Loss of Actrt1 in non-hematopoietic stroma suppresses pathological angiogenesis and tumor progression

Tumor progression and therapeutic resistance depend not only on tumor cells but also on the tumor vasculature, a central component of the tumor microenvironment (TME). Accordingly, normalization and remodeling of tumor vessels represent a promising therapeutic strategy, highlighting the urgent need for approaches that selectively and efficiently modulate this compartment. Actrt1 (ArpT1) is an actin-related protein implicated in ciliogenesis, but its roles in the TME are unknown. Here, we show that Actrt1 drives tumor progression through non-hematopoietic cells, with endothelium as a principal site of action. Actrt1 knockout (Actrt1^-/-) mice displayed significantly reduced growth of B16F1 and MC38 tumors and improved survival. Bone-marrow chimeras localized this phenotype to the host non-hematopoietic compartment; Actrt1^-/- marrow in WT hosts did not confer protection, whereas WT marrow in Actrt1^-/- hosts did. Immunofluorescence detected Actrt1 in CD31⁺ tumor vessels in vivo. Developmental retinal vascularization was preserved, but endothelial sprouting from Actrt1^-/- aortic rings was reduced, and recovery after hindlimb ischemia was delayed. In Matrigel-tumor plugs, gross vascular ingrowth was decreased. Single-cell RNA-seq of the GFP^- CD45^- stromal fraction resolved 14 clusters; the endothelial fraction was unchanged in proportion, yet Actrt1 deficiency shifted its transcriptome toward immaturity. Histology showed shorter, discontinuous CD31⁺ vessels. Together, our data indicate that Actrt1 promotes tumor growth through non-hematopoietic endothelium by sustaining sprouting and vessel maturation, while developmental angiogenesis remains intact. Targeting Actrt1 may therefore restrain tumor growth by impairing maladaptive angiogenesis and could complement strategies for vascular normalization.