Characterising Changes in Lipid Homeostasis as an Anti-Cancer Mechanism of Action of Methylthio-DADMe-Immucillin-A

Methylthio-DADMe-immucillin-A (MTDIA) is an 86 picomolar inhibitor of 5’-methylthioadenosine phosphorylase (MTAP) that harbours anti-cancer efficacy. MTAP salvages S -adenosylmethionine (SAM) from 5’methylthioadenosine (MTA), a toxic metabolite produced during polyamine biosynthesis. Changes in MTAP expression and increased polyamine levels have been implicated in cancer growth and development, which makes MTAP an attractive target for anti-cancer therapeutics. MTAP inhibition leads to MTA accumulation and a reduction of intracellular SAM pools. Since SAM is involved in the synthesis of various lipids, we hypothesise that MTDIA will have profound effects on the lipidomic profile of treated cells. Using thin-layer chromatography and mass spectrometry, changes in the lipidome caused by MTDIA indicates an association between MTDIA-induced cell toxicity and lipid homeostasis dysregulation. MTDIA treatment caused a reduction in phosphatidylinositol (PI) and a preferential change in acyl chain specificity of PI lipid species. Fluorescent microscopy suggests that this aberrance in PI homeostasis also disrupts the phosphoinositide kinase/phosphatase network vital for cellular signaling mechanisms. As reactive oxygen species (ROS) utilise the phosphoinositide network to control mTOR and Sch9 for cellular growth and differentiation, flow cytometry analyses demonstrated a reduction in ROS levels when cells were treated with MTDIA. Similarly, mTOR inhibition via rapamycin also exhibited the same changes in ROS levels. Together, these results indicate that the anti-cancer efficacy of MTDIA is attributable to the disruption of lipid homeostasis that causes downstream effects on the phosphoinositide network and ROS-mTOR axis, which are crucial for cell growth.