The notch target gene HEYL modulates metastasis forming capacity of colorectal cancer patient-derived spheroid cells in vivo.

BACKGROUND: While colorectal cancer (CRC) patients with localized disease have a favorable prognosis, the five-year-survival rate in patients with distant spread is still below 15%. Hence, a detailed understanding of the mechanisms regulating metastasis formation is essential to develop therapeutic strategies targeting metastasized CRC. The notch pathway has been shown to be involved in the metastatic spread of various tumor entities; however, the impact of its target gene HEYL remains unclear so far. METHODS: In this study, we functionally assessed the association between high HEYL expression and metastasis formation in human CRC. Therefore, we lentivirally overexpressed HEYL in two human patient-derived CRC cultures differing in their spontaneous metastasizing capacity and analyzed metastasis formation as well as tumor cell dissemination into the bone marrow after xenotransplantation into NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG) mice. RESULTS: HEYL overexpression decreased tumor cell dissemination and the absolute numbers of formed metastases in a sub-renal capsular spontaneous metastasis formation model, addressing all steps of the metastatic cascade. In contrast, metastatic capacity was not decreased following intrasplenic xenotransplantation where the cells are placed directly into the blood circulation. CONCLUSION: These results suggest that HEYL negatively regulates metastasis formation in vivo presumably by inhibiting intravasation of metastasis-initiating cells.

Genetic subclone architecture of tumor clone-initiating cells in colorectal cancer.

A hierarchically organized cell compartment drives colorectal cancer (CRC) progression. Genetic barcoding allows monitoring of the clonal output of tumorigenic cells without prospective isolation. In this study, we asked whether tumor clone-initiating cells (TcICs) were genetically heterogeneous and whether differences in self-renewal and activation reflected differential kinetics among individual subclones or functional hierarchies within subclones. Monitoring genomic subclone kinetics in three patient tumors and corresponding serial xenografts and spheroids by high-coverage whole-genome sequencing, clustering of genetic aberrations, subclone combinatorics, and mutational signature analysis revealed at least two to four genetic subclones per sample. Long-term growth in serial xenografts and spheroids was driven by multiple genomic subclones with profoundly differing growth dynamics and hence different quantitative contributions over time. Strikingly, genetic barcoding demonstrated stable functional heterogeneity of CRC TcICs during serial xenografting despite near-complete changes in genomic subclone contribution. This demonstrates that functional heterogeneity is, at least frequently, present within genomic subclones and independent of mutational subclone differences.

Succession of transiently active tumor-initiating cell clones in human pancreatic cancer xenografts.

Although tumor-initiating cell (TIC) self-renewal has been postulated to be essential in progression and metastasis formation of human pancreatic adenocarcinoma (PDAC), clonal dynamics of TICs within PDAC tumors are yet unknown. Here, we show that long-term progression of PDAC in serial xenotransplantation is driven by a succession of transiently active TICs producing tumor cells in temporally restricted bursts. Clonal tracking of individual, genetically marked TICs revealed that individual tumors are generated by distinct sets of TICs with very little overlap between subsequent xenograft generations. An unexpected functional and phenotypic plasticity of pancreatic TICs in vivo underlies the recruitment of inactive TIC clones in serial xenografts. The observed clonal succession of TIC activity in serial xenotransplantation is in stark contrast to the continuous activity of limited numbers of self-renewing TICs within a fixed cellular hierarchy observed in other epithelial cancers and emphasizes the need to target TIC activation, rather than a fixed TIC population, in PDAC.

Patient-derived xenografts of gastrointestinal cancers are susceptible to rapid and delayed B-lymphoproliferation.

Patient-derived cancer xenografts (PDX) are widely used to identify and evaluate novel therapeutic targets, and to test therapeutic approaches in preclinical mouse avatar trials. Despite their widespread use, potential caveats of PDX models remain considerably underappreciated. Here, we demonstrate that EBV-associated B-lymphoproliferations frequently develop following xenotransplantation of human colorectal and pancreatic carcinomas in highly immunodeficient NOD.Cg-Prkdcscid Il2rgtm1Wjl /SzJ (NSG) mice (18/47 and 4/37 mice, respectively), and in derived cell cultures in vitro. Strikingly, even PDX with carcinoma histology can host scarce EBV-infected B-lymphocytes that can fully overgrow carcinoma cells during serial passaging in vitro and in vivo. As serial xenografting is crucial to expand primary tumor tissue for biobanks and cohorts for preclinical mouse avatar trials, the emerging dominance of B-lymphoproliferations in serial PDX represents a serious confounding factor in these models. Consequently, repeated phenotypic assessments of serial PDX are mandatory at each expansion step to verify "bona fide" carcinoma xenografts.

