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The human hepatocarcinoma-intestine-pancreas/pancreatic-associated protein (HIP/PAP) gene was previously identified because of its increased expression in primary liver cancers and during the acute phase of pancreatitis. In normal tissues, HIP/PAP is expressed both in endocrine and exocrine cells of the intestine and pancreas. HIP/PAP is a lactose binding C-type lectin which acts as an adhesion molecule for rat hepatocytes. The aim of the work was to study the HIP/PAP secretory pathway and to produce high levels of HIP/PAP in the milk of lactating transgenic mice. In view of its lactose C-type lectin properties, we have studied the consequences of the expression of HIP/PAP on mammary epithelial cells. In homozygous mice, production reached 11.2 mg.mL-1 of milk. High levels of soluble and pure HIP/PAP (18.6 mg) were purified from 29 mL of milk. The purified protein was sequenced and the N-terminal amino acid of the mature HIP/PAP was identified as Glu27, thus localizing the site of cleavage of the signal peptide. The HIP/PAP transgene was only expressed in the mammary gland of lactating transgenic mice. HIP/PAP was detected by immunofluorescence in the whole gland, but labelling was heterogeneous between alveolar clusters, with strongly positive sparse cells. Using immuno electron microscopy, HIP/PAP was observed in all the compartments of the secretory pathway within the mammary epithelial cells. We provide evidence that HIP/PAP is secreted through the Golgi pathway. However, the number of distended Golgi saccules was increased when compared to that found in wild-type mouse mammary cells. These modifications could be related to HIP/PAP C-type lectin specific properties.

Milk protein synthesis is initiated by prolactin and a glucocorticoid. In the rabbit, prolactin alone is sufficient. However, glucocorticoids potentiate the action of prolactin. The stimulatory effect of glucocorticoids was evaluated after injections of hydrocortisone acetate alone or associated with prolactin by measurements of (a) the total RNA and DNA content of mammary glands, (b) the lactose synthetase activity, (c) casein synthesis, and (d) the concentration of casein mRNA in total cellular RNA and in polysomal RNA by hybridization with its cDNA. The glucocorticoid, totally inactive alone, proved to have a stimulatory effect proportional to the dose injected when prolactin was present. This effect was more evident with low doses of prolactin. Glucocorticoids proceeded by amplifying the capacity of prolactin to enhance the concentration of casein mRNA available for translation. A parallel effect of glucocorticoids on translation of casein mRNA was suspected. Glucocorticoids injected with low doses of prolactin were unable to mimic all the effects of high doses of prolactin alone.