PmrAB, the two-component system of Acinetobacter baumannii, controls the phosphoethanolamine modification of lipooligosaccharide in response to metal ions.

Acinetobacter baumannii is highly resistant to antimicrobial agents, and XDR strains have become widespread. A. baumannii has developed resistance to colistin, which is considered the last resort against XDR Gram-negative bacteria, mainly caused by lipooligosaccharide (LOS) phosphoethanolamine (pEtN) and/or galactosamine (GalN) modifications induced by mutations that activate the two-component system (TCS) pmrAB. Although PmrAB of A. baumannii has been recognized as a drug resistance factor, its function as TCS, including its regulatory genes and response factors, has not been fully elucidated. In this study, to clarify the function of PmrAB as TCS, we elucidated the regulatory genes (regulon) of PmrAB via transcriptome analysis using pmrAB-activated mutant strains. We discovered that PmrAB responds to low pH, Fe2+, Zn2+, and Al3+. A. baumannii selectively recognizes Fe2+ rather than Fe3+, and a novel region ExxxE, in addition to the ExxE motif sequence, is involved in the environmental response. Furthermore, PmrAB participates in the phosphoethanolamine modification of LOS on the bacterial surface in response to metal ions such as Al3+, contributing to the attenuation of Al3+ toxicity and development of resistance to colistin and polymyxin B in A. baumannii. This study demonstrates that PmrAB in A. baumannii not only regulates genes that play an important role in drug resistance but is also involved in responses to environmental stimuli such as metal ions and pH, and this stimulation induces LOS modification. This study reveals the importance of PmrAB in the environmental adaptation and antibacterial resistance emergence mechanisms of A. baumannii. IMPORTANCE: Antimicrobial resistance (AMR) is a pressing global issue in human health. Acinetobacter baumannii is notably high on the World Health Organization's list of bacteria for which new antimicrobial agents are urgently needed. Colistin is one of the last-resort drugs used against extensively drug-resistant (XDR) Gram-negative bacteria. However, A. baumannii has become increasingly resistant to colistin, primarily by modifying its lipooligosaccharide (LOS) via activating mutations in the two-component system (TCS) PmrAB. This study comprehensively elucidates the detailed mechanism of drug resistance of PmrAB in A. baumannii as well as its biological functions. Understanding the molecular biology of these molecules, which serve as drug resistance factors and are involved in environmental recognition mechanisms in bacteria, is crucial for developing fundamental solutions to the AMR problem.

Serum osteopontin predicts the response to atezolizumab plus bevacizumab in patients with hepatocellular carcinoma.

BACKGROUND: Combination therapy with anti-programmed death-ligand 1 and anti-vascular endothelial growth factor (VEGF) antibodies has become the standard treatment for un-resectable hepatocellular carcinoma (uHCC). We aimed to identify predictive circulating biomarkers for the outcome/response of the combination therapy in uHCC patients. METHODS: This prospective multicenter study enrolled 70 patients with uHCC who received atezolizumab and bevacizumab (Atez/Bev). We evaluated 47 circulating proteins in sera before and after 1 and 6 weeks of Atez/Bev therapy by multiplex bead-based immunoassay and ELISA. As controls, we analyzed the sera from 62 uHCC patients before treatment of lenvatinib (LEN) and healthy volunteers (HVs). RESULTS: The disease control rate was 77.1%. Median progression-free survival (PFS) was 5.7 months (95% confidence interval [CI] = 3.8-9.5). The pretreatment levels of osteopontin (OPN), angiopoietin-2, VEGF, S100-calcium-binding protein A8/S100-calcium-binding protein A9, soluble programmed cell death-1, soluble CD163, and 14 cytokines/chemokines were higher in patients with uHCC than in HVs. Among these, pretreatment OPN levels were higher in PD group than in non-PD group for Atez/Bev. The PD rate was higher in high OPN group than in low OPN group. Multivariate analysis identified high pretreatment OPN and high α-fetoprotein levels as independent predictors of PD. In the sub-analysis of Child-Pugh class A patients, PFS was also shorter in the high OPN group than in the low OPN group. Pretreatment OPN level was not associated with treatment response for LEN. CONCLUSION: High serum OPN levels were associated with poor response to Atez/Bev in patients with uHCC.