Phenotypic differentiation does not affect tumorigenicity of primary human colon cancer initiating cells.

Within primary colorectal cancer (CRC) a subfraction of all tumor-initiating cells (TIC) drives long-term progression in serial xenotransplantation. It has been postulated that efficient maintenance of TIC activity in vitro requires serum-free spheroid culture conditions that support a stem-like state of CRC cells. To address whether tumorigenicity is indeed tightly linked to such a stem-like state in spheroids, we transferred TIC-enriched spheroid cultures to serum-containing adherent conditions that should favor their differentiation. Under these conditions, primary CRC cells did no longer grow as spheroids but formed an adherent cell layer, up-regulated colon epithelial differentiation markers, and down-regulated TIC-associated markers. Strikingly, upon xenotransplantation cells cultured under either condition equally efficient formed serially transplantable tumors. Clonal analyses of individual lentivirally marked TIC clones cultured under either culture condition revealed no systematic differences in contributing clone numbers, indicating that phenotypic differentiation does not select for few individual clones adapted to unfavorable culture conditions. Our results reveal that CRC TIC can be propagated under conditions previously thought to induce their elimination. This phenotypic plasticity allows addressing primary human CRC TIC properties in experimental settings based on adherent cell growth.

Targeting of KRAS mutant tumors by HSP90 inhibitors involves degradation of STK33.

Previous efforts to develop drugs that directly inhibit the activity of mutant KRAS, the most commonly mutated human oncogene, have not been successful. Cancer cells driven by mutant KRAS require expression of the serine/threonine kinase STK33 for their viability and proliferation, identifying STK33 as a context-dependent therapeutic target. However, specific strategies for interfering with the critical functions of STK33 are not yet available. Here, using a mass spectrometry-based screen for STK33 protein interaction partners, we report that the HSP90/CDC37 chaperone complex binds to and stabilizes STK33 in human cancer cells. Pharmacologic inhibition of HSP90, using structurally divergent small molecules currently in clinical development, induced proteasome-mediated degradation of STK33 in human cancer cells of various tissue origin in vitro and in vivo, and triggered apoptosis preferentially in KRAS mutant cells in an STK33-dependent manner. Furthermore, HSP90 inhibitor treatment impaired sphere formation and viability of primary human colon tumor-initiating cells harboring mutant KRAS. These findings provide mechanistic insight into the activity of HSP90 inhibitors in KRAS mutant cancer cells, indicate that the enhanced requirement for STK33 can be exploited to target mutant KRAS-driven tumors, and identify STK33 depletion through HSP90 inhibition as a biomarker-guided therapeutic strategy with immediate translational potential.

Distinct types of tumor-initiating cells form human colon cancer tumors and metastases.

Human colon cancer harbors a small subfraction of tumor-initiating cells (TICs) that is assumed to be a functionally homogeneous stem-cell-like population driving tumor maintenance and metastasis formation. We found unexpected cellular heterogeneity within the TIC compartment, which contains three types of TICs. Extensively self-renewing long-term TICs (LT-TICs) maintained tumor formation in serial xenotransplants. Tumor transient amplifying cells (T-TACs) with limited or no self-renewal capacity contributed to tumor formation only in primary mice. Rare delayed contributing TICs (DC-TICs) were exclusively active in secondary or tertiary mice. Bone marrow was identified as an important reservoir of LT-TICs. Metastasis formation was almost exclusively driven by self-renewing LT-TICs. Our results demonstrate that tumor initiation, self-renewal, and metastasis formation are limited to particular subpopulations of TICs in primary human colon cancer. We identify LT-TICs as a quantifiable target for therapies aimed toward eradication of self-renewing tumorigenic and metastatic colon cancer cells.