Emerging Function of Prolactin-Inducible Protein—Is This Important Tear Protein Found in Alzheimer’s Disease?

Alzheimer’s disease is characterized by a chronic, long-term neurodegenerative process and an increasing need for easily accessible biomarkers that enable early diagnosis and disease monitoring. For this reason, tears have attracted growing interest as a potential source of such biomarkers, and prolactin-inducible protein is a candidate tear protein of mechanistic interest whose clinical value remains to be established as a biomarker of Alzheimer’s disease. The literature indicates that prolactin-inducible protein is physiologically present in the lacrimal apparatus. Proteomic studies in patients with Alzheimer’s disease have repeatedly demonstrated decreased levels of prolactin-inducible protein in tears, typically accompanied by reduced concentrations of other proteins associated with normal lacrimal gland function. Although the evidence remains inconclusive, these findings suggest that alterations in prolactin-inducible protein levels may reflect lacrimal gland dysfunction related to neurodegenerative processes, autonomic dysregulation, and inflammation. Nevertheless, the lack of specificity of prolactin-inducible protein for Alzheimer’s disease, as well as the influence of various factors on its concentration, limit its value as a standalone biomarker. The most plausible approach is the incorporation of prolactin-inducible protein into multimarker panels, which could enable improved patient stratification and assessment of lacrimal gland dysfunction in Alzheimer’s disease.