Abstract 209A: Involvement of pRb-E2F pathway in green tea extract-induced growth inhibition of human myeloid leukemia cells

Abstract

Abstract It has long believed that green tea has many health benefits. However, the clinical research has not provided conclusive evidence on these benefits especially on cancer prevention and treatment. In basic research, there are also many inconsistent reports on the biological activity of green tea extract and its major chemical components (catechins) with both inhibitory and stimulatory effects of EGCG on various cancer cells being reported. The stimulatory or inhibitory activity is often linked to either activation or inactivation of receptor tyrosine kinase. In this study, we present evidence that green tea extract and its major chemical components, Epigallocatechin-3-gallate (EGCG), epigallocatechin (ECG), and epicatechin (EC) inhibit growth of two human myeloid leukemia cells (TF-1a and MV4-11) through the regulation of pRb synthesis and pRb-E2F complexes. Addition of green tea extract to the culture of the myeloid leukemia cells significantly inhibited their proliferation with a substantial portion of cell death and decreased their colony forming cells in soft-agar culture. EGCG, ECG, and EC showed a similar inhibitory effect on these cells with the inhibitory efficiency varied with each catechin and the dose applied. 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 examined by Western blot and immunoprecipitation, respectively. 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 determined by EMSA assay. Since both TF-1a and MV4-11 cell are factor-independent cell lines, the upregulation of pRb-E2F complexes and downregulation of DNA binding activity by green tea extract is most likely through a receptor tyrosine kinase-independent pathway. In summary, this study reveals that green tea extract, EGCG, ECG, and EC inhibited growth in human myeloid leukemia cells via regulating pRb synthesis and the formation of pRb-E2F complexes, leading to the inhibition of the E2F DNA binding activity. In addition, we found that that the stem/progenitor cells derived from the leukemia cell lines tested are more sensitive to the inhibitory effect of green tea extract. We propose that as a beverage one should not consume high amounts of green tea (extract) due to its possible cytotoxic or apoptotic effect, and its significant inhibitory effect on stem/progenitor cells, although the biological effect of green tea extract in vivo could be quite different from in vitro. However, the concentrated green tea extract may have potential for clinical investigation as an inducer of cancer cell death. Citation Format: Darrell Henry, Sebastien Brumaire, Gabriela Alvarez, Beatriz Alvarez, Xiaotang Hu. Involvement of pRb-E2F pathway in green tea extract-induced growth inhibition of human myeloid leukemia cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 209A.