Microengineered tumor models: insights & opportunities from a physical sciences-oncology perspective

Abstract

Prevailing evidence has established the fundamental role of microenvironmental conditions in tumorigenesis. However, the ability to identify, interrupt, and translate the underlying cellular and molecular mechanisms into meaningful therapies remains limited, due in part to a lack of organotypic culture systems that accurately recapitulate tumor physiology. Integration of tissue engineering with microfabrication technologies has the potential to address this challenge and mimic tumor heterogeneity with pathological fidelity. Specifically, this approach allows recapitulating global changes of tissue-level phenomena, while also controlling microscale variability of various conditions including spatiotemporal presentation of soluble signals, biochemical and physical characteristics of the extracellular matrix, and cellular composition. Such platforms have continued to elucidate the role of the microenvironment in cancer pathogenesis and significantly improve drug discovery and screening, particularly for therapies that target tumor-enabling stromal components. This review discusses some of the landmark efforts in the field of micro-tumor engineering with a particular emphasis on deregulated tissue organization and mass transport phenomena in the tumor microenvironment.

Figure 1. Biological and physical changes inherent to the tumor microenvironment

Tumors are characterized by aberrant tissue organization (A) and mass transport (B). While normal epithelia form sheets that adhere to a layer of basement membrane, which is anchored to the underlying stroma, malignant transformation perturbs this architecture and leads to the development of a 3D tumor mass. During this process, basement membrane integrity is compromised and ECM composition, conformation, and mechanical properties are altered due in part to transformation of normal fibroblasts into cancer-associated fibroblasts (A). Excessive tumor cell proliferation results in diffusion-limited oxygen and waste transport and consequential development of hypoxia and acidosis. This induces secretion of pro-angiogenic factors including VEGF that activate neovascularization via angiogenesis and recruitment of bone marrow progenitors. These newly formed vessels, in turn, enable tumor growth and metastasis by providing vascular conduits for nutrients and extravasated circulating tumor cells, respectively (B).

Figure 2. Microfabrication techniques to recapitulate vascular networks in vitro

Cylindrical 3D networks can be incorporated into hydrogels using carbohydrate glass as sacrificial templates. Labeled human umbilical vein endothelial cells (HUVECs) seeded into these channels form perfusable vascular networks that can be induced to sprout (arrowheads), while incorporation of a second labeled cell type (e.g.10T1/2 cells) into the hydrogel bulk permits recapitulation of stromal tissue. Scale bars represent 1 mm and 200 µm (Miller 2012) (A). Similarly, lithographically patterned microfluidic devices may be used to generate functional vascular networks within remodelable hydrogels. In particular, incorporation of pericytes (indicated by α-SMA staining) into the bulk leads to the recruitment of these cells to HUVEC-lined vessels (indicated by CD31 staining) and their corresponding stabilization. Scale bar, 100 µm (Zheng 2012) (B). Microdevices composed of two parallel endothelial cell-coated channels (GFP-labeled HUVECs in top and dsRed-labeled HUVECs in bottom channel) separated by an invadable collagen matrix allow studies of vascular anastomosis in response to VEGF gradients as well as interstitial flow (dashed arrow) and axial shear flow (solid arrow). Scale bar, 100 µm (Song 2012) (C). Microvascular structures generated in microfluidic type I collagen gels using a pull-through technique can be stabilized by cyclic AMP (cAMP)-elevating agents as shown via perfusion of endothelial cell-coated vessels with fluorescently labeled BSA; *indicates focal leaks. Scale bar, 100 µm (Wong 2010) (D). Culture of stromal cells (10T1/2) and endothelial cells (HMVEC) in adjacent channels separated by a 3D collagen scaffold allows studying the effect of bi-directional paracrine signaling on invasion and sprouting behavior of these cells, respectively. 10T1/2 invaded at an enhanced rate in the presence of HMVECs. Additionally, HMVECs formed sprouts towards the blank collagen scaffold located in the left channel (red arrowheads), whereas vessels were stabilized in regions directed towards 10T1/2s (Chung 2009) (E). Images are modified and reproduced with permissions from the publishers.

Figure 3. Integration of microengineered tumor cultures with “body-on-a-chip” models

Cancer progression and therapy are controlled by systemic interactions. For example, endocrine signaling between tumors and distant tissues such as brain, bone, lungs, and liver regulate the tropism of certain cancers to particular sites (e.g. breast cancer to bone). Additionally, the efficacy of anti-cancer therapies is regulated by systemic distribution and metabolism in various tissues and organs such as adipose tissue, kidney, heart and liver. Therefore, connecting multiple organ components via vascular conduits in ‘cancer on a chip’ models has the potential to improve our understanding of metastatic homing mechanisms of tumor cells as well as drug therapy (A–B). Already, microscale cell culture analogs have been used to improve the predictability of pharmacokinetic-pharmacodynamic models in vitro (Sung 2010) (C). This platform, comprising bone, liver, and tumor compartments, was used to assess the toxicity of 5-fluorouracil in a 3D tissue context with multi-organ interactions. The combination of in vitro experimental data and mathematical PB-PK modeling may improve screening for drug development, as well as inform our understanding of cancer pathogenesis during therapy. Images are reproduced with permission from the publisher.