Share, but unequally: A plausible mechanism for emergence and maintenance of intratumor heterogeneity

TL;DR

This paper presents a quantitative evolutionary model explaining how intratumor heterogeneity (ITH) emerges and persists through unequal sharing of growth factors, with implications for understanding tumor growth and developing therapies.

Contribution

It introduces a novel model linking ITH to unequal allocation of public goods and validates it with experimental data on pancreatic cancer and glioblastoma.

Findings

ITH requires unequal sharing of growth factors.

Surplus exogenous growth factors promote competition.

ITH persists in a narrow resource range.

Abstract

Intratumor heterogeneity (ITH), referring to coexistence of different cell subpopulations in a single tumor, has been a major puzzle in cancer research for almost half a century. The lack of understanding of the underlying mechanism of ITH hinders progress in developing effective therapies for cancers. Based on the findings in a recent quantitative experiment on pancreatic cancer, we developed a general evolutionary model for one type of cancer, accounting for interactions between different cell populations through paracrine or juxtacrine factors. We show that the emergence of a stable heterogeneous state in a tumor requires an unequal allocation of paracrine growth factors ("public goods") between cells that produce them and those that merely consume them. Our model provides a quantitative explanation of recent {\it in vitro} experimental studies in pancreatic cancer in which insulin growth factor (IGF-II) plays the role of public goods. The calculated phase diagrams as a function of exogenous resources and fraction of growth factor producing cells show ITH persists only in a narrow range of concentration of exogenous IGF-II. Remarkably, maintenance of ITH requires cooperation among tumor cell subpopulations in harsh conditions, specified by lack of exogenous IGF-II, whereas surplus exogenous IGF-II elicits competition. Our theory also quantitatively accounts for measured {\it in vivo} tumor growth in glioblastoma multiforme (GBM). The predictions for GBM tumor growth as a function of the fraction of tumor cells are amenable to experimental tests. The mechanism for ITH also provides hints for devising efficacious therapies.