Small cell lung cancer: model of circulating tumor cell tumorospheres in chemoresistance

Abstract

Small cell lung cancer (SCLC) represents 15% of lung cancers and is characterized by early dissemination, development of chemoresistance and a poor prognosis. A host of diverse drugs failed invariably and its mechanisms of global chemoresistance have not been characterized so far. SCLC represents the prototype of an aggressive and highly metastatic tumor which is ultimately refractory to any treatment. High numbers of circulating tumor cells (CTCs) allowed us to establish 5 CTC cell lines (BHGc7, 10, 16, 26 and UHGc5) from patients with recurrent SCLC. These cell lines exhibit the typical SCLC markers and CTCs of all patients developed spontaneously large multicellular aggregates, termed tumorospheres. Ki67 and carbonic anhydrase 9 (CAIX) staining of tumorosphere sections revealed quiescent and hypoxic cells, respectively. Accordingly, comparison of the chemosensitivity of CTC single cell suspensions with tumorospheres demonstrated increased resistance of the clusters against chemotherapeutics commonly used for treatment of SCLC. Therefore, global chemoresistance of relapsing SCLC seems to rely on formation of large tumorospheres which reveal limited accessibility, lower growth fraction and hypoxic conditions. Since similar tumor spheroids were found in other tumor types, SCLC seems to represent a unique tumor model to study the association of CTCs, metastasis and drug resistance.

Circulating Tumor Cells in the Parallel Invasion Model Supporting Early Metastasis

“…These 3D-structures are highly chemoresistant compared to the same cells as single cell suspensions. [12] Although the CTC lines can be kept in culture for years, formation of apoptotic fragments is observed, similar to CTCmat detectable in the circulation. In conclusion, SCLC CTCs circulate in high numbers, lack EMT and CTC phenotypes and have high metastatic potential.…”

“…Although the primary tumor in SCLC is largely chemosensitive and the CTCs are responsive, in relapsed patients the tumor is broadly chemoresistant. [12] Although, persistent high CTC counts in response to chemotherapeutic treatments seem to indicate drug resistance of the resident lesions, a drop of the CTC count may still not be representative for the tumor as whole. Furthermore, detection of drug resistance via CTC count in clinical decision making is without effect in the unavailability of alternative effective treatment regimens.…”

“…[92] For example, the SCLC CTCs proved to be chemosensitive but the tumors which have relapsed exhibit a broad drug resistance. [12,96] Therefore, successful reduction or elimination of the CTCs is not certified to correlate with response of the resident lesions. In 65% of cases of colorectal cancer, lymphatic and distant metastases were derived from independent subclones in the primary tumor, whereas in 35% of cases they shared a common origin.…”

“…One solution may be the in vitro expansion of viable CTCs and there genetic and phenotypic characterization. [12] Surrogate marker of response Systemic anticancer therapy targets tumor cells that have been shed from the primary lesion to settle elsewhere and which are undetectable by clinical imaging and inaccessible to surgery. [24] The use of primary tumors to predict therapy response is based largely on the linear progression model in which tumor ontogeny proceeds within the original tumor microenvironment and, consequently, the primary tumor determines the molecular characteristics of DTCs.…”

“…However, small cell lung cancer (SCLC) is distinguished by excessive amounts of CTCs in advanced stage and this exceptional case allowed us to establish 6 permanent CTC lines in vitro which could be employed to study several aspects of the characteristics of CTCs. [12] Some aspects of CTCs were deduced from a classical model of tumor dissemination which comprises gain of invasive properties of selected clones of tumor cells, intravasation linked to epithelial-mesenchymal transition (EMT), distribution via blood and the lymphatic system and, finally, reversion of EMT via MET and extravasation for colonization at distal sites. [4] Consequently, metastasis would be a relatively late event in tumor development after invasive growth, elicitation of blood vessel extensions and intravasation after a time-consuming crossing of the surrounding tumor stroma.…”