The α folate receptor is highly activated in malignant pleural mesothelioma

Objective: To determine whether the folate receptor gene is overexpressed in malignant pleural mesothelioma. Methods: Differential display analysis was performed with fresh frozen RNA obtained from normal lung, pleura, and mesothelioma. Sixty differentially expressed genes were identified and characterized. One gene that was overexpressed in mesothelioma versus normal tissue was the human alpha folate receptor. In situ hybridization with antisense probes designed on the basis of the sequence of the folate receptor was performed with frozen sections from 61 patients (33 epithelial and 28 mixed or sarcomatoid tumors) with malignant pleural mesothelioma. The controls included normal pleura, normal lung, other cancers, and sense controls for all of the tumors. Northern analysis with a folate receptor probe and immunohistochemical analysis with anti-alpha folate receptor antibodies were also performed. Results: Forty-four (72%) of the 61 mesothelioma tumors were found to have between 2-fold and 4-fold higher mRNA expression of the folate receptor when compared with the control tissues. The histologic type of the tumor did not affect the rate of folate receptor activation. Northern analysis and immunohistochemical experiments confirmed these findings. Conclusions: A majority of mesothelioma tumors examined overexpress the alpha folate receptor protein when compared with normal adjacent tissues. This finding may help explain the observations that antifolate drugs have activity in the treatment of mesothelioma. It also encourages further study of folate receptor-related treatment strategies in this malignancy.