For five decades, scientists have known about a notorious cancer-causing enzyme called Src. But it has always been assumed that the cancer-associated tyrosine kinase only appeared on the inside of cells, where it sent signals that fueled tumor growth, and stayed hidden from the immune system.
Now, researchers at the University of California, San Francisco (UCSF), have discovered that Src also appears like a flag on the surface of bladder, colorectal, breast, pancreatic, and probably many other tumor cells. As cancer cells furiously divide, they produce a lot of garbage. In healthy cells, the trash gets broken down. But in tumors, the recycling system gets overwhelmed, and the cells expel some of their trash. This pushes the SRC onto the surface of the cell, where it is visible to potential therapies, like antibodies.
The investigators showed that targeting this extracellular membrane-associated Src (eSRc) on the surface of cancer cells using antibodies that carried radioactive payloads or summoned immune cells killed the cancer cells, shrinking tumors in mice. The new target, the team suggests, could apply to up to half of all tumors.
“No one thought to look for it on the outside, said Jim Wells, PhD, professor of pharmaceutical chemistry at UCSF. “Our discovery enables us to test proven immunotherapies on this new tumor target.” Wells is senior author of the team’s published paper in Science, titled “Autophagolysosomal exocytosis inverts Src kinase onto the cell surface in cancer.” In their report, the investigators concluded that their studies “… support eSrc as a highly attractive cancer-associated antigen for antibody-based therapies.”
In the 1970s, UCSF’s J. Michael Bishop, MD, and Harold Varmus, MD, identified SRC—the gene containing the instructions for building the Src enzyme—as the very first oncogene. It launched the modern field of cancer genetics and won the researchers a Nobel Prize in 1989. “Overexpression of the proto-oncogene Src is common to a wide variety of cancers,” the authors wrote in their newly published study. “Src kinase is essential for cell viability and was one of the first identified regulators of cell proliferative activity … Understanding the biological consequences and mechanisms of Src activity is important for developing therapeutics that may benefit a wide variety of cancer patients.”
Ever since, scientists have tried to block the Src enzyme with drugs that slip inside cells. But the therapies have not worked well because they disable Src in both cancerous and healthy cells, which need the enzyme to function.
To understand how Src reached the cell surface, the scientists tracked the protein in cancer cells grown in petri dishes. They found that Src was getting caught up in the cell’s overactive disposal system. Cells normally trap waste in small sacs that they break down and reuse. But in fast-growing cancer cells, the system can become overloaded. Instead of being digested, the sacs containing waste fuse with the cell membrane and dump their contents outside the cell. “… we found that Src, the master regulatory kinase typically anchored by its N-myristoyl lipid to the inner leaflet of the plasma membrane, was noncanonically translocated to the cell surface through autophagolysosomal exocytosis (ALE),” the authors explained.
Corleone Delaveris, PhD, first author of the paper, added, “We saw that SRC was getting swept out onto the outer membrane, where it sat exposed like a red flag.” Delaveris carried out the work as a post-doctoral researcher in Wells’ lab and is now at Inversion Therapeutics.
The researchers found that SRC was present on the surface of bladder tumor cells taken from patients at UCSF, but it was not present on healthy bladder tissue or on immune cells. This suggests it is specific enough to steer cancer-killing antibodies to the right target. “In this work, we describe a distinctive consequence of Src overexpression coupled with ALE that leads to the display of eSrc on the outer cell membrane of cancer cell lines, cell line xenografts, syngeneic tumor models, colorectal cancer and bladder cancer patient–derived xenografts, and primary tumor samples from bladder cancer patients,” the team noted.
In collaboration with UCSF professor of radiology Michael Evans, PhD, the team then aimed experimental radioactive antibodies at SRC in mice that had been implanted with human tumor cells. They found that these antibodies accumulated in the cells. They also engineered antibodies to help immune cells recognize and kill human cancer cells in mice. The authors reported. “…we demonstrated the translational potential of eSrc as a diagnostic and therapeutic cancer antigen using an anti-Src antibody formulated as a bispecific T cell engager (TCE), an antibody-drug conjugate (ADC), or a radioligand therapeutic (RLT).”
Wells added, “We went all the way from the discovery to developing two preclinical therapies that target SRC—and they worked. It’s truly exciting.” UCSF has licensed the antibodies and related molecules to Inversion Therapeutics to explore their therapeutic potential.
