Protein tyrosine kinase activity and its substrates in rat intestinal microvillus membranes

Protein tyrosine phosphatase SAP-1 protects against colitis through regulation of CEACAM20 in the intestinal epithelium

Intestinal epithelial cells contribute to regulation of intestinal immunity in mammals, but the detailed molecular mechanisms of such regulation have remained largely unknown. Stomach-cancer-associated protein tyrosine phosphatase 1 (SAP-1, also known as PTPRH) is a receptor-type protein tyrosine phosphatase that is localized specifically at microvilli of the brush border in gastrointestinal epithelial cells. Here we show that SAP-1 ablation in interleukin (IL)-10-deficient mice, a model of inflammatory bowel disease, resulted in a marked increase in the severity of colitis in association with up-regulation of mRNAs for various cytokines and chemokines in the colon. Tyrosine phosphorylation of carcinoembryonic antigen-related cell adhesion molecule (CEACAM) 20, an intestinal microvillus-specific transmembrane protein of the Ig superfamily, was greatly increased in the intestinal epithelium of the SAP-1-deficient animals, suggesting that this protein is a substrate for SAP-1. Tyrosine phosphorylation of CEACAM20 by the protein tyrosine kinase c-Src and the consequent association of CEACAM20 with spleen tyrosine kinase (Syk) promoted the production of IL-8 in cultured cells through the activation of nuclear factor-κB (NF-κB). In addition, SAP-1 and CEACAM20 were found to form a complex through interaction of their ectodomains. SAP-1 and CEACAM20 thus constitute a regulatory system through which the intestinal epithelium contributes to intestinal immunity.

Mechanical strain rapidly redistributes tyrosine phosphorylated proteins in human intestinal Caco-2 cells

Repetitive strain stimulates proliferation and modulates differentiation in human Caco-2 intestinal epithelial cells via tyrosine kinase activity. We therefore sought to characterize strain modulation of tyrosine phosphorylation in Caco-2 cells. Immunoblotting for phosphotyrosine demonstrated that repetitive strain (10 cpm, 10% strain) rapidly increased tyrosine phosphorylation of 125-, 70-, 60-, and 50-kDa bands in the soluble fraction by 94+/-31, 145+/-21, 365+/-46, and 1240+/-240%, respectively (p<0.05, n=4). However, strain decreased tyrosine phosphorylated band intensity of the 125-, 70-, 60-, and 50-kDa proteins in the particulate fraction by 81+/-17, 70+/-23, 79+/-7, and 59+/-23%, respectively (p<0.05, n=4). The decreased band intensity in the particulate fraction was not due to decreased tyrosine kinase activity because strain equally increased tyrosine kinase activity in both soluble and particulate fractions. Cyclic strain at a physiologically relevant amplitude and frequency appears to modulate the subcellular distribution of tyrosine phosphorylated proteins in human Caco-2 intestinal cells.

Developmental regulation of epidermal growth factor receptor kinase in rat intestine

BACKGROUND/AIMS

Intraluminal epidermal growth factor (EGF) may regulate intestinal growth and function. The ontogeny, localization, and phosphorylation of the EGF receptor in rat small intestine were studied.

METHODS

EGF-receptor phosphorylation was assayed by antiphosphotyrosine Western blot after EGF administration in vivo and EGF incubation to everted sacs in vitro. EGF-receptor abundance and localization were assayed by Western blot and immunofluorescence using anti-EGF-receptor antibodies.

RESULTS

In vivo, orogastric EGF enhanced EGF-receptor phosphorylation in newborn rat jejunum and liver. However, intraluminal EGF had no effect on EGF-receptor phosphorylation in adult intestine or liver. In vitro, mucosal EGF stimulated a fourfold increase in EGF-receptor phosphorylation in suckling jejunum but not in weanling or adult jejunum. In Western blot, EGF-receptor abundance was similar in 22-day-old fetal, 8-day-old suckling, and adult jejunum. In both crypt and villus, EGF-receptor was localized to the basolateral membrane but not to the apical membrane in fetal, suckling, and adult jejunum.

