Late-phase and sustained activation of p44/42(MAPK) has been reported to be a critical factor in cell mitogenesis. We therefore hypothesized that p44/42(MAPK) is involved in mannosyl-rich glycoprotein-induced mitogenesis in bovine airway smooth-muscle cells (ASMC). Treatment of adherent ASMC with beta-hexosaminidase A (Hex A, 50 nM), an endogenous mannosyl-rich glycoprotein, resulted in a late-onset (30-min) activation of p44/42(MAPK) that lasted for 4 h. Activation of p44/42(MAPK) induced by Hex A was inhibited by an 18-mer phosphorothioate-derivatized antisense oligonucleotide (1-5 microM) directed to human p44(MAPK); the mitogen-activated protein kinase kinase (MEK1) inhibitor PD98059 (5 microM); the p42(MAPK) inhibitor Tyrphostin AG-126 (0.2 microM); the farnesyl transferase inhibitors SCH-56582 (10 microM) and FPT III (10 miroM), which inhibit p21Ras activation; and Calphostin C (0.2 microM), an inhibitor of protein kinase C. These agents also inhibited Hex A-induced cell proliferation in bovine ASMC. These data suggest that Hex A activates p44/42(MAPK) in a p21Ras- and PKC-dependent manner and that this activation mediates Hex A- induced mitogenesis in bovine ASMC.

Airway smooth muscle proliferation is a key component of airway wall remodelling that occurs as a consequence of inflammation in asthma. Studies were conducted to examine the effect of dexamethasone on airway smooth muscle cell (ASMC) proliferation in vitro. Dexamethasone (25-250 nM) significantly inhibited DNA synthesis and cell division induced by beta-hexosaminidase A (Hex A, 50 nM) in bovine ASMC. The inhibitory effect of dexamethasone on DNA synthesis was variable depending on the growth factors: significant effect was observed on Hex A and insulin; no significant effect was observed on epidermal growth factor and fetal bovine serum.

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beta-Hexosaminidases A & B, inflammatory marker enzymes, are mannosyl-rich glycoproteins that are implicated in acute asthma. At physiologically and pathologically relevant concentrations (nM), beta-Hexosaminidases act as potent mitogens of bovine airway smooth muscle cells. This mitogenic action is mediated via 175 kD mannose recognizing receptors that have been isolated from bovine airway smooth muscle cells and human bronchial smooth muscle. There seems to be an involvement of multiple signal transduction pathways in this process and this mitogenic effect is cell density dependent. Moreover, beta-Hexosaminidases are protease resistant. Thus, beta-Hexosaminidases may serve as key inflammatory mediators critical to airway remodeling process.