Targeting autophagy: a promising approach for the treatment of breast cancer brain metastases

Abstract

Patients with breast tumors that metastasize to the brain have limited treatment options and a very poor prognosis. More effective therapeutic strategies are desperately needed for this patient population. Recent evidence demonstrates that brain metastases arising from breast tumors display altered energy production that results in enhanced autophagy. Preclinical studies have shown that genetically or pharmacologically disrupting the autophagy pathway significantly decreases the brain metastatic burden, resulting in improved animal survival and increased sensitivity to lapatinib. These findings pave the way for the development of novel strategies targeting autophagy for breast cancer patients with brain metastatic disease.

Approximately 20–30% of women diagnosed with breast cancer will ultimately develop brain metastases (1). Several subtypes of breast cancer exhibit a particularly high propensity for brain metastasis including triple-negative breast cancer patients (ER-, PR-, HER2 unamplified) and those with HER2 amplification. While trastuzumab has shown promise in treating primary breast tumors of the latter subtype, its inability to penetrate the blood-brain barrier (BBB) limits its effectiveness in preventing or treating brain metastases. The poor penetration of trastuzumab and other standard drugs across the BBB creates tremendous treatment challenges for the physicians caring for patients with metastatic brain disease. The management of patients with metastatic HER2+ breast cancer also remains a major clinical challenge as up to 55% of cases eventually develop brain metastases (2). Collectively across subtypes, only 20% of patients that develop brain metastases from breast cancer will achieve 5-year survival. Improving long-term survival for these patients is dependent upon achieving a better understanding of which processes that contribute to breast cancer brain metastogenesis are truly outcome determinant.

During metastogenesis, tumor cells encounter significant stress that may lead to autophagy activation to drive pro-survival metabolic reprogramming. Indeed, preclinical studies using an AKT-driven tumor model demonstrated that autophagy is preferentially triggered in tumor cells during the early phases of carcinogenesis when nutrients and oxygen are limited and vasculature is immature. Autophagy may also play an important role in promoting the establishment of metastatic lesions and maintaining residual tumor burden following surgical resection or other therapeutic intervention as cancer cells in these situations face analogous challenges regarding nutrient and oxygen supply. In agreement with autophagy’s potential role in metastasis, upregulation of autophagy has been reported during hypoxia, metabolic stress, loss of cell-extracellular matrix (ECM) contact, and in dormant cells (3). Moreover, high levels of the autophagy marker LC3B have been linked to invasion, proliferation, poor outcome, and metastasis in patients with breast cancer (4). These collective findings indicate that inhibition of autophagy may be a novel and effective approach to prevent and treat breast cancer metastasis.

Our recent study conducted global metabolic profiling on brain-seeking variants (231-BR and 231-BR-HER2) of MDA-MB-231 cells to identify potential metabolic differences that could be targeted therapeutically (5). The study revealed that the 231-BR and 231-BR-HER2 cells have distinctly different energy metabolism profiles, which is consistent with a previous study that investigated the phenotypic differences between BCM2 breast cancer cells and its brain metastatic variant (6). Interestingly, the brain metastatic cells displayed lower basal ATP levels than the parental cells and correspondingly exhibited increased phosphorylation of AMPK, a key regulator of multiple metabolic pathways. Along with upregulated AMPK phosphorylation and diminished energy capacity, the brain metastatic cells demonstrated characteristics of increased autophagy compared to the parental cells. It is possible that the upregulation of autophagy exhibited by these cells may have resulted as an adaptation to specific stress conditions in the microenvironments that breast cancer cells are exposed to during the metastogenic process prior to their establishment in the brain. Since brain metastases exhibit a long latency period averaging 2–3 years after their initial diagnosis, it is possible that autophagy acts as an essential mechanism that empowers cells to survive during their journey from their primary site to the brain. It is also possible that the clonal expansion of cells with features of autophagy-linked metabolic reprogramming could be driven, in part, by the selective pressure of the agents/modalities used in treating primary breast tumors.

Our findings show that autophagy plays an important role in driving this process as genetic inhibition of autophagy by ATG7 silencing significant blunts the development of brain metastases (5). While inhibition of autophagy decreased the presence of both large and micrometastases, the effect was more pronounced with respect to the burden of large metastases. This suggests that autophagy may be especially important for the formation and survival of larger lesions. In support of this concept, a prior study also found that ATG7-mediated autophagy was required for mesenchymal-epithelial transition and the formation of pulmonary metastasis (7). However, it was noted that deletion of ATG7 had only a marginal effect on the tumor growth of primary breast cancers. Collectively, these results suggest that primary and metastatic breast tumors may exhibit a differential dependence upon autophagic activity and that autophagy inhibition may be best utilized to strategically eliminate cells with metastatic potential and/or target established metastases.

Treatment with lapatinib, a dual EGFR/HER2 inhibitor, exhibits activity against HER2 overexpressing breast tumors. Importantly, this agent can cross the BBB, which makes it an option for patients with HER2+ breast cancer brain metastases. Despite its BBB-penetrating properties, lapatinib therapy has demonstrated modest efficacy against established brain metastases (8). Upregulation of autophagy has been well established as a mechanism that promotes resistance to many different classes of anticancer agents including kinase inhibitors such as lapatinib (9, 10). Consistent with this observation, we demonstrated that lapatinib treatment induces autophagy in HER2+ breast cancer cells and that inhibition of this process via ATG7 silencing or with hydroxychloroquine (HCQ) significantly augments its pro-apoptotic and anti-brain metastatic activity (5). Based on our collective findings combined with HCQ’s demonstrated ability to cross the BBB, further evaluation of its safety and efficacy for therapy of breast cancer brain metastases in combination with tyrosine kinase inhibitors (lapatinib and tucatinib) or antibody-drug conjugates (trastuzumab-emtansine; T-DM1 and trastuzumab-deruxtecan; T-DXd) is warranted (Figure 1). Since the majority of patients with HER2+ breast cancer do not initially present with brain metastases, the addition of an autophagy inhibitor to current treatment regimens may be especially impactful in preventing metastatic disease.

Figure 1. Inhibition of autophagy disrupts breast cancer brain metastases.

Autophagy is triggered by hypoxia, metabolic stress, and targeted agents such as lapatinib, which contributes to brain metastasis. Agents that inhibit autophagy such as HCQ and ROC-325 can reduce breast cancer brain metastasis and enhance the efficacy of conventional therapy.