B7-H3 promotes triple-negative breast cancer progression by sustaining SP1-dependent HSPA8 expression

Triple-negative breast cancer (TNBC) lacks durable therapeutic targets. B7-H3 (CD276) is frequently overexpressed in TNBC, but its tumor-cell-intrinsic signaling and actionable downstream dependencies remain unclear. We integrated public datasets, TNBC tissue microarrays, multiplex immunofluorescence, RNA sequencing, and genetic perturbation assays. Promoter-reporter assays, SP1 rescue experiments, and pharmacological p38 MAPK inhibition were used to investigate the relationship among B7-H3, SP1, and HSPA8. The effect of adding HSPA8 knockdown to B7-H3 silencing was assessed in contemporaneously generated experimental groups. An anti-B7-H3 antibody combined with the HSP70-family inhibitor VER155008 was evaluated in syngeneic 4T1 tumors. B7-H3 was elevated in TNBC and associated with poor survival. B7-H3 overexpression enhanced migration, invasion, and clonogenic growth and reduced apoptosis, whereas B7-H3 knockdown produced the opposite effects. RNA sequencing identified HSPA8 as a candidate B7-H3-associated effector, and HSPA8 depletion attenuated the malignant phenotypes induced by B7-H3 overexpression. B7-H3 knockdown reduced p38 MAPK phosphorylation, SP1 expression, and HSPA8 expression, whereas pharmacological p38 MAPK inhibition produced similar changes. SP1 re-expression partially restored HSPA8 expression and clonogenic growth and attenuated apoptosis in B7-H3-silenced cells. Adding HSPA8 knockdown to B7-H3 silencing further inhibited malignant phenotypes compared with B7-H3 knockdown alone. In vivo, the combination of anti-B7-H3 treatment and VER155008 produced the greatest observed reduction in tumor burden among the four treatment groups. B7-H3 promotes malignant TNBC phenotypes and supports SP1-dependent HSPA8 expression. The phosphorylation and pharmacological data are consistent with the involvement of p38 MAPK but do not establish it as an obligatory intermediate in a linear signaling cascade. Combined targeting of B7-H3 and the HSP70 chaperone family warrants further investigation.