Hyaluronan, fluid pressure, and stromal resistance in pancreas cancer

Abstract

Pancreatic ductal adenocarcinomas (PDAs) are notoriously aggressive and resistant to treatment. They distinguish themselves further by their robust fibroinflammatory, or desmoplastic, stromal reaction and degree of hypovascularity. Recent findings have revealed multiple mechanisms of stromal complicity in disease pathogenesis and resistance. In this review, we focus on altered physicomechanics as one mechanism of what we term as 'stromal resistance' in PDA. Extremely high interstitial fluid pressures and a dense extracellular matrix combine to limit the delivery and distribution of therapeutic agents. We discuss the genesis and consequences of altered fluid dynamics in PDA and strategies to restore them.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1

The desmoplastic stroma in PDA. Both mouse (top) and human (bottom) PDA display robust deposition of ECM and activated pancreatic stellate cells. Masson’s trichrome reveals robust collagen content in PDA (blue) while a more complex Movat’s pentachrome staining highlights the presence of GAGs and mucins (blue) co-localised with collagen (turquoise/green). Histochemistry with hyaluronic acid binding protein (HABP) confirms the abundance of HA in PDA and immunohistochemistry for α-SMA identifies activated PSC, or myofibroblasts. Scale bars=50 μm.

Figure 2

Fluid and solid mechanics in distinct physiological states. (A–C) Masson’s trichrome staining of normal pancreas, engrafted pancreas cancer cells, and autochthonous PDA. (A) Normal pancreata are composed largely of three epithelial compartments, namely ductal (d), acinar (ac), and islet cells (is), as well as ample functional vasculature and a scarce ECM. In tumours engrafted from purified carcinoma cells (B), the ECM is modest and numerous patent vessels are present. In contrast, autochthonous PDA (C) is dominated by a robust desmoplasia and a largely collapsed vasculature resulting in extremely limited perfusion. (D–F) Schema for distinct states (for illustrative purposes only and not drawn to scale). (D) In normal tissue, the interstitial fluid pressure is low and dependent upon the vascular pressure and the oncotic gradient. (E) Tumour implants of isolated cancer cells or lines possess a moderate IFP that is also related to vascular pressure. (F) In autochthonous PDA, the IFP is very high and the vasculature already collapsed. While the exact mechanisms of the elevated IFP remain to be elucidated, our results allow us to formulate a number of testable hypotheses for the genesis and maintenance of these pressures and mechanics. After vascular collapse, free and HA-bound fluid is trapped in the interstitial space (initially at pressures that result from communication with the vascular space before collapse, e.g., ∼20 to 50 mm Hg plus additional swelling pressures transmitted though the fluid during and shortly after collapse). Following vascular collapse, we further hypothesise that the pressure in this now compartmentalised fluid continues to increase, contributed to by a combination of events in the solid components of the tumour. First, solid stress continues to increase and acts on adjacent fluid through ongoing ECM production, tumour cell and fibroblast proliferation, and immune cell infiltration; as these components increase the density of the tumour, fluid pressure will correspondingly rise. Second, we propose that cells, activated by tumour-expanding pressures, resist this deformation by increasing cellular contractile force to actively compact the tumour, further elevating fluid pressure. As a consequence of these events, fluid pressure in PDA is extremely high. We suggest that digestion of HA by treatment with PEGPH20 liberates bound water and also relaxes the hydrogel being actively counterposed by mechanical forces. As pressures begin to drop, expanding vessels permit mobilisation of excess fluid into the circulation. The rush of free fluid and relaxation of physical constraints may also now permit some direct leaking of fluid out of the tumour. For (D–F), ductal epithelial/PDA cells are shown in aqua, stellate cells in brown, macrophages in green, and T cells in orange. Collagen is illustrated in green and HA in yellow. Arrows indicate fluid pressures and small blue circles indicate water molecules. Scale bars, 25 μm for (A–C).