A team of researchers at Tokyo Metropolitan University have discovered that osmolytes such as mannitol, which are used to treat increased intraocular or intracranial pressure, can cause kidney damage by inducing hyperosmotic stress that leads to epithelial-mesenchymal transition (EMT) of the tubular epithelial cells. These findings were reported in PLoS One online.

High intraocular or intracranial pressure puts enormous pressure on organs and can cause tremendous damage. In the eye, ocular hypertension can lead to glaucoma and eventually permanent vision loss. Senior author Naoya Sakamoto said that osmolytes like mannitol are a lifesaving treatment for those conditions.

"Mannitol is a common chemical given intravenously to tackle excess pressures in both the eye and brain," Sakamoto told BioWorld Science. "It draws out excess fluid from high pressure compartments and reduces the pressure. However, it is known that this lifesaving treatment has some side effects, including potential acute kidney failure. But we did not know why."

Sakamoto is an associate professor in the Department of Mechanical Systems Engineering where he works on the biomechanics of vascular systems.

EMT involves the loss of cell polarity and cell-cell adhesion of epithelial cells, which then gain migratory and invasive properties to become mesenchymal stem cells that can differentiate into a variety of cell types. EMT is a major event in tumor progression with metastatic expansion, and the generation of chemoresistant tumor cells with stem cell properties.

In acute and chronic kidney injuries, EMT of tubular epithelial cells (TECs) is a also major mechanism leading to renal fibrosis.

Sakamoto's team saw that when they were treated with mannitol, rat TECs underwent a change in their skeletal structure, inducing a transformation that can lead to renal failure.

Hyperosmotic stress, induced by mannitol, led to a "reorganization of actin structure in the TECs, with elevated incorporation of alpha-SMA (smooth muscle actin) and reorganization of focal adhesions. This leads to EMT, with decreased expression of epithelial markers like E-cadherin," Sakamoto said.

The authors also noted a concomitant increase in the mRNA expression of EMT markers Snail and Twist. According to Sakamoto, "since mannitol cannot traverse the membrane of the TECs, the cells shrank due to osmotic pressure, and it was this shrinkage that impacted the cytoskeleton causing rearrangements of focal adhesions, accompanied by recruitment of alpha-SMA into the filaments." Sakamoto added that, although it was known that hyperosmotic stress induced depolymerization and rearrangement of actin filaments, this was the first study that explored the role of hyperosmolarity in focal adhesions. The authors then showed that cotreatment with a Rho-associated protein kinase (ROCK) inhibitor Y-27632 prevented rearrangement of focal adhesions, even under hyperosmotic conditions.

Sakamoto emphasized that his team's findings show that the rearrangement of focal adhesions, in response to hyperosmotic mannitol, plays a key role in EMT in proximal tubular cells. According to Sakamoto, "hyperosmotic stress generates mechanical stress and presents a potential risk factor for induction of EMT in proximal tubular cells."

Sakamoto believes that ROCK inhibitors like Y-27632 could prove to be an effective therapeutic strategy to prevent hyperosmotic stress-induced EMT. Translated into treatments, this may lead to enhancement in the therapeutic value of common osmolytes as a lifesaving treatment.