Neurotrophin receptors expression and JNK pathway activation in human astrocytomas

Neurotrophins are growth factors that regulate cell growth, differentiation and apoptosis in the nervous system. Their diverse actions are mediated through two different transmembrane – receptor signaling systems: Trk receptor tyrosine kinases (TrkA, TrkB, TrkC) and p75^NTR neurotrophin receptor. Trk receptors promote cell survival and differentiation while p75^NTR induces, in most cases, the activity of JNK-p53-Bax apoptosis pathway or suppresses intracellular survival signaling cascades. Robust Trk activation blocks p75^NTR -induced apoptosis by suppressing the JNK-p53-Bax pathway. The aim of this exploratory study was to investigate the expression levels of neurotrophin receptors, Trks and p75^NTR, and the activation of JNK pathway in human astrocytomas and in adjacent non-neoplastic brain tissue.

Formalin-fixed paraffin-embedded serial sections from 33 supratentorial astrocytomas (5 diffuse fibrillary astrocytomas, WHO grade II; 6 anaplastic astrocytomas, WHO grade III; 22 glioblastomas multiforme, WHO grade IV) were immunostained following microwave pretreatment. Polyclonal antibodies against TrkA, TrkB, TrkC and monoclonal antibodies against p75^NTR and phosphorylated forms of JNK (pJNK) and c-Jun (pc-Jun) were used. The labeling index (LI), defined as the percentage of positive (labeled) cells out of the total number of tumor cells counted, was determined.

Moderate to strong, granular cytoplasmic immunoreactivity for TrkA, TrkB and TrkC receptors was detected in greater than or equal to 10% of tumor cells in the majority of tumors independently of grade; on the contrary, p75^NTR receptor expression was found in a small percentage of tumor cells (~1%) in some tumors. The endothelium of tumor capillaries showed conspicuous immunoreactivity for TrkB receptor. Trk immunoreactivity seemed to be localized in some neurons and astrocytes in non-neoplastic tissue. Phosphorylated forms of JNK (pJNK) and c-Jun (pc-Jun) were significantly co-expressed in a tumor grade-dependent manner (p < 0.05). Interestingly, a statistically significant (p < 0.05) reverse relationship between Trk receptors LIs and pc-Jun/pJNK LIs was noted in some glioblastomas multiforme.

In the context of astrocytomas, Trk receptors (TrkA, TrkB, TrkC) expression may promote tumor growth independently of grade. Furthermore, activation of JNK pathway may contribute to progression towards malignancy. Considering the fact that regional tumor heterogeneity may be a limiting factor for immunohistochemical studies, the significance of the reverse relationship between Trk receptors and pc-Jun/pJNK LIs with respect to biological behavior of human astrocytomas requires further evaluation.