At least in breast, endocrine resistant cancers very rarely lose expression of ERalpha. To use hormone receptor-negative cells as models of endocrine resistance is indeed inadequate. In patients, endocrine-resistant breast cancers can grow in the presence of aromatase inhibitors and often fulvestrant. I would call these tumors estrogen-insensitive. Some endocrine-resistant cells that express ERalpha are resistant to estrogen deprivation and 4-hydroxytamoxifen but still require ERalpha (and they are fulvestrant-sensitive). Whether appropriate in vitro models of endocrine resistance should be fulvestrant sensitive or insensitive is rather controversial. Personally, I derive my definitions from what type and stage of human disease I am trying to model. In the clinic, fulvestrant is only modestly effective but it does not cure endocrine-resistant disease. Therefore, it is not unreasonable to use fulvestrant-resistant cells to model fulvestrant-resistant human disease.

Sorry, I mean only receptor positive cels

In that case, I don't think the reviewer's criticism is justified, based on clinical evidence...You may want to point out that fulvestrant has very limited efficacy in the clinical setting, even when combined with AIs.

Konstantin:

This is a misunderstanding of category definitions. Hormone-insensitive cells in the literature refer to cells that are unresponsive to any endocrine therapy (SERM (like tamoxifen or toremifene), AI (letrozole, etc.), antiestrogen (fulvestrant), AR inhibitors (like bicalutamide)). Your definition does not capture this sense of hormone-insensitive. Furthermore, your definition would be considered too "promiscuous" since proliferation in the absence of exogenous hormone, your criterion, is satisfied by cells that are jointly HER2-positive and ER-negative and PR-negative, which of course can and do proliferate independent of any hormonal assault, or hormonal milieu.

Given this semantic confusion, the reviewer is correct on the consensus understanding of "hormone-insensitive": true hormone-insensitive cells, definitionally lacking responsivity to endocrine therapeutics (whether for breast or prostate cell lines), are an inappropriate model of hormone resistance, since such a model requires at least the theoretical potential of responsivity coupled with acquired endocrine-therapeutic resistance. With hormone-insensitive cells being intrinsically unresponsive, they cannot model hormone resistance. Thus, the triple negative MDA-MB-231 breast cancer cell line is strictly hormone-insensitive but it also would be an irrelevant model of hormone resistance since it is intrinsically hormone unresponsive. The principle is that any rational model of hormone resistance must support, and exhibit, the potential for acquired hormone-therapeutic resistance.

Finally, I note that robust evidence supports that fulvestrant is non-inferior in efficacy to aromatase inhibitors (AIs), themselves demonstrably superior to all SERMs (tamoxifen, etc.), and furthermore can be deployed to help overcome acquired hormone resistance. The confusion here may be the use of sub-optimal dosing at 250 mg, but now that 500 mg dosing has been FDA approved, the true strength of this exceptional antiestrogen is beginning to be appreciated (the final 2014 CONFIRM results show a 19% reduction in mortality from the double dose over 250 mg). Admittedly, FULV+AI combination regimens have not shown clinically relevant advantage over monotherapy, but as to this poor response additive to AI therapy, FACT and other trials have only tested the sub-optimal dosing of 250 mg. Finally, data suggest (Kaniklidis C. Fulvestrant loading, tumor markers, and ongoing trials. Comm Oncol 2007) that fulvestrant deployment is often abandoned prematurely (it requires running out to at least 12 - 14 weeks, as late response is not uncommon).

Lucio and Constantine,

Thank you very much for your answers. Now, let’s test if I understand everything correctly:

1. cells expressing appropriate steroid receptor and requiring exogenous steroid hormone for growth and proliferation are hormone sensitive

2. cells expressing appropriate steroid receptor and not requiring exogenous steroid hormone for growth and proliferation but still sensitive to antiestrogen (fulvestrant) are hormone independent

2. cells expressing appropriate steroid receptor and not requiring exogenous steroid hormone for growth and proliferation and which resistant (unresponsive) to any endocrine therapy are hormone resistant (or hormone insensitive)

Konstantin:

Below I modify and clarify your restatements:

[Konstantin Panov: ] Thank you very much for your answers. Now, let’s test if I understand everything correctly:

1. cells expressing appropriate steroid receptor and requiring exogenous steroid hormone for growth and proliferation are hormone sensitive

[Constantine Kaniklidis: ] COMMENTARY:

This is close. What's needed is a refinement: HR-positive (hormone-receptor positive, that is, ER and/or PR expressing) cells that are PREDOMINANTLY and differentially responsive to endocrine (hormonal) therapy, due to the fact that the primary drivers of HR-positive disease are receptor expression-driven (ER, PR, AR - and certain receptors elements, either coactivators and corepressors, therein, as Lucio shrewdly pointed out above concerning ER-alpha - his posting is required reading here), are definitionally hormone-sensitive.

