Discovery of this basic transport mechanism opens a new door for future research on related compounds. The finding has important implications for the fight against cancer and other diseases.
The research, which appears in a recent issue of the journal Molecular Cell (Vol. 18, No. 3), explains for the first time how the cancer-fighting vitamin A derivative retinoic acid (RA) gains entry into a cell's nucleus.
When vitamin A enters a cell's cytoplasm (the portion that lies between the outer membrane and the nucleus), it can be converted to RA, a member of a group of compounds that enter a cell's nucleus and play a role in triggering transcription. This is a basic process for relaying genetic information and switching genes on and off. In this role, RA can inhibit tumor growth. In fact, past clinical trials have shown that RA can help treat leukemia, head, neck and breast cancer. RA and its synthetic derivatives may also be useful in treatment of diabetes, arteriosclerosis and emphysema. Unfortunately, conventional treatments using RA require high, toxic doses, and tumors can develop resistance to the treatment.
Noa Noy, a professor of nutritional sciences at Cornell, and Richard Sessler, the paper's lead author and a graduate student in Noy's lab, wanted to learn how RA is transported into the cell's nucleus. The chemical structure of RA makes it hydrophobic, meaning it is barely soluble in water. But the path from the cell cytoplasm, where RA is made, to the nucleus requires passage through water, a difficult journey for a hydrophobic compound. For RA to rapidly enter a cell's nucleus, it must catch a ride on a water-soluble protein called cellular retinoic acid-binding protein type II (CRABP-II). This protein was discovered o ver two decades ago, but until recently scientists had no idea what it did.
Much like a doorway that requires a pass code to enter, CRABP-II can only move into a cell's nucleus if its amino acids are organized in a certain sequence, called a nuclear localization signal (NLS). However, CRABP-II does not have a recognizable NLS. Researchers have long wondered how proteins without an NLS enter a cell's nucleus.
By comparing the 3-D structures of the CRABP-II protein before and after it comes in contact with RA, Noy and Sessler made a startling discovery: When exposed to RA, three amino acids on the CRABP-II molecule flip their positions, exposing positive charges. Combined with the way the molecule is folded, this area suddenly looks like a classical NLS.
"This discovery creates a precedent for many other proteins that don't have an NLS, and it solves a mystery that has been in the literature for a long time," Noy says. "It explains a basic mechanism of how this protein, CRABP-II, gets into the nucleus, where it can act to suppress tumors."
CRABP-II is a member of a group of proteins called intracellular lipid binding proteins (ILBP), which don't have a recognizable NLS. But now researchers have something new to look for -- folds in a molecule's structure and amino acids that flip when exposed to a hormone or a drug. Noy currently is working with an ILBP that she has pinpointed as a pro-carcinogen -- it promotes cancer. Armed with new tools and knowledge, she hopes to figure out how to suppress the ability of this protein to move to the nucleus and promote cancer, perhaps by blocking the hormone that switches on its NLS.
In previous experiments with mice, Noy and her colleagues showed that by increasing CRABP-II levels within cells, tumor growth rates slow dramatically. The protein transfers RA rapidly and efficiently to the cell's nucleus. In this way, tumor growth may be inhibited using naturally occurring levels of RA, as oppo sed to the toxic doses currently administered.
"If you can provide a bus to get retinoic acid into the nucleus more efficiently, then you enhance its ability to act as an anti-carcinogen," says Noy. "It's a rapid transport advantage."
Her findings emphasize the importance of efforts in structural genomics, the study of the folding motifs in proteins, which allows researchers to compare 3-D structures of seemingly unrelated proteins. "Advances in structural genomics will allow you to predict what the protein does," Noy says.