Phosphorylated DHX9 inhibited the progression of lung adenocarcinoma by regulating R-loops mediated DNA damage.

ObjectivesWe investigated the interactions among DHX9, phosphorylated DHX9, R-loops, and DNA damage to clarify the mechanism by which phosphorylated DHX9 inhibited lung adenocarcinoma progression.MethodsUsed PC-9 and 2BS cells divided into control, siDHX9, OE-DHX9, siDHX9 + OE-RNase H1, OE-PKA, DHX9-S279A, 6-22 Amide, and DHX9-S279A + OE-RNase H1 groups. Assays included quantitative real-time polymerase chain reaction (qRT-PCR), WB (DHX9, γH2AX, Rad51, pCtIP), EdU/CCK-8 (proliferation), TUNEL/flow cytometry (apoptosis), comet assay (DNA damage), CldU/IdU (replication), DRIP-qPCR (R-loops). Nude mice xenografts (control, siDHX9, DHX9-S279E, DHX9-S279A) assessed tumor growth, Ki67, R-loops, DNA damage, and replication.ResultsDHX9 was highly expressed in multiple cancer tissues and lung cancer cell lines, with higher messenger RNA levels in PC-9 than in 2BS cells. Compared with PC-9, siDHX9 reduced proliferation and increased apoptosis, while OE-DHX9 exerted opposite effects. siDHX9 increased DNA damage (with corresponding changes in γH2AX, Rad51, and pCtIP levels), reduced replication (rescued by OE-RNase H1), and elevated R-loops; OE-DHX9 showed opposite effects on damage and R-loops. OE-PKA increased R-loops and damage, and reduced replication, while DHX9-S279A or 6-22 Amide decreased these and 6-22 Amide also increased replication versus PC-9/OE-PKA. DHX9-S279A increased proliferation, with DHX9-S279A + OE-RNase H1 further enhancing this and reducing apoptosis. In vivo, siDHX9 and DHX9-S279E reduced tumor volume/mass and Ki67, increased R-loops, damage, and γH2AX/Rad51/pCtIP, and inhibited replication; DHX9-S279A showed opposite effects versus these groups, with no significant tumor difference versus PC-9 and higher replication versus both.ConclusionsPhosphorylated DHX9 might enhance DNA damage by suppressing R-loop resolution, ultimately inhibiting the proliferation of lung adenocarcinoma cells.