Involvement of pRb-E2F pathway in green tea extract-induced growth inhibition of human myeloid leukemia cells.

Both inhibitory and stimulatory effect of EGCG on cancer cells have been reported, which often is linked to receptor tyrosine kinase signaling. In this study, we present evidence that green tea extract and its chemical component, Epigallocatechin-3-gallate (EGCG), inhibit growth of human myeloid leukemia cells through the regulation of pRb synthesis and formation of pRb-E2F complexes. Addition of green tea extract to the culture of TF-1a and MV4-11 myeloid leukemia cells significantly inhibited their proliferation with a substantial portion of cell death being observed. The green tea extract and EGCG had no significant effect on the expression of G1 CDKs and the CDK inhibitors but downregulated the formation of pRb-CDKs. Surprisingly, the expression of pRb was markedly upregulated while the phosphorylation of pRb downregulated. The upregulation of pRb was blocked by pre-treatment with cycloheximide, a protein synthesis inhibitor, suggesting a requirement of protein synthesis. In agreement with these results, pRb-E2F complexes were upregulated and E2F DNA binding activity decreased. Since both TF-1a and MV4-11 are factor-independent cell lines, the upregulation of pRb-E2F complexes and inhibition of DNA binding activity by green tea extract is most likely through a receptor tyrosine kinase-independent pathway. We also found that the stem/progenitor cells derived from these two leukemia cell lines are more sensitive to the inhibitory effect of green tea extract. Our result suggests that concentrated green tea extract and EGCG may have potential for clinical investigation as an inducer of cancer cell death.