Cell-cycle triggers might be cancer drug targets

Cyclin D proteins not required for development of tissues, as previously believed

In an experiment that appears to refute current theory, Dana-Farber Cancer Institute scientists have found that removing three key proteins believed essential to cell division and growth had little impact on normal tissue development of a mouse embryo. These same proteins, when overly active, have been linked to cancer cell proliferation.

With one significant exception, the absence of proteins called cyclin D1, D2, and D3 seemed to have no deleterious effect on development of the tissues and organs of laboratory mouse embryos. “D-type cyclins” are molecules that sense growth signals from the cell’s environment and, when appropriate, switch on cell division and growth. But when the system is faulty, the cyclins over-respond to the growth signals and can cause cancerous growth. The discovery that these proteins aren’t indispensable lends encouragement to an idea that blocking overactive cyclins could halt the growth of cancer.

In the Aug. 20 issue of Cell, lead author Katarzyna Kozar, MD, and senior author Peter Sicinski, MD, PhD, report on developing the first mouse embryos to date in which all three D-type cyclins were absent or “knocked out.” It had been thought that at least one cyclin was required for an embryo to be viable and its tissues to form normally. Yet the “triple-knockout” mouse embryos followed a normal course of cell division and proliferation until as late as 13.5 days, when most tissues and organs are already formed. A typical mouse pregnancy last 18 days.

(In a companion paper in Cell, researchers from Spain report similar findings involving protein kinases called CDK4 and CDK6, which are molecular partners of the D-cyclins. Embryonic mice with both CDK molecules knocked out had normal tissue development as well.)

The unexpectedly viable embryos contradict theory and previous laboratory experiments. The only abnormality in the triple knockout mice was a deficiency of blood-forming cells, causing them to be pale and anemic, and was ultimately fatal. But the Dana-Farber researchers can’t yet say if this would create problems for an anti-cyclin cancer therapy in patients. “It’s not known whether D cyclins are required for blood cell generation in adults,” said Sicinski. “We are addressing this in ongoing experiments.”

The blood system issue aside, the results remove a major theoretical objection to developing drugs that would inhibit, or block, overactivity of cyclin D proteins, according to the study’s authors. Excess cyclin D production has been seen in tumors of the breast, head and neck and stomach, and in some blood cancers.

The main function of the D cyclins is a linking one. They respond to signals outside the cell and turn the cell-cycle machinery on when needed, enabling the cell to adjust to changes in environment. How this process is carried out in the mouse embryos with no D-type cyclins is a puzzle, says Sicinski who is also an associate professor at Harvard Medical School. “There must be alternative mechanisms that allow the cell to respond to the environment when the D-type cyclins are missing,” he speculates.

But the blood-forming cells may lack such an alternative pathway, said Sicinski, and it is highly likely that cancer cells don’t, making them vulnerable to future drugs aimed at D-cyclins, or their partners, the CDK kinases.

In the Cell report, the researchers say they attempted to induce cancer in cells taken from embryos lacking the cyclins, and they remained stubbornly normal. By contrast, cells that contained the cyclins were easily made cancerous by the insertion of cancer-causing oncogenes.

That was what the scientists had hoped they would see, based on experiments reported by Sicinski in 1995 showing that mice lacking cyclin D1 had very little breast tissue and, when crossed with cancer-prone mice, had offspring that were largely protected against breast cancer.

It was those and other findings that raised the prospect of cancer therapy using drugs to block D1 and other cyclins. “Now we think it should be safe to target the cyclins with drugs that would be designed specifically to inhibit them,” Sicinski says.