**1. Introduction**

Melanoma is one of the most malignant and aggressive types of cancer worldwide, and the identification of new targets for treating melanoma is urgently needed. Melanoma cells have been reported to constitutively express fibroblast growth factor 2 (FGF2) [1,2], which is an autocrine factor and promotes the proliferation, angiogenesis, and metastasis of melanoma cells. Thus, FGF2 is commonly considered one of the potential targets for treating melanoma, and may be used with other targets to synergistically enhance therapeutic e fficacy [3].

FGF2, which belongs to the FGF family, participates in a variety of physiological and pathological processes both in vitro and in vivo, including cellular survival, di fferentiation, proliferation,

angiogenesis, adhesion, skeletal formation, and wound healing [4–6]. In terms of function, FGF2 first binds to two fibroblast growth factor receptors (FGFRs), and then recruits the heparan sulfate (HS) chain(s) of membrane-anchored heparan sulfate proteoglycan (HSPGs) for assembly into a ternary complex (FGF–HS–FGFR). The FGF–HS–FGFR complex results in receptor dimerization, with subsequent autophosphorylation of specific tyrosine residues that a ffects multiple downstream signal transduction pathways [7–10]. Meanwhile, FGF2 and FGFR-1 complexes can enter into the nucleus, where they engage with a variety of sequence-specific transcription factors and further regulate the release and activity of matrix metalloproteinase 2 (MMP-2) and matrix metalloproteinase 9 (MMP-9), as well as the expression of proteins involved in the epithelial–mesenchymal transition [11,12].

Among the FGF–HS–FGFR complex, the basic amino acid residues of FGF2–FGFR2 form a positively charged cluster, which can attract negatively-charged HS chains for incorporation. Exogenous sulfated carbohydrates like heparin and its mimetic derivatives have long been studied for their competition with HS chains, to break the formation of the FGF2–HS–FGFR1 ternary complex and physiologically disrupt the function of cancer cells [13]. Several heparin mimics have been proven to be e ffective in blocking the formation of the FGF2–HS–FGFR1 ternary complex in vitro; however, their potential use might be limited by their potency, pharmacokinetic defect, and safety profile. Therefore, it would be an e ffective strategy to explore FGF2 inhibitors from existing drugs to facilitate cancer treatment.

Propylene glycol alginate sodium sulfate (PSS) is a heparin-like drug that was approved by the China Food and Drug Administration (CFDA) over 30 years ago to treat hyperlipidemia and ischemic cardio-cerebrovascular diseases [14]. PSS is obtained from alginate polysaccharide of Laminaria with multiple chemical modifications. PSS is composed of mannuronic acid (M) and guluronic acid (G) disaccharide repeat units and sulfates occurring at the C-2 or C-3 position of the sugar moiety, with a substitution at the C-6 position by propylene glycol. PSS has the following structural characteristics: an M/G ratio above 1.5, a molecular weight of 15–20 kDa, and an organic sulfur content of 9–14% [15].

Based on our previous research, it is well known that PSS and its fractions exert e ffects on anti-coagulation-related activities and anti-selectin activities [15–17]. Wu et al. reported that PSS and its oligosaccharides could significantly stimulate FGF2-induced cell proliferation in FGFR1c-expressing BaF3 murine pro-B cell line [18], suggesting that the potential binding e ffect occurs between PSS and FGF2. However, no studies have investigated the e ffects of PSS on FGF2-mediated functional regulation on a highly metastatic B16-F10 melanoma model and the related tumor microenvironment. As PSS possesses similar structural and bioactive properties to heparin, we hypothesized that PSS might exhibit an inhibitory e ffect on FGF2-mediated cellular proliferation, invasion, migration, or angiogenesis, as well as related downstream signaling.
