Two
interferon-mediated
enzyme activities, the
protein kinase and
pppA (2'p5'A)n
synthetase (2-5A synthetase) were used to assess the presence and action of
interferon on HeLa
tumor cells in athymic nude mice. The
protein kinase is manifested by the phosphorylation of endogenous
proteins with a molecular weight of 67,000 and 72,000 in mouse and human cells, respectively. Treatment of HeLa
tumor-bearing mice with mouse
interferon (alpha and beta) resulted in enhanced levels of
2-5A synthetase and
protein kinase (Mr 67,000) activities in the spleen and lung while there were no apparent effects on HeLa cells. In these HeLa
tumor cells of human origin, the
2-5A synthetase and
protein kinase (Mr 72,000) activities were enhanced considerably only
after treatment of mice with human fibroblastic (
beta) interferon. When HeLa
tumor-bearing mice were given
injections of polyadenylate-polyuridylate or with polyinosinylate-polycytidylate, then the
2-5A synthetase and the
protein kinase activities were enhanced in
tumor cells [
protein kinase] as well as in the different tissues [
protein (Mr 67,000)
kinase] of mice since both mouse and human
interferons were produced under these conditions. These results indicate a direct action of
interferon on homologous
tumor cells, and furthermore they indicate that
tumor cells in an organism may themselves produce
interferon and respond to their own
interferon.