Mechanisms of Altered Control of Proliferation by Cyclic Amp/Protein Kinase a During Mammary Tumor Progression

摘要

We hypothesize that alterations in the regulation of growth by cAMP during mammary tumor progression are related to MAP kinase (ERK, JNK, p38) signaling modules known to be affected by cAMP and pertussis toxin (PT)- sensitive G proteins. Mammary epithelial cells from normal mouse mammary glands were compared to ovarian-independent mouse mammary tumors (OIT) in serum-free, collagen gel cell culture. The role of cAMP-pathway responsive transcription factors in PT or PD98O59 (a specific ERK pathway inhibitor) inhibition of cAMP mitogenesis showed that CREB activation is not targeted by these inhibitors. Thus, other PT-sensitive or ERK-dependent cAMP pathways are involved. Further examination of lysophosphatidic acid (LPA) signaling showed that it can stimulate CREB and ATF2 phosphorylation, thus interacting with the cAMP pathway. The p38 inhibitor, SB2O2l9O (SB), was used to evaluate the importance of the PT- responsive p38 pathway in cAMP mitogenesis. Surprisingly it alone was mitogenic, activated ERK and JNK kinase activity and stimulated p38 phosphorylation. It synergized with LPA and cAMP in stimulating proliferation or normal mammary epithelium but inhibited the proliferation of OITs. These novel data implicate p38 as playing a central role in proliferation control. 11-89, a cAMP-dependent protein kinase A (PKA) inhibitor also stimulated proliferation of normal mammary epithelium but not OIT and could potentiate cAMP, and LPA mitogenesis suggesting that nonPKA pathways are critical for cAMP mitogenesis. Thus, while cAMP alone does not significantly stimulate MAP kinase activity it appears to participate in multiple kinase networks to affect proliferation. Proliferation of mammary epithelium is controlled by multiple pathways and it is the balance between positive and negative signals that determines the final proliferative response of the cells. These networks are disrupted at many points during mammary tumorigenesis.