Abstract B24: De novo and acquired resistance to first-line standard therapy in colorectal cancer: from cell lines to metastatic tumors

Introduction: Personalized medicine (PM) is a concept that has raised high expectations amongst scientists, clinicians, and patients. An emerging approach is to examine tumor biopsy material for genomic changes that are known targets of currently available therapeutic agents, with the assumption that a clinical benefit will be observed if the target is inhibited. While striking anecdotal reports are predictable from this approach, the clinical impact of these agents is limited by the inevitable development of therapeutic resistance. Our focus is on the design of parallel research programs using both in vitro and in vivo strategies, in an effort to delay or inhibit resistance. We present here preliminary data for a signature of resistance to standard first-line treatment - fluorouracil, folinic acid, oxaliplatin and bevacizumab (FOLFOX/B) using cell line models of resistance to this regimen. In parallel, we are conducting a prospective study to identify biomarkers of clinical resistance to first-line therapy in patients with metastatic colorectal cancer (CRC) (NCT00984048).

Methods: Ten established CRC cell lines were treated with FOLFOX/B and categorized as resistant or sensitive based on IC50 values. In parallel, patients who consented to an initial biopsy and one at disease progression following an initial response were identified as intrinsically resistant or as having acquired resistance during treatment. CRC cell lines that were initially sensitive were rendered resistant to mimic the acquired resistance in patients, by serial passages with gradual increases in concentration of the combination regimen. We compared microarray data from three sensitive and three resistant cell lines.

Results: We found a different expression pattern from microarray data comparing sensitive and resistant cell lines, thereby indicating a potential signature of resistance to FOLFOX/B. Interestingly, we found that the Src family kinase Lyn was overexpressed in resistant cells lines. Treating cells with non-cytotoxic concentration of dasatinib, a dual Src family kinase and Abl inhibitor, sensitized both the parental sensitive cells and the cells with acquired resistance to FOLFOX/B, thereby suggesting that combination treatment with dasatinib may be effective in delaying or inhibiting resistance. We have thus far collected needle core biopsies from liver metastases from forty patients who agreed to partake in this multi-center trial. Eligible patients have confirmed metastatic CRC, measurable disease, and consent to three needle-core biopsies (NCBs) of a non-resectable liver metastasis before treatment and at resistance, as well as serial blood collection throughout the study. Using standard operating procedures developed for this trial, we were able to both preserve morphology and obtain high-quality genomic material from biopsy tissue. We will determine if the resistance signature and overexpression of Lyn observed in the resistant CRC cell lines are similarly demonstrated in patients that were intrinsically resistant to FOLFOX/B.

Conclusions: We have designed parallel in vitro and in vivo experiments to study resistance to standard first-line treatment for mCRC. These studies provide insight on metastatic signatures of resistance and suggest combination therapies to delay or inhibit therapeutic resistance in patients.

A new paradigm for the regulation of the mammalian ribosomal RNA genes

Ribosome assembly occurs co-transcriptionally on the rRNA genes. This process requires the co-ordinated expression and assembly of many hundreds of proteins and is finely tuned to cell and organism growth. Co-ordinate regulation of the rRNA genes and the ribosomal protein genes is therefore essential for high-fidelity ribosome assembly. Recent work shows that rRNA gene transcription is regulated at the level of elongation via the mitogen-activated protein kinase pathway. We argue that this may provide an explanation for the high fidelity of ribosome assembly.

A housekeeper with power of attorney: the rRNA genes in ribosome biogenesis

Ribosome biogenesis centres both physically and functionally on the activity of the ribosomal RNA (rRNA) genes. Ribosome assembly occurs co-transcriptionally on these genes, requires the coordinated expression and assembly of many hundreds of proteins and is finely tuned to cell and organism growth. This review presents contemporary understanding of the mode and the means of rRNA gene transcription and how growth factors, oncogenes and tumour suppressors regulate this transcription. It is argued that transcription elongation is a key mechanism regulating rRNA gene transcription. This unorthodox view provides a logical framework to explain the co-transcriptional phase of ribosome biogenesis.

An immediate response of ribosomal transcription to growth factor stimulation in mammals is mediated by ERK phosphorylation of UBF

Ribosomal transcription in mammals is regulated in response to growth, differentiation, disease, and aging, but the mechanisms of this regulation have remained unresolved. We show that epidermal growth factor induces immediate, ERK1/2-dependent activation of endogenous ribosomal transcription, while inactivation of ERK1/2 causes an equally immediate reversion to the basal transcription level. ERK1/2 was found to phosphorylate the architectural transcription factor UBF at amino acids 117 and 201 within HMG boxes 1 and 2, preventing their interaction with DNA. Mutation of these sites inhibited transcription activation and abrogated the transcriptional response to ERK1/2. Thus, growth factor regulation of ribosomal transcription likely acts by a cyclic modulation of DNA architecture. The data suggest a central role for ribosome biogenesis in growth regulation.