The team led by David H. Ellison, M.D. “ whose findings are described in a paper being published today (NOV. 1) in the Journal of Clinical Investigation “ likens the switch to a rheostat that modulates the balance of salt and potassium in the kidney, thereby raising or lowering blood pressure.

When the switch malfunctions, the group suggests, high blood pressure or hypertension occurs, as it does when certain mutations in the WNK kinase protein complex are present. Those genetic defects cause a disease called familial hyperkalemic hypertension (FHHt), also called pseudohypoaldosteronism type 2 or Gordon's syndrome. The OHSU group and others have focused on FHHt, which is rare, in a search for clues to how blood pressure is regulated in the more common form of high blood pressure, known as essential hypertension, often labeled the silent killer.

Hypertension affects at least 50 million Americans and untold millions around the world and is a major cause of heart attacks, strokes and kidney failure. The root cause is unknown in 95 percent of cases. If the study's conclusions are borne out in further research, they can lead to better targeted and more effective drugs for the disease, said Ellison, a professor of medicine in the OHSU School of Medicine and head of its Division of Nnephrology and Hypertension.

It is not widely understood by the general public that hypertension is most often a kidney disease, said Ellison. If we can figure out the ways the kidney adjusts salt excretion, we can devise methods to prevent hypertension, cure it or design better treatments for it. Our findings in this study get us a step closer, we think.

Ellison and his colleagues, Chao-Ling Yang, M.D., and Xiaoman Zhu, M.D., M.S., focused in the study on the complex interactions between the WNK1, WNK3 and WNK4 kinases in regulating NCC, a protein that normally keeps salt in the body. They explain for the first time that WNK 3 plays a key role in this process and that none of the WNK kinases act alone but function as a unit.

These WNKs form a protein signaling complex, said Ellison. All three WNKs talk to each other. Only when you understand how they work together and talk to each other can you understand the real biology of the disease. The complex acts as a rheostat-controlled amplifier that modulates the activity of NCC, the salt transporter gene, in response to physiological needs. The disease really is caused by a glitch in communications between the different WNKs regulating NCC.

Protein kinases constitute one of the largest human gene families and are key regulators of cell function. There are 518 of them “ referred to as the human kinome “ and they coordinate a wide variety of complex biological functions. The WNK kinases, which were discovered in 2000, have been a subject of intense interest among medical researchers since 2001 when a group at the Yale University School of Medicine found a link between this class of kinases and FHHt. Ellison and his group subsequently found that mutations in WNK1 and WNK4 cause this disease by modulating NCC activity.

The current OHSU study explains how aldosterone, a hormone produced in the adrenal gland, can have different effects on sodium and potassium balance at different times. The hormone sometimes increases salt absorption and at other times increases potassium excretion, but how it knows which role to play has been a mystery.

We think the answer is the WNK kinases, which switch aldosterone from a sodium chloride (salt) -retaining hormone to a potassium-wasting hormone, said Ellison. When you inherit a mutation in one of the WNK kinases the switch gets turned in the wrong direction. The switching mechanism explains for the first time why eating a high potassium diet lowers blood pressure. High potassium not only stimulates aldosterone secretion but also modulates WNK kinase activity; together aldosterone and certain WNK kinases cause the kidney to rid itself of potassium rather than reabsorbing salt.

The OHU study also breaks new ground in refining the explanation of how WNK mutations cause FHHt.

We showed that the way the mutations cause the disease is with the participation of WNK3, said Ellison. Unlike WNK4, which inhibits NCC, the salt cotransporter, WNK3 has a stimulative effect. If there's more WNK3, you'll have more salt reabsorption, and if there's more WNK4, you'll have less. What also happens is that WNK4 normally inhibits WNK3, but mutant WNK4 blocks this effect, thereby generating more active WNK3, increasing salt transport and causing the disease.

ohsu/

"These proteins enter the cell nucleus to turn on transcription of the EPO gene," Yeh says. They found that SENP1 controls EPO production by regulating one particular HIF protein, HIF1a. "When there isn't any SENP1, HIF1a is very unstable," he says. "It is not detectable in the embryo, compared to an embryo that has the SNEP1 gene."

It was already known that SUMO plays a role in the hypoxia process, Yeh adds. "We know that when you lower oxygen, HIF1a enters the cell's nucleus, and is quickly modified by SUMO."

But they discovered that there was one more step before HIF1a becomes active, producing EPO proteins to make more blood cells, and other proteins like VEGF that build more blood vessels to seek new sources of oxygen. They found that SENP1 needs to snip SUMO from SUMO-modified HIF1a before HIF1a can be active in transcription.

But that still didn't explain why HIF1a was missing in the nucleus of cells without SENP1. That led them to another, surprising finding - that if SENP1 does not clip off SUMO from SUMO-modified HIF1a when it is inside the nucleus, that SUMO then acts like ubiquitin, targeting destruction of HIF1a.

"This is the first example that SUMOylation of a protein can lead to its destruction," Yeh says. "That goes against the dogma we all believed in: that SUMO can change the location of a cell, but not degrade it. SUMO can do everything under the sun, including what ubiquitin can do. This vastly increases the functions of SUMOylation."

All this makes sense as far as cancer is concerned, Yeh says. HIF1a expression plays a role in many cancers and to date SENP1 has also found to be over-produced in prostate cancer. "This tells us that SENP1 is potentially involved in the overall regulation of tumorigenesis."

If true, Yeh says, that suggests it could become an Achilles heel for cancer. "These findings imply that you could inhibit SENP1 in tumors and let SUMO target HIF1a for destruction," Yeh says. "If tumors can't grow, these cancers could not continue to build a blood supply and grow and thrive."

mdanderson/