Background

Small cell lung cancer (SCLC) is an aggressive tumour acquiring resistance and showing early metastasis. It is widely accepted that SCLC is of neuroendocrine phenotype, yet epithelial markers are also present. Recent studies highlight the importance of heterogeneity in SCLC (1). Our aims were to investigate heterogeneity using SCLC cell lines and to consider how irradiation affects heterogeneity in a SCLC xenograft model.

Method

Cloned DMS79 and CORL24 SCLC cells were stained for pro-opiomelanocortin (POMC), neural-cell adhesion molecule (N-CAM), neuron specific enolase (NSE), synaptophysin, chromogranin A (CgA) and cytokeratin (CK). For xenografts, DMS79 cells were subcutaneously injected into nude mice (5x10⁶/mouse). Tumours of 250mm³ were irradiated at 20Gy over 10 days. POMC was determined by ELISA and qPCR.

Results

DMS79 and CORL24 cells stained positively for POMC, NSE, N-CAM and CgA, but were also positive for CK. The majority of cells within the suspended spheroids stained positive for all markers but heterogeneity was observed especially when considering N-CAM and CK characteristics. DMS79 xenografts stained positive for both POMC and CK. In irradiated xenografts, there was evidence of distinct neuroendocrine heterogeneity in tumour cells. In addition, POMC expression and protein levels were greatly reduced. Invasive cells in the primary tumour were strongly positive for both POMC and CK.

Conclusion

Cloned SCLC cells showed a strong neuroendocrine phenotype combined with epithelial characteristics. In xenografts, the neuroendocrine and epithelial markers in invasive cells suggest both phenotypes contribute to metastasis. In the irradiated xenografts, there is marked heterogeneity with areas of positive and negative neuroendocrine staining but with similar levels of CK staining. This, along with the decrease in POMC expression and protein levels in the irradiated tumours, suggests that neuroendocrine cells could be more susceptible to irradiation.