The genetic heterogeneity of pheochromocytomas and paragangliomas (PPGLs) conceivably offers the opportunity to develop an individual treatment strategy for each patient. Therefore, a better understanding of the biomolecular features associated with different mutations is necessary to improve patient care especially after development of metastatic disease. The MYC/MAX (MYC-associated factor X) complex has a central involvement within pathways regulating phenotypic features of PPGLs; however, its role in PPGL progression and malignancy is unknown.
The present study used the rat pheochromocytoma cell line, PC12, lacking Max (PC12-EV) and its counterpart cell line re-expressing functional Max (PC12-MAX) to investigate how the MYC/MAX complex influences pro-metastatic behavior. Additionally, two small molecule inhibitors targeting MYC/MAX complex formation (MYRA-A) or suppressing c-Myc transcription (JQ1) were used. Re-expression of Max resulted in an increased n-Myc expression while c-Myc expression was unaffected. Both cell lines showed a comparable doubling time (PC12-MAX: 34.5h; PC12-EV: 32.8h). Treatment with 10 µM JQ1 repressed doubling time significantly (PC12-MAX: 42.3h; PC12-EV: 47.4h), whereas MYRA-A only delayed cell growth in the first 72h. PC12-MAX cells showed significantly lower cell motility (scratch assay, P<0.001) compared to control cells. Treatment with JQ1 decreased the motility in both cell lines (P<0.001). Moreover, re-expression of Max had no impact on migration but reduced the invasion capacity (P<0.001), as determined by Boyden Chamber assays. JQ1 diminished cell migration significantly, whereas the invasion capacity of both cell lines was unaffected. In contrast, treatment with MYRA-A had no influence on pro-metastatic behavior.
In conclusion, re-expressing Max resulted in a less aggressive cellular phenotype. Moreover, suppression of c-Myc transcription by JQ1 diminished pro-metastatic behavior in presence or absence of Max. Improved understanding of the MYC/MAX convergence point and involved molecular pathways could be useful for the development of novel treatments against metastatic PPGLs.