A Gn-RH analog alone, or guanosine thiotriphosphate (GTP-gamma-S) alone, caused a remarkable loss of phosphotyrosine from a 35-kD protein of the membranes; incubation with a Gn-RH analog and GTP-gamma-S produced a further dephosphorylation of this endogenous protein. The Gn-RH analog buserelin stimulated the PTP activity of the membranes in a dose-dependent manner (P < 0.01). GTP-gamma-S enhanced the stimulatory action of Gn-RH on PTP; GDP-gamma-S reversed the Gn-RH action. A similar stimulation of PTP was observed (P < 0.01) when carcinoma tissue slices were exposed to Gn-RH analog in vivo prior to assay in vitro.

CONCLUSIONS

Activation of PTP by Gn-RH stimulated the loss of phosphotyrosine from endogenous proteins through GTP-binding protein within plasma membrane isolated from Gn-RHR-expressing ovarian carcinoma. The antimitogenic action of the hormone may occur by counteracting tyrosine phosphorylation to promote cell growth.

Unique and selective mitogenic effects of vanadate on SV40-transformed cells

Vanadate and insulin both function as unique complete mitogens for SV40-transformed 3T3T cells, designated CSV3-1, but not for nontransformed 3T3T cells. The mitogenic effects induced by vanadate and insulin in CSV3-1 cells are mediated by different signaling mechanisms. For example, vanadate does not stimulate the tyrosine phosphorylation of the insulin receptor beta-subunit nor the 170 kDa insulin receptor substrate-1. Instead, vanadate induces a marked increase in tyrosine phosphorylation of 55 and 64 kDa proteins that is not observed in insulin-stimulated CSV3-1 cells. Perhaps most interestingly, vandate-induced mitogenesis is associated with the selective induction of c-jun and junB expression without significantly inducing c-fos or c-myc. Furthermore, treatment of CSV3-1 cells with genistein abolishes the effects of vanadate on protein tyrosine phosphorylation and c-jun induction. These and related data suggest that modulation of protein tyrosine phosphorylation and c-jun and junB expression may serve the critical roles in mediating vandate-induced mitogenesis in SV40-transformed cells.

Early signalling mechanism in colonic epithelial cell response to gastrin

The hormone gastrin exerts a growth-promoting effect on gastrointestinal cells. The molecular mechanisms by which colonic epithelial cells respond to gastrin are still poorly understood. In this study, we demonstrate a novel feature of the action of gastrin on normal colonic cells, namely the rapid phosphorylation on tyrosine of phospholipase C gamma 1 (PLC gamma 1). Tyrosine phosphorylation of PLC gamma 1, elicited by gastrin, was transient, concentration-dependent, and was abrogated by pretreating the colonic cells with the gastrin-receptor antagonist proglumide, the tyrosine kinase inhibitor genistein, and by removal of the tyrosine phosphatase inhibitor orthovanadate from the isolation buffer. Tyrosine phosphorylation of PLC gamma 1 correlated with the time- and concentration-dependent decrease in the mass of membrane phosphatidylinositol 4,5-bisphosphate (PIP2) and the increase in the epithelial concentration of inositol 1,4,5-trisphosphate (IP3). Likewise, the stimulated increase in IP3 was also prevented by proglumide and genistein. Gastrin induced a definite but transient increase in the intracellular concentration of free Ca2+ [Ca2+]i, and increased membrane-translocation of immunoreactive alpha- and beta-protein kinase C. The data thus indicate that gastrin elicits at least one signalling cascade, through rapid tyrosine phosphorylation of PLC gamma 1, leading to the activation of a PIP2-specific PLC pathway.

Transforming growth factor-alpha increases tyrosine phosphorylation of microtubule-associated protein kinase in a small intestinal crypt cell line (IEC-6)

The small intestinal crypt cell line (IEC-6) is an undifferentiated, untransformed, mitotically active cell used in this study to determine the effect of transforming growth factor-alpha (TGF-alpha) on tyrosine phosphorylation levels of cellular proteins. Thymidine incorporation increased maximally after addition of 2 ng/ml TGF-alpha for 24 h. At the same dose, TGF-alpha induced the tyrosine phosphorylation of proteins with approximate molecular masses of 42, 44, 52, 80, 150 and 175 kDa as shown by Western blots treated with anti-phosphotyrosine antibody. The most intense phosphorylation was seen in the 42 kDa (p42) and 44 kDa (p44) proteins, which were identified as two isoforms of microtubule-associated protein kinase (MAPK). This phosphorylation was seen as early as 5 min post stimulation and was dose dependent. Both p42 and p44 were found in the nucleus after stimulation, although a basal level of unphosphorylated protein was present before stimulation. The observed tyrosine phosphorylation of p42 and p44 was inhibited by genistein, a tyrosine kinase inhibitor, and tyrphostin 23, an epidermal growth factor receptor tyrosine kinase inhibitor. We conclude that MAPK is tyrosine phosphorylated in response to TGF-alpha stimulation of IEC-6 cells.