Zinc transport and metallothionein secretion in the intestinal human cell line Caco-2.

Caco-2, a human cell line, displays several biochemical and morphological characteristics of differentiated enterocytes. Among these is the ability to transport zinc from the apical to the basal compartment. This process was enhanced following exposure by the apical compartment to increasing concentrations of the metal. High pressure liquid chromatography fractionation of the media obtained from cells labeled with radioactive zinc showed that metallothioneins (MTs), small metal-binding, cysteine-rich proteins), were present in the apical and basal media of controls as well as in cells grown in the presence of high concentrations of zinc. Following exposure to the metal, the levels of Zn-MTs in the apical medium increased, while in the basal compartment the greatest part of zinc appeared in a free form with minor changes in the levels of basal MTs. Metabolic labeling experiments with radioactive cysteine confirmed the apical secretion of MTs. A stable transfectant clone of Caco-2 cells (CL11) was selected for its ability to express constitutively high levels of the mouse metallothionein I protein. This cell line showed an enhanced transport of the metal following exposure to high concentrations of zinc and a constitutive secretion of the mouse metallothionein I protein in the apical compartment. Together, these findings strongly support the hypothesis of a functional role between the biosynthesis and secretion of MTs and the transport of zinc in intestinal cells.

Evidence that thyroglobulin has an associated protein kinase activity correlated with the presence of an adenosine triphosphate binding site.

19S Thyroglobulin (Tg), a dimeric glycoprotein with an M(r) of 660,000, was extracted from rat, bovine, and human (goitrous) thyroid tissues, as well as from culture medium of FRTL5 rat thyroid cells. Subjected to rigorous purification procedures, all Tg preparations showed a protein kinase activity that is able to phosphorylate serine residues in vitro. Further characterization of this enzymatic activity revealed that Tg has the specificities of a protein kinase A when Kemptide was used as specific substrate and after analysis of cAMP-stimulated phosphotransferase activity in all Tg preparations tested. Furthermore, Tg contains a specific and saturable ATP binding site, as evidenced by specific binding with the ATP affinity analog p-fluorosulfonylbenzoyl 5'-p-adenosine (FSO2BzAdo). Limited proteolysis of FSO2[14C]BzAdo-labeled human goiter Tg with clostripain gave rise to a M(r) 64,000 amino terminal polypeptide carrying almost all of the label. The analysis of this fragment revealed two sequences that may play an analogous phosphate-loop (P-loop) function, even though their primary sequences differ slightly from the classical A consensus sequence or P-loop. The ATP binding site of Tg may be functionally associated with the presence of a protein kinase A phosphorylating activity linked to the Tg itself, because the addition of the analog is able to inhibit the enzymatic activity in a dose-dependent fashion. Contamination with non-Tg phosphorylase cannot be ruled out at present, even though the amount of ATP that Tg was able to bind did not significantly change during the various purification steps. Together these findings favor the hypothesis that the Tg molecule contains a protein kinase activity that is capable of autophosphorylation.

Two abnormal thyroglobulin-like polypeptides are produced from Afrikander cattle congenital goiter mRNA.

In vitro translation of the mRNA from the congenital goiter tissue of Afrikander cattle directs the synthesis of two (Mr congruent to 250,000 and congruent to 75,000, respectively) abnormal thyroglobulin-like polypeptides. Common features of these polypeptides are the following: 1) both are immunoprecipitated by purified, anti-thyroglobulin antibody; 2) the signal sequence is present on their nascent polypeptide chains, and 3) their synthesis is apparently directed by the same high molecular weight mRNA. However, the goiter thyroglobulin mRNA shows, by Northern blot analysis, a slightly reduced size (congruent to 30 S) as compared to the mRNA from normal, bovine, thyroid tissue (33 S). Furthermore, a 10- to 15-fold decrease (as assessed by dot blot hybridization) in relative mRNA concentration was observed. Thus, it can be concluded that the genetic defect in congenital goiter of Afrikander cattle results in the synthesis of an altered mRNA which is present in decreased amount and directs the synthesis of two abnormal thyroglobulin-like polypeptides.

Construction of recombinant plasmids containing rat thyroglobulin mRNA sequences.

Two plasmids containing rat thyroglobulin cDNA sequences have been constructed and characterized. A plasmid with a 500-bp insert (pRT6) was isolated and identified as thyroglobulin-specific on the basis of the tissue specificity of the inserted sequence and of its ability to retain thyroglobulin mRNA on a nitrocellulose filter. The cDNA insert in pRT6 was subsequently used to screen a rat thyroid cDNA library constructed with large cDNA. A plasmid was found containing a 1700-bp insert. The polarity and the fidelity of the insert is demonstrated by S1 mapping.

The segregation into microsomal vesicles and core-glycosylation in vitro of a 300-kDa rat thyroglobulin subunit.

Translation of rat thyroid mRNA in a cell-free protein synthesis system derived from rabbit reticulocytes results in synthesis of a 300-kDa thyroglobulin polypeptide [C.G. Alvino et al. (1982) FEBS Lett. 137, 307-313]. In the presence of dog pancreas microsomal membranes this polypeptide is segregated into the microsomal vesicles and core-glycosylated, as shown by increased protection against proteolytic treatment and binding to concanavalin-A-Sepharose. The segregation process appears to be strictly coupled to glycosylation, i.e. no detectable amounts of unglycosylated chains are found in the vesicles, and both processes only occur cotranslationally. The glycosylated protein exhibits lower electrophoretic mobility with respect to its non-glycosylated form and comigrates with a glycosylated 330-kDa polypeptide found in rat thyroid epithelial cells cultured in vitro [F.S. Ambesi-Impiombato et al. (1980) Proc. Natl Acad. Sci. USA, 77, 3455-3459]. Both glycosylated and non-glycosylated forms are immunoprecipitated by antibodies to 19-S rat thyroglobulin. Some features of the tyroglobulin polypeptide segregation and glycosylation have been studied by synchronized translation of rat thyroid mRNA: (a) the signal sequence seems to be located at the amino-terminal portion of the nascent polypeptide chain; (b) the distribution of the carbohydrate units is mostly clustered in the amino-terminal half of the protein as monitored by posttranslational analysis of the polypeptides synthesized in the presence of pancreatic membranes during competition experiments carried out with the detergent Nonidet P40.