A CARDIOTONIC STEROID BUFALIN-INDUCED DIFFERENTIATION OF THP-1 CELLS - INVOLVEMENT OF NA-ATPASE INHIBITION IN THE EARLY CHANGES IN PROTOONCOGENE EXPRESSION(, K+)
S. Numazawa et al., A CARDIOTONIC STEROID BUFALIN-INDUCED DIFFERENTIATION OF THP-1 CELLS - INVOLVEMENT OF NA-ATPASE INHIBITION IN THE EARLY CHANGES IN PROTOONCOGENE EXPRESSION(, K+), Biochemical pharmacology, 52(2), 1996, pp. 321-329
Human monocytic leukemia THP-1 cells were induced to differentiate int
o macrophage-like cells by treatment with cardiotonic steroid bufalin,
which was previously shown to interact with the Na+, K+-ATPase with s
imilar kinetics to ouabain, a specific inhibitor of the enzyme. This i
nduction of differentiation was characterized by loss of proliferation
, cell adherence, increased ability to reduce Nitro Blue tetrazolium (
NBT), and increased expression of interleukin 1 beta (IL-1 beta). Duri
ng this process, bufalin downregulated c-myb and c-myc expressions and
induced c-fos and Egr-1 transcripts. Ouabain also caused similar chan
ges in protooncogene expression and induced phenotypic markers of diff
erentiated cells at concentrations comparable to bufalin. The 12-O-tet
radecanoyl phorbol-13-acetate resistant THP-1 cell variant, which was
unresponsive to this agent as to growth inhibition and proto-oncogene
expression, responded to bufalin. The finding that protein kinase inhi
bitor H7 failed to inhibit bufalin-mediated c-fos induction further su
pports the theory that the signal transduction machinery caused by buf
alin is separable from the phorbol ester. The cytotoxic effect of high
doses of bufalin apparently disappeared in the medium where Na+ was r
eplaced with choline ions. Furthermore, bufalin failed to induce c-fos
expression and to downregulate c-myb transcripts in the low-Na+ mediu
m. These findings indicate that an increased intracellular Na+ concent
ration resulting from the Na+, K+-ATPase inhibition possibly triggers
the change in proto-oncogene expression evoked by bufalin.