Iron deficiency leads to repression of a non-canonical methionine salvage pathway in Schizosaccharomyces pombe

The methionine salvage pathway (MSP) regenerates methionine from 5'-methylthioadenosine (MTA). Aerobic MSP consists of six enzymatic steps. The mug14⁺ and adi1⁺ genes that are involved in the third and fifth steps of the pathway are repressed when Schizosaccharomyces pombe undergoes a transition from high- to low-iron conditions. Results consistently show that methionine auxotrophic cells (met6Δ) require iron for growth in the presence of MTA as the sole source of methionine. Inactivation of the iron-using protein Adi1 leads to defects in the utilization of MTA. In the case of the third step of the pathway, co-expression of two distinct proteins, Mta3 and Mde1, is required. These proteins are interdependent to rescue MTA-dependent growth deficit of met6Δ cells. Coimmunoprecipitation experiments showed that Mta3 is a binding partner of Mde1. Meiotic met6Δ cells co-expressing mta3⁺ and mde1⁺ or mta3⁺ and mug14⁺ produce comparable levels of spores in the presence of MTA, revealing that Mde1 and Mug14 share a common function when co-expressed with Mta3 in sporulating cells. In sum, our findings unveil several novel features of MSP, especially with respect to its regulation by iron and the discovery of a non-canonical third enzymatic step in the fission yeast.

Human acireductone dioxygenase (HsARD), cancer and human health: Black hat, white hat or gray?

Multiple factors involving the methionine salvage pathway (MSP) and polyamine biosynthesis have been found to be involved in cancer cell proliferation, migration, invasion and metastasis. This review summarizes the relationships of the MSP enzyme acireductone dioxygenase (ARD), the ADI1 gene encoding ARD and other gene products (ADI1GP) with carcinomas and carcinogenesis. ARD exhibits structural and functional differences depending upon the metal bound in the active site. In the penultimate step of the MSP, the Fe²⁺ bound form of ARD catalyzes the on-pathway oxidation of acireductone leading to methionine, whereas Ni²⁺ bound ARD catalyzes an off-pathway reaction producing methylthiopropionate and carbon monoxide, a biological signaling molecule and anti-apoptotic. The relationship between ADI1GP, MSP and polyamine synthesis are discussed, along with possible role(s) of metal in modulating the cellular behavior of ADI1GP and its interactions with other cellular components.

The essential role of methylthioadenosine phosphorylase in prostate cancer

Prostatic epithelial cells secrete high levels of acetylated polyamines into the prostatic lumen. This distinctive characteristic places added strain on the connected pathways, which are forced to increase metabolite production to maintain pools. The methionine salvage pathway recycles the one-carbon unit lost to polyamine biosynthesis back to the methionine cycle, allowing for replenishment of SAM pools providing a mechanism to help mitigate metabolic stress associated with high flux through these pathways. The rate-limiting enzyme involved in this process is methylthioadenosine phosphorylase (MTAP), which, although commonly deleted in many cancers, is protected in prostate cancer. We report near universal retention of MTAP expression in a panel of human prostate cancer cell lines as well as patient samples. Upon metabolic perturbation, prostate cancer cell lines upregulate MTAP and this correlates with recovery of SAM levels. Furthermore, in a mouse model of prostate cancer we find that both normal prostate and diseased prostate maintain higher SAM levels than other tissues, even under increased metabolic stress. Finally, we show that knockdown of MTAP, both genetically and pharmacologically, blocks androgen sensitive prostate cancer growth in vivo. Our findings strongly suggest that the methionine salvage pathway is a major player in homeostatic regulation of metabolite pools in prostate cancer due to their high level of flux through the polyamine biosynthetic pathway. Therefore, this pathway, and specifically the MTAP enzyme, is an attractive therapeutic target for prostate cancer.