The majority of cancer cells have a unique metabolic addiction to methionine in contrast to normal cells. This “methionine-dependent” phenotype describes the inability of cancer cells to proliferate in methionine stress conditions, where methionine has been replaced with its metabolic precursor, homocysteine, in the growth media. Methionine-dependence is implicated as a signature of cancer and upon spontaneous reversion to a methionine-independent phenotype, cells lose anchorage independent growth, a hallmark of tumorigenicity. Comparing methionine-dependent and -independent cell lines is critical for a clear interpretation of our results as we investigate the molecular mechanisms behind methionine-dependence and malignancy. For this purpose, we use the methionine-dependent, triple negative, breast cancer cell line MDA-MB-468 and derived a methionine-independent variant, MDA-MB-468res-R8. With this breast cancer cell pair we are able to compare cancer and normal-like cells due to their similar genetic backgrounds, proliferation rates, and media requirements.
Previous studies on methionine-dependence identified a reduction in the synthesis of S-adenosylmethionine (SAM) during methionine stress. SAM is the principal methyl donor in the cell and, interestingly, cancer cells have been shown to have higher methylation activity as compared to normal cells. Therefore, the studies presented in this thesis focus on the early response to methionine stress and how the resultant decrease in SAM is communicated throughout the cell. We used mass spectroscopy methods to understand the metabolic response to methionine stress in both cell lines over a 24 hour period. We observed methionine stress induced oxidative stress in both cell types resulting in a redirection of homocysteine metabolism toward synthesis of the antioxidant glutathione. Additionally, lipidomic analyses indicated a complete reprogramming of lipid synthesis in the methionine-dependent cell line that may be caused by irreversible oxidative damage and limited SAM availability for the synthesis of SAM-dependent lipids such as phospho- and sphingolipids.
To further understand how SAM levels are communicated to initiate a cellular response for protection of cell integrity, we focused on SAM as a co-factor for substrate methylation. SAM is a universal methyl donor and can serve as a co-factor for protein, lipid, chromatin, and RNA methylation events. Particularly, we focus on mRNA 5’ cap methylation and it’s influence on protein translation as a possible mechanism to communicate SAM levels. We employed both human cell lines and yeast strains to develop a methyl cap purification method for use in high-throughput RNA sequencing and have thus far identified unique, gene-specific responses to SAM depletion in regards to methyl cap stability. Continuing our efforts to fully understand this unique, metabolic requirement of cancer will allow us to shed light on an undefined molecular area essential for the development and use for cancer therapeutics in methionine-dependent cancers.