Metastatic outgrowth encompasses COL-I, FN1, and POSTN up-regulation and assembly to fibrillar networks regulating cell adhesion, migration, and growth

Abstract

Although the outgrowth of micrometastases into macrometastases is the rate-limiting step in metastatic progression and the main determinant of cancer fatality, the molecular mechanisms involved have been little studied. Here, we compared the gene expression profiles of melanoma lymph node micro- and macrometastases and unexpectedly found no common up-regulation of any single growth factor/cytokine, except for the cytokine-like SPP1. Importantly, metastatic outgrowth was found to be consistently associated with activation of the transforming growth factor-beta signaling pathway (confirmed by phospho-SMAD2 staining) and concerted up-regulation of POSTN, FN1, COL-I, and VCAN genes-all inducible by transforming growth factor-beta. The encoded extracellular matrix proteins were found to together form intricate fibrillar networks around tumor cell nests in melanoma and breast cancer metastases from various organs. Functional analyses suggested that these newly synthesized protein networks regulate adhesion, migration, and growth of tumor cells, fibroblasts, and endothelial cells. POSTN acted as an anti-adhesive molecule counteracting the adhesive functions of FN1 and COL-I. Further, cellular FN and POSTN were specifically overexpressed in the newly forming/formed tumor blood vessels. Transforming growth factor-beta receptors and the metastasis-related matrix proteins, POSTN and FN1, in particular, may thus provide attractive targets for development of new therapies against disseminated melanoma, breast cancer, and possibly other tumors, by affecting key processes of metastasis: tumor/stromal cell migration, growth, and angiogenesis.

Figure 1

POSTN gene expression in different normal cells, tissues, and melanoma cells. A: POSTN expression in normal, micrometastatic, and macrometastatic LNs. POSTN and β-actin (ACTB) cDNA levels were measured by qRT-PCR in triplicate for each sample. POSTN/ACTB ×10³ was calculated as (cDNA POSTN/cDNA ACTB) ×10³. Horizontal bars represent the mean expression values. B: Expression of POSTN C-terminal splice-variants in different primary cells, cell lines, and tissues as detected by semiquantitative RT-PCR. ACTB served as a control. C: Schematic drawing of the C-terminal structures of the POSTN splice variants identified. The last nucleotide in exon 16 starts a new codon, resulting in one new triplet of nucleotides (which causes a single amino acid change, marked by asterisks) in the beginning of the next exon compared with the full-length protein. The sizes of exons are indicated above the full-length transcript.

Figure 2

Genes expressed coordinately with POSTN in metastatic LNs. A: Genes showing coordinate expression with POSTN in melanoma metastases. First, genes with a <100 mean difference and a <1.5 fold-change between micro- and macrometastatic LNs were filtered off. The remaining 2211 probe sets were analyzed by hierarchical gene clustering in nine melanoma LN micro- and 13 macrometastases. B: Genes showing coordinate expression with POSTN in breast cancer metastases. Genes with a <100 mean difference and a <1.5 fold-change between normal LNs and breast cancer LN metastases were filtered off. The remaining 4547 probe sets were analyzed by hierarchical gene clustering in 11 breast cancer metastases.

Figure 3

Immunohistochemical staining of POSTN, FN1, VCAN, and PCOL-I in melanoma and breast cancer LN metastases. A–P: POSTN, FN1, VCAN, and PCOL-I staining in control LNs (A, E, I, and M), melanoma metastases (B, C, F, G, J, K, N, and O) with (C, G, K, and O) showing the boxed area from (B, F, J, and N, respectively), and breast cancer metastases (D, H, L, and P). POSTN, VCAN, and PCOL-I are shown in red and FN1 in brown. Note the specific organization of the PCOL-I-synthesizing fibroblasts along the fibrillar structures surrounding the melanoma cell nests in (O). Scale bars = 200 μm.

Figure 4

Immunohistochemical staining of POSTN, FN1, and phosphorylated SMAD2 in melanoma LN micro- and macrometastases. A–L: POSTN, FN1, and pSMAD2 staining in melanoma micrometastases from three patients (A–C, E–G, and I–K) and in a melanoma macrometastasis (D, H, and L). FN1 is shown in brown and other proteins in red. Scale bars = 100 μm (A–C, E–G, and I–K) and 200 μm (D, H, and L).

Figure 5

Immunohistochemical staining of POSTN, cFN, COL-I, von Willebrand factor, and CD31 in melanoma and breast cancer LN metastases. A–D: POSTN (A and C) and cFN (B and D) staining in breast cancer metastases (Scale bars = 400 μm). E–N: POSTN, cFN, COL-I, von Willebrand factor, and CD31 staining in control LNs (E–I) and melanoma metastases (J–N; panels L and N are from the same block as J, K, and M but not consecutive sections) (Scale bars = 100 μm). A–N: cFN is shown in brown and other proteins in red.

Figure 6

Confocal fluorescence imaging and three-dimensional reconstructions of a melanoma metastasis section stained for FN1, POSTN, and COL-I. A and B: Triple staining of FN1, POSTN, and COL-I showing their colocalization in an abundant fibrillar network (A) and in a crossing point of the network (B). C: Double staining of FN1 and POSTN showing their colocalization in a ring-like fibrillar structure. Scale bars = 100 μm.

Figure 7

Channel-like organization of the fibrillar protein structures around melanoma cell nests and interactions of FN1, POSTN, and COL-I in vivo. A: Confocal image of a melanoma metastasis triple-stained for FN1, POSTN, and MLANA, showing organization of the fibrillar tubular structures formed by FN1 and POSTN around melanoma cell nests. B: Confocal images of the spatial organizations/interactions of FN1, POSTN, and COL-I in the fibrillar strand structures. Scale bars = 100 μm.

Figure 8

The ability of various ECM molecules to support adhesion of different types of cells. A and B: Effect of ECM protein coatings on adhesion of primary fibroblasts, HMVECs, and melanoma cells (A), and the cell lines WM793, WM239, and MBA-MD-231 (B). C: Adhesion of WM239 cells expressing control, POSTN, and FN1 shRNAs on uncoated tissue culture plates.

Figure 9

POSTN combined with FN1 or COL-I increases cell migration. A: Effect of coatings of cFN and COL-I alone and combined with full-length POSTN and POSTN fragment on migration of WM793 melanoma cells through transwell inserts. B: Effect of coatings of cFN and COL-I alone and combined with full-length POSTN on migration of HMVECs through transwell inserts.

Figure 10

FN1 is required for cell growth. A: Primary human embryonic fibroblasts (4 × 10⁴) transduced with lentiviral particles encoding control, POSTN, or FN1 shRNAs were grown for 20 days. Mean values of the cell number in two parallel cultures are shown. Variation between the two cultures was <12% in all cases. B: Inhibition of growth of WM793 cell and primary fibroblast (1:1) co-cultures by FN1 L8 antibody.