Presently, there is no effective treatment for
glioblastoma, the most malignant and common
brain tumor.
Growth factors are potential targets for therapeutic strategies because they are essential for
tumor growth and progression.
Peptidylglycine alpha-amidating monooxygenase is the
enzyme producing alpha-amidated bioactive
peptides from their inactive
glycine-extended precursors. The high expression of
peptidylglycine alpha-amidating monooxygenase mRNA in
glioblastoma and
glioma cell lines points to the involvement of alpha-amidated
peptides in tumorigenic growth processes in the brain. After screening of amidated
peptides, it was found that human
glioblastoma cell lines express high levels of
adrenomedullin (AM)
mRNA, and that immunoreactive AM is released into the culture medium. AM is a multifunctional regulatory
peptide with mitogenic and angiogenic capabilities among others. Real-time quantitative
reverse transcriptase-polymerase chain reaction analysis showed that AM
mRNA was correlated to the
tumor type and grade, with high expression in all
glioblastomas analyzed, whereas a low expression was found in
anaplastic astrocytomas and barely detectable levels in low-grade
astrocytomas and
oligodendrogliomas. In the present study we also demonstrate the presence of
mRNA encoding the putative AM
receptors, calcitonin receptor-like receptor/receptor activity-modifying protein-2 and -3 (CRLR/RAMP2; CRLR/RAMP3) in both
glioma tissues and
glioblastoma cell lines and further show that exogenously added AM can stimulate the growth of these
glioblastoma cells in vitro. These findings suggest that AM may function as an autocrine
growth factor for
glioblastoma cells. One way to test the autocrine hypothesis is to interrupt the function of the endogenously produced AM. Herein, we demonstrate that a polyclonal antibody specific to AM, blocks the binding of the
hormone to its cellular receptors and decreases by 33% (P < 0.001) the growth of U87
glioblastoma cells in vitro. Intratumoral administration of the anti-AM antibody resulted in a 70% (P < 0.001) reduction in subcutaneous U87 xenograft weight 21 days
after treatment. Furthermore, the density of vessels was decreased in the antibody-treated
tumors. These findings support that AM may function as a potent autocrine/paracrine
growth factor for human
glioblastomas and demonstrate that inhibition of the action of AM (produced by
tumor cells) may suppress
tumor growth in vivo.