CAF-derived lactate sustains prostate carcinoma progression by promoting the establishment of an immunosuppressive microenvironment and enhancing tumor cells mitochondrial metabolism

Leukocyte infiltration plays a role in controlling tumour development and is now considered one of the hallmarks of cancer. During the early stages of tumour progression, tumour-infiltrating lymphocytes (TILs) inhibit tumour growth through a process called immunoediting. However, cancer cells are able to shape the surrounding microenvironment to escape from the immunological control. Indeed, at later stages of malignancy, cancer cells promote the establishment of an immunosuppressive microenvironment in order to evolve towards malignancy. Cancer-associated fibroblasts (CAFs) are the major cellular stromal component of many solid tumors. In prostate cancer, CAFs establish a metabolic symbiosis with prostate cancer cells: CAFs, exploiting glycolysis upon tumor cell exposure, secrete lactate that is utilized by prostate cancer cells to fuel oxidative phosphorylation (OXPHOS) in order to sustain tumor progression. Since the creation of an immunosuppressive environment is crucial for tumor progression and lactate has an important role in our prostate cancer model, our aim is to understand if other interactions between TME components, such as CAFs:lymphocytes crosstalk, could be influenced by this metabolite. Our data show that CAFs-released lactate induces a significant increase in the percentage of T regulatory cells (Tregs) with a concomitant reduction of effector Th1 cells. The role of lactate in immunemodulation was confirmed by the treatment with the MCT1 inhibitor AR-C155858 that impairs lactate influx and counteracts both Treg upregulation and Th1 reduction. We also defined the molecular mechanism by which lactate drives this immunemodulation: when uploaded by Th1 cells, it induces Sirtuin 1 activation and the consequent deacetylation of the transcription factor T-bet, leading to the impairment of Th1 cells stability. When uploaded by Th0 cells, it promotes NF-kB activity, which in turn, enhances Foxp3 expression, thereby driving Treg polarization. Our results also show that lactate-induced immunemodulation is then responsible for triggering epithelial-mesenchymal transition in prostate cancer cells via the activation of the TLR8/miR-21 axis, resulting in the enhancement of their invasiveness and aggressive features. Hence, our data reveal the establishment of a metabolic-based relationship among stromal and immune cells in order to promote an immune suppressive environment that sustains cancer malignancy. On the other hand, we also wanted to better clarify the metabolic regulation of CAFs:tumor cells crosstalk in order to investigate how lactate is involved in the metabolic reprograming of prostate cancer cells. In this study, we report that lactate uptake alters the NAD+/NADH ratio in the cancer cells, which culminates with SIRT1-dependent PGC-1α activation and subsequent enhancement of mitochondrial mass and activity. The high exploitation of mitochondria results in tricarboxylic acid cycle deregulation, accumulation of oncometabolites and in the altered expression of mitochondrial complexes, responsible for superoxide generation. In addition to the enhancement of the endogenous mitochondria activity, cancer cells also hijack CAFs-derived dispensable but functional mitochondria through the formation of cellular bridges, a phenomenon that we observed in both in vitro and in vivo prostate cancer models. Our work reveals a crucial function of tumor mitochondria as the energy sensors and transducers of CAF-dependent metabolic reprogramming and underscores the reliance of prostate cancer cells on CAF catabolic activity and mitochondria trading. Hence, this study suggest a crucial role for stromal-derived lactate in enhancing prostate cancer malignancy, acting through two different strategies: i) by inducing a metabolic reprogramming directly on tumor cells, with the enhancement of mitochondrial metabolism through both endogenous mitochondria re-education and exogenous mitochondria acquisition; ii) by concurring to the establishment of an immunesuppressive environment, through the upgrading of the pro-tumoral Treg compartment and the concomitant reduction of the anti-tumor Th1 subset.