Selective Targeting of Leukemic Cell Growth in Vivo and in Vitro Using a Gene Silencing Approach to Diminish S-Adenosylmethionine Synthesis

Abstract

We exploited the fact that leukemic cells utilize significantly higher levels of S-adenosylmethionine (SAMe) than normal lymphocytes and developed tools that selectively diminished their survival under physiologic conditions. Using RNA interference gene silencing technology, we modulated the kinetics of

S-adenosylmethionine limitation induces p38 mitogen activated protein kinase and triggers cell cycle arrest in G1

“…Recent studies in breast cancer cells suggest that SAM rather than methionine might be the limiting metabolite defining addiction to this metabolic pathway (Booher et al, 2012). Consistent with this idea, targeting methionine adenosyltransferase (MAT) with either a small molecule inhibitor or shRNA induces apoptosis in leukemia cells (Attia et al, 2008;Jani et al, 2009). The mechanisms of how cells induce cell cycle arrest and apoptosis in response to SAM limitation is currently unknown, but experiments in the yeast and breast cancer cell models indicate loss of pre-replication complexes from chromatin as a contributing factor (Booher et al, 2012;Su et al, 2005).…”

“…This idea is reminiscent of the SAM checkpoint proposed in the model organism S. cerevisiae (Kaiser et al, 2006). Consistent with this notion, targeted inhibition of MAT with a chemical inhibitor or shRNA knockdown was shown to inhibit leukemia cell proliferation and induce apoptosis (Attia et al, 2008;Jani et al, 2009), suggesting not only the existence of a SAM checkpoint in mammalian cells but also its potential as a therapeutic target. Furthermore, recent reports indicate breast and prostate cancer cells suffer a G1 cell cycle arrest when cultured in medium where methionine is replaced with its metabolic precursor homocysteine (Booher et al, 2012;Lu and Epner, 2000), probably as a consequence of reduced flux through the homocysteinemethionine-SAM metabolic axis, which results in insufficient SAM to support cell cycle progression (Booher et al, 2012).…”

Downregulation of Cdc6 and pre-replication complexes in response to methionine stress in breast cancer cells

“…Targeted inhibition of the regulatory subunit of MAT reduced SAM levels and strongly induced apoptosis and growth inhibition of leukemic cells. 16,17 Given such high demand for methionine and SAM by cancer cells, it is conceivable that replacement of methionine by homocysteine curtails flux through methyl group metabolism reducing methionine to a level insufficient for the high methionine requirement of cancer cells, thus affecting a cell cycle arrest and apoptosis.…”

“…[18][19][20] Furthermore, earlier studies showed that reducing SAM synthesis blocks proliferation of leukemic cells. 16,17 Therefore, we hypothesized that methionine dependence is a reflection of limiting SAM availability caused by reduced flux through the methionine metabolic pathway in Met-Hcy+ conditions. Consistent with our hypothesis, SAM supplementation restored proliferation of MDAMB468 cells in Met-Hcy+ medium (Fig.…”

Crystallography captures catalytic steps in human methionine adenosyltransferase enzymes

“…For example, mutations involving the MAT1A gene have links with hepatic MAT deficiency which leads to hypermethioninemia (6,7). Expression of MATα2 confers a growth advantage in cells and is important for differentiation and apoptosis (1) in diseases such as human hepatocellular carcinoma (5), colon cancer (8), and leukemia (9).…”