We describe a biological model for tumor initiation and progression based on heterogeneous stromal production of diffusive factors that mediate epithelial transformation. The aim was to account for a striking set of observations in which heterogeneity (but not homogeneity) of stromal fibroblastic cells was associated with tumorigenesis. Given obtained biological data for rates of epithelial transformation, the model predicted ranges for the relative fold production of factors. Given experimental measures of factor production, the model reported whether the factor was consistent with the biological model and predicted its diffusive range.
Given the ternary observations of the tissue recombination allografting experiments, it seemed unlikely that alternative hypotheses to the proposed mechanism (such as the epithelial progression does not occur in at least two steps or that these steps are not mediated by the stroma) would hold true. Certainly, the mechanism may be more complex, with added layers of interaction. Much literature supports that alterations in the stroma alone, independent of genetic events in the epithelium, can promote progression of neighboring epithelium to cancer (6
). The step-wise nature of human carcinoma progression has been highlighted by the Knudson “Two Hit Hypothesis” (38
). Paracrine-mediated cancer initiation and progression likely involves at least two distinct steps perpetuated by two or more factors produced by at least two independent cell types. Fibroblasts represent a heterogeneous cell population, some of which express FSP-1 and accordingly were knocked out for Tgfbr2 expression in the Tgfbr2fspKO
mice. At least two fibroblastic cell sub-populations could be distinguished by CD90 expression (17
) and we found CD90 expression coincided with heterogeneous Smad2 activity loss in human PCa associated fibroblasts. Similar fibroblastic heterogeneity in TGF-β responsiveness occurs in Tgfbr2fspKO
mouse prostates and is likely functionally essential for the ensuing cancer progression.
The model is valid to the extent that stromal interactions were dominated by diffusive signaling. This biologically informed computational model was independent of specific paracrine factor production. M1
could represent individual or a combination of factors that result in epithelial proliferation and invasion, respectively. The computational model provided a quantitative measure of lower bounds for dominant factor production in terms of n
-fold relative production. On the contrary, interactions are known to occur via extracellular matrix components and cell contacts. It is also possible that one step is dominantly mediated by a diffusive factor while the other is not. Observing an absence of epithelial transformation within the predicted diffusive range of the factor or the persistence of epithelial transformation after the introduction of barriers to diffusion could invalidate the model. An experiment to simultaneously test the candidacy of an M1
factor and the model would be to apply the factor exogenously with a steady concentration and measure the radius of epithelial transformation. The model is not explicitly time resolved as only cell response to steady state concentrations of signaling factors is considered, which was reasonable as steady state concentrations establish at fast time scales compared to cellular responses (17
). However, activities occurring at intermediate time scales, such as the gradual erosion of the basement membrane as a diffusion barrier in response to tumorigenic factors and epithelial effects on the stroma (for example, epithelial contribution to maintaining fibroblastic heterogeneity), could not be addressed by a steady-state model. Resolving the diffusion of paracrine factors over time to follow these intermediate activities, including other mechanisms of cellular communication, and a 3-D representation of prostatic tissue are obvious extensions for a more detailed computational model of prostatic cancer progression.
With respect to details of factor production and number, the computational model is flexible and may be modified to accommodate any set of experimental observations regarding factor production for a report of the viability of the model with these factors. Here, the model was modified to accommodate non-linear, cooperative levels of M2 production as observed in SDF-1 expression. Inputting the production levels phenomenologically, showed that SDF-1 was consistent with the model M2. However, modeling the experimentally observed SDF-1 levels as a function of stromal heterogeneity supported heterotypic stromal communication, for example through additional signaling factors M3 and M4. However, it is particularly true for heterotypic stromal interactions that this communication need not be diffusive, such as through juxtacrine signaling.
The dichotomous view of TGF-β action as a tumor-suppressor in benign tissues and tumor-promoting in initiated cancer can involve the fibroblasts. TGF-β is tumor-suppressive in WT fibroblasts since it represses the secretion of Wnt-3a (and other Stat3 induced paracrine factors) (6
). TGF-β promotes SDF-1 production by fibroblasts and the expression of its receptor, CXCR4, in epithelial cells (33
). The TGF-β non-responsive fibroblasts associated with cancerous epithelia express factors like IL-1β (M3
) to further SDF-1 production. Such TGF-β responsive heterotypic fibroblastic and stromal-epithelial signaling would support PCa onset and progression ().