Structural and bioenergetic changes in tumour spheroids during growth
Keywords: Tumour spheroids

Multicellular tumour spheroids (TS) are an in vitro model of avascular tumours, and have been widely used to investigate tumour growth, metabolism and hypoxia. The geometry of the TS lends itself to mathematical representation, and theoretical models of TS growth and the development of hypoxia are abundant. With some notable exceptions however, these models have been developed independently of the biological data collection process and are overwhelmingly based upon data from multiple sources. Thus, whilst mathematical modeling has the potential to help explain and guide biological experiments, without reliable data it is unlikely to live up to this expectation.
In this thesis, a combination of experimental and theoretical approaches was used to characterize the relationship between proliferation, hypoxia and metabolism during the growth of TS derived from the DLD-1 human colon adenocarcinoma cell line. Experimental data were collected over the entire period of TS growth, generating a high volume of predominantly imaging data. To facilitate the extraction of quantitative information from this, a suite of image analysis software, which is readily applicable to other data sets, was developed. During growth, the DLD-1 TS maintained a macroscopic spherical geometry but at the microscale level the TS boundary was increasingly irregular, with TS disintegrating rapidly after 20 days. Immunofluorescence (IF) studies showed that hypoxia developed soon after TS initiation, followed by the characteristic onset of necrosis. Reduced proliferation was found to be concomitant with the development of hypoxia, although some cells retained proliferative capacity even under severely hypoxic conditions. Towards the end of culture, TS were primarily comprised of severely hypoxic and necrotic cells, a probable cause of disintegration.
Mathematical simulation of oxygen gradients in TS using literature-based values for the maximal rate of oxygen consumption was used to estimate the partial oxygen pressure (pO2) at which the IF marker of hypoxia was bound. Assuming a spatially-invariant rate of oxygen consumption, the model predicted that the onset of hypoxic binding occurs at pO2 levels similar to those reported in the literature, however the onset of necrosis was overestimated. Mathematical simulations predicted that oxygen consumption decreases as TSs increase in size, supporting previous observations.
The Warburg Effect, where glucose metabolism is favoured even under aerobic conditions, is a hallmark of tumours. Although development of the glycolytic phenotype during TS growth was observed in the form of an elevated activity of the lactate dehydrogenase V (LDHV) enzyme, the activity and expression of other glycolytic enzymes, such as hexokinase II (HKII), was unaltered. Whilst the spatial distribution of HKII was unrestricted throughout the TS's viable fraction, LDHV expression was elevated in regions of hypoxia, suggesting constant adaptation of tumour cells to their microenvironment.
In addition to the above findings, the data generated have been collected and analysed in the context of the requirements of theoretical modelling at each step; thus, they can be used to parameterise and inform more sophisticated models of tumour metabolism.