Many studies have investigated how morphine influences tumor cell growth. Some studies [
4] reported that when morphine is administered at a concentration greater than 10 μM, tumor cell proliferation is inhibited. One study found that high concentrations of morphine reduced MCF-7 cell growth in nude mice [
4]. A similar result was obtained when morphine treatment prevented neuroblastoma SH-SY5Y cells from differentiating [
5]. Although many studies have shown that high concentrations of morphine can inhibit the proliferation of tumors, the mechanism remains unclear. The involvement of opioid receptors remains debatable. It was shown that opioids inhibit T47D cell growth, the mechanism of which was mediated through κ- and δ-opioid receptors [
6]. Another aspect of morphine which may contribute to its antigrowth effect is via apoptosis promotion. Morphine activates c-JunN-terminal kinase, which generates reactive oxygen species (ROS), inducing the release of cytochrome c and caspase-9/3 by enhancing the pro-apoptotic protein Bim and reducing the anti-apoptotic protein Bcl-2 [
7]. In addition to this mitochondrial pathway, morphine can also promote Fas-mediated apoptosis [
8]. Apoptosis of MCF-7 cells was shown to be mediated by a novel δ-2 receptor, p53, and caspase-independent pathway [
9]. Activation of the κ-opioid receptor (KOR) promotes apoptosis via a phospholipase C pathway in the CNE2 human epithelial tumor cell line [
10]. There are obvious differences in the number of necrotic cells induced by morphine between different cell lines. For example, the number of necrotic cells in an MCF-7 cell line is higher than that in HL-60 and A549 cell lines [
11]. This difference is largely dependent on how the cell lines react to morphine.
Despite this, there is also accumulating evidence for the growth-promoting effect of morphine. Sergeeva et al. [
12] reported that morphine activates the proliferation of myeloid K562 and T-lymphoma Yurkat cells. Sabrina et al. [
13] reported that clinically relevant concentrations of morphine can increase breast cancer progression. Recently, it was reported that morphine can inhibit cisplatin-induced apoptosis [
14]. Some studies have confirmed that taking low doses of morphine can increase the growth of the tumor cell; however, the mechanism is still unclear. Some researchers have indicated that the μ-opioid receptor (MOR) may have a significant role in this mechanism. Mathew et al. [
15] showed that compared to wild-type controls, μ-opioid receptor-knockout (MORKO) mice did not have significant tumor growth when injected with Lewis lung cancer cells. Further, after infusion of methylnaltrexone, a MOR antagonist, wild-type mice treated with Lewis lung cancer cells had a significant reduction, up to 90%, in tumor growth [
15]. It has been pointed out that morphine can stimulate the mitogen-activated protein kinase (MAPK) or Erk pathways to regulate cell cycle progression by binding to the MOR [
16]. Morphine can also stimulate the degranulation of mast cells to promote the release of neuropeptide substance P (SP) [
17,
18], which has been shown to enhance the proliferation of tumor cells through the tachykinin 1 (NK-1) receptor [
19]. The anti-apoptosis effect of morphine is involved. As naloxone cannot reverse this effect, a non-opioid receptor-mediated signaling pathway is implicated. Further reports suggest that morphine inhibits the generation of ROS and prevents the DOX-mediated activation of caspase-3, release of cytochrome c, and changes in Bax and Bcl-2 protein expression [
20]. In addition, several researchers have focused their research efforts on Survivin, a member of the inhibitor of apoptosis family. It was shown that morphine can enhance renal cell carcinoma (RCC) growth by promoting the expression of Survivin [
21].