In the present study, we showed that p53 deficiency in colon cancer cells suppressed autophagy and promoted subsequent activation of fibroblasts. Suppression of the autophagy-related protein, ATG2B, by exosomes derived from TP53-deficient colon cancer cells suppressed autophagy in fibroblasts. Furthermore, we also identified specific miRNAs in exosomes derived from TP53-deficient cancer cells that can suppress expression of ATG2B in fibroblasts.
The role of autophagy in tumor progression is still controversial21–25, particularly in the tumor stromal cells. It has been reported that autophagy in tumor stroma promotes tumor growth by supplying nutrients, including amino acids11,26. Another report also described the pro-tumorigenic effect of autophagy in CAFs by protecting against oxidative stress10; therefore, autophagy in CAFs promotes tumor growth. However, some studies have reported that the suppression of autophagy in fibroblasts can induce their activation27. For instance, myofibroblast differentiation in the pulmonary fibroblasts was promoted by suppression of autophagy and contributed to the pathogenesis of chronic obstructive pulmonary disease28. The present study showed that suppression of autophagy in colon fibroblasts co-existing with TP53-deficient colon cancer cells can induce activation of fibroblasts. Autophagy in tumor stromal fibroblasts may play diverse roles in different phases of tumor progression, such as in intratumoral nutrient supply or myofibroblast differentiation during a desmoplastic reaction. We believe that the suppression of autophagy in fibroblasts induced by cancer cells may be involved in their activation; fibroblast activation plays a pivotal role in the transition of normal tissue fibroblasts into CAFs and formation of tumor stroma. CAFs have higher expression of activation markers and tumor growth factors than normal fibroblasts29; our study showed that normal fibroblasts with suppressed autophagy had acquired properties similar to that of CAFs. This is the first report validating the association between the suppression of autophagy in colon fibroblasts and activation and transition of normal tissue fibroblasts into CAFs.
Normal tissue fibroblasts can acquire CAF-like phenotypes in response to soluble secretions from cancer cells, including exosomes18,30,31. Exosomes communicate information on myriad proteins, mRNAs and miRNAs between secreting and recipient cells, and hence, have an impact on the expression of various genes in recipient cells that governs the conversion of normal fibroblasts into CAFs18,31. Many target genes and related factors, such as TP53, sphingosine kinase 1, E-cadherin, TGFβ1, and senescence are involved in the activation of fibroblasts induced by cancer cell-derived exosomes18,31−35. CAFs have a heterogeneous cell population with varying origins, phenotypes, and functions in the tumor stroma29. Different processes are associated with the transition of normal tissue fibroblasts into CAFs; herein, we focused on the significance of autophagy in activated fibroblasts and identified a specific miRNA in exosomes that can suppress autophagy and induce fibroblast activation with CAF-like phenotypes. Although the detailed mechanisms of miRNA sorting into exosomes have not been elucidated, several reports have indicated that TP53 expression in donor cancer cells can modify their inherent exosomal miRNA profiles and affect gene expression in surrounding cells18,36. After analysis of the miRNAs sorted into exosomes in donor cancer cells with p53 deficiency, we discovered that miR-4534 suppressed ATG2B and activated fibroblasts via suppression of autophagy.
A previous report indicated that miR-4534 was over-expressed in prostate cancer tissues and showed oncogenic effects by downregulating the tumor suppressor gene, PTEN37; however, most of its functions remain unclear. ATG2B is an autophagy-related protein that can regulate the transfer of lipids for autophagosome formation38,39. The present report demonstrates a novel and important function of miR-4534 as a regulator of autophagy via suppression of ATG2B in the tumor stroma during colorectal tumorigenesis. To reveal the clinical usefulness of miR-4534, a better understanding of its roles and further research are required.
In conclusion, loss of p53 function in colon cancer cells may promote the activation of surrounding fibroblasts through the suppression of autophagy. miR-4534 in the exosomes of TP53-deficient cancer cells activated fibroblasts by suppressing ATG2B. Therefore, we propose that the suppression of autophagy by specific miRNAs in exosomes can play an important role in the transition of normal tissue fibroblasts into CAFs and subsequent stroma-mediated tumor growth.