Interferon induces two double-stranded RNA-dependent enzymatic activities: an oligoisoadenylate synthetase that converts ATP to ppp(A2'p).5'A, and a protein phosphokinase. We have explored the level and inducibility of these two enzymes in a human cell line (HEC-1) totally insensitive to both the antiviral and the anticellular actions of interferon. The activities of both enzymes are high in untreated cells and only minor changes occur after treatment with interferon, even at high concentrations. Interferon-treated HEC-1 cells do not contain an inhibitor of the oligoisoadenylate synthetase activity. The products of this HEC-1 oligoisoadenylate synthetase consist mainly of dimers, trimers, and tetramers as found in other cell lines after interferon treatment. The synthetase level is unaffected by treating the cells with anti-interferon antiserum, indicating that the results cannot be explained by a spontaneous low production of interferon by these cells. Furthermore, virus multiplication is not inhibited, even after treatment with interferon. These observations suggest that either the two enzymatic activities do not suffice for the establishment of an antiviral state in vivo or that a regulatory control mechanism, lost in these cells and common for both enzymes, is required for the expression of the antiviral action of interferon. This might explain both the constitutivity of the two enzymes and the interferon resistance observed.

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In rat hepatoma cells the synthetic glucocorticoid dexamethasone causes a 3-fold increase in the activity of the plasma membrane enzyme alkaline phosphodiesterase I (oligonucleate 5'-nucleotidohydrolase, EC 3.1.4.1). The data are consistent with an induction phenomenon mediated by the glucocorticoid receptor involved in tyrosine aminotransferase induction. The Glucocorticoids modify several properties of cellular plasma membranes. One of the earliest effects of these hormones in lymphoid cells is an inhibition of hexose, nucleoside, and amino acid uptake (1). Glucocorticoid hormones also decrease hexose uptake by fibroblasts (2) and by adipocytes (3). In rat hepatoma tissue culture (HTC) cells, glucocorticoids promote cell adhesiveness and the reappearance of surface antigens present in normal hepatocytes (4). These hormones also inhibit the release of plasminogen activator (5) and the uptake of aminoisobutyric acid (6, 7), and they decrease the number of cell surface microvilli (8). Such modifications of HTC cell membrane functions have been considered as part of a program of glucocorticoid-induced reversal of the transformed phenotype (8). The biochemical reactions underlying these effects have not been elucidated.Glucocorticoid hormones appear to produce many of their physiological effects through a stimulation of the activity of specific enzymes in target cells, most often as a result of an increase in their rate of synthesis (9). This prompted us to examine whether enzymes located in the plasma membrane could be under glucocorticoid control, an effect that might be related to one or more of the phenomena described above. This possibility was investigated in HTC cells wherein the interaction of steroids with intracellular receptors and the ensuing induction of tyrosine aminotransferase (L-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5) have been extensively studied as a model of glucocorticoid action.(10). In this system the hormonal response is very specific in that the number of glucocorticoidsensitive gene products is quite limited (11). Only seven proteins are reproducibly induced, including two unidentified membrane proteins. We show here that, in HTC cells, glucocorticoid hormones specifically increase the activity of alkaline phosphodiesterase I (oligonucleate 5'-nucleotidohydrolase, EC 3.1. Enzyme Assays. For determination of alkaline phosphodiesterase I, frozen HTC cells were sonicated for 10 sec (Branson Sonifier) in an aqueous solution of Triton X-100 (1 g/liter). Enzyme activity was determined as described by Beaufay et al. (15) by incubating the sample for 10 min at 250C in a mixture containing 0.1 M glycine-NaOH (pH 9.6), 2 mM zinc acetate, and 3 mM p-nitrophenylthymidine 5'-phosphate (from Sigma or Boehringer Mannheim). The reaction rate was constant for at least 20 min and the assay was linear with protein concentration up to 120 ,ug per assay. For determination of tyrosine aminotransferase activity, cells were sonicated as described above in 50 mM potassium phosphate buffer...

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L1210 cells resistant to the antiviral and anticellular effects of interferon are not inducible for the 2,5A synthetase and for the protein kinase activities. Cloning of one resistant L1210 strain has revealed heterogeneity of the cell population with respect to their antiviral and anticellular response as well as protein kinase and 2,5A synthetase inducibility. The defect in the response of truly interferon-resistant L1210 cells appears to reside at an early step of interferon action.