Discovery raises possibility of safer, more selective diabetes drugs
Bruce Spiegelman, PhD
A new understanding of insulin resistance and the action of diabetes
drugs such as Avandia and Actos could pave the way for improved
medications that are more selective and safer, say scientists from
Dana-Farber Cancer Institute and The Scripps Research Institute.
"Our findings strongly suggest that good and bad effects of these
drugs can be separated by designing second-generation drugs that focus
on the newly uncovered mechanism," said Bruce Spiegelman, PhD, of Dana-Farber, senior author on a report appearing in the July 22 issue of Nature.
Avandia and Actos, known generically as rosiglitazone and
pioglitazone, are widely used to counteract the obesity-related
abnormalities in insulin response that lead to diabetes.
The drugs act on a master regulatory protein called PPAR-gamma,
primarily in fat cells, which governs genes involved in the body's
response to insulin.
Obesity resulting from a high-fat diet alters the function of
PPAR-gamma and disrupts the expression of those insulin response genes,
including adipsin and adiponectin. Avandia and Actos work by binding to
PPAR-gamma and reversing the gene expression changes.
The drugs were believed to work by stimulating or "agonizing" the
PPAR-gamma receptor, causing it to rev up some genes and dampen the
activity of others.
In the Nature report, however, the researchers say they have
identified "an entirely new and surprising mechanism by which
PPAR-gamma can control whole-body insulin sensitivity."
It is mainly through this mechanism, they found, that the diabetes
drugs counteract insulin resistance — not their agonist effect on
PPAR-gamma. Moreover, they say, agonism of PPAR-gamma may be largely
responsible for the harmful drug side effects.
The newly identified pathway linking obesity and insulin response
involves cdk5, a protein kinase, or molecular "switch." When cdk5 is
activated by the development of obesity in mice, it causes a chemical
change in PPAR-gamma called phosphorylation.
In contrast to agonism of PPAR-gamma, phosphorylation has a narrow
effect, disrupting a smaller set of genes that lead to insulin
In addition to agonizing PPAR-gamma, Avandia and Actos also block the
phosphorylation of PPAR-gamma by cdk5. It's this latter effect that
accounts for most of the drugs' anti-diabetic benefits, the authors
"Agonism may not be therapeutically necessary and likely results in a lot of the toxicities," Spiegelman said.
The strength of various drugs' agonist effects on PPAR-gamma doesn't
correlate with how well they work, the researchers observe; instead, it
is their ability to block cdk5 phosphorylation that counts.
In support of this assertion, the paper describes the researchers'
findings from patients treated with Avandia in a German clinical trial.
It showed that improvements in insulin sensitivity were tightly
correlated with decreased phosphorylation of PPAR-gamma.
"I think this is a really important finding, and potentially very
timely in light of the current discussions about Avandia," commented
Jeffrey Flier, MD, Dean of the Harvard Medical School, a leading
researcher in obesity, insulin resistance, and diabetes.
"It may motivate pharmaceutical companies to take another look at
compounds acting through PPAR-gamma that were taken to various stages of
development but put on hold because they did not demonstrate strong
agonism of PPAR-gamma," Flier said.
"People may have been focusing on the wrong outcomes."
Avandia and Actos belong to a relatively new class of compounds
called thiazolidinediones, the first medications that can reverse
insulin resistance. They have been widely used to treat Type 2 diabetes
since being approved in 1999.
However, in recent years they have been linked in some patients to
heart attacks, heart failure, and strokes. Thousands of lawsuits have
been filed against the maker of Avandia, and the US Food and Drug
Administration is currently weighing whether it should be taken off the
The research was supported by grants from the National Institutes of Health.
The paper's first author is Jang-Hyun Choi, PhD, of Dana-Farber. The
co-authors are Alexander S. Banks, PhD, Jennifer L. Estall, PhD, Shingo
Kajimura, PhD, Pontus Bostrom, MD, PhD, Dina Laznik, Jorge L. Ruas,
PharmD, PhD, Dana-Farber; Patrick R. Griffin, PhD, Michael J. Chalmers,
PhD, and Theodore M. Kamenecka, PhD, Scripps Research Institute,
Jupiter, Fla.; and Matthias Bluher, MD, University of Leipzig, Germany.