Background
Global obesity rates are continuing to rise [
1,
2]. Obesity significantly increases the risk for the development of hormone receptor positive breast cancer in postmenopausal individuals [
3‐
5]. Breast cancer patients with obesity have a significantly worse prognosis and overall survival regardless of menopausal status or tumor subtype [
6]. Further, breast tumors from patients with obesity demonstrated higher levels of desmoplasia, which is characterized by increased alpha-smooth muscle actin (SMA) positive cancer-associated fibroblasts (CAF) and collagen deposition, than breast tumors from lean patients [
7], suggesting that obesity also impacts the breast tumor microenvironment.
Weight loss ameliorates multiple health conditions associated with obesity, and epidemiological studies have shown that weight loss may decrease the risk for breast cancer in women with obesity [
8,
9]. Breast tissue is a depot of subcutaneous adipose tissue, and a hallmark of obesity is the recruitment of macrophages to form crown-like structures (CLS) to remove lipid and necrotic adipocytes [
10]. In patients treated with bariatric surgery for weight loss, macrophage populations appear to switch from an inflammatory to an alternatively activated phenotype in subcutaneous white adipose tissue [
11,
12], which may enhance tissue repair [
13]. The impact of weight loss on adipose tissue fibrosis is less clear [
14,
15]. Changes in inflammation and adipose tissue fibrosis following weight loss have not been investigated in the mammary gland. Further, limited mouse models have examined how weight loss affects tumor growth and the tumor microenvironment. The effects of weight loss prior to tumor formation on the resulting mammary tumor microenvironment have yet to be examined.
In obesity, mammary adipose tissue is associated with increased collagen deposition and stiffness surrounding adipocytes [
7,
16] and the emergence of SMA
+ myofibroblasts [
7]. We have shown that fibrocytes are increased in obesity and contribute to fibrosis in the mammary gland [
17]. Fibrocytes, which originate in the myeloid progenitor cell population of the bone marrow, have attributes of both macrophages and myofibroblasts and are associated with diseases characterized by inflammation and fibrosis [
18,
19]. Fibrocytes have been identified in tissues using combinations of markers including CD34, CD11b, CD45, SMA, and collagen I [
20,
21]. Recent single cell RNA sequencing studies have identified a role for fibrocytes in the pathogenesis of lung tumors [
22,
23]. In a tumor model of inflammation associated with obesity, we identified elevated numbers of fibrocytes in early-stage mammary tumors [
24]. In human breast tissue, CD34
+ cells have been detected in the extracellular matrix surrounding breast lobules and low grade ductal carcinoma in situ (DCIS) but not when SMA
+ myofibroblasts were increased surrounding high grade DCIS and invasive ductal carcinoma [
25,
26], which is suggestive of differentiating fibrocytes. However, the role of fibrocytes in altering the breast tumor microenvironment in obesity has not been examined.
Here, we investigate how weight loss impacts mammary gland inflammation and collagen fibrosis, as well as tumor growth and development of the tumor microenvironment, using a diet-induced obesity mouse model. We observed that weight loss resolves CLS and reduces fibrocytes within the mammary gland but does not change total numbers of macrophages or collagen deposition. Tumors that develop in the mammary glands of formerly obese mice have a tumor microenvironment more similar to tumors from lean mice. Interestingly, fibrocytes were decreased in tumors from obese mice. However, transplant of estrogen receptor alpha (ERα)+ TC2 tumor cells mixed with myeloid progenitor cells from the bone marrow of obese mice into the mammary glands of both lean and obese mice leads to lasting increases in collagen and CAF within tumors. Together, these results suggest that weight loss prior to tumor formation reduces desmoplasia within tumors, potentially through reduced numbers of fibrocytes.
