DNA methylation-independent regulation of p16 in epithelial cells during mouse mammary gland development.

p16 (INK4a) is a known negative regulator of the cell cycle acting up-stream of Rb to inhibit cellular growth. While the contribution of p16 to the tumorigenic process has been extensively studied, little is known about its role in the cellular differentiation process of normal cells. p16 expression in mammary gland epithelial cells and its possible mediation by DNA methylation was explored. The mammary glands from female mice (mus musculus) at distinct developmental stages (virgin, day 10 of lactation and day 3 of involution) were used. The expression pattern of p16 and the DNA methylases, DNMT1, 3a and 3b was investigated by semi-quantitative RT-PCR, in situ hybridization (ISH) and immunohistochemistry. The p16 methylation status was assesed by methylation-specific PCR (MSP). p16 was differentially expressed during distinct developmental stages and its transcriptional regulation was DNA methylation-independent, which was also corroborated by the expression pattern of the three known DNA methyltransferases (DNA MTase). The p16 expression level was elevated during involution compared to the corresponding expression level during lactation. p16 is differentially expressed during normal mammary gland development, which is not mediated by DNA methylation.