Specifically, the study found that PROX1 is turned on in "double-negative" prostate cancer—tumors that have low androgen receptor activity—as well as in neuroendocrine prostate cancer, an extremely aggressive subtype where the expression of the androgen receptor is lost entirely. In additional experiments, the team demonstrated a clear inverse relationship: when PROX1 levels are high, androgen receptor levels are low. In fact, when the researchers artificially introduced PROX1 into prostate cancer cells in the lab, it effectively turned off the androgen receptor, mimicking the transition seen in the most difficult-to-treat patients.
"We think PROX1 is regulating the androgen receptor," Alumkal explained. "It may be one explanation for why the androgen receptor gets turned off when tumors undergo lineage plasticity and transition away from the typical glandular prostate cancer identity."
This realization provided a clear target for intervention, but targeting PROX1 directly presented a major hurdle. As a transcription factor, PROX1 lacks the type of "binding pockets" that traditional small-molecule drugs use to latch onto a protein. Transcription factors are notoriously difficult to target with drugs, often described by scientists as "undruggable." To overcome this, the Michigan team decided to look at the molecular "company" that PROX1 keeps—the other proteins it interacts with to carry out its function.
By mapping the proteins that bind to PROX1, the researchers found that it frequently partners with histone deacetylases, commonly known as HDACs. This discovery provided a breakthrough moment for the team. "We felt like this was guilt by association," Alumkal said. "We hypothesized that HDACs might cooperate with PROX1 and that targeting HDACs might be like targeting PROX1."
This was a particularly promising lead because HDACs are already well-known in the world of cancer research. Several HDAC inhibitors have already been approved by the U.S. Food and Drug Administration (FDA) for the treatment of other types of cancer, such as certain lymphomas and multiple myeloma. While they had not yet found a primary role in the treatment of prostate cancer, the connection to PROX1 suggested they might be the "missing link" needed to treat aggressive subtypes.
The team tested this hypothesis by treating PROX1-expressing prostate cancer cells with HDAC inhibitors. The results were striking. The drugs not only inhibited the activity of the cells but actually led to the depletion of the PROX1 protein itself. As the levels of PROX1 dropped, the cancer cells began to die. The effect of the drugs was nearly identical to the results the team achieved when they used sophisticated genetic methods to "knock out" or eliminate the PROX1 gene entirely.
