Evidence for the expression of a primitive intestinal-like alkaline phosphatase in the intestinal 407 cell line

Expression of a heterodimeric (placental-intestinal) hybrid alkaline phosphatase in KB cells

A hybrid heterodimeric alkaline phosphatase expressed in KB cells, consisting of placental and intestinal (fetal) subunits, was purified by use of two different immunoaffinity columns using the monoclonal antibodies 2HIMS-1 and HPMS-1. The closely related subunits were found to yield a dimeric active enzyme glycosylated as the mature heterodimeric forms. This enzyme displays intermediate properties to the placental and intestinal (fetal) isozymes with regard to heat stability, inhibition patterns with amino acids and amino acid derivatives, as well as reactivity with monoclonal antibodies specific for human alkaline phosphatase isozymes. Peptide fragments obtained from the hybrid enzyme after cyanogen bromide cleavage belong to either the placental or intestinal (meconial) isozyme as evaluated by SDS polyacrylamid gel electrophoresis, and the N-terminal amino acid sequences, corresponding to the placental and intestinal subunits, can be identified in the peptide fragments. By N-glycanase digestion or tunicamycin treatment, the molecular mass of the subunits was reduced to 62 kDa compared to 69 kDa for the native ones. The results confirm that some cell lines can synthesize hybrid alkaline phosphatases.

Purification and partial characterization of intestinal-like alkaline phosphatase in rabbit kidney

Two types of alkaline phosphatase (AP) isozymes in rabbit kidney, a major intestinal-like type and a minor tissue-unspecific type, have been identified. The former enzyme was purified from rabbit kidney by immunoaffinity chromatography using monoclonal anti-human intestinal AP antibody. The purified enzyme yielded a single protein band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the apparent molecular size of its monomer subunit was found to be 72,000. Three amino acid residues within the first 16 N-terminal amino acid residues were different in purified AP and human intestinal AP. Although the rabbit enzyme possessed some peptide bands identical to those of human adult intestinal AP after Staphylococcus aureus V8 protease digestion, the enzyme did not react with monoclonal antibody against human adult intestinal AP alone, whereas it did react with monoclonal antibody against both human adult and fetal intestinal APs. The affinity of the enzyme for concanavalin A was identical to that of the fetal intestinal AP, but different from that of the adult enzyme. These results indicate that the antigenicity and certain properties of purified rabbit AP are more like those of human fetal intestinal AP or Kasahara isozyme, so-called intestinal-like AP, than like human adult intestinal AP.

Biosynthesis of alkaline phosphatase during differentiation of the human colon cancer cell line Caco-2

The human colon cancer cell line Caco-2 undergoes spontaneous enterocytic differentiation during growth and expresses a number of brush-border membrane-associated hydrolases typical of a differentiated phenotype. Among these is the enzyme alkaline phosphatase, which is frequently used as a marker of cell differentiation in colon cancer cells. Since the biochemical processes regulating the expression of alkaline phosphatase during cell differentiation are only poorly understood, we examined the biosynthesis and processing of alkaline phosphatases in undifferentiated (0-day confluent) and differentiated (14-day confluent) Caco-2 cells. It was found that both cell phenotypes expressed a single, heat-labile intestinal-like enzyme, which undergoes similar post-translational processing and glycosylation. Although the rate of enzyme synthesis and alkaline phosphatase messenger ribonucleic acid was 5-6-fold higher in differentiated cells, the degradation rates in both cell types were similar with a half-life of approximately 10 days. These results suggest that the increase in alkaline phosphatase activity during Caco-2 cell differentiation is caused by changes in the synthetic rate and that the low turnover rates facilitate accumulation of the enzyme. Furthermore, these studies demonstrate that Caco-2 cells are useful for examining the molecular and biochemical events involved in the differentiation of the small intestinal epithelium.