Stathmin gene expression in mammary gland and in Nb2 cells.

Mammary gland growth occurs essentially during pregnancy and induction of milk synthesis is triggered at parturition. Prolactin is mammogenic in vivo but only marginally in vitro. Prolactin induces milk synthesis in vivo and in cultured mammary cells. Prolactin is also strictly required for the multiplication of the rat lymphoid Nb2 cells. Stathmin is an ubiquitous and highly conserved phosphoprotein which seems to be involved in the intracellular mechanisms which trigger cell multiplication and differentiation. In the present study, the concentration of stathmin mRNA has been evaluated during the pregnancy-lactation-weaning cycle in mouse and rabbit. Stathmin mRNA appeared at its highest level during pregnancy and it was almost undetectable during lactation. Prolactin injected into mid-pregnant rabbits induced milk synthesis and this effect was not accompanied by any modification of stathmin mRNA concentration. In cultured primary rabbit mammary cells, prolactin induced casein gene expression without any alteration of stathmin mRNA concentration. In Nb2 cells, prolactin induced a progressive increase of stathmin mRNA concentration. This effect was not significant until after 4 h of prolactin action. These data suggest that stathmin is involved in mammary and Nb2 cell multiplication but may not be necessary for mammary cell differentiation.