Carcinoid tumors are rare cancers with an incidence of 1.5 cases per 100,000 people in the United States.[1
] These tumors typically arise from neuroendocrine tissue in the digestive tract or lungs. While slow growing, carcinoids frequently metastasize and secrete hormones that often cause debilitating symptoms and a poor quality of life. Unfortunately, there are limited therapeutic options for patients with these tumors, and surgical resection is the only viable cure for patients with benign disease. For patients with metastatic disease, treatments are even less effective. Thus, new treatment modalities must be discovered.
We have previously shown that Notch1 plays an important role in growth inhibition and hormonal suppression in neuroendocrine tumors.[7
] Until recently, there were no known pharmacologic activators of the Notch1 pathway in neuroendocrine tumors. We have demonstrated that both VPA and SBHA are potent activators of Notch1 and associated with growth inhibition and hormonal suppression in GI and pulmonary carcinoid[9
], medullary thyroid cancer[16
], small cell lung cancer[18
], and pheochromocytoma cell lines.[15
] Clearly, this is a viable therapeutic target for neuroendocrine tumors. We have also recently shown that the GSK-3ß pathway is important in neuroendocrine tumors: lithium inhibits the GSK-3ß pathway, limits hormonal secretion, decreases neuroendocrine markers, and inhibits growth.[11
] Whether the combination of the two drugs could be therapeutically useful is not known. The combination of multiple drugs acting on different pathways or mechanisms is common in the treatment of several cancers. Serving as the basis for our study, we asked if there was a more efficient method to limit growth and hormonal secretion in carcinoid tumors.
Consistent with our previous results, treatment with the HDAC inhibitors VPA and SBHA upregulated Notch1 in both GI and pulmonary carcinoid cells, suggesting that the observed effects on hormonal secretion and growth inhibition are due, in part, to Notch1 activity. We also successfully inhibited the GSK-3ß pathway with lithium; likewise, GSK-3ß must also contribute to limiting hormonal secretion and growth inhibition. We observed that HDAC inhibitors could upregulate Notch1, lithium could inhibit GSK-3ß, and the combination of the drugs does not affect the other pathway. Therefore, we set out to see if we could utilize lower doses of these drugs to achieve similar effects to that of either lithium or HDAC inhibitors alone.
We were successful in limiting hormonal secretion with lower doses of both drugs in combination. In GI carcinoid cells, we utilized either lower-dose VPA or SBHA with less lithium to achieve a similar decrease in CgA to that of the drugs alone at higher doses. In pulmonary carcinoid cells, the lower-dose combination was able to decrease CgA levels more than that of any drug alone. These data suggest that hormonal secretion can be limited effectively by lower-dose combination therapy, which serves as an important proof of concept. A similar effect was shown on growth: with the same doses in both GI and pulmonary carcinoid cell lines, lower-dose combination therapy limited growth as well as higher doses of the drugs. Moreover, this growth inhibition was shown to occur through cell cycle arrest. In aggregate, these results show that lower-dose combination therapy is a viable therapeutic option for neuroendocrine tumors.
How these seemingly independent pathways may interact is not entirely clear. Clearly, these drugs all have an effect on neuroendocrine hormone secretion and cellular proliferation. These mechanistic details represent a possible future avenue of investigation. Additionally, the cause of different effects of the HDAC inhibitors is not clear: different levels of Notch1 activation seem to result in similar growth inhibition, which could be explained by other properties of the compounds. Also, pulmonary carcinoid cells appear to be less sensitive to the HDAC inhibitors for unclear reasons. This study lays the groundwork for eventual exploration of targeting multiple pathways in an in vivo model of neuroendocrine tumors.
In conclusion, combination therapy in carcinoid cells with either VPA or SBHA and lithium chloride effectively up-regulates Notch1, inhibits GSK-3ß, suppresses hormonal secretion, and reduces growth through cell cycle arrest. Significantly, these effects were achieved with lower doses when used in combination than when any of the three drugs were used alone. Thus, targeting both the Notch1 and GSK-3ß signaling pathways simultaneously may be an effective treatment modality with the potential benefit of equal efficacy in the treatment for carcinoid tumors.