Prolactin-induced
protein (PIP) is a small secreted
glycoprotein carrying several N-linked
carbohydrate chains. The expression of PIP is generally restricted to cells with apocrine properties. It was found in apocrine glands of the axilla, vulva, eyelid, ear canal, and seminal vesicle. Being a secretory
protein, PIP is present in seminal plasma, saliva, lacrimal fluid, tears, sweat gland secretion. Little is known about the biological role of PIP. It binds to numerous
proteins, however, in most cases the biological role of such interactions is poorly understood. A notable exception is its binding to
CD4 receptors present on the surface of T lymphocytes, macrophages, and spermatozoa. The available data suggest that PIP can have immunomodulatory functions and plays an important role in cell-mediate adoptive immunity. PIP binds to bacteria from several genera, which suggests that this
glycoprotein may participate also in innate immunity and protection of hosts against microbial
infections. Increased levels of PIP were found in several types of human
cancer (prostate, sweat and
salivary gland cancers). It is especially common in
breast cancer, however, data on the expression of PIP in normal and cancerous
breast cancer tissues are to some degree conflicting. In early studies, it was shown that PIP is absent or its expression is very low in normal breast epithelium, whereas in breast
cancers PIP is frequently expressed and present in large amounts. On the other hand, later study showed that expression of PIP is lower in advanced apocrine
carcinomas and invasive
carcinomas than in, respectively, in situ
carcinomas and adjacent normal tissue. The most recent study revealed that PIP gene expression decreased gradually along with higher stage and grade of
breast cancer. In agreement with these data, it was shown that that low levels or the lack of PIP expression are associated with a worse response of
breast cancer cells to
chemotherapy. It was proposed that PIP plays important role in the development and progression of
breast cancer. However, its role in these processes is both unclear and controversial. In this review, the role of PIP in both physiological processes and
carcinogenesis is discussed.