CONCLUSIONS

Mucosal EGF stimulates EGF-receptor phosphorylation in immature but not in mature intestine. These differences are not explained by EGF-receptor abundance or localization and suggest that the greater mucosal permeability of the immature gut may allow EGF to bind to the EGF-receptor in the basolateral membrane to mediate its effects in suckling rat intestine.

Ontogenic regulation of phospholipase C-gamma 1 activity and expression in the rat small intestine

BACKGROUND/AIMS

The postnatal rat small intestine undergoes major morphological, biochemical, and physiological changes during weaning. Phospholipase C-gamma 1 (PLC gamma 1), a tyrosine kinase substrate of the epidermal growth factor receptor (EGFR) hydrolyzes phosphatidylinositol-4,5-bisphosphate to products that may serve as mediators of growth and development. The aim of this study was to define developmental changes in intestinal PLC gamma 1 expression, catalytic activity, and growth factor regulation of PLC gamma 1.

METHODS

Immunodetection was used to compare the expression and tyrosine phosphorylation state of PLC gamma 1, EGFR, phosphatidylinositol 3-kinase (PI 3-kinase), ras guanosine triphosphatase activating protein (GAP), and src homologous collagen-like protein (SHC) in the postnatal rat intestine.

RESULTS

The catalytic activity and expression of PLC gamma 1 markedly increased during weaning. Significant EGF-induced increases in the activity and tyrosine phosphorylation of PLC gamma 1 occurred in weanling but not suckling animals. EGFR and SHC expression were increased in weanling compared with suckling and adult animals; however, differences in expression of PI 3-kinase and GAP did not occur during weaning.

CONCLUSIONS

The expression and catalytic activity of rat intestinal PLC gamma 1 are greatest during weaning. A functional consequence is the age-dependent modulation of EGF regulation of PLC gamma 1 tyrosine phosphorylation state and catalytic activity. This is the first in vivo demonstration of EGF-dependent tyrosine phosphorylation of PLC gamma 1 in normal animal tissue.

Tyrosine protein kinase activity in renal brush-border membranes

Tyrosine protein kinase (TPK) activity was detected in rat renal brush-border membranes (BBM) using poly(Glu80Na,Tyr20) as a substrate. Maximal TPK activity required prior detergent dispersion of the membranes with 0.05% Triton X-100 and the presence of vanadate, a potent inhibitor of phosphotyrosine protein phosphatases, in the phosphorylation medium. Optimal conditions for measurement of TPK activity were 10 mM of MgCl2 and MnCl2, at 30 degrees C and pH 7.0. TPK activity was inhibited by genistein, with a IC50 value of 15 microM, while no inhibition was observed in the presence of 1-(5-isoquinolinesulfonyl)-2-methyl-piperazine dihydrochloride (H7), an inhibitor of serine-threonine kinases. TPK activity was enriched 4-fold in the BBM fraction relative to cortex homogenate. It was co-enriched with BBM enzyme markers, but not with those of the basolateral membrane (BLM). The endogenous substrates of TPK in brush-border and basolateral membranes were determined by Western blot analysis using an antiphosphotyrosine monoclonal antibody (PY20). Various phosphotyrosine-containing proteins were found in the BBM (31, 34, 46, 50, 53, 72, 90, 118 and 170 kDa) and in the BLM (37, 48, 50, 53, 72, 90, 130 and 170 kDa). Addition of exogenous insulin receptor to BBM and BLM increased the phosphorylation of most of the substrates. Solubilization of the TPK activity from BBM with 0.5% CHAPS and subsequent gel filtration on Superdex 75 yielded two peaks of tyrosine protein kinase activity with apparent molecular masses of 49 and 66 kDa. These results provide evidence for a non-receptor TPK activity associated with the renal tubular luminal membrane.