Discussion
Obesity is associated with poor breast cancer prognosis [
6]. While weight loss improves outcomes for other health conditions, little is known about how weight loss prior to tumor formation potentially impacts the growth and microenvironment of mammary tumors. Our studies suggest that while weight loss did not completely reduce the rate of tumor growth, the microenvironment of the resulting tumors was less fibrotic and immunosuppressive than tumors from obese mice. While fibrocytes appear to be less frequent in growing tumors, their presence in the mammary gland prior to tumor formation may promote the rapid formation of CAF in the early tumor microenvironment. We observed that mixing of myeloid progenitor cells from the bone marrow of obese mice with TC2 tumors cells resulted in significantly increased collagen deposition in tumors of both lean and obese mice. Fibrocytes may promote fibrotic changes in resident fibroblasts and adipose-derived stromal cells to become CAF in the developing tumor microenvironment, leading to more desmoplastic tumors observed clinically in breast cancer patients with obesity [
7].
Within the mammary glands of non-tumor-bearing mice, weight loss did not resolve the increased collagen deposition around mammary ducts, indicating that fibrosis may be a longer-lasting microenvironmental condition than inflammation due to macrophages in CLS. These results are consistent with human studies of fibrosis in subcutaneous and visceral fat following weight loss through bariatric surgery [
14,
15]. The continued presence of elevated collagen in the mammary glands of formerly obese mice may reflect slower tissue remodeling of mature collagen fibers within the mammary gland [
45].
Consistent with a decrease in myeloid progenitor cells in the bone marrow, we observed a decrease in fibrocytes within the mammary glands of formerly obese mice. Multiple signals have been shown to enhance fibrocyte recruitment into fibrotic conditions including chemokine (C-X-C motif) ligand 12 (CXCL12) and platelet-derived growth factor receptor [
46‐
49]. Adipose tissue expression of CCL2 is increased in obesity [
50], and we have shown that loss of CCR2 signaling reduces fibrocytes within the obese mammary gland [
17]. Weight loss has been shown to decrease circulating levels of CCL2 [
51], which may lead to the decreased recruitment of fibrocytes that we observed in the mammary glands of formerly obese mice. However, little is known about how fibrocyte numbers are regulated in the myeloid progenitor cell population within the bone marrow. Inflammatory cytokines, including interleukin (IL)-6, tumor necrosis factor alpha (TNFα), and IL-1β, have been shown promote the expansion of myeloid progenitor cells [
52,
53] and are produced by both adipocytes and macrophages in obesity [
54‐
56]. Direct effects of these cytokines on fibrocytes or fibrocyte progenitor cells has not been examined.
While we observed that weight loss reduced CLS, which are a histological marker for local inflammation [
57], the total CD11b
+ cell population, which includes macrophages, was not reduced. Studies of macrophages within different adipose tissue depots have demonstrated that the macrophage population is heterogeneous, depending on microenvironment conditions [
58‐
60]. Macrophages that form CLS may be functionally distinct from macrophages in other locations within adipose tissue [
61]. Macrophages can also acquire a metabolically-activated phenotype [
62] due to removal of lipid from dying adipocytes [
63]. In visceral fat during weight loss, macrophage populations shift to include those with a phagocytotic function, which may participate in tissue remodeling [
59,
64]. Further, in the obesity-resistant Balb/c strain, mice that switched from a HFD to a LFD had F4/80
+ macrophages that remained elevated in the mammary glands [
65], which may suggest that exposure to a HFD may also play a role in the macrophage populations present. The estrus cycle also contributes to immune cells regulation in the mammary gland [
66]. Since we did not examine immune cell populations at a synchronized point in the estrus cycle, this limitation may have contributed to variability we observed in our study. CD11b is expressed at various levels by multiple different cell types [
67], and additional gene expression experiments and expanded flow cytometry markers are required to characterize the distinct populations of CD11b
+ cells and their localization in the mammary glands of lean, obese, and formerly obese mice.
