Heat shock protein 72 supports extracellular matrix production in metastatic mammary tumors
This study identified tumorigenic processes most dependent on murine HSP72 in the MMTV-PyMT mammary tumor model, which give rise to spontaneous mammary tumors that exhibit HSP72-dependent metastasis to the lung. RNA-seq expression profiling of Hspa1a/Hspa1b (Hsp72) WT and Hsp72 -/- primary mammary tumors discovered significantly lower expression of genes encoding components of the extracellular matrix (ECM) in Hsp72 knockout mammary tumors compared to WT controls. In vitro studies found that genetic or chemical inhibition of HSP72 activity in cultured collagen-expressing human or murine cells also reduces mRNA and protein levels of COL1A1 and several other ECM-encoding genes. In search of a possible mechanistic basis for this relationship, we found HSP72 to support the activation of the TGF-ß – SMAD3 signaling pathway and evidence of SMAD3 and HSP72 co-precipitation, suggesting potential complex formation. Human COL1A1 mRNA expression was found to have prognostic value for HER2+ breast tumors over other breast cancer subtypes, suggesting a possible human disease context where targeting HSP72 may have a therapeutic rationale. Analysis of human HER2+ breast tumor gene expression data using a gene set comprising ECM and protein folding-related genes as an input to the statistical learning algorithm, Galgo, found a subset of these genes that can collectively stratify patients by relapse-free survival, further suggesting a potential interplay between ECM and protein-folding genes may contribute to tumor progression. Overall design: To investigate tumorigenic processes dependent on murine HSP72, we performed expression profiling on spontaneous murine mammary tumors of HSP72 WT and HSP72 knockout mice expressing the PyMT oncogene under the control of the MMTV promoter.  Differential expression analysis was performed to identify mRNAs differentially expressed between HSP72 WT and HSP72 knockout mammary tumors.