Scientists Discover New Therapeutic Target for Some Breast Cancers

A protein that pumps calcium out of cells also moonlights as a signal to get massive quantities of the stuff to flow in, according to Johns Hopkins scientists. Their discovery of this surprisingly opposite function, reported Oct. 1 in Cell, highlights the link between calcium and cancer and holds the promise of a new therapeutic target for certain breast cancers.

The Hopkins study, a good example, the scientists say, of the value of following unexpected "detours" in biomedical research, focused on the enigmatic molecular machines known as SPCA2 that are found in very high levels in human breast cancer cells. Historically, SPCA2 were assumed to be redundant and less essential versions of better-known calcium pumps that scavenge calcium inside of cells everywhere in the body and store it away.

Mingye Feng, a graduate student of physiology in the Johns Hopkins University School of Medicine, learned that wasn't necessarily the case as he conducted a standard control experiment and put the gene that codes for the SPCA2 pump into an ordinary human cell. He expected that if the pump were functional, calcium levels in the cell would decrease and if it were not, the levels wouldn't change much or at all. Instead, the calcium levels rose dramatically.

"Rather than push this surprising turn of events under the rug, Feng kept probing," says Rajini Rao, Ph.D., a professor of physiology in the Johns Hopkins University School of Medicine. "Eventually, what turned up was this very unusual calcium signaling mechanism."

Experiments showed that SPCA2 actually moves from its normal location inside cells to the cell surface, where it interacts with porelike openings called calcium channels. SPCA2 activated the channels, essentially calling all calcium into the cells.

"This signaling role was overriding its pumping and scavenging function," Rao explains. "By overwhelming the pump's ability to put away the calcium, the net effect was an elevation of calcium."

Rao speculates that nature evolved this unusual mechanism because under certain conditions, tissues need to secrete a lot of calcium. One
example: lactating mammary glands during breast feeding.

"Human milk is extremely high in calcium, and all that calcium gets there because SPCA2, along with an elaborate network of other proteins, is turned on during lactation," says Rao. "SPCA2's normal purpose, we think, is to signal calcium channels to open so lots and lots of calcium comes into the cells of mammary tissue, where it is packaged and pumped out to the milk."

Efficient as the process is, however, it's also susceptible to misregulation, Rao says. Further studies by her team found that in cells taken from human breast tumors, the SPCA2 gene - normally turned off except during lactation - is on.

"When regulation of SPCA2 goes wrong, that's when you have breast cancer," Rao says, probably because in breast tumor cells, the lack of regulation of the pump/signaling mechanism lets vast amounts of calcium into the cells, which stimulates the cell cycle, and triggers high levels of proliferation.