VEGFR-1 expression levels predict occurrence of disseminated tumor cells in the bone marrow of patients with esophageal carcinoma

Blocking angiogenesis by inhibiting VEGF represents an established therapeutic strategy in many cancers. The role of placental growth factor (PlGF) and of its receptor VEGFR-1 in tumor biology remain more elusive. Currently, humanized monoclonal antibodies against PlGF are studied in early phase clinical trials because PlGF inhibition blocked murine tumor growth and angiogenesis. In contrast to mice exclusively expressing one PlGF isoform (PlGF-2), humans can produce four PlGF isoforms (PlGF1-4). Surprisingly nothing is yet known about expression of all four PlGF isoforms in human cancer, because until now mostly total PlGF levels or PlGF-1/2 were analyzed without discriminating further. In this study we determined mRNA expression levels of PlGF1-4 and of VEGFR-1 by QRT-PCR in human esophageal tumor tissue and investigated whether gene expression levels correlate with clinical data. PlGF-1 and -2 were expressed in virtually all analyzable tumors, whereas PlGF-3 and -4 were present in tumors of 59 and 74 % of patients, respectively. MRNA Expression levels of all four splice variants correlated with each other. In contrast, PlGF-1 and -2 mRNA expression was lower in esophageal control tissue and PlGF-3 and -4 mRNA were undetectable. VEGFR-1 was expressed by more than 80 % of patients. Interestingly, VEGFR-1 expression levels significantly correlate with presence of disseminated tumor cells (DTCs) in bone marrow. Patients with DTCs exhibit lower VEGFR-1 mRNA expression than patients without DTCs. Pending validation in other types of cancer, expression levels of VEGFR-1 might be useful as surrogate marker for DTCs.