The folate receptor alpha is frequently overexpressed in osteosarcoma samples and plays a role in the uptake of the physiologic substrate 5-methyltetrahydrofolate.
OBJECTIVE
Two major systems exist for folate cell entry: the reduced folate carrier (RFC) and the folate receptor (FR). Although defective RFC-mediated transport was frequently identified as a mechanism of methotrexate (MTX) resistance in osteosarcoma, the status of FR and its role in this disease are unknown.


METHODS
mRNA for FR alpha was measured in 107 osteosarcoma specimens using quantitative reverse transcription-PCR and was related to RFC expression. The effect of FR alpha overexpression on MTX resistance and natural folate uptake was studied using FR alpha non-expressing osteosarcoma 143B cells transfected with FR alpha cDNA in comparison with those transfected with sense or antisense RFC in the same genetic background.


RESULTS
Eighty-four samples (78.5%) had detectable FR alpha mRNA, and 29.9% had higher levels than the ovarian cancer cell line SKOV-3. No correlation was found between mRNA levels of FR alpha and RFC (r(2)=0.002). FR alpha overexpression had minor effects on the transport of MTX and sensitivity to this drug. Among the transfected 143B sublines, only the 143B-FR alpha was able to uptake 5-methyltetrahydrofolate when the extracellular concentration was reduced to 2 nmol/L, which conferred a growth advantage in physiologic folate concentrations compared with vector-only-transfected cells. Importantly, this was not similarly achieved by RFC overexpression.


CONCLUSIONS
This study suggests that FR alpha plays a role in the uptake of 5-methyltetrahydrofolate when the concentration gradient is insufficient for RFC-mediated transport. FR alpha overexpression is unlikely secondary to the decreased RFC expression in osteosarcoma.
Yang
Rui
R
Department of Pediatrics and Molecular Pharmacology, The Albert Einstein College of Medicine, The Children's Hospital at Montefiore, Bronx, New York 10467, USA.
Kolb
E Anders
EA
Qin
Jing
J
Chou
Alexander
A
Sowers
Rebecca
R
Hoang
Bang
B
Healey
John H
JH
Huvos
Andrew G
AG
Meyers
Paul A
PA
Gorlick
Richard
R
D000818:Animals; D000964:Antimetabolites, Antineoplastic / Q000494:pharmacology; D001859:Bone Neoplasms / Q000235:genetics / Q000378:metabolism*; D002352:Carrier Proteins / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D045744:Cell Line, Tumor; D019008:Drug Resistance, Neoplasm; D058973:Folate Receptors, GPI-Anchored; D005492:Folic Acid / Q000378:metabolism; D005493:Folic Acid Antagonists / Q000494:pharmacology; D006801:Humans; D008727:Methotrexate / Q000494:pharmacology; D012516:Osteosarcoma / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D013763:Tetrahydrofolates / Q000378:metabolism*; D023041:Xenograft Model Antitumor Assays
D000964:Antimetabolites, Antineoplastic; D002352:Carrier Proteins; D058973:Folate Receptors, GPI-Anchored; D005493:Folic Acid Antagonists; D012333:RNA, Messenger; D011956:Receptors, Cell Surface; D013763:Tetrahydrofolates; D005492:Folic Acid; C005984:5-methyltetrahydrofolate; D008727:Methotrexate