“These findings may lead to a new target for chemoresistant cancer cells,” said Ruth W. Craig, PhD, professor of Pharmacology Toxicology, Geisel School of Medicine at Dartmouth, Hanover, NH, who is primary author of the peer reviewed article. “These cells are resistant to multiple types of standard chemotherapeutic agents because of over-expression of Myeloid Cell Leukemia-1 (Mcl-1), however, Mcl-1 expression plummets when we inhibit one particular enzyme and then cancer cells subsequently die.”
The Mcl-1 protein is frequently over-expressed in cancer; it is present not only in leukemia and lymphoma but also in a host of solid tumors. While Mcl-1 is expressed in a highly-controlled fashion in normal cells, its over-expression and lack of destruction maintains the viability of cancer cells and renders them resistant to chemotherapy. When high levels of this protein are maintained, the patient’s cancer cells survive multiple types of drug treatment.
The research found that an enzyme that removes phosphate groups from Mcl-1 is critical in terms of maintaining its expression in cancer. This enzyme, known as protein phosphatase 2A (PP2A), can be inhibited to stop the removal of phosphate groups from a regulatory motif in Mcl-1 referred to as the PEST region (enriched with amino acids Proline, glutamic acid, Serine, and Threonine). Inhibition of the removal of phosphate groups, such as at Threonine-163 and Serine-159, targets the Mcl-1 protein for rapid destruction and, shortly thereafter, the cancer cells die.
“PP2A is a complex multi-subunit enzyme and we hope to identify more specifically which form of PP2A is involved in dephosphorylating Mcl-1,” said Craig. “This could give a more specific way of causing Mcl-1 destruction.”