Meflin-Positive Cancer-Associated Fibroblasts Inhibit Pancreatic Carcinogenesis

Abstract

Cancer-associated fibroblasts (CAF) constitute a major component of the tumor microenvironment. Recent observations in genetically engineered mouse models and clinical studies have suggested that there may exist at least two functionally different populations of CAFs, that is, cancer-promoting CAFs (pCAF) and cancer-restraining CAFs (rCAF). Although various pCAF markers have been identified, the identity of rCAFs remains unknown because of the lack of rCAFspecific marker(s). In this study, we found that Meflin, a glycosylphosphatidylinositol-anchored protein that is a marker of mesenchymal stromal/stem cells and maintains their undifferentiated state, is expressed by pancreatic stellate cells that are a source of CAFs in pancreatic ductal adenocarcinoma (PDAC). In situ hybridization analysis of 71 human PDAC tissues revealed that the infiltration of Meflin-positive CAFs correlated with favorable patient outcome. Consistent herewith, Meflin deficiency led to significant tumor progression with poorly differentiated histology in a PDAC mouse model. Similarly, genetic ablation of Meflin-positive CAFs resulted in poor differentiation of tumors in a syngeneic transplantation model. Conversely, delivery of a Meflin-expressing lentivirus into the tumor stroma or overexpression of Meflin in CAFs suppressed the growth of xenograft tumors. Lineage tracing revealed that Meflin-positive cells gave rise to a-smooth muscle actin-positive CAFs that are positive or negative for Meflin, suggesting a mechanism for generating CAF heterogeneity. Meflin deficiency or low expression resulted in straightened stromal collagen fibers, which represent a signature for aggressive tumors, in mouse or human PDAC tissues, respectively. Together, the data suggest that Meflin is a marker of rCAFs that suppress PDAC progression. Significance: Meflin marks and functionally contributes to a subset of cancer-associated fibroblasts that exert antitumoral effects.

Cancer‐associated fibroblasts that restrain cancer progression: Hypotheses and perspectives

“…Furthermore, these CAF were found to also be positive for Gli1, a transcription factor critical for Shh signaling, consistent with previous findings that describe the cancer‐promoting effect of Shh deletion in cancer cells . We have also shown that Meflin is specifically expressed in PSC within the normal pancreas . Hence, it is feasible to hypothesize that Meflin + PSC proliferate to become rCAF during tumorigenesis of PDAC (Figure 2A).…”

“…As mentioned above, Meflin + CAF weakly express α‐SMA, suggesting that in the previous study, in which all proliferating α‐SMA + CAF were genetically depleted, a portion of Meflin + CAF were also eradicated (Figure 2A). Together, the data, including data not described here, suggest that Meflin + α‐SMA low/− CAF represent rCAF in PDAC. Furthermore, these CAF were found to also be positive for Gli1, a transcription factor critical for Shh signaling, consistent with previous findings that describe the cancer‐promoting effect of Shh deletion in cancer cells .…”

“…Together, the data, including data not described here, suggest that Meflin + α‐SMA low/− CAF represent rCAF in PDAC. Furthermore, these CAF were found to also be positive for Gli1, a transcription factor critical for Shh signaling, consistent with previous findings that describe the cancer‐promoting effect of Shh deletion in cancer cells . We have also shown that Meflin is specifically expressed in PSC within the normal pancreas .…”

“…A recent study by our group revealed that a CAF subset expressing Meflin, a glycosylphosphatidylinositol‐anchored protein, acts to suppress tumor progression in a PDAC mouse model and human PDAC . Examination of α‐SMA and Meflin expression by in‐situ hybridization (ISH) revealed that both genes are seemingly mutually exclusive (Figure 1B).…”

“… However, quantification of the ISH signals and single‐cell transcriptome data analysis indicated that Meflin + CAF were weakly positive for α‐SMA, while α‐SMA + CAF were weakly positive for Meflin (Figure 2A). Interestingly, the notion that Meflin + CAF function to inhibit PDAC progression has been supported by ISH findings in human PDAC tissue sections and comparison with patient outcomes, together with results from a study that knocked out the immunoglobulin superfamily containing leucine‐rich repeat ( Islr ) gene encoding Meflin in mice causing genetic depletion of Meflin + CAF, and overexpression of Meflin in CAF in xenografted tumors in mice . As mentioned above, Meflin + CAF weakly express α‐SMA, suggesting that in the previous study, in which all proliferating α‐SMA + CAF were genetically depleted, a portion of Meflin + CAF were also eradicated (Figure 2A).…”