Similar to the CD11b
+ cell population in the mammary gland, the CD11b
+ cell population in mammary tumors is heterogeneous. The growing mammary tumor may affect both the types and functions of myeloid cells within the tumor microenvironment [
68]. Further, through cytokine secretion, mammary tumors can increase proliferation of specific populations of myeloid cells within bone marrow [
69,
70], which may change the composition of myeloid cells recruited into tumors. Although we observed an increase in fibrocytes within the CD11b
+ cell population in the mammary glands of obese non-tumor-bearing mice, fibrocytes were no longer enhanced within this population in the tumor microenvironment. Cytokines and growth factors secreted by tumor cells may shift the differentiation of immature myeloid cells into MDSC and tumor-associated macrophages [
71]. Consistent with this idea, we observed increased Gr-1
+ MDSC in the tumors of obese mice, and the immunosuppressive environment was reflected in significantly reduced CD8
+ T cells. Granulocyte–macrophage colony-stimulating factor is one cytokine that has been implicated in promoting the differentiation of MDSC from immature myeloid cells within tumors [
72,
73]. While MDSC are elevated in tumors, it is likely that multiple factors contribute to an immunosuppressive microenvironment. Following treatment with anti-F4/80 antibodies to deplete macrophages, both obese and lean mice had elevated levels of CD8
+ T cells within tumors [
36]. Further work is necessary to identify how obesity alters the recruitment and function of immune cells within tumors contributing to immunosuppression.
Recent work suggests that fibrocytes play an important role in lung tumor growth and generation of metastases [
22]. When transplanted with gastric cancer cells, fibrocytes promoted the growth of larger, more fibrotic tumors [
42]. Following transplantation of myeloid progenitor cells with TC2 tumor cells, we did not observe significant promotion of tumor growth compared to the other groups. However, collagen deposition was significantly increased in the tumors transplanted with myeloid progenitor cells in both obese and lean mice. While we hypothesized that fibrocytes differentiated into myofibroblasts within the tumors, we did not detect GFP
+ cells in tumors from any group. These data could suggest that fibrocytes had an impact in tumors early during growth and recruited limited fibrocytes from circulation as the tumors grew. Interestingly, we observed a significant increase in SMA
+ CAF in obese, but not lean mice. Given the number of cytokines associated with fibrocyte recruitment and differentiation that are upregulated in obesity [
46‐
49,
74‐
76], fibrocytes from CD11b
+CD34
+ cells could be longer lived in obese mice, leading to increased SMA
+ CAFs in tumors of obese mice. Transplanted fibrocytes may also induce the differentiation or recruitment of other cells to become CAFs, as has been observed in culture in other contexts [
43,
44]. It is also possible that this experiment was limited by technical challenges such as rejection of GFP
+ immune cells by recipient mice. Further insight into the role of fibrocytes in mammary tumor growth may be gained through lineage tracing studies as well examining tumors at earlier time points during tumor growth.
Weight loss is a commonly recommended intervention for obesity and may reduce breast cancer risk [
8,
9]. Multiple methods for weight loss are clinically used, and each method for weight loss may impact the mammary gland and the resulting tumor microenvironment in divergent ways. In a recent study that explored four different methods for weight loss, mice that lost weight through low-fat calorie restriction, Mediterranean-style calorie restriction, and intermittent-calorie restriction had reduced tumor growth and diminished expression of genes associated with epithelial-to-mesenchymal transition following injection with EO771 mammary tumor cells compared to mice fed a HFD or formerly obese mice that lost weight through switching to a non-restricted LFD [
77]. In a mouse model using bariatric surgery to induce weight loss, mammary tumors that developed formerly obese mice in the surgical group had higher expression of genes associated with an inflammatory response and improved responses to anti-PD-L1 immune checkpoint therapy [
78]. Together these studies suggest that the method for weight loss may have a long-term impact on the biology of tumors that may develop at a later time point. It is also possible that changes in the mammary microenvironment due to weight loss may have divergent effects on different subtypes of breast cancer. In models of triple negative breast cancer, mice that lost weight due to decreased consumption of a HFD developed tumors with aggressive characteristics that were more similar to tumors from obese mice [
77,
79]. Consistent with our study, weight loss did not decrease the latency to tumor formation compared to obese mice [
77,
79]. Here we show that weight loss prior to ERα
+ tumor formation limits desmoplasia and immunosuppression within mammary tumors. Since increased desmoplasia, including SMA
+ stromal cells, is associated with worse survival in breast cancer patients [
80,
81], our results suggest that weight loss prior to ERα
+ mammary tumor formation may ameliorate the effects of obesity on the tumor microenvironment. Understanding how different methods of weight loss alter the microenvironment and biology of tumors may lead to improved prevention strategies for women at high risk for breast cancer.
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