Mounting Pressure in the Microenvironment: Fluids, Solids, and Cells in Pancreatic Ductal Adenocarcinoma

Abstract

The microenvironment influences the pathogenesis of solid tumors and plays an outsized role in some. Our understanding of the stromal response to cancers, particularly pancreatic ductal adenocarcinoma, has evolved from that of host defense to tumor offense. We know that most, although not all, of the factors and processes in the microenvironment support tumor epithelial cells. This reappraisal of the roles of stromal elements has also revealed potential vulnerabilities and therapeutic opportunities to exploit. The high concentration in the stroma of the glycosaminoglycan hyaluronan, together with the large gel-fluid phase and pressures it generates, were recently identified as primary sources of treatment resistance in pancreas cancer. Whereas the relatively minor role of free interstitial fluid in the fluid mechanics and perfusion of tumors has been long appreciated, the less mobile, gel-fluid phase has been largely ignored for historical and technical reasons. The inability of classic methods of fluid pressure measurement to capture the gel-fluid phase, together with a dependence on xenograft and allograft systems that inaccurately model tumor vascular biology, has led to an undue emphasis on the role of free fluid in impeding perfusion and drug delivery and an almost complete oversight of the predominant role of the gel-fluid phase. We propose that a hyaluronan-rich, relatively immobile gel-fluid phase induces vascular collapse and hypoperfusion as a primary mechanism of treatment resistance in pancreas cancers. Similar properties may be operant in other solid tumors as well, so revisiting and characterizing fluid mechanics with modern techniques in other autochthonous cancers may be warranted.

Keywords: Drug Delivery; Gel Fluid; Hyaluronan; Interstitial Pressure; Pancreatic Ductal Adenocarcinoma.

Figure 1

IFP can be measured by a number of techniques. (A) Perforated capsules of 20 to 30 mm in diameter and varying shapes can be implanted into tissues and allowed to equilibrate over 2 to 3 weeks. The capsules fill with interstitial fluid and proteins, and the pressure can be measured by inserting a needle into the inner cavity. (B) The WN technique consists of a hypodermic needle (0.6 mm, outer diameter) with a ~3-mm-long side hole situated about 5 mm from the needle tip. Nylon fibers are pulled through the needle and connected through fluid-filled tubing to a pressure transducer. (C) The PC is the smallest probe, with a 0.33-mm outer diameter. It is optimized for measurements in the pressure range from −50 the device is to +300 mm Hg and is the only device capable of measuring pressures associated with the immobile (gel) fluid phase.

Figure 2

Xenograft and allograft models do not faithfully recapitulate the epithelial or stromal properties of autochthonous tumors. Histologic and histochemical analyses of tumors from KPC mice and human PDA (autochthonous) compared with allograft and xenograft tumors from mice and human PDA cell lines, respectively (transplantable). Important differences can be seen in basic histological features (by H&E stain), total collagen content (trichrome), HA content, and vascular patency (CD31) showing collapsed vessels characteristic of autochthonous PDA (arrows) and patent vessels found in transplantable tumors (arrowheads). Scale bar = 25 µm.

Figure 3

Determinants of interstitial pressure in pancreas cancer. Factors that contribute to interstitial pressure in PDA range from pure fluid to pure solid. Most pressure derives from a viscoelastic, HA-dominated, immobile gel-fluid phase that cannot be measured with conventional devices for measuring fluid pressure.

Figure 4

Transmission of fluid, solid, and gel fluid pressures in tissues. (A) The classic depiction of tissue pressures (adapted from Guyton et al with permission) considers 2 broad categories: fluid and solid. Gel fluid is categorized as a solid. (B) In an alternative model proposed here, pressure sources are instead described according to the manner in which they transmit force (ie, vectorially or hydraulically).

Figure 5

Contributions of fluid and solid stress in normal tissue and PDA. (A) In normal tissues, pressures exerted on blood vessels are generated from the combination of IFP and solid stress from the ECM. Fluid pressures from free fluid and the HA-bound, gel-fluid phase are distinct from the pressures generated by solid elements such as collagen and fibroblasts in the stroma. Most interstitial fluid found in normal tissues is bound to HA and other GAGs. (B) The interstitial phase is greatly expanded in the transition from normal epithelium to preinvasive and invasive PDA. This transition is characterized by increased deposition of HA and other ECM components, culminating in an invasive disease with extremely high IFP and significant areas of vascular collapse. (C) A competing framework instead models HA as a solid element with IFP being generated solely by free fluid.

Figure 6

Fluid pressures in the 2-phase model of the tissue interstitium. At steady state, the sum of the hydrostatic and osmotic fluid pressures in the intravascular (c), interstitial free fluid (i), and interstitial gel fluid (g) compartments are in equilibrium.