Tyrosine Kinase Signal Modulation: A Matter of H₂O₂ Membrane Permeability?

Abstract

H 2 O 2 produced by extracellular NADPH oxidases regulates tyrosine kinase signaling inhibiting phosphatases. How does it cross the membrane to reach its cytosolic targets? Silencing aquaporin-8 (AQP8), but not AQP3 or AQP4, inhibited H 2 O 2 entry into HeLa cells. Re-expression of AQP8 with silencing-resistant vectors rescued H 2 O 2 transport, whereas a C173A-AQP8 mutant failed to do so. Lowering AQP8 levels affected H 2 O 2 entry into the endoplasmic reticulum, but not into mitochondria. AQP8 silencing also inhibited the H 2 O 2 spikes and phosphorylation of downstream proteins induced by epidermal growth factor. These observations lead to the hypothesis that H 2 O 2 does not freely diffuse across the plasma membrane and AQP8 and other H 2 O 2 transporters are potential targets for manipulating key signaling pathways in cancer and degenerative diseases.

Stress Regulates Aquaporin-8 Permeability to Impact Cell Growth and Survival

“…These data confirmed that H 2 O 2 acts as a rheostat of tyrosine kinase signaling and demonstrated that AQP8 guarantees efficient cytosolic import of H 2 O 2 likely produced by NOXes. AQP8 silencing inhibited, in part, the transport of H 2 O 2 through the ER membrane, but had little if any effect on mitochondria (6), where a fraction of AQP8 has been reported to localize (9). In other cell types, AQP3 (18,39) and possibly different channels can allow efficient H 2 O 2 transport.…”

“…Recently, members of the aquaporin protein family (AQP), initially described as bidirectional water transporters (1), have been found to channel also H 2 O 2 (7). We have recently shown that aquaporin-8 (AQP8) is essential to allow effective transport of H 2 O 2 across the plasma membrane, thus potentiating tyrosine phosphorylation induced by growth factors (6), a notion later corroborated by others (57). These data confirmed that H 2 O 2 acts as a rheostat of tyrosine kinase signaling and demonstrated that AQP8 guarantees efficient cytosolic import of H 2 O 2 likely produced by NOXes.…”

Transit of H2O2 across the endoplasmic reticulum membrane is not sluggish

“…This topological problem is solved by membrane-embedded aquaporin channels (AQPs). By serving as H 2 O 2 pores they facilitate the formation of local areas with an elevated H 2 O 2 concentration on both sides of the plasma membrane [13][14][15][16].…”

“…AQP8 fulfills a major function in the transport of H 2 O 2 at the plasma membrane [13]. In addition, knockdown of AQP8 strongly diminishes the entry of exogenous H 2 O 2 into the ER of plasma membrane-permeabilized cells [13].…”

Composition of the redox environment of the endoplasmic reticulum and sources of hydrogen peroxide

“…Hydrogen peroxide molecule is similar to water, thus it is not surprising that the water channel aquaporins can also facilitate the traffic of hydrogen peroxide across cellular membranes [129]. Specific aquaporins -peroxiporinsare especially engaged in hydrogen peroxide transport [129,130], and some representatives of these hydrogen peroxide-channels are present in the ER membrane. However, the actual direction of hydrogen peroxide flux has not been elucidated.…”