Image courtesy of AFIP
New research suggests the vitamin A derivative all-trans retinoic acid (ATRA) inhibits multiple oncogenic pathways and, at the same time, eliminates cancer stem cells by degrading the Pin1 enzyme.
Investigators said this discovery explains how ATRA successfully treats acute promyelocytic leukemia (APL), and it likely has implications for the treatment of other aggressive or drug-resistant cancers.
The team detailed their discovery in Nature Medicine.
“Pin1 changes protein shape through proline-directed phosphorylation, which is a major control mechanism for disease,” said study author Kun Ping Lu, MD, PhD, of Beth Israel Deaconess Medical Center at Harvard Medical School in Boston, Massachusetts.
“Pin1 is a common, key regulator in many types of cancer and, as a result, can control over 50 oncogenes and tumor suppressors, many of which are known to also control cancer stem cells.”
Until now, agents that inhibit Pin1 have been developed mainly through rational drug design. These inhibitors have proven active against Pin1 in the test tube, but, when tested in a cell model or in vivo, they are unable to efficiently enter cells to successfully inhibit Pin1 function.
In this new work, the investigators decided to take a different approach to identify Pin1 inhibitors. They developed a mechanism-based, high-throughput screen to identify compounds that were targeting active Pin1.
“We had previously identified Pin1 substrate-mimicking peptide inhibitors,” said Xiao Zhen Zhou, MD, also of Beth Israel Deaconess Medical Center.
“We therefore used these as a probe in a competition binding assay and screened approximately 8200 chemical compounds, including both approved drugs and other known bioactive compounds.”
To increase screening success, the investigators chose a probe that specifically binds to the Pin1 enzyme active site very tightly, an approach that is not commonly used for this kind of screen.
“Initially, it appeared that the screening results had no positive hits, so we had to manually sift through them looking for the one that would bind to Pin1,” Dr Zhou said. “We eventually spotted cis retinoic acid, which has the same chemical formula as all-trans retinoic acid but with a different chemical structure.”
It turned out that Pin1 prefers binding to ATRA, and cis retinoic acid needs to convert to ATRA in order to bind Pin1.
ATRA in APL and other cancers
ATRA was first discovered for the treatment of APL in 1987. It was originally thought that ATRA was treating APL by inducing cell differentiation, causing cancer cells to change into normal cells by activating the cellular retinoic acid receptors.
But these new findings suggest that is not the mechanism that is actually behind ATRA’s successful outcomes in treating APL.
“While it has been previously shown that ATRA’s ability to degrade the leukemia-causing fusion oncogene PML-RAR causes ATRA to stop the leukemia stem cells that drive APL, the underlying mechanism has remained elusive,” Dr Lu said.
“Our new, high-throughput drug screening has revealed the ATRA drug target, unexpectedly showing that ATRA directly binds, inhibits, and ultimately degrades active Pin1 selectively in cancer cells. The Pin1-ATRA complex structure suggests that ATRA is trapped in the Pin1 active site by mimicking an unreleasable enzyme substrate. Importantly, ATRA-induced Pin1 ablation degrades the fusion oncogene PML-RAR and treats APL in cell and animal models as well as in human patients.”
The investigators discovered that ATRA-induced Pin1 ablation inhibits triple-negative breast cancer growth as well. The drug proved active in human cells and in animal models, simultaneously turning off many oncogenes and turning on many tumor suppressors.
The team said these results provide a rationale for trying to extend ATRA’s half-life and for developing more potent, Pin1-targeted ATRA variants for cancer treatment.
“The current ATRA drug has a very short half-life of only 45 minutes in humans,” Dr Lu said. “We think that a more potent Pin1 inhibitor will be able to target many ‘dream targets’ that are not currently druggable.”
“ATRA appears to be well tolerated, with minimal side effects, and offers a promising new approach for targeting a Pin1-dependent, common oncogenic mechanism in numerous cancer-driving pathways in cancer and cancer stem cells. This is especially critical for treating aggressive or drug-resistant cancers.”
