CD44 as a Central Integrator of Inflammation and Fibrosis: From Molecular Signaling to Environmental Modulation

CD44, a multi-isoform adhesion receptor for hyaluronic acid (HA), plays a crucial role in regulating cell interactions with the extracellular matrix, cell migration, differentiation, and survival in both physiological and pathological contexts. Accumulating experimental evidence suggests that CD44 is not merely a passive marker of mesenchymal cell activation but rather an active signaling hub driving fibrosis in many organs, including the lung, skin, heart, and liver. Its involvement in fibroblast differentiation into myofibroblasts, as well as induction of the invasive phenotype of these cells, shows striking analogies to the mechanisms of epithelial-to-mesenchymal transition (EMT) known from cancer progression. In this paper, we discuss both the molecular mechanisms of CD44-dependent signaling (including through EGFR, MAPK/ERK, CaMKII, lipid rafts, and Smad) and the influence of its modulation (knockout, antibodies, blockade of HA synthesis) on the course of fibrosis in in vitro and in vivo models. In addition, we present the influence of environmental pollutants-such as heavy metals, particulate matter, endocrine disruptors, and microplastics-on the activation of the HA-CD44 axis in connective tissue, with particular emphasis on their role in the induction of chronic inflammation, EMT, and extracellular matrix deposition. The collected evidence suggests that CD44 serves as a central integrator of inflammatory and fibrogenic signals, and its pharmacological modulation may represent a novel therapeutic strategy for treating fibrotic diseases and chronic inflammatory conditions.