Expression of enzymes of covalent protein modification during regulated and dysregulated proliferation of mammary epithelial cells: PKA, PKC and NMT.

: Three proteins are functionally interlinked in the targeting of protein phosphorylation catalyzed by the C-subunit of PKA: PKA itself, AKAPs and NMT. Furthermore, in a variety of biological contexts, mechanisms exist whereby PKA and PKC are able to modulate the activity of one another. We have investigated the expression and subcellular distribution of these proteins in two models of mammary cell proliferation and differentiation--the normal rat mammary gland during pregnancy and lactation and human breast tissue before and after malignant transformation. Modulation of PKA does not acutely affect activity or sub-cellular distribution of PKC in mammary acini, nor does modulation of PKC acutely affect PKA activity or subcellular distribution. Therefore, the co-ordinate expression of these two protein kinases in normal and cancerous mammary epithelial cells and the greater basal activation level of them both accompanying increased mitogenic activity, which we have reported, does not result from short-term cross-talk between them. Although basal and total levels of PKA diminish in rodent mammary epithelial cells during the transition from proliferative to secretory functional mode, the level of expression of AKAPs increases. The expression of two apparently mammary-specific and mostly membrane-associated AKAPs is tightly linked to the onset and maintenance of differentiated function in rat mammary tissue. Paradoxically, the probable analogues of these two AKAPs in human mammary tissue are hyperexpressed when normal epithelial cells transform to a cancer phenotype--conventionally regarded as a process involving a degree of dedifferentiation. Mammary AKAP hyperexpression in breast cancers is accompanied by increases in the levels of total and basal PKA. One mechanism whereby NMT is targeted to membranes, via interaction with ribosomal proteins, has recently been elucidated. Our data support the contention that the localization of NMT is an important variable in the regulation of cellular proliferation, but they do not characterize the mechanisms whereby the differential targeting of NMT is achieved. As yet we lack a full tool-kit with which to examine NMT either to draw firm conclusions regarding the identity of particular isoforms found in particular sub-cellular locations or to define the relationships between these different molecular variants. However, it is technically possible to transfect cells with inducible NMT expression constructs engineered in such a way that the recombinant, catalytically competent, NMT that they encode is targeted either to membranes or to cytosol: an exploration of the effects of such transfections on cellular proliferation would afford a critical test of the mechanistic involvement of NMT in the control of mitogenesis.