Interplay of the transcription factor MRTF-A and matrix stiffness controls mammary acinar structure and protrusion formation

Background: Ongoing differentiation processes characterize the mammary gland during sexual development and reproduction. In contrast, defective remodelling is assumed to be causal for breast tumorigenesis. We have shown recently that the myocardin-related transcription factor A (MRTF-A) is essential for forming regular hollow acinar structures. Moreover, MRTF-A activity is known to depend on the biochemical and physical properties of the surrounding extracellular matrix. In this study we analysed the mutual interaction of different matrix stiffnesses and MRTF-A activities on formation and maintenance of mammary acini.

Methods: Human MCF10A acini and primary mature organoids isolated from murine mammary glands were cultivated in 3D on soft and stiff matrices (200-4000 Pa) in conjunction with the Rho/MRTF/SRF pathway inhibitor CCG-203971 and genetic activation of MRTF-A.

Results: Three-dimensional growth on stiff collagen matrices (> 3000 Pa) was accompanied by increased MRTF-A activity and formation of invasive protrusions in acini cultures of human mammary MCF10A cells. Differential coating and synthetic hydrogels indicated that protrusion formation was attributable to stiffness but not the biochemical constitution of the matrix. Stiffness-induced protrusion formation was also observed in preformed acini isolated from murine mammary glands. Acinar outgrowth in both the MCF10A acini and the primary organoids was partially reverted by treatment with the Rho/MRTF/SRF pathway inhibitor CCG-203971. However, genetic activation of MRTF-A in the mature primary acini also reduced protrusion formation on stiff matrices, whilst it strongly promoted luminal filling matrix-independently.

Conclusion: Our results suggest an intricate crosstalk between matrix stiffness and MRTF-A, whose activity is required for protrusion formation and sufficient for luminal filling of mammary acini.

 

Comments:

In this study, the researchers investigated the interaction between matrix stiffness and the myocardin-related transcription factor A (MRTF-A) in the formation and maintenance of mammary acini, which are hollow structures in the mammary gland. They used human MCF10A cells and primary organoids isolated from murine mammary glands for their experiments.

The researchers cultivated the acini and organoids in three-dimensional (3D) culture on matrices with varying stiffness levels, ranging from soft to stiff (200-4000 Pa). They also manipulated the activity of MRTF-A using a pathway inhibitor (CCG-203971) and genetic activation techniques.

The findings of the study demonstrated that when the acini were grown on stiff collagen matrices with a stiffness exceeding 3000 Pa, there was an increase in MRTF-A activity and the formation of invasive protrusions. This protrusion formation was found to be influenced by matrix stiffness rather than the biochemical composition of the matrix itself. The researchers further confirmed this observation by using different coating materials and synthetic hydrogels.

The stiffness-induced protrusion formation was also observed in preformed acini isolated from murine mammary glands, suggesting that the phenomenon is not limited to the MCF10A cell line. Treatment with the Rho/MRTF/SRF pathway inhibitor partially reversed the acinar outgrowth, indicating the involvement of this signaling pathway in the process. However, genetic activation of MRTF-A in the mature primary acini reduced protrusion formation on stiff matrices while promoting luminal filling, regardless of matrix stiffness.

In conclusion, the study suggests that there is a complex interplay between matrix stiffness and MRTF-A activity. MRTF-A activity is necessary for protrusion formation in mammary acini, and it is sufficient to promote luminal filling of the acini. These findings contribute to our understanding of the role of matrix properties and transcription factors in mammary gland development and potentially in breast tumorigenesis.