Study of small proline-rich proteins (SPRRs) in health and disease: a review of the literature

Epithelial tissues are specialized to protect underlying tissues from environmental influences such as physical and chemical agents, infection by invasive microorganisms as well as water and heat loss. They are grouped into simple, transitional and stratified epithelia, which line the cavities and surfaces of structures throughout the body, and also form glands, separate compartments, regulate the exchange of molecules and act as sensory organs. Stratified epithelia such as the epidermis and the gingival and hard palate mucosa are in constant renewal, with cells proliferating in the lower layers, while the intermediate stratum and outermost layers undergo a tissue-specific process of differentiation to form a protective cornified barrier. This review focuses on a subclass of structural proteins, the small proline-rich proteins (SPRRs), which constitute cornified cell envelope precursors. Several studies have suggested that the SPRRs are related to increased epithelial proliferation and to malignant processes. Hence, we also review the literature for more extensive and in-depth profile of these proteins in cancer and other diseases. The understanding of SPRR functions has advanced in recent years, but many important questions about their role in pathophysiological processes remain unanswered, which stimulate new studies and approaches.