Unexpected connection

The discovery of insulin in the 1920s; marked the breakthrough in the almost 3,500-year-long mystery of diabetes; a disease first described in ancient Egyptian papyruses. Until its discovery, physicians struggled to explain; how symptoms such as sugary urine, constant thirst and frequent urination; could lead to ailments ranging from blindness; and nerve damage to coma and death. Over the past century, scientists have detailed the hormone’s central role as a regulator of blood sugar; mapped its cell-signaling pathways and established its involvement in diabetes; and a staggering array of other chronic conditions; including neurodegeneration, cardiovascular disease and cancer.
Still, many aspects of insulin signaling remain unclear; particularly its long-term effects on cells; and there are currently no effective cures for the hundreds of millions of people; around the world living with diabetes. Now, researchers from Harvard Medical School have made key new insights; into the molecular behavior of insulin. Reporting online in Cell, they describe an unexpected mechanism by which insulin triggers changes to the expression of thousands of genes throughout the genome.

A new field of study on the insulin receptor

Their analyses show that the insulin receptor—a protein complex at the cell surface; physically transports to the cell nucleus after it detects and binds insulin. Once there, it helps initiate the expression of genes involved; in insulin-related functions and diseases. This process was impaired in mice with insulin resistance. The results outline a set of potential therapeutic targets for insulin-related diseases; and establish a wide range of future avenues of research on insulin signaling; including potential clues toward the underlying biological mechanisms; that differentiate type 1 and type 2 diabetes.

“Our findings open the door for a new field of study on the insulin receptor; a remarkable protein complex expressed in almost all cells; and implicated in major chronic diseases that affect hundreds of millions of people,” said senior study author John Flanagan, professor of cell biology at HMS. “Understanding the fundamental mechanisms of how cells work can help us design new drugs or improve existing ones, and the insulin receptor certainly has potential for tremendous returns on investment,” Flanagan added.

Produced by specialized cells in the pancreas, the hormone insulin serves as the main signal to cells to absorb glucose from the bloodstream and begin the production and metabolism of carbohydrates, fats and proteins. This process is essential for normal cell function, growth and nutrient storage. Dysfunctions in insulin signaling give rise to a number of serious chronic diseases. In type 1 diabetes, pancreatic cells fail to produce enough insulin, and in type 2 diabetes—the far more common form of the condition—cells become resistant to insulin.

Strange bedfellows

So, without proper insulin signaling, glucose accumulates in the blood where it damages tissues and organs. However, insulin resistance has also been; implicated in neurodegenerative diseases such as Alzheimer’s and Parkinson’s, and excessive insulin signaling contributes to a variety of cancers. So, Flanagan and colleagues were broadly interested in studying how cell surface receptors communicate with the interior of a cell and performed screens to identify proteins associated with the insulin receptor. Meanwhile, their experiments suggested that one of the most prominent such proteins is RNA polymerase, an enzyme responsible for transcribing DNA into RNA—the first step in gene expression.

However, this was unexpected, said Flanagan, because RNA polymerase functions inside the nucleus of a cell—far away from surface of the cell where the insulin receptor is located. Additional analyses revealed an unexpected explanation. So, the team found that after the insulin receptor binds insulin, it is physically transported from the cell surface to the nucleus via a yet unidentified mechanism. Once there, it binds to RNA polymerase on chromatin—the protein-DNA complex that cells use to store their genomes.