TLR4 and cancer: Though lactate receptor HCAR1 signaling via cyclic adenosine monophosphate (cAMP) and protein kinase A (PKA) has been observed in neuronal cells [24], and HCAR1 was shown to mediate macrophage and monocyte inflammatory response via arrestin β-2, toll-like receptor 4 (TLR4), and NLRP3 inflammasome [26], lactate-stimulated HCAR1 signaling in cancer cells has not been defined.