Human Breast Cancer Xenograft Model Implicates Peroxisome Proliferator–activated Receptor Signaling as Driver of Cancer-induced Muscle Fatigue

Abstract

Purpose: This study tested the hypothesis that a patientderived orthotopic xenograft (PDOX) model would recapitulate the common clinical phenomenon of breast cancer-induced skeletal muscle (SkM) fatigue in the absence of muscle wasting. This study additionally sought to identify drivers of this condition to facilitate the development of therapeutic agents for patients with breast cancer experiencing muscle fatigue. Experimental Design: Eight female BC-PDOX-bearing mice were produced via transplantation of tumor tissue from 8 female patients with breast cancer. Individual hind limb muscles from BC-PDOX mice were isolated at euthanasia for RNA-sequencing, gene and protein analyses, and an ex vivo muscle contraction protocol to quantify tumor-induced aberrations in SkM function. Differentially expressed genes (DEG) in the BC-PDOX mice relative to control mice were identified using DESeq2, and multiple bioinformatics platforms were employed to contextualize the DEGs. Results: We found that SkM from BC-PDOX-bearing mice showed greater fatigability than control mice, despite no differences in absolute muscle mass. PPAR, mTOR, IL6, IL1, and several other signaling pathways were implicated in the transcriptional changes observed in the BC-PDOX SkM. Moreover, 3 independent in silico analyses identified PPAR signaling as highly dysregulated in the SkM of both BC-PDOX-bearing mice and human patients with early-stage nonmetastatic breast cancer. Conclusions: Collectively, these data demonstrate that the BC-PDOX model recapitulates the expected breast cancer-induced SkM fatigue and further identify aberrant PPAR signaling as an integral factor in the pathology of this condition.

Skeletal muscle reprogramming by breast cancer regardless of treatment history or tumor molecular subtype

“…This indicates that the ERPR, TP, and TN groups share a greater number of DEGs in skeletal muscle than one would expect if the DEGs were independent of subtype and in contrast, muscle from HER2 patients does not exhibit the same similarity to the other subtypes in terms of shared DEGs ( Figure 3C). Collectively, these data demonstrate that transcriptional responses in skeletal muscle of patients with ERPR, TP and TN tumors are highly similar, in support of previous data from our laboratory 14,15 . Furthermore, the transcriptional responses in muscles from patients with HER2/neu-overexpressing tumors partially overlap with the other subtypes, but exhibit a significant contrast to the other 3 subtypes, suggesting that this tumor type is associated with a unique transcriptional adaptation within skeletal muscle.…”

“…These proteins have demonstrated roles in whole-body energy regulation, are critical regulators of mitochondrial function in multiple tissues, and are targets of multiple FDA-approved agents in the treatment of type 2 diabetes and hyperlipidemia. Among the three PPAR isoforms, we identified PPARG as a key regulator in BC-induced muscle dysfunction observed in PDOX mice 15 . PPARG is a ligand-activated nuclear receptor that, upon activation by a variety of endogenous and synthetic lipids, forms complexes with retinoid X receptor (RXR) and cofactors such as the peroxisome proliferator-activated receptor-γ coactivator 1α (PGC1α), and stimulates transcription of downstream genes.…”

“…Our laboratory's interest in the PPAR family of proteins arose from our previous RNAsequencing analysis of muscle from BC patients and PDOX-bearing mice, which our group found to recapitulate the clinical phenotype of increased muscle fatigue without muscle atrophy or bodyweight loss 15 . These proteins have demonstrated roles in whole-body energy regulation, are critical regulators of mitochondrial function in multiple tissues, and are targets of multiple FDA-approved agents in the treatment of type 2 diabetes and hyperlipidemia.…”

“…When the 43 overlapping pathways were ranked by how similarly all subtypes appeared in terms of magnitude and directionality of dysregulation, LXR/RXR signaling stood out in its strong, consistent inhibition across all BC subtypes ( Figure 4D), and the related TR/RXR pathway was also identified as consistently significantly dysregulated. These pathways are closely related to and often integrated with the PPAR/RXR signaling pathway, which our laboratory has previously identified as a likely upstream regulator of muscle fatigue in BC patients 14,15 .…”

“…Our laboratory has recently reported that skeletal muscle of women with BC exhibits a distinct gene expression signature that is not dependent on molecular subtype 14 . Furthermore, we have identified signaling via the metabolic regulators of the peroxisome-proliferator activated 5 receptor (PPAR) family as potential key mediators of fatigue in women with BC and female mice bearing BC patient-derived orthotopic xenografts (PDOXs) 15 . Our previous analyses did not include women with primary tumors that overexpressed HER2/neu in the absence of estrogen receptor (ER) and progesterone receptor (PR) expression.…”