Interaction of visceral adiposity and age in shaping the human breast tumor microenvironment

Background: Preclinical studies suggest that aging and adiposity synergistically impair anti-tumor immunity, but most evidence comes from triple negative breast cancer (TNBC) models. In humans, visceral adipose tissue (VAT), a metabolically active, pro-inflammatory depot not captured by body mass index, has been linked to alterations in the breast tumor microenvironment (TME), with patterns varying by molecular intrinsic subtype. However, the joint impact of aging and adiposity on the human breast TME remains poorly understood. Methods: We included 1,377 women diagnosed with primary, stage II-III breast cancer who received treatment at Kaiser Permanente Northern California between 2005 and 2015 and had an abdominal computed tomography (CT) scan within 6 months of diagnosis and before systemic therapy. We selected a stratified random sample enriching for HER2+ and ER- tumors and isolated RNA from treatment-naïve specimens collected at biopsy or excision, verified RNA quality, and performed NanoString BC360™ profiling to determine PAM50 subtype and quantify TME-related gene expression levels. VAT area (cm 2 ) was quantified from CT scans at the third lumbar vertebra. We examined the differential gene expressions associated with high vs. low VAT index (VAT area scaled to height; ≥42.5 vs <42.5 cm 2 /m 2 ) stratified by age as a proxy for menopausal status (≤55 vs >55) and PAM50 subtype. Results: Patients were on average 56±13 years old at diagnosis. Most had overweight (30%, BMI 25-<30) or obesity (38%, BMI 30+) and stage II (58%) cancer. The PAM50 subtype distribution was: Basal-like (31%), HER2-enriched (25%), Luminal B (22%), and Luminal A (22%). VAT was associated with differential TME gene expression only among patients aged >55: In Basal-like tumors, high VAT was associated with upregulation of CDKN2A, a cell cycle inhibitor that can drive senescence, which in turn, promotes immune modulation, treatment resistance, and poor prognosis in TNBC. In Luminal A tumors, low VAT showed increased expression of genes associated with metabolic adaptation (PCK1) and differentiation regulators (ACVR1C, WIF1, SOCS2), whereas high VAT was linked to SPP1, a pro-inflammatory glycoprotein also related to senescence. Conclusion: Excess VAT modifies breast TME gene expression in post-menopausal patients with Basal-like and Luminal A breast cancers. The differential expression patterns were suggestive of metabolic shifts and inflammatory reprogramming that could lead to an aggressive, pro-inflammatory and/or immunosuppressive breast TME in menopausal patients, negatively impacting treatment response. These findings highlight the interplay of aging and visceral adiposity in breast cancer biology and prognosis.