AQP9 expression and phosphorylation affect arsenic uptake and tolerance in human liver hepatocellular cells

Background Little is known about the signaling mechanisms involved in arsenic tolerance or detoxification, although the water channel protein aquaporin-9 (AQP9) and components of the mitogen-activated protein kinase (MAPK) pathway have putative roles. Elucidation of the mechanisms of resistance in mammalian cells would be helpful in developing effective, arsenic-based therapeutic strategies. Methods The association between AQP9 and arsenic accumulation and tolerance was investigated in arsenic-sensitive human liver hepatocellular cells (HepG2) and arsenic-resistant HepG2 (AsHepG2) cells. Results The IC50 value for arsenic of AsHepG2 cells (15.59 µM) was significantly higher than that of HepG2 cells (7.33 µM; Ρ < 0.05), and AsHepG2 cells accumulated significantly low levels of arsenic after treatment with sodium arsenite (NaAsO2; Ρ < 0.01). Arsenic accumulation in AsHepG2 cells reached a plateau 6 h after treatment, while in HepG2 cells it continued to increase throughout the experimental period. Additionally, intracellular arsenic accumulation in AQP9-overexpressing AsHepG2 cells significantly increased within 10 h of treatment, whereas in HepG2 cells it increased throughout the experimental period. The level of AQP9 protein in AsHepG2 cells decreased in a concentration-dependent manner, but was not markedly different in HepG2 cells. Furthermore, after NaAsO2 treatment the level of phosphorylated AQP9 and p38 was significantly increased with time in the two cell lines. Partial inhibition of p38 activity by the specific inhibitor SB203580 did not significantly affect AQP9 protein expression or phosphorylation. Conclusion AQP9 expression and its state of phosphorylation are closely related to arsenic uptake and may regulate cellular arsenic tolerance by reducing its uptake rate. p38 may have a limited role in the regulation of AQP9 phosphorylation in AsHepG2 cells.

A Bifan Motif Shaped by ArsR1, ArsR2, and Their Cognate Promoters Frames Arsenic Tolerance of Pseudomonas putida

Prokaryotic tolerance to inorganic arsenic is a widespread trait habitually determined by operons encoding an As (III)-responsive repressor (ArsR), an As (V)-reductase (ArsC), and an As (III)-export pump (ArsB), often accompanied by other complementary genes. Enigmatically, the genomes of many environmental bacteria typically contain two or more copies of this basic genetic device arsRBC. To shed some light on the logic of such apparently unnecessary duplication(s) we have inspected the regulation—together and by separate—of the two ars clusters borne by the soil bacterium Pseudomonas putida strain KT2440, in particular the cross talk between the two repressors ArsR1/ArsR2 and the respective promoters. DNase I footprinting and gel retardation analyses of Pars1 and Pars2 with their matching regulators revealed non-identical binding sequences and interaction patterns for each of the systems. However, in vitro transcription experiments exposed that the repressors could downregulate each other’s promoters, albeit within a different set of parameters. The regulatory frame that emerges from these data corresponds to a particular type of bifan motif where all key interactions have a negative sign. The distinct regulatory architecture that stems from coexistence of various ArsR variants in the same cells could enter an adaptive advantage that favors the maintenance of the two proteins as separate repressors.

A molecular switch in sulfur metabolism to reduce arsenic and enrich selenium in rice grain

Rice grains typically contain high levels of toxic arsenic but low levels of the essential micronutrient selenium. Anthropogenic arsenic contamination of paddy soils exacerbates arsenic toxicity in rice crops resulting in substantial yield losses. Here, we report the identification of the gain-of-function arsenite tolerant 1 (astol1) mutant of rice that benefits from enhanced sulfur and selenium assimilation, arsenic tolerance, and decreased arsenic accumulation in grains. The astol1 mutation promotes the physical interaction of the chloroplast-localized O-acetylserine (thiol) lyase protein with its interaction partner serine-acetyltransferase in the cysteine synthase complex. Activation of the serine-acetyltransferase in this complex promotes the uptake of sulfate and selenium and enhances the production of cysteine, glutathione, and phytochelatins, resulting in increased tolerance and decreased translocation of arsenic to grains. Our findings uncover the pivotal sensing-function of the cysteine synthase complex in plastids for optimizing stress resilience and grain quality by regulating a fundamental macronutrient assimilation pathway.