Changes in the cytologic distribution of heparin/heparan sulfate interacting protein/ribosomal protein L29 (HIP/RPL29) during in vivo and in vitro mouse mammary epithelial cell expression and differentiation

HIP/RPL29 is a small, highly basic, heparin/heparan sulfate interacting protein identical to ribosomal protein L29 and present in most adult epithelia. In the present study, we show that mouse HIP/RPL29 is ubiquitously present in adult mammary epithelia and is significantly increased during pregnancy and lactation. We observed for the first time that HIP/RPL29 intracellular expression and distribution varies, depending on the growth/differentiation state of the luminal epithelium. HIP/RPL29 was detected at low levels in mammary glands of virgin animals, increased markedly during lactation, and was lost again during involution. HIP/RPL29, preferentially found in the expanded cytoplasm of mature epithelial cells secreting milk, is present also in the nucleus of proliferating and differentiating ductal and alveolar elements. We used COMMA-D cells as an in vitro model for mammary-specific differentiation and examined similar intracellular redistribution of HIP/RPL29 associated with functional differentiation. However, no changes in HIP/RPL29 expression levels were detected in response to lactogenic hormones. Finally, the cellular distribution of HIP/RPL29 in both nuclear and cytoplasmic compartments was confirmed by transfecting a normal mammary epithelial cell line, NMuMG, with a fusion protein of HIP/RPL29 and EGFP. Collectively, these data support the idea that HIP/RPL29 plays more than one role during adult mammary gland development.