Cellular Signals Underlying β-Adrenergic Receptor Mediated Parotid Gland Enlargement

Objective: Stimulation of β-adrenergic receptors (β-ARs) is known to cause salivary gland enlargement. We have previously shown that the non-selective β-AR agonist isoproterenol (ISO) stimulates mitogenic signaling in a salivary cell line via epidermal growth factor receptor (EGFR) and cAMP dependent activation of extracellular signal-regulated protein kinases (ERK1/2) (Am J Physiol 288: C1357, 2005). We now determine the signaling pathways activated during ISO mediated salivary gland enlargement in vivo. Methods: Rats were injected with ISO (20 μg/gm ip) twice daily, and at various times (0h, 1h, 8h, 24h, 72h) the parotid glands were dissected and prepared for immunoblot and/or immunohistochemistry analysis of β-AR responsive signals. Results: ISO injection caused an early increase in the activated form of ERK1/2 (phospho-ERK1/2, pERK1/2) in parotid gland (at 1 and 8h), and the level of pERK1/2 remained slightly elevated at 24 and 72h; total ERK1/2 levels paralleled pERK1/2 changes. Phospho-EGFR (pEGFR) levels gradually increased over the entire period of ISO treatment, whereas total EGFR levels remained unchanged. Analysis of β-AR subtypes following ISO treatment revealed that β1-AR protein expression was initially reduced and returned to the pretreatment (control) level by 24h; β2-AR expression gradually increased to 3 times the control level over 72h. The expression of proliferating cell nuclear antigen (PCNA) increased in parallel with β2-AR changes. The cAMP response element binding protein (CREB) increased at 1h, with apparent nuclear translocation of active phospho-CREB. Conclusion: In rat parotid glands, ISO injection induced an early increase/activation of ERKs and CREB, followed by gradual increases in pEGFR and β2-AR accompanied by elevation in PCNA. Early transient decline in β1-AR levels suggests a role for this receptor subtype in initiating cell signals leading to gland enlargement. Temporal and causal relationships among signals mediating β-AR induced salivary enlargement require further investigation (VA Merit Review and NIH/NIDCR R21DE15381).