Palladin mediates stiffness-induced fibroblast activation in the tumor microenvironment

Abstract

Mechanical properties of the tumor microenvironment have emerged as key factors in tumor progression. It has been proposed that increased tissue stiffness can transform stromal fibroblasts into carcinoma-associated fibroblasts. However, it is unclear whether the three to five times increase in stiffness seen in tumor-adjacent stroma is sufficient for fibroblast activation. In this study we developed a three-dimensional (3D) hydrogel model with precisely tunable stiffness and show that a physiologically relevant increase in stiffness is sufficient to lead to fibroblast activation. We found that soluble factors including CC-motif chemokine ligand (CCL) chemokines and fibronectin are necessary for this activation, and the combination of C-C chemokine receptor type 4 (CCR4) chemokine receptors and β1 and β3 integrins are necessary to transduce these chemomechanical signals. We then show that these chemomechanical signals lead to the gene expression changes associated with fibroblast activation via a network of intracellular signaling pathways that include focal adhesion kinase (FAK) and phosphoinositide 3-kinase (PI3K). Finally, we identify the actin-associated protein palladin as a key node in these signaling pathways that result in fibroblast activation.

Figure 1

Collagen-I hydrogel stiffness can be tuned with PEG-diNHS cross-linking. (A) Collagen-I gels form by physical cross-linking between fibers. Hydrogel stiffness can be increased by introducing more rigid cross-links between collagen-I and PEG-diNHS. (B) Stiffness of collagen-I (COL) and collagen-I PEG-diNHS hydrogels (PEG) without cells was measured by rheometry. Young’s modulus was 209.6 Pa ± 4.8 for soft COL gels (n = 6) and 770.5 Pa ± 22.3 for stiff PEG gels (n = 12), corresponding to values reported for normal and tumor adjacent stroma (1,2). Diamonds in the box and whisker plots represent the mean, and the midline is the 50th percentile. The top and bottom bar represent the 90th and 10th percentile, respectively, and the top and bottom boxes represent the 75th and 25th percentile, respectively. (C) Stiffness of hydrogels with encapsulated cells. The elastic modulus of COL gels with cells was 289.2 Pa ± 12.7 (n = 6) and of PEG gels with cells was 769.0 Pa ± 35.7 (n = 6). (D) Stiffness of hydrogels over time. Hydrogels with encapsulated cells were cultured for 3 days. Elastic modulus of COL gels at day 3 was 262.0 Pa ± 20.0 (n = 9) and of PEG gels at day 3 was 716.8 Pa ± 44.6 (n = 9). All Young’s modulus values were obtained by converting measured bulk modulus with the formula E = 2G(1 + v), with v as 0.1 (43). Asterisk indicates significant difference. Student’s t-test p values < 0.05. Error bars show standard error of the mean. To see this figure in color, go online.

Figure 2

Hydrogel structure and compaction. (A) Representative second harmonic generation images of collagen-I organization in soft (COL) or stiff (PEG) hydrogels. Scale bar: 50 μm. (B) Quantification of skewness from second-harmonic generation images, normalized with collagen day 0 and 50% percentile at 0. Forty-two images from each gel were quantified, with six gels per condition. Skewness decreases as fibril organization increases (48). Box and whisker plots as in Fig. 1B. Asterisk indicates significant difference from day 0. Student’s t-test p values < 0.0001. (C) Hydrogel compaction over 3 days, CM (COL n = 41; PEG n = 39). Asterisk indicates significant difference from day 0. Student’s t-test p values < 0.005. Error bars show standard error of the mean.

Figure 3

ECM stiffness representative of the tumor microenvironment can lead to fibroblast activation. (A) Representative images of fibroblasts in soft (collagen-I) or stiff (collagen-I PEG-diNHS) hydrogels (stained with phalloidin to label f-actin). Scale bar: 20 μm. (B) Quantification of the number of projections from the cell body and cell aspect ratio (COL n = 12; PEG n = 11). (C) Semi-quantitative immunocytochemistry of αSMA, palladin, intracellular FN-EDA, TGFβ, and extracellular FN-EDA. All values were normalized to cells in RSM COL (n = 3). Asterisk indicates significant difference between soft and stiff gels. (D) qRT-PCR of markers of fibroblast activation. All levels were normalized to expression levels in cells in collagen-I hydrogels in complete media (n = 3). Asterisk indicates significant difference between cells in soft COL gels and in stiff PEG gels, Student’s t-test values < 0.05. Error bars show standard error of the mean. To see this figure in color, go online.

