CAMP‐Phosphodiesterase PDE4D as a Target for Colon Cancer Therapy

Abstract

Type 4 cyclic nucleotide phosphodiesterases (PDE4s) are part of a superfamily of isoenzymes that hydrolyze and inactivate the second messenger cAMP. Recent studies have shown that treatment with non‐selective PDE4 inhibitors impairs cancer cell proliferation and tumor growth, cell migration, epithelial to mesenchymal transition and tumor angiogenesis in various types of cancers; suggesting that PDE4s represent a potential target for the development of novel cancer therapeutics. However, non‐selective PDE4 inhibitors are known to induce a number of side effects, including nausea and emesis, diarrhea and weight loss that limit their clinical utility. The PDE4 family comprises four subtypes/genes, PDE4A to D. As each plays unique and non‐overlapping physiological and pathophysiological roles in the body, targeting individual PDE4 subtypes may serve to dissect the therapeutic benefits from the side effects of non‐selective PDE4 inhibitors. To this end, we have explored the role of PDE4 and individual PDE4 subtypes in five human colon cancer cell lines. Treatment with the archetypal non‐selective PDE4 inhibitor Rolipram ablated the chief portion of total cAMP hydrolytic activity in cell extracts prepared from all five colon cancer cell lines, suggesting that PDE4s are the main cAMP‐PDE expressed in these cells. Treatment with non‐selective PDE4 inhibitors such as Rolipram significantly reduced the viability of colon cancer cells in the presence of low doses of the adenylyl cyclase activator forskolin; confirming the potential of PDE4 as a therapeutic target. In addition, treatment with Rolipram attenuated cancer cell migration assessed in wound scratch assays. Western blotting and immunoprecipitation with PDE4 subtype‐selective antibodies revealed that PDE4 activity and protein in colon cancer cells is largely due to PDE4D, with little to none of the other PDE4 subtypes (PDE4A‐C) being expressed. If colon cancer cells exclusively express PDE4D, then the effect of non‐selective PDE4 inhibitors on cell viability/motility must necessarily have been mediated via inhibition of PDE4D, thus identifying PDE4D as the preferred target for development of a therapeutic. We will be testing this hypothesis by assessing the effect of selective PDE4D inactivation on various cancer cell functions. The ultimate goal is to determine whether selective inhibition of PDE4D retains the cancer therapeutic benefits, but exhibits an improved safety profile compared to the non‐selective PDE4 inhibitors available to date.