The distinction of exogenous versus endogenous is immaterial (the internal aromatization process for example can produce estrogens from internal androgens). In addition, growth and proliferation in hormone-sensitive cells is not exclusively dependent on steroid hormone malignant processes (for example, pro-VEGF angiogenic processes are contributors, as are several oncogenes, in multiple tumor cells regardless of their hormone sensitivity).

Furthermore, HR-positive cells and strictly hormone-sensitive cells can still be therapeutically responsive to non-endocrine therapies (like certain chemotherapies and targeted biological therapies). What is definitionally critical for hormone-sensitivity of HR-positive cells is that the most efficacious therapeutic interventions be endocrine therapies, whether SERMS, or aromatase inhibitors (AIs), or pure antiestrogens (fulvestrant), or AR inhibitors (bicalutamide), which does not rule out a modest potential benefit from cytotoxic or biological interventions. So hormone-sensitivity is a matter of optimal therapeutics for targeting HR-positive disease, while HR-positivity is a matter of the malignant drivers being founded in certain receptor overexpression (ER and/or PR and/or AR).

2. cells expressing appropriate steroid receptor and not requiring exogenous steroid hormone for growth and proliferation but still sensitive to antiestrogen (fulvestrant) are hormone independent.

[Constantine Kaniklidis: ] COMMENTARY:

The distinction is not clinically relevant: fulvestrant as a pure antiestrogen only differs from SERMS and AIs via the mode of antiestrogen activity: whereas SERMS competitively bind via ligands the estrogen and progesterone receptors, for example, therefore preempting such binding by estrogens and progesterones, and AIs inhibit the production of estrogens by inhibiting the process (aromatization) of the conversion of androgen to estrogens, fulvestrant is said to "destroy" the estrogen receptor in the sense that it changes the conformational shape of the estrogen receptor, degrading it so that the associated ligand (partner for a receptor), estrogen, cannot interlock or bind with the degraded receptor.

2. cells expressing appropriate steroid receptor and not requiring exogenous steroid hormone for growth and proliferation and which resistant (unresponsive) to any endocrine therapy are hormone resistant (or hormone insensitive)

[Constantine Kaniklidis: ] COMMENTARY:

Konstantin, as I noted above, the issue of dependency on endogenous versus exogenous versus endogenous challenge is not relevant here. What matters is the presence, within the class of HR-positive cells, of cells with therapeutic resistance to endocrine therapies, these then being the hormone-RESISTANT (NOT hormone-insensitive) cells.

[Constantine Kaniklidis: ] THE DEVIL IN THE DETAILS

Preclinical studies [1-5] , have already shown that de novo and/or acquired hormone resistance is associated with increased levels of different kinases like EGFR, MAPK and AKT, as well as secondary to bidirectional crosstalk across ER and HER2 signaling pathways [6,7], among still other pro-resistance mechanisms.

REFERENCES

1. Schiff R., Reddy P., Ahotupa M., et al. Oxidative stress and AP-1 activity in tamoxifen-resistant breast tumors in vivo. J Natl Cancer Inst (Bethesda) 2000; 92: 1926-1934.

2. Shou J., Weigel NL, Osborne CK, Schiff R. Onset of endocrine resistance in breast cancer is associated with increased active p38 MAPK . 24th San Antonio Breast Cancer Symposium 2001. Abstract 254.

3. Masamura S, Santner SJ, Heitjan DF, Santen RJ. Estrogen deprivation causes estradiol hypersensitivity in human breast cancer cells. J Clin Endocrinol Metab 1995; 80:2918-2925.

4. Nicholson RI., Gee JMW, Barrow D, Pamment JS, Knowlden JM, McClelland R. Endocrine resistance in breast cancer can involve a switch towards EGFR signaling pathways and a gain of sensitivity to an EGFR-selective tyrosine kinase inhibitor, ZD1839 . AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics 1999.

5. Schiff R, Massarweh S, Shou J, Osborne CK. Breast cancer endocrine resistance: how growth factor signaling and estrogen receptor coregulators modulate response. Clin Cancer Res 2003; 9(1 Pt 2):447S-54S.

6. Osborne CK, Schiff R. Mechanisms of endocrine resistance in breast cancer. Annu Rev Med 2011 Feb 18; 62:233-47.

7. Giuliano M, Trivedi MV, Schiff R. Bidirectional Crosstalk between the Estrogen Receptor and Human Epidermal Growth Factor Receptor 2 Signaling Pathways in Breast Cancer: Molecular Basis and Clinical Implications. Breast Care (Basel) 2013; 8(4):256-262.

For example, prostate cancer cell lines PC-3 and Du145 do not express androgen receptor (AR) and they are androgen independent cells instead of LNCaP cells. In case of castration resistant prostate cancer, prostate cancer cells still grow and metastasize in absence of the hormone. Some cells have different mutation in AR and became very resistant to drug treatment. The same scenario for breast cancer cells. Based on ER expression, PR and HER, triple negative cells are very resistant to the drug treatment. Overall, you should look for the best cell model that can fit your experimental design!