Scavenger Receptor A1 Prevents Non-Small Cell Lung Cancer Metastasis by Suppressing Macrophage Serum Amyloid A1 Expression

Metastasis is the key determinant of poor prognosis for advanced-stage NSCLC patients. Although an important contributor to metastasis is cross-talk between tumor-associated macrophages (TAMs) and tumor cells, its regulation is not fully understood. Expressed primarily in macrophages, scavenger receptor A1 (SR-A1) has been associated with lung tumorigenesis. Here, the mechanistic basis for the involvement of SR-A1 in lung cancer prognosis was investigated using population genetics, transcriptomics, and functional analyses. SR-A1 genetic variants were investigated for possible association with survival of advanced-stage NSCLC patients in the Harvard Lung Cancer Study cohort. Two SNPs (rs17484273, rs1484751) in SR-A1 were significantly associated with poor overall survival of NSCLC patients. Further, data from The Cancer Genome Atlas showed a considerable down-regulation of SR-A1 in lung tumor tissues. The association of SR-A1 with prognosis was validated in animal models in the context of lung cancer metastasis. Macrophages derived from SR-A1 knockout mice accelerated metastasis in a lung cancer mouse model. On the other hand, tumor cell seeding, migration, and invasion as well as macrophage accumulation in lung cancer tissue were enhanced in SR-A1 knockout mice. Furthermore, SR-A1 deficiency promoted up-regulation of serum amyloid A1 (SAA1) in macrophages, which appeared to be mediated by MAPK/Ikappa-B/NF-kappaB signaling. Further, SAA1 exposure promoted tumor cell invasion and macrophage migration in vitro and in vivo, but these effects were blocked by administration of an anti-SAA1 antibody. These findings suggest that SR-A1 may suppress lung cancer metastasis by down-regulating SAA1 production in macrophages. In the experimental metastasis model, 3 × 105 LLC cells were re-suspended in 200 uL phosphate-buffered saline (PBS) and injected into the tail vein of 8-week-old WT and SR-A1 KO female mice (B6.Cg-Msr1tm1Csk/J) to induce the pulmonary metastasis. Mice were euthanized after three weeks and the lungs were collected for analysis. Total RNA was extracted, purified, and assessed by formaldehyde agarose gel electrophoresis and quantitated spectrophotometrically. cDNA labeled with a fluorescent dye (Cy3-dCTP) was produced by Eberwine’s linear RNA amplification method and subsequent enzymatic reaction. Array hybridization, washing, scanning, and data analysis were carried out according to NimbleGen Expression user’s guide and performed at Capital Biotech Corporation (Beijing, China).