October 18, 2010
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 Johns 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,” said Rajini Rao, a professor of physiology in the Johns Hopkins School of Medicine. “Eventually, what turned up was this very unusual calcium signaling mechanism.”
Experiments showed that SPCA2 actually move from their normal location inside cells to the cell surface, where they interact 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 said. “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 breastfeeding.
“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,” Rao said. “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 said, 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.
When the researchers studied human breast cancer cell lines in which they knocked down expression of the SPCA2 pump, they saw calcium levels fall, along with a loss of tumorlike properties such as rapid growth and loss of contact inhibition by other cells. On the other hand, if they inserted the gene coding for the SPCA2 pump into noncancerous human breast cells, these cells behaved like tumor cells.
In another set of experiments, they injected the breast cancer cells into mouse flanks, where they formed tumors. However, when they injected the mice with breast cancer cells in which the SPCA2 gene was knocked out, those cells failed to form tumors, suggesting that the SPCA2 protein is a trigger in breast cancer.
This newly discovered mechanism may provide an underlying cause for the microcalcifications (deposits of calcium) in breast tissue that, detectable by mammograms, may signal cancer, Rao says.
“No one knew why these appear or where they are coming from,” Rao added. “But our work shows that these calcium-handling proteins are very misregulated in breast cancer.”
In the future, Rao says, drugs might be used to disrupt the pump-channel interaction and block proliferation of breast cancer cells. “Some of the best-selling drugs in the world target channel and transporter proteins that reside in cell membranes because they are so available and accessible,” she said.
The study was supported by the National Institutes of Health and by the National Health and Medical Research and Cancer Council Queensland.
Johns Hopkins authors of the study, in addition to Rao and Feng, are Nguyen Nguyen, Sharon Leitch, Yingyu Wang, Sabina Muend and Saraswati Sukumar. Other authors are Paraic A. Kenny, of the Albert Einstein College of Medicine; and Desma M. Grice, Helen M. Faddy, Sarah J. Roberts-Thomson and Gregory R. Monteith, all of the University of Queensland, Brisbane, Australia.