Upregulation of calcium-sensing receptor and mitogen-activated protein kinase signalling in the regulation of growth and differentiation in colon carcinoma

In the present study, we demonstrate that Ca2+-induced growth inhibition and induction of differentiation in a line of human colon carcinoma cells (CBS) is dependent on mitogen-activated protein (MAP) kinase signaling and is associated with upregulation of extracellular calcium-sensing receptor (CaSR) expression. When CBS cells were grown in Ca2+-free medium and then switched to medium supplemented with 1.4 mM Ca2+, proliferation was reduced and morphologic features of differentiation were expressed. E-cadherin, which was minimally expressed in nonsupplemented medium, was rapidly induced in response to Ca2+ stimulation. Sustained activation of the extracellular signal-regulated kinase (ERK) occurred in Ca2+-supplemented medium. When an inhibitor of ERK activation (10 microM U0126) was included in the Ca2+-supplemented culture medium, ERK-activation did not occur. Concomitantly, E-cadherin was not induced, cell proliferation remained high and differentiation was not observed. The same level of Ca2+ supplementation that induced MAP kinase activation also stimulated CaSR upregulation in CBS cells. A clonal isolate of the CBS line that did not upregulate CaSR expression in response to extracellular Ca2+ was isolated from the parent cells. This isolate failed to produce E-cadherin or undergo growth inhibition/induction of differentiation when exposed to Ca2+ in the culture medium. However, ERK-activation occurred as efficiently in this isolate as in parent CBS cells or in a cloned isolate that underwent growth reduction and differentiation in response to Ca2+ stimulation. Together, these data indicate that CaSR upregulation and MAP kinase signalling are both intermediates in the control of colon carcinoma cell growth and differentiation. They appear to function, at least in part, independently of one another.