Sensitivity to Romidepsin Cytotoxicity Depends on STAT3-Dependent Metabolic Reprogramming of Cancer Cells

Introduction: Acute myeloid leukemia (AML) is the most common acute leukemia in adults and accounts for approximately 80% of cases in this group. Even with the best treatment currently available, there is a very high rate of relapse and mortality. The five-year survival rates are only 24% with relapse rates up to 78%. Accordingly the search for a better understanding of the molecular biology and genetics underlying AML is essential in our quest to find treatments that achieve a better overall survival. Signal transduction and activator of transcription protein 3 (STAT3) has been implicated in many human cancers, including AML; up to 50% of AML patients display enhanced tyrosine phosphorylation of STAT3. Constitutively active STAT3 has also been correlated with shorter disease free survival in AML patients. Accumulating clinical, genetic and biochemical data implicate STAT3 in the majority of human malignancies, indicating that it is essential for tumor initiation and/or maintenance. The best understood tumorigenic activity of STAT3 depends on its tyrosine phosphorylation, mostly due to response to increased secretion of cytokines or mutations in oncogenic tyrosine kinases. However, serine phosphorylation of STAT proteins in the absence of tyrosine phosphorylation or increased cytokine signaling has also been implicated in human leukemias. Moreover, it has recently been found that STAT3 contributes to tumor metabolic reprogramming in a cytokine-independent manner through a novel mitochondrial function that was noted to cause myeloproliferative disease-like in mice models.

Methods: We conducted a global analysis of metabolites whose abundance in cancer cells depended on the presence of mitochondrial STAT3, assessed by LC/HPLC coupled to mass spectrometry. This analysis identified glutathione (GSH) levels as being dependent on mitochondrial STAT3, along with multiple intermediaries in the GSH biosynthetic pathway. GSH provides essential reducing capacity in cells and its levels are often elevated in cancer. Romidepsin (Depsipeptide) is an HDAC inhibitor that must be converted to an active form by disulfide reduction via glutathione, originally characterized for its cytotoxicity against Ras-transformed cells. We have found enhanced Romidepsin toxicity in STAT3-containing cancer cells, which correlates with the presence of increased GSH and reduced reactive oxygen species (ROS). We found that the enzymes in the GSH synthetic pathway are coordinately regulated by STAT3. We also found that Romidepsin increases STAT3 expression at both the mRNA and the protein levels and we are analyzing the mechanism by which STAT3 is affected by Romidepsin.

Conclusion: Enhanced Romidepsin toxicity was found in STAT3-containing cancer cells. These STAT3-containinng cells were noted to have higher glutathione synthase expression, which plays a major role in the GSH production in the gamma glutamyl cycle. This could potentially explain the higher GSH levels noted in the presence of STAT3-contianing cells. These studies hopefully will aid in the design of cancer therapeutics that target STAT3-dependent processes that contribute to oncogenesis and could lead to have better treatments available in myeloid diseases.