Figure 4

Depletion of soluble growth factors blocks fibroblast activation. (A) Representative images of fibroblasts grown in soft (collagen-I) or stiff (collagen-I PEG-diNHS) hydrogels with reduced serum media (RSM) (stained with phalloidin to label f-actin). Scale bar: 20 μm. (B) Quantification of the number of projections from the cell body and cell aspect ratio (COL n = 8; PEG n = 7). (C) Hydrogel compaction in reduced serum media (COL n = 69; PEG n = 61). Asterisk indicates significant difference. Student’s t-test p values < 0.005. (D) Semi-quantitative immunocytochemistry for αSMA, palladin, FN-EDA, TGFβ, and extracellular FN-EDA comparing cells grown in CM with those grown in RSM in soft (COL) or stiff (PEG) gels (n = 3). (E) qRT-PCR for markers of fibroblast activation comparing cells grown in CM with those grown in RSM (n = 3). Asterisk indicates significant difference between COL and PEG conditions. Hash represents significant difference between RSM and CM conditions. Student’s t-test values < 0.05. Error bars show standard error of the mean. To see this figure in color, go online.

Figure 5

Both serum CCL growth factors and fibronectin are necessary for FAK clustering and fibroblast activation. (A) Cells grown in complete media (10% serum) show punctate clusters of FAK, whereas FAK labeling is diffuse in cells grown in reduced serum media (1% serum). (B) Inhibition of CCR4 receptor with a CCR4 antagonist drug in CM blocks FAK clustering. Reintroduction of a cocktail of recombinant CCL growth factors does not rescue FAK clustering in RSM conditions. (C) Fibronectin addition to RSM does not rescue FAK clustering, but addition of fibronectin and CCLs together in RSM condition does recover punctate FAK staining. Scale bar: 20 μm. (D) qRT-PCR of markers of fibroblast activation in cells treated with CCR4 receptor antagonist in CM (n = 3). Expression level of each marker is seen in RSM conditions shown with diamond markers for comparison. (E) qRT-PCR of markers of fibroblast activation in cells in RSM hydrogels supplemented with recombinant CCL growth factors (n = 3). (F) qRT-PCR of markers of fibroblast activation in cells in RSM hydrogels supplemented with fibronectin (n = 3). Asterisk indicates significant difference from control. Student’s t-test values < 0.05. All qPCR data normalized to COL condition. (G) Mean standard deviation of FAK staining intensity in COL and PEG hydrogels (n = 5). Images with bright and dark spots (clusters and unlabeled background) have higher standard deviation than those with more uniform labeling. Asterisk indicates significant difference from CM. ANOVA p-values < 0.05. Error bars are standard error of the mean. (H) qRT-PCR of markers of fibroblast activation in cells grown in RSM supplemented with both human serum fibronectin and CCL cocktail (n = 3). Asterisk indicates significant difference from RSM control. Student’s t-test values < 0.05. Error bars show standard error of the mean. To see this figure in color, go online.

Figure 6

β1 and β3 integrins, FAK, and PI3K all play roles in control of fibroblast activation in stiff hydrogels. (A) β1 integrin activity was inhibited with a blocking antibody and the expression of markers of fibroblast activation was measured by qRT-PCR (n = 3). (B) β3 integrin activity was inhibited with a blocking antibody and the expression of markers of fibroblast activation was measured by qRT-PCR (n = 3). (C) Focal adhesion kinase (FAK) activity was blocked with FAK Inhibitor 14 drug and the expression of markers of fibroblast activation was measured by qRT-PCR (n = 3). (D) Phosphoinositide 3-kinase (PI3K) activity was inhibited with LY 294002 and the expression of markers of fibroblast activation was measured by qRT-PCR (n = 3). (E) Cells were treated with a combination of FAK and PI3K inhibitors and the expression of markers of fibroblast activation was measured by qRT-PCR (n = 3). (F) Cells were treated with a combination of β1 integrin blocking antibody and PI3K inhibitor and the expression of markers of fibroblast activation was measured by qRT-PCR (n = 3). Asterisk indicates significantly different increase. Hash indicates significantly different decrease. All data is normalized to COL control. Student’s t-test values < 0.05. All error bars signify standard error of the mean.

Figure 7

Palladin localization is controlled by integrins. (A) Palladin localized to the nucleus when β1 integrin was blocked and localized to the cytoplasm when β3 integrin was blocked (n = 3). (B) Quantification of the change in localization relative to the amount present in controls. Asterisk indicates significant difference from control. Student’s t-test values < 0.005. All error bars signify standard error of the mean. (C) Representative images of palladin nuclear localization.

Figure 8

Model of palladin, αSMA and FAPα regulation. Palladin is regulated through integrin interactions in focal adhesions as well as growth factor and nonreceptor tyrosine kinase signaling. Palladin has both f-actin and non-f-actin interactions and plays a role in the regulation of αSMA and FAPα gene expression. To see this figure in color, go online.