Obesity is associated with increased incidence and severity of triple negative breast cancer (TNBC). About 65% of the US population is obese or overweight. Obesity is a major modifiable risk factor for breast cancer and is responsible for approximately 20% of cancer deaths. In addition to its role in breast cancer pathogenesis, obesity is recognized as a marker of poor prognosis in pre- and post-menopausal women with TNBC. Indeed, obese women generally have larger, higher-grade tumors and lymph node involvement as well as a higher risk of distant recurrence. Several studies have focused on understanding the mechanism(s) by which obesity promotes ER+ breast tumor progression; however, its role in TNBC has been less explored. Since basal-like breast cancers (estrogen, progesterone and her2 receptor negative) are one of the most aggressive and invasive cancer subtypes lack targeted therapies, understanding the biological processes that link obesity and TNBC has important clinical applications for prognosis and treatment.

Macrophages play key roles in obesity-associated breast cancer. Mechanisms by which obesity leads to a worse TNBC prognosis are not well understood. One clue to its action is that obesity causes chronic inflammation, and macrophage infiltration into adipose tissue is a key mediator of this inflammation. While it is well established that tumor-associated macrophages are key effector cells that promote TNBC, the breast also contains adipose tissue macrophages (ATMs), whose impact on tumorigenesis has largely been ignored.

ATMs are an attractive mechanistic link between obesity and TNBC for several reasons. First, it is well accepted that obesity promotes ATM accumulation and inflammation in breast fat. Second, ATMs are the predominant type of macrophage in the breast during early tumorigenesis, and may therefore regulate effects of obesity on tumor initiation. Indeed, recent studies showed that obesity, through its action on ATMs, promotes stromal vascularization and angiogenesis even before the formation of cancer. Third, ATMs may regulate metastasis because the majority of invasion events occur at the tumor-host border, which in the case of breast cancer is adipose tissue. These findings raise the possibility that ATMs play an important role in tumorigenesis.

A novel macrophage phenotype found in obese patients could link obesity to breast cancer. In recently published work from the Becker lab (Kratz et al., Cell Metab., 2014), we showed that metabolic dysfunction promotes a unique pro-inflammatory phenotype in visceral adipose tissue of obese patients. We refer to these macrophages as ‘metabolically activated’ (MMe).

Our work tested the hypothesis that obesity promotes metabolic activation of mammary ATMs, which in turn, potentiate tumor initiation and aggressiveness in patients with TNBC.

We have reached the following milestones over the two-year period of this grant:

1. Using mammary adipose tissue samples from 10 lean and 10 obese women, we provided evidence that obesity promotes ATMs to adopt an MMe phenotype in mammary fat. We obtained similar findings in obese mice. These findings establish the presence of MMe macrophages in the primary tumor site of obese women and of obese mice.

2. We demonstrated that MMe macrophages (generated in vitro or isolated from mammary fat of obese mice or humans) release factors that promote tumor initiation and tumor aggressiveness of multiple human TNBC tumor cells both in vitro and in vivo. These findings establish the MMe macrophage as a novel pro-tumorigenic macrophage subtype.

3. We identified a signaling pathway within MMe macrophages that causes them to induce tumor initiation and tumor metastasis. These findings provide a target to antagonize the pro-tumorigenic actions of MMe macrophages during obesity.

4. We are finalizing our project by genetically ablating this signaling pathway in macrophages specifically, and determining whether mice lacking this signaling pathway are resistant to obesity-induced tumorigenesis. We expect to obtain these decisive data in the next few months.

Collectively, our findings provide strong support for the hypothesis presented in our grant. We are currently writing a manuscript on our work, which we believe will serve as the foundation for additional grant support of greater scope.

We would like to thank the Cancer Research Foundation for supporting our work and look forward to sharing updates regarding our project going forward.

Obesity is associated with increase incidence and severity of triple negative breast cancer. About 65% of the US population is obese or overweight. Obesity is a major modifiable risk factor for breast cancer and is responsible for approximately 20% of cancer deaths. In addition to its role in breast cancer pathogenesis, obesity is recognized as a marker of poor prognosis in pre- and post-menopausal women with TNBC. Indeed, obese women generally have larger, higher-grade tumors and lymph node involvement as well as a higher risk of distant recurrence. Several studies have focused on understanding the mechanism(s) by which obesity promotes ER+ breast tumor progression; however, its role in TNBC has been less explored. Since basal-like breast cancers (estrogen, progesterone and her2 receptor negative) are one of the most aggressive and invasive cancer subtypes lack targeted therapies, understanding the biological processes that link obesity and TNBC has important clinical applications for prognosis and treatment.

Macrophages play key roles in obesity-associated breast cancer. Mechanisms by which obesity leads to a worse TNBC prognosis are not well understood. One clue to its action is that obesity causes chronic inflammation, and macrophage infiltration into adipose tissue is a key mediator of this inflammation. While it is well established that tumor-associated macrophages are key effector cells that promote TNBC, the breast also contains adipose tissue macrophages (ATMs), whose impact on tumorigenesis has largely been ignored.

ATMs are an attractive mechanistic link between obesity and TNBC for several reasons. First, it is well accepted that obesity promotes ATM accumulation and inflammation in breast fat. Second, ATMs are the predominant type of macrophage in the breast during early tumorigenesis, and may therefore regulate effects of obesity on tumor initiation. Indeed, recent studies showed that obesity, through its action on ATMs, promotes stromal vascularization and angiogenesis even before the formation of cancer. Third, ATMs may regulate metastasis because the majority of invasion events occur at the tumor-host border, which in the case of breast cancer is adipose tissue. These findings raise the possibility that ATMs play an important role in tumorigenesis.

A novel macrophage phenotype found in obese patients could link obesity to breast cancer. In recently published work from the Becker lab (Kratz et al., Cell Metab., 2014), we showed that metabolic dysfunction promotes a unique pro-inflammatory phenotype in visceral adipose tissue of obese patients. We refer to these macrophages as ‘metabolically activated’ (MMe).

Our Cancer Research Foundation funded work tested the hypothesis that obesity promotes metabolic activation of mammary ATMs, which in turn, potentiate tumor aggressiveness in patients with TNBC.

Our progress in the first year was focused on 3 key areas:

1. We provided evidence that obesity promotes ATMs to adopt an MMe phenotype in human and murine mammary adipose tissue. These findings were important since they established the presence of MMe macrophages in the primary tumor site of obese women.
2. We demonstrated that MMe macrophages release factors that promote aggressiveness of multiple human TNBC tumor cells both in vitro and in vivo. These findings established the MMe macrophage as a novel pro- tumorigenic macrophage subtype.
3. We identified a mechanism that may explain how MMe macrophages induce a more aggressive tumor cell phenotype. This mechanistic work enables us to interfere with MMe macrophage dysfunction, and explore this approach as a novel anti-TNBC therapeutic.

Collectively, our findings have established an excellent foundation for attaining the long-term goal we originally defined in our proposal to the Cancer Research Foundation: “to delineate specific mechanisms by which obesity worsens breast cancer progression, which can be leveraged to develop novel anti-cancer therapeutics that target macrophages.”