Apolipoprotein E promotes invasion in oral squamous cell carcinoma.

Oral squamous cell carcinoma (OSCC) patients generally have a poor prognosis, due to the invasive nature of these tumors. In comparing transcription profiles between OSCC tumors with a more invasive (WPOI 5) versus a less invasive (WPOI 3) pattern of invasion, we identified a total of 97 genes that were overexpressed at least 1.5-fold in the more invasive tumor subtype. The most functionally relevant genes were assessed using in vitro invasion assays with an OSCC cell line (UM-SCC-1). Individual siRNA knockdown of 15 of these 45 genes resulted in significant reductions in tumor cell invasion compared to a non-targeting siRNA control. One gene whose knockdown had a strong effect on invasion corresponded to Apolipoprotein E (APOE). Both matrix degradation and the number of mature invadopodia were significantly decreased with APOE knockdown. APOE knockdown also resulted in increased cellular cholesterol, consistent with APOE's role in regulating cholesterol efflux. APOE knockdown resulted in decreased levels of phospho-ERK1/2, phospho-JNK, and phospho-cJun, as well as decreased AP-1 activity. Expression of matrix metalloproteinase 7 (MMP7), an AP-1 target, was also significantly decreased. Our findings suggest that APOE protein plays a significant role in OSCC tumor invasion due to its effects on cellular cholesterol and subsequent effects on cell signaling and AP-1 activity, leading to changes in the expression of invasion-related proteins including MMP-7.