Research in my laboratory is focused on the role of polyamines in cancer cell proliferation and ways to exploit their metabolism and function as antiproliferative and chemopreventive targets. Polyamines are naturally occurring polycationic alkylamines that are absolutely required for eukaryotic cell growth and differentiation. Their intracellular concentrations (millimolar) are maintained in a narrow range through the actions of a highly regulated and specific transport system and a rapidly responding metabolic pathway. The first rate-limiting step in the production of polyamines is the decarboxylation of ornithine by the highly inducible and short-lived enzyme, ornithine decarboxylase (ODC). Increased ODC activity and new polyamine synthesis have frequently been associated with the neoplastic phenotype. The catabolism of the polyamines is controlled by the rate limiting enzyme spermidine/spermine N1-acetyltransferase (SSAT) that my laboratory has cloned and continues to study. This enzyme is highly controlled at the level of transcription, translation and is stabilized by the natural polyamines and antitumor polyamine analogues. We have also demonstrated that the phenotype-specific antitumor activity of specific polyamine analogues is associated with a superinduction of this enzyme in response to treatment.
Another enzyme cloned in my laboratory is the inducible FAD-dependent, spermine oxidase (SMO) that is also a polyamine catabolic enzyme that produces H2O2 as one of its products. SMO is induced by many inflammatory stimuli including bacterial infection and inflammatory cytokines. As an example, Helicobacter pylori, a causative agent of gastric cancer, highly induces SMO resulting in sufficient H2O2 production to produce oxidative DNA damage. More recently we have demonstrated that SMO is induced by a toxigenic strain of Bacteriodes fragilis, a bacteria that is associated with colon cancer. In an animal model of colon cancer induced by B. fragilis, the Casero lab demonstrated inhibiting SMO significantly reduced tumor formation. Thus it appears that SMO may be one of the molecular links between inflammation and carcinogenesis and as such represents a potential target for chemopreventive intervention.
During the studies that lead to the discovery of SMO, we also identified the sequence of a related FAD-dependent oxidase that was later identified as lysine specific demethylase 1 (LSD1). LSD1 is an important chromatin-remodeling enzyme that demethylates mono- and dimethyl lysine 4 of histone 3 (H3K4me1 & H3K4me2). As these histone marks are associated with active transcription, LSD1 has the ability to broadly repress gene transcription. LSD1 activity is involved with the inappropriate silencing of several tumor suppressor gene involved in the etiology and progression of cancer. As LSD1 is structurally and functionally homologous to SMO, we hypothesized that certain polyamine analogues would inhibit LSD1 and lead to the re-expression of inappropriately silenced genes. We have now demonstrated that this is, in fact, the case and are pursuing this strategy to develop agents that may be useful in the treatment of neoplastic disease.
Consequently, polyamine metabolism and function, and related pathways present a target rich environment against which therapeutic and chemopreventive strategies may be developed.