Researchers have identified a non-traditional pathway for stimulating a cancer-promoting protein into the cell nucleus that could be a potential combination therapy for esophageal cancer. The finding suggests a resistance mechanism for new drugs that attack the Hedgehog pathway.

In the March 20 issue of the journal Cancer Cell, researchers at The University of Texas MD Anderson Cancer Center reveal that the mTOR molecular pathway stimulates the activity of the Gli1 protein in the development and progression of esophageal cancer.

Senior author of the study, Mien-Chie Hung, Ph.D., vice president for basic research, professor and chair of MD Anderson’s Department of Molecular and Cellular Oncology, explained:

“The Hedgehog pathway is the established, or canonical, pathway for activating Gli1. We’ve shown a clear-cut mechanism to link all non-canonical activation of Gli1 through a single pathway, mTOR.

Crosstalk between these two pathways is a challenge, but our experiments showed a combination of the mTOR inhibitor RAD-001 (Afinitor®) and the Hedgehog inhibitor GDC-0449 (Erivedge®) steeply reduced the tumor burden in a mouse model of esophageal adenocarcinoma.”

The U.S. Food and Drug Administration (FDA) has approved both drugs for use in other types of cancer.

After examining 107 tissue samples of human esophageal cancer, the researchers found that 87 (81.3%) had a marker of Gli1 activated by Hedgehog and 80 (74.8%) had a marker of mTOR promotion of Gli1.

According to the researchers less than 20% of individuals suffering from esophageal cancer (one of the most aggressive forms of cancer) survive for 5 years. Furthermore, they highlight that since the 1980s, the disease has become more prevalent in the U.S by 5% to 10% each year. Obesity and inflammation are believed to contribute to this increased incidence.

In order to show how mTOR and Hedgehog, both involved in esophageal and other types of cancers, converge on Gli1, the team conducted experiments with cell lines, human tumor samples and mouse models.

Gli1 is a transcription factor – a protein that enters the cell nucleus where it attaches and stimulates other genes. Usually a protein called SuFu attaches to Gli1 at a specific region preventing it from entering the nucleus.

According to Hung, the Hedgehog pathway stimulates a signaling protein called Smoothened (SMO), which prevents SuFu from attaching to Gli1. SMO allows Gli1 to enter the nucleus and activate a variety of genes, including Hedgehog activators.

In January the FDA approved GDC-0449 for treatment of metastatic basal cell carcinoma, inhibits SMO. Mutations in the Hedgehog pathway power basal cell carcinoma, however in clinical trials conducted to treat cancers, such as pancreas and ovarian, resistance to SMO inhibitors has emerged.

Hung explained:

“We now believe the mTOR pathway is one
source of this resistance.”

The researchers first conducted a series of experiments with Tumor Necrosis Factor Alpha (TNFa), an inflammatory protein associated to development of esophageal cancer, and discovered that TNFa activates Gli1 via the mTOR pathway by:

  • Stimulating the kinase S6K1, which binds to phosphate group to Gli1. This attachment prevents phosporylated Gli1 from binding to SuFu.
  • With SuFu inhibited, the phosphorylated version of Gli1 enters the nucleus and stimulates genes.

In order to identify the presence of phosphorylated Gli1, the researchers developed an antibody which could provide a biomarker of cancer resistance to Hedgehog inhibitors.

They treated mice with esophageal cancer with RAD-001, GDC-0449 or both and found that the Hedgehog inhibitor GDC-0449 reduced tumor volume by 40%, while the mTOR inhibitor RAD-001 alone had virtually no effect. However, when combined RAD-001 and GDC-0449 reduced tumor volume by 90%.

According to Hung, human trials of the RAD-001 and GDC-0449 combination for esophageal and other cancers could be guided by the antibody for phosphorylated Gli as well as the presence of plain Gli1, which would suggest that both drugs are needed.

Previous studies conducted by other labs suggest that the AKT and MAPK/ERK also stimulate the Hedgehog pathway. Hung and colleagues demonstrate that AKT and ERK, which both stimulate the mTOR pathway, seem to activate Gli1 through phosphorylation of S6K1 and Gli1.

Written by Grace Rattue