Hedgehog signaling negatively co-regulates BH3-only protein Noxa and TAp73 in TP53-mutated cells

Area of Science:

• Oncology
• Molecular Biology
• Cancer Signaling

Background:

• The Hedgehog (Hh) signaling pathway is implicated in various cancers, including embryonal pediatric tumors like rhabdomyosarcoma (RMS) and medulloblastoma (MB).
• TP53 mutations are common in these aggressive tumors, affecting cellular response to damage and apoptosis.
• Understanding the regulatory mechanisms of proapoptotic proteins within Hh-driven tumors is crucial for developing targeted therapies.

Purpose of the Study:

• To investigate the effect of Hedgehog (Hh) pathway inhibition on the expression of the proapoptotic protein Noxa in TP53-mutated pediatric tumor cells.
• To elucidate the role of Gli1, a key transcription factor in the Hh pathway, in regulating Noxa and TAp73 expression.
• To identify potential downstream mediators, such as EGR1, involved in the Hh pathway's control of Noxa and TAp73.

Main Methods:

• Pharmacological inhibition of the Hh pathway using GANT61, a Hedgehog pathway inhibitor (HPI).
• Genetic manipulation of Gli1 expression using siRNA for silencing and overexpression techniques.
• Quantitative analysis of mRNA and protein levels for Noxa, TAp73, and EGR1 using molecular biology techniques.
• Assessment of the functional role of TAp73 and EGR1 in Noxa induction through knockdown experiments.

Main Results:

• Pharmacological repression of Hh signaling by GANT61 induced Noxa expression in TP53-mutated RMS and MB cells.
• Genetic silencing of Gli1 increased Noxa mRNA and protein levels, while Gli1 overexpression decreased them.
• Gli1 knockdown elevated TAp73 levels, whereas Gli1 overexpression reduced TAp73 levels; however, TAp73 knockdown did not inhibit GANT61-induced Noxa.
• mRNA levels of EGR1 correlated with Noxa and TAp73, and EGR1 silencing reduced Noxa and TAp73 expression, indicating EGR1's regulatory role.

Conclusions:

• Gli1 appears to repress the expression of Noxa and TAp73, potentially mediated through the transcription factor EGR1.
• These findings suggest a novel regulatory axis within Hh-driven pediatric tumors.
• The identified pathway could be therapeutically exploited to sensitize Hh-driven tumors to chemotherapy.