Metabolic hormones in saliva: origins and functions

Abstract

The salivary proteome consists of thousands of proteins, which include, among others, hormonal modulators of energy intake and output. Although the functions of this prominent category of hormones in whole body energy metabolism are well characterized, their functions in the oral cavity, whether as a salivary component, or when expressed in taste cells, are less studied and poorly understood. The respective receptors for the majority of salivary metabolic hormones have been also shown to be expressed in salivary glands (SGs), taste cells, or other cells in the oral mucosa. This review provides a comprehensive account of the gastrointestinal hormones, adipokines, and neuropeptides identified in saliva, SGs, or lingual epithelium, as well as their respective cognate receptors expressed in the oral cavity. Surprisingly, few functions are assigned to salivary metabolic hormones, and these functions are mostly associated with the modulation of taste perception. Because of the well-characterized correlation between impaired oral nutrient sensing and increased energy intake and body mass index, a conceptually provocative point of view is introduced, whereupon it is argued that targeted changes in the composition of saliva could affect whole body metabolism in response to the activation of cognate receptors expressed locally in the oral mucosa.

Keywords: biochemistry/physiology; medicine.

Fig. 1. Schematic representation of a submandibular salivary gland

Acinar cells exhibit a predominant exocrine regulated secretory pathway (RSP) across the apical membrane and into the luminal space accumulating in saliva (depicted by block arrows). The endocrine constitutive secretory pathway (CSP) from acinar cells delivers peptides across the basolateral membrane into the interstitial space and into the bloodstream (depicted by dashed line arrows). The transport of peptides from plasma into the saliva includes both transcellular and paracellular mechanisms of passive diffusion, active transport and ultrafiltration (collectively depicted by uninterrupted line arrows). White adipose tissue (WAT) is shown within the interstitium as a possible source of salivary leptin and adiponectin.

Fig. 2. Schematic representation of a taste bud

The diagram shows an onion-like taste bud structure embedded into the lingual epithelia. Taste receptor cell (TRCs) types are shown in the inset.