Thrombospondin-4 from cancer-associated fibroblasts downregulates ERBB2 expression in gastric cancer cells

The intra-tumoral heterogeneity of HER2 expression is associated with resistance to anti-HER2 therapy in HER2-positive gastric cancers (GCs). We previously reported that thrombospondin-4 (THBS4) is overexpressed in cancer-associated fibroblasts (CAFs) in the GC microenvironment and is associated with GC remodeling. To clarify the relationship between CAFs and the intra-tumoral heterogeneity of HER2 in GC, the effect of CAFs on HER2 expression was investigated in GC cells. Two HER2-positive GC cell lines (NCI-N87 and OE19) and two pairs of gastric CAFs were used. The effect of fibroblasts on HER2 expression in cancer cells was analyzed by immunohistochemical staining and reverse transcription-polymerase chain reaction. THBS4 siRNA was used for knockdown assays. The effects of Herceptin or gabapentin, a THBS4 receptor inhibitor, on subcutaneous tumors were examined in nude mice. CAFs and THBS4 recombinant significantly downregulated HER2 (ERBB2) expression in GC cells. THBS4 siRNA and gabapentin significantly inhibited the HER2-decreasing activity in CAFs. In vivo, CAFs suppress HER2 expression of subcutaneous GC tumors and induce Herceptin resistance. Gabapentin overcomes CAF-induced Herceptin resistance. THBS4 from CAFs downregulated HER2 (ERBB2) expression in GC cells. Thus, THBS4 receptor inhibitors may be useful in preventing the acquisition of resistance to anti-HER